This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Copyright Permission Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Gromada, J. Articles by Wollheim, C. B. Search for Related Content PubMed PubMed Citation Articles by Gromada, J. Articles by Wollheim, C. B.

-Cells of the Endocrine Pancreas: 35 Years of Research but the Enigma Remains

Novartis Institutes for BioMedical Research (J.G.), Cambridge, Massachusetts 02139; and Department of Cell Physiology and Metabolism (I.F., C.B.W.), University Medical Centre, 1211 Geneva 4, Switzerland

Correspondence: Address all correspondence and requests for reprints to: Jesper Gromada, Novartis Institutes for BioMedical Research, 100 Technology Square, Cambridge, Massachusetts 02139. E-mail: jesper.gromada{at}novartis.com

	Abstract

Top Abstract I. Introduction II. Islet Endocrine Cell... III. Paracrine, Autocrine, and... IV. Autonomic Regulation of... V. Transcriptional Control of... VI. The Glucagon Gene:... VII. {alpha}-Cell Stimulus... VIII. {alpha}-Cell... IX. Summary and Conclusions Note Added in Proof References

 
Glucagon, a hormone secreted from the -cells of the endocrine pancreas, is critical for blood glucose homeostasis. It is the major counterpart to insulin and is released during hypoglycemia to induce hepatic glucose output. The control of glucagon secretion is multifactorial and involves direct effects of nutrients on -cell stimulus-secretion coupling as well as paracrine regulation by insulin and zinc and other factors secreted from neighboring ß- and -cells within the islet of Langerhans. Glucagon secretion is also regulated by circulating hormones and the autonomic nervous system. In this review, we describe the components of the -cell stimulus secretion coupling and how nutrient metabolism in the -cell leads to changes in glucagon secretion. The islet cell composition and organization are described in different species and serve as a basis for understanding how the numerous paracrine, hormonal, and nervous signals fine-tune glucagon secretion under different physiological conditions. We also highlight the pathophysiology of the -cell and how hyperglucagonemia represents an important component of the metabolic abnormalities associated with diabetes mellitus. Therapeutic inhibition of glucagon action in patients with type 2 diabetes remains an exciting prospect.

I. Introduction

II. Islet Endocrine Cell Composition

A. Islet microcirculation

B. Junctional communication between -cells

C. Islet innervation

III. Paracrine, Autocrine, and Hormonal Regulation of Glucagon Secretion

A. Insulin

B. Zinc

C. GABA

D. Glutamate

E. Somatostatin

F. Ghrelin

G. GLP-1

H. Glucagon

IV. Autonomic Regulation of Glucagon Secretion

V. Transcriptional Control of Pancreatic -Cell Development

VI. The Glucagon Gene: Transcriptional Control and Proglucagon Processing

VII. -Cell Stimulus Secretion Coupling

A. Ion channels present in the -cell plasma membrane

B. Regulation of electrical activity in rat -cells

C. Regulation of electrical activity in mouse -cells

D. Metabolism of the -cell

E. Intracellular Ca²⁺ homeostasis

F. Regulation of exocytosis of glucagon-containing granules

G. Pharmacology

VIII. -Cell Pathophysiology and the Treatment of Diabetes

IX. Summary and Conclusions

	I. Introduction

Top Abstract I. Introduction II. Islet Endocrine Cell... III. Paracrine, Autocrine, and... IV. Autonomic Regulation of... V. Transcriptional Control of... VI. The Glucagon Gene:... VII. {alpha}-Cell Stimulus... VIII. {alpha}-Cell... IX. Summary and Conclusions Note Added in Proof References

 
THE HISTORY OF glucagon begins with that of insulin. In 1921, when F. Banting and C. Best tested their first pancreatic extracts in depancreatized dogs, they observed that insulin-induced hypoglycemia was preceded by a transient, rather mild hyperglycemia, and they thought that this unwanted effect was due to epinephrine release (1). Murlin et al. (2) must be credited with the discovery of glucagon in 1923, because they suggested that the early hyperglycemic effect of the pancreatic extracts was due to a contaminant with glucogenic properties that they also proposed to call "glucagon," or the mobilizer of glucose. In a classical paper published in 1948, Sutherland and de Duve (3) established the -cells of the pancreas as being the source of glucagon. At about the same time, Foá et al. (4, 5, 6) conducted elegant cross-circulation experiments in anesthetized dogs and suggested that hypoglycemia was triggering the release of glucagon by the pancreas. The description by Unger et al. (7, 8, 9) between 1959 and 1962 of a glucagon RIA made it possible to investigate the physiology of glucagon and its role in various diseases and disorders. Glucagon is a sensitive and timely regulator of glucose homeostasis in vivo in both animals and humans. Small doses of glucagon are sufficient to induce rapid but transient glucose elevations consistent with its role as a counterregulatory hormone (10, 11, 12). To increase blood glucose, glucagon promotes hepatic glucose output by stimulating glycogenolysis and gluconeogenesis and by decreasing glycogenesis and glycolysis in a concerted fashion via multiple mechanisms.

The physiological defenses against falling plasma glucose concentrations include decreased pancreatic ß-cell insulin secretion as well as increased glucagon and adrenomedullary epinephrine secretion (13) (Fig. 1). Under normal physiological conditions, the intraislet paracrine and endocrine interactions, especially reduced intraislet insulin and zinc release from the pancreatic ß-cells, promote glucagon secretion. A direct stimulatory effect of low glucose on the -cell seems of little physiological importance and, at least for the rat -cell, high glucose augments glucagon release (14, 15). In addition to peripheral glucose sensing, an essential role of glucose-responsive neurons in the ventromedial hypothalamus (VMH) has been established for the regulation of glucagon secretion and for glucose homeostasis (16, 17, 18). All of these defense mechanisms are compromised in type 1 diabetes and advanced type 2 diabetes. This involves absent insulin response resulting from ß-cell failure and loss of glucagon response. This is probably caused by loss of the decrement in intraislet insulin and/or zinc that normally results in enhanced glucagon secretion. In the setting of absent insulin and glucagon responses, attenuated epinephrine responses cause the clinical syndrome of defective glucose counterregulation that is associated with a much greater risk of severe hypoglycemia (19).

View larger version (37K): [in this window] [in a new window]   FIG. 1. Physiological defenses against hypoglycemia. It is important to note that the decreased insulin secretion and increased glucagon release are lost and the increase in epinephrine is often attenuated in type 1 and advanced type 2 diabetes. AA, Amino acid.

	II. Islet Endocrine Cell Composition

Top Abstract I. Introduction II. Islet Endocrine Cell... III. Paracrine, Autocrine, and... IV. Autonomic Regulation of... V. Transcriptional Control of... VI. The Glucagon Gene:... VII. {alpha}-Cell Stimulus... VIII. {alpha}-Cell... IX. Summary and Conclusions Note Added in Proof References

 
Pancreatic -cells were discovered in 1907 as histologically distinct cells from the ß-cells of the islet of Langerhans (20). The -cells are one of four distinct polypeptide-secreting islet cell types: glucagon-secreting -cells, insulin-producing ß-cells, somatostatin-releasing -cells, and pancreatic polypeptide (PP)-secreting cells. Recently, ghrelin-producing cells have also been observed in the islet (21, 22, 23). A recent study has shown that ß-, -, and -cells are scattered throughout the human islet (24). Thus, human islets do not show the anatomical subdivisions like rodent islets where the ß-cells are concentrated in the core of the islet, and - and -cells are located in the mantle (24). The cytoarchitecture of the human islet, where most of the ß-cells (71%) showed associations with other endocrine cells, suggest unique paracrine interactions. Most of the ß-, -, and -cells in human islets were aligned along blood vessels with no particular order or arrangement, suggesting that islet microcirculation likely does not determine the order of paracrine interactions (24). This would contrast with the situation in the rat islet (25) but needs to be demonstrated directly. In type 1 diabetic patients, where ß-cells are lost, -cells comprise approximately 75% of the total cell number (26), although the absolute mass (and therefore the ratio) of the -, -, and PP cells does not appear to be altered (27). However, in type 2 diabetes, -cell hyperplasia occurs (27), whereas the ß-cell mass is probably reduced as a result of increased apoptosis (28). Early studies also associated an increase in -cell number with age (29).

A. Islet microcirculation
Anatomical studies have shown that islets are densely vascularized, with at least one arteriole supplying every islet (30). This would permit simultaneous exposure of islets to changes in arterial milieu and rate of flow (31). In vivo studies in rat using fluorescent microscopy to monitor the flow of microspheres or albumin in a single islet located in the head of the pancreas indicated that the blood supplied by the arteriole flows first into capillaries located in one pole of the islet mantle, then traverses the islet core either directly or via the mantle to the opposite pole of the islet, where it exits via venules (32). Such a direction of flow would provide for paracrine actions of both - and -cell secretory products on downstream ß-cells and subsequent effects of ß-cell secretory products on cells located in the mantle of the venular pole. However, this possibility is not supported by physiological studies in the isolated pancreata of rats and humans, where perfusion with antisomatostatin antibodies (somatostatin is an inhibitor of both insulin and glucagon secretion) in the anterograde direction had no effect on insulin or glucagon secretion (33, 34, 35). In contrast, an increase in both glucagon and insulin secretion was observed when the antibody was perfused through the pancreata in the retrograde direction, suggesting that the -cells are in fact downstream of both - and ß-cells. This was supported by early anatomical studies in the rat islet showing arterial supply first to the ß-cell enriched core (25). In a separate study, perfusion of the human pancreas with an antibody against somatostatin in the anterograde direction had a very small but significant stimulatory effect on insulin secretion, and also glucagon secretion, the latter in low glucose conditions only, in support of the pole-to-pole direction of blood flow (36). A thorough anatomical examination of islet microvascular flow in different regions of the pancreas is now timely. This is particularly pertinent in the case of the mouse, where information is severely lacking, and may provide answers to the conflicting findings from the early anatomical vs. physiological studies.

B. Junctional communication between -cells
We have noted that, when measured as a percentage of content, the amount of glucagon released from fluorescence-activated cell sorted (FACS) -cells is much greater than that from intact islets (7.6 ± 0.9%, n = 9, 30-min incubation; vs. 0.53 ± 0.1%, n = 7, 1-h incubation, respectively, in the presence of 2.5 mM glucose). A high rate of basal glucagon secretion was also observed from dispersed (unsorted) islet cells (6.1 ± 1.1% of content, n = 4, 30-min incubation), indicating that the loss of intercellular contacts underlies the increased rate of basal secretion, rather than the absence of or reduction in exposure to paracrine inhibitory factors (I. Franklin and C. B. Wollheim, unpublished data). In isolated -cells, the increased basal secretion rate was not reduced by the removal of extracellular Ca²⁺ (14). To investigate whether junctional communication exists between neighboring -cells and whether these contacts are indeed required for normal rates of basal glucagon release, as previously observed for insulin secretion in ß-cells (37, 38), we attempted to reform contacts between cells by reaggregating the isolated -cell fraction. Reaggregating the cells caused a 10-fold reduction in the rate of glucagon release in basal glucose conditions, without altering the response to the secretagogue pyruvate, indicating that intercellular contacts may be necessary for normal basal secretion. Doubling the density of cells per well from 20,000 to 40,000 did not alter the rate of basal glucagon release, demonstrating that the altered rate of glucagon release from the reaggregated cells was not due to an increase in the local concentration of secreted autocrine factors. Immunocytochemical analysis of the reaggregated cell fraction did not reveal positive staining for either of the gap junction proteins connexin 36 or connexin 43 (I. Franklin and C. B. Wollheim, unpublished data), although the involvement of other members of the connexin family in the formation of gap junctions between rat -cells cannot be excluded. Connexin expression has been detected in FACS-isolated non-ß-cells from rats and mice (39, 40). Gap junctions, composed of connexin 36, are required for normal glucose-induced insulin secretion by rat ß-cells (38, 41). An understanding of the mechanisms regulating basal rates of glucagon secretion may lead to the identification of novel drug targets for controlling hyperglucagonemia in diabetes.

C. Islet innervation
Pancreatic islets are richly innervated to enable autonomic regulation of endocrine cell hormone secretion. The most extensively studied are the sympathetic (adrenergic) and parasympathetic (cholinergic) nerves, which can project deeply into the islet, but other types of sensory neurons have also been detected, including GABAergic nerve bodies (42). The sympathetic nervous system is activated by hypoglycemia or exercise stress, causing the release of norepinephrine and other neurotransmitters as well as neuropeptides into the islet (for review, see Ref. 43). These agents trigger glucagon secretion from -cells. In the case of norepinephrine and epinephrine, this occurs via the ₁- and ß-adrenoceptors (44, 45). There is evidence to suggest that autonomic blockade to prevent sympathetic activation results in a blunted -cell response to hypoglycemia in different species, including man (for review, see Ref. 43), although this point is still debated with a large body of evidence to the contrary. For example, the denervated dog pancreas (Ref. 46 and references therein) and the denervated human pancreas (47) secrete glucagon in response to hypoglycemia.

Norepinephrine and epinephrine inhibit secretion from ß- and -cells. There is some evidence that hyperglycemia will suppress sympathetic neuronal activity, reducing islet glucagon output (for review, see Ref. 48). The islets are also richly supplied with parasympathetic fibers originating from intrapancreatic, cholinergic ganglia (43, 49). Activation of the parasympathetic nervous system during hyperglycemia triggers insulin secretion from ß-cells, as well as somatostatin and PP release. At least four different neurotransmitters (acetylcholine, vasoactive intestinal polypeptide, pituitary adenyl cyclase-activating polypeptide, and gastrin-releasing polypeptide) of the parasympathetic system can stimulate glucagon secretion from islet -cells (for review, see Ref. 43), although their relative contribution to enhancing glucagon release in vivo remains to be elucidated.

	III. Paracrine, Autocrine, and Hormonal Regulation of Glucagon Secretion

Top Abstract I. Introduction II. Islet Endocrine Cell... III. Paracrine, Autocrine, and... IV. Autonomic Regulation of... V. Transcriptional Control of... VI. The Glucagon Gene:... VII. {alpha}-Cell Stimulus... VIII. {alpha}-Cell... IX. Summary and Conclusions Note Added in Proof References

 
Since the early 1970s, the mechanism underlying the regulation of glucagon secretion by glycemia has puzzled scientists. The debate continues whether -cells directly sense and respond to fluctuations in plasma glucose or whether the response is mediated by the autonomic nervous system and/or the paracrine/endocrine effects of secretory products from other islet cell types. Currently, a large body of research favors the latter "paracrine/endocrine" hypothesis. Although neuronal modulation of -cell activity is certainly operative, this is probably secondary to the inhibitory effects of ß-cell secretory products during hyperglycemia (50) and is unlikely to underlie the dysregulated -cell activity associated with the onset of type 1 diabetes, when ß-cell function begins to fail (51).

Clinical studies have shown that in type 1 diabetes, where normal ß-cell function is markedly impaired, glucose (administered either iv or orally) can actually stimulate glucagon secretion (52). A similar finding was reported for the alloxan-treated diabetic dog (53). In vitro, glucose can stimulate glucagon secretion from the dog pancreas when perfused in the retrograde direction (when ß-cells are down-stream of -cells) (54). Glucagon secretion from FACS-purified rat -cells is stimulated rather than inhibited by glucose (14, 15). In intact mouse islets, in which physiological glucose concentrations (like in rat islets) suppress glucagon secretion, supraphysiological glucose levels were recently reported to stimulate glucagon secretion (55). In accord with the observation that the absence of ß-cell secretory products leads to -cell hyperactivity, the glucagon secretory response to hypoglycemia is blunted in infants suffering from hyperinsulinemic hypoglycemia (56). Moreover, whereas pyruvate stimulates glucagon but not insulin secretion in intact rat islets, the -cell secretory response is lost after transduction of the ß-cells with the monocarboxylate transporter 1 (MCT-1). This is explained by the engineered pyruvate stimulation of ß-cell exocytosis leading to suppression of glucagon secretion (57). Likewise, malate and succinate stimulated glucagon secretion in -cells engineered to express the Na⁺-dependent dicarboxylate transporter-1, whereas the concomitant expression of Na⁺-dependent dicarboxylate transporter-1 in ß-cells within intact islets prevented glucagon release (58).

Thus, caution should be taken when interpreting the results of glucagon secretion experiments because -cells appear to be highly sensitive to ß-cell secretory products. This is true for clonal -cell lines that rarely display a pure phenotype (59). It is also pertinent for intact or dispersed islets, analyzed in a superfusion system, where the influence of "contaminating" ß-cell secretory products cannot be excluded. In all of these cases, high glucose continues to be associated with a reduction in -cell activity (60). Of interest, an early investigation into the effects of starvation on hormone release from the perfused rat or mouse pancreas revealed an alternative mechanism for the inhibition of glucagon release by high glucose that was independent of the usual requirement for ß-cell secretory products (61). This may reflect an adaptive mechanism involving the local neural network, -cells, or even the -cells themselves.

We use the terms "paracrine/endocrine" to encompass local interactions between different cells within the same islet. These interactions occur via the interstitium (paracrine) and/or the microcirculatory system (endocrine). It remains very difficult to discern between these two conduits when discussing intraislet signaling, although the release of ß-cell secretory products into the interstitium has been documented (62). Numerous factors modulate glucagon secretion, but in this review we will focus only on the major physiological regulators.

A. Insulin
Insulin has long been considered the most likely candidate of the plethora of ß-cell products secreted in response to high glucose to have an inhibitory paracrine effect on -cell activity (63). Particularly convincing are those studies where the action of endogenous insulin is: 1) blocked by antibody inclusion in in vitro studies in rat (14, 63, 64) and human (36), resulting in increased glucagon secretion (these data do not exclude the involvement of other paracrine factors in the regulation of glucagon release); 2) lost in islets of sulfonylurea receptor subunit 1 (SUR1)-deficient mice (SUR1 is a component of ATP-sensitive K⁺-channels), which no longer display glucose-stimulated insulin secretion or suppression of glucagon release (65, 66); or 3) diminished in alloxan diabetic minipigs, leading to a reduction of postprandial insulin-driven suppression of glucagon secretion (67). In addition, exogenously added insulin has been shown to inhibit glucagon secretion from: 1) the perfused pancreas of streptozotocin-treated rats (68); 2) alloxan-treated diabetic dogs (53, 69) and streptozotocin-treated hamsters (70); 3) islets of streptozotocin-treated guinea pigs (71); 4) isolated rat -cells (14, 60); and 5) rodent islets in low but not high glucose conditions (14, 60).

In humans, it has been reported that intraislet hyperinsulinemia prevents the glucagon response to hypoglycemia (despite an intact autonomic response) (72). It was also shown that suppression of baseline insulin secretion, abolishing the decrement in intraislet insulin during the induction of hypoglycemia, reduced the glucagon response to hypoglycemia (73). Furthermore, increasing the decrement in insulin secretion improves glucagon responses to hypoglycemia in advanced type 2 diabetes (74). These observations lead to the ß-cell "switch-off" hypothesis, suggesting that a sudden cessation of insulin secretion from the ß-cells into the portal circulation of the islet during hypoglycemia is a necessary signal for the glucagon response from downstream -cells. Support for the ß-cell switch-off hypothesis has also been obtained in streptozotocin-treated rats (75) as well as in isolated rat and human islets and islets from streptozotocin-administered rats (76). It has been argued that in several of these studies supraphysiological concentrations of exogenous insulin were required to see an effect on glucagon release. However, a physiological concentration of insulin (0.3 mU/ml) was sufficient to inhibit glucagon secretion (by about 30%) from the rat pancreas perfused in the retrograde direction in the presence of 5 mM glucose, a condition in which downstream effects of endogenous insulin are negligible (54).

Recent investigations have provided some insight into the mechanism by which insulin inhibits -cell glucagon secretion. The transcript encoding the insulin receptor is highly abundant in rat -cells, similar to another major insulin target tissue, the liver (14). Insulin receptors are also expressed in TC6 and In R1 G9 cells, and glucagon secretion was decreased with the addition of insulin in both cell types (77). Insulin transiently inhibits electrical activity and glucagon secretion in isolated rat -cells, most probably by a signaling pathway that triggers activation of ATP-sensitive K⁺-channels (K_ATP-channel) and thus membrane hyperpolarization (14). This is supported by evidence in mouse -cells where insulin activates K_ATP-channels by markedly reducing the sensitivity of the channels to ATP, an effect that is mediated via the phosphatidylinositol 3-kinase (PI3K) signaling pathway (78). Exogenously added insulin also inhibits the spontaneous Ca²⁺ oscillations observed under low glucose conditions in -cells from dissociated mouse islets (60). A similar effect was observed in TC1–9 cells and was prevented by the inclusion of the PI3K inhibitor wortmannin (60). Insulin is also known to activate K_ATP-channels in isolated mouse ß-cells (79) and in rodent hypothalamic neurons by PI3K-dependent pathways (80, 81). Insulin has been reported to activate GABA_A receptors in the -cells by receptor translocation via an AKT kinase-dependent pathway. This leads to membrane hyperpolarization in the -cell and suppression of glucagon secretion (82). The relative contribution of K_ATP-channel activation and GABA_A receptor translocation and activation to insulin-induced inhibition of glucagon release is currently unknown. Consistent with the idea that the inhibitory effect of insulin on glucagon secretion involves modulation of ion channel activity rather than granular exocytosis, epinephrine-evoked glucagon secretion from isolated rat -cells was inhibited by exogenous insulin via a cAMP-independent mechanism (83).

B. Zinc
A relative newcomer to the islet paracrine signaling scene is the metal ion Zn²⁺. Zinc cocrystallizes with insulin in ß-cell granules (84, 85, 86) and is secreted from rat (14) and mouse (87) ß-cells on exposure to high glucose (1 pmol/islet/h, a 6.5-fold increase over basal levels) (14). High local concentrations of Zn²⁺ (µM) are therefore anticipated within the islet microvasculature during hyperglycemia. The possibility that ß-cell-derived Zn²⁺ could have inhibitory effects on rat -cell glucagon release was first raised in 2003 (57) when it was found that the Zn²⁺ chelator Ca²⁺-EDTA permitted a stimulatory effect of the mitochondrial substrate monomethylsuccinate on glucagon secretion in the perfused rat pancreas, without altering the stimulated rate of insulin secretion. This was substantiated by the demonstration that exogenous Zn²⁺ reversibly inhibited pyruvate- or glucose-stimulated glucagon release from isolated rat -cells, and a mechanism for this effect has now been provided (14). Zn²⁺ can reversibly activate K_ATP-channels (EC₅₀ = 2.2 µM) in isolated rat -cells, concordantly reducing electrical activity and glucagon secretion.

Zn²⁺ has been shown to play a similar role in the central nervous system (CNS), where low concentrations can activate K_ATP-channels to restore membrane potential and inhibit glutamate release in rat neurons, playing a potentially neuroprotective role (88). Activation of K_ATP-channels in the ß-cell lines RINm5F and INS-1E by Zn²⁺ (EC₅₀ = 1.7 µM) has also been demonstrated (89, 90), and the site of action has recently been located to several histidine residues on the extracellular side of the SUR1 subunit of the K_ATP-channel (90). Interestingly, exogenously added Zn²⁺ (30 µM) has no effect on glucagon release from mouse islets in static incubations in the presence of low or high glucose (60), although Zn²⁺ (0.3–3 µM) appears to have an inhibitory effect on glucagon release from the perfused mouse pancreas in low glucose conditions (I. Franklin and C. B. Wollheim, unpublished observations). The effect of Zn²⁺ on glucagon release from human -cells remains to be investigated. The apparent convergence of the mechanisms behind both insulin and Zn²⁺ inhibition of glucagon secretion on K_ATP-channel activity is of interest and supports the idea that modulation of ion channel activity would permit paracrine signals to have a rapid and precise effect on hormone secretion (91).

C. GABA
-Aminobutyric acid (GABA) is a major nonpeptidal neurotransmitter that inhibits neuronal firing in the CNS, where it acts on two types of receptors (92). Activation of GABA_A receptors typically induces an inward Cl^– current that causes cell inhibition by hyperpolarization. Activation of GABA_B receptors usually leads to cell inhibition through closure of Ca²⁺-channels or opening of K⁺-channels (92). GABA is also produced in the ß-cells of the endocrine pancreas, where it is taken up into synaptic-like microvesicles and secreted in a regulated manner (for review, see Ref. 93). Although the circumstances under which ß-cell GABA is released only began to be elucidated recently (94, 95), it has been reported that rat islet GABA directly inhibits glucagon secretion in static assays by hyperpolarizing the -cell plasma membrane after activation of the GABA_A receptor (96). Exogenous GABA inhibits arginine-induced glucagon secretion in mouse (97) and guinea pig islets (98) as well as in isolated rat -cells (15) and TC6 cells (99). GABA_A receptors have been identified in guinea pig and rat -cells but not in rat ß- and -cells (96, 98). GABA_A receptors are heteromultimeric channels typically composed of two , two ß, and a varying third subunit. RT-PCR analysis detected transcripts of ₁ and ₄ as well as ß_1–3 GABA_A receptor subunits in rat -cells (96). It has been proposed that activation of the -cell GABA_A-receptor channel by GABA released from adjacent ß-cells in response to glucose may provide a mechanism for paracrine control of glucagon secretion (96, 98). However, when purified rat ß-cells were incubated over 24 h, high glucose concentrations inhibited GABA release into the medium, thus dissociating GABA from the stimulation of insulin secretion (100). The action of glucose is due to inhibition of mitochondrial GABA transaminase and is probably immediate (101). These results argue against a primary role for ß-cell GABA in glucose suppression of glucagon secretion in islets.

GABA_B receptor subunits have also been detected in preparations of purified rat -cells using RT-PCR (94). However, the GABA_B receptor agonist baclofen affected neither glucagon secretion from isolated rat islets nor epinephrine-stimulated exocytosis in single rat -cells (102). Further experimentation is required to determine whether protection from GABA receptor activation can alleviate inhibition of glucagon secretion during high glucose conditions in the perfused pancreas and in vivo to substantiate the physiological importance of intraislet GABA in the suppression of glucagon secretion.

D. Glutamate
L-Glutamate is the major excitatory neurotransmitter in the CNS. Endocrine cells of pancreatic islets possess glutamatergic signaling features of their own (103). -, ß-, and -Cells, to varying extents, appear to have the ability to produce, secrete, and respond to L-glutamate. L-Glutamate is produced in ß-cell mitochondria by the enzyme glutamate dehydrogenase and by cytosolic glutaminase as well as transaminase reactions in both cellular compartments. In addition to an intracellular signaling role (104), ß-cell glutamate probably accumulates in secretory vesicles and could therefore be an important intercellular signaling molecule. Vesicular glutamate transporter subtype 1 and subtype 2 have been detected in -cell secretory granules (105, 106), and indeed a parallel increase in secretion of glutamate and glucagon from intact rat islets, in response to low glucose, has been reported (106). Ca²⁺-dependent exocytosis of L-glutamate has also been observed in TC6 cells (107). Non-ß-cells have the ability to take up extracellular glutamate via a high-affinity glutamate/aspartate transport system (108, 109).

Many of the subunits that comprise the large family of glutamate receptors have been identified in the different islet cells. -Cells express both ionotropic receptors, including the AMPA and kainite subtypes (110), and metabotropic receptors (105, 111). Ionotropic glutamate receptors function as Na⁺ (but not Ca²⁺)-conducting ion channels, and their activation would therefore modulate plasma membrane potential and glucagon secretion. G protein-coupled metabotropic receptors modulate cellular cAMP levels in neurons and are typically associated with autocrine feedback inhibition of L-glutamate signaling. ß-Cells in rat and human islets also express a wide range of glutamate receptor subtypes (105, 112, 113, 114, 115, 116), and ionotropic receptor subunits have also been detected in rat -cells (117).

Given the potential complexity of intercellular glutamate signaling in the islets, what role can we anticipate glutamate playing in the regulation of -cell glucagon secretion? Activation of AMPA or kainite ionotropic glutamate receptors in low-glucose conditions stimulates glucagon release from the perfused rat pancreas (118). Conversely, at stimulatory glucose concentrations, AMPA and kainite trigger insulin secretion in vivo (119), in the perfused rat pancreas (120), and in isolated rodent islets (109, 113). In dispersed rat islets, AMPA and kainite cause membrane depolarization and Ca²⁺ influx through voltage-dependent Ca²⁺ channels (110, 113). These findings lead us to predict that glutamate cosecreted with glucagon would have a positive feedback effect on glucagon release. Similarly, we can expect glutamate released from ß-cells to promote -cell exocytosis. Although, as discussed elsewhere, ß-cell activation is synonymous with inhibition of glucagon release, any stimulatory effect of ß-cell glutamate may be masked by the inhibitory effect of other cosecreted factors, including insulin and Zn²⁺. Surprisingly, stimulation of -cell metabotropic receptors inhibited glucagon secretion from rat islets in low-glucose conditions (105), in accord with similar findings by other groups (111). The apparently opposing effects of ionotropic and metabotropic glutamate receptor activation on -cell glucagon secretion add another layer of complexity to the puzzle of intraislet glutamate signaling. Studies are now required to firmly establish the conditions under which glutamate is released from purified primary islet cells. Follow-up analyses of glutamate (and its receptor subtype-specific analogs) effects on electrical activity, cytosolic Ca²⁺ levels, and hormone secretion should be performed in isolated purified cells. Eventually, tissue-specific glutamate receptor mutagenesis studies in more intact systems such as the perfused rat pancreas may yield definitive information on the role of islet glutamate as a paracrine/autocrine effector of glucagon secretion.

E. Somatostatin
Somatostatin is a peptide hormone produced by neural and endocrine tissues, including the -cells of the endocrine pancreas. It is well established that somatostatin is a potent inhibitor of glucagon and insulin secretion in rat, dog, and human (121, 122). Somatostatin might inhibit - and ß-cell activity via the local islet microcirculatory network and/or the interstitium. These possibilities are discussed below.

Of the five distinct somatostatin receptor subtypes characterized to date, all are coupled to inhibitory G proteins and display a tissue-specific pattern of expression (123). Somatostatin receptors are expressed by islet - and ß-cells, with some conserved differences in subtype specificity. The significance of the subtype specific pattern of expression is unknown. Human, rat, and mouse -cells predominantly express the type 2 somatostatin receptor (124, 125, 126, 127, 128). In the case of human and rat ß-cells, they express predominantly type 1 (124) or type 5 (127) somatostatin receptors, respectively, in preference to type 2. However, there probably is not any absolute cell-type specificity for any particular receptor subtype, and indeed some receptor subtypes appear common to all islet endocrine cells, e.g., somatostatin receptor 5 in human islets (124).

There are at least three different mechanisms by which -cell somatostatin receptor activation leads to inhibition of glucagon secretion. Electrophysiological recordings showed that somatostatin activated G protein-coupled K⁺-channels in single rat and mouse -cells, causing hyperpolarization of the plasma membrane and inhibition of electrical activity (129, 130). The ability of somatostatin to inhibit -cell exocytosis appears dependent on the activity of a pertussis toxin-sensitive G protein (G_i2) and also calcineurin (131). Somatostatin receptor activation also inhibits adenylate cyclase activity, thereby reducing cAMP levels and protein kinase A (PKA)-stimulated glucagon secretion (132, 133, 134). Specific agonists for the type 2 somatostatin receptor were found to selectively inhibit glucagon secretion from mouse (135) and rat islets without affecting insulin release (136). Moreover, islets of transgenic mice unable to express the somatostatin receptor type 2 exhibited a 2-fold increase in arginine/K⁺-stimulated glucagon secretion, whereas basal glucagon release and glucose-induced insulin secretion did not differ from control in static assays (137). In animal models of type 2 diabetes in the nonfasted state, circulating glucagon and glucose levels were decreased after treatment with a somatostatin receptor subtype-2 agonist. In the fasted state, the agonist lowered blood glucose by approximately 25% (135). The somatostatin receptor subtype-2 agonist did not have any effects on glucagon or glucose levels in somatostatin receptor 2-deficient mice (135). This indicates that the type 2 receptor is required for the action of somatostatin on mouse glucagon secretion. These studies suggest that the inhibitory effects of somatostatin on glucagon secretion are direct and are mediated by the type 2 somatostatin receptor in rodents. However, recent data suggest that more than one somatostatin receptor subtype is likely to be involved in the regulation of glucagon secretion from rat islets (138).

Early studies in the perfused rat pancreas with antibodies against insulin, glucagon, or somatostatin led to the concept of ß directional flow of the islet microcirculation (34). The observation that perfusion with somatostatin antibody in the anterograde direction had no effect on glucagon (or insulin) secretion, but that retrograde perfusion with the antibody increased glucagon release, argued against local -cell action on -cells mediated via the circulatory system. A similar elegant study in the perfused human pancreas arrived at the same conclusion (35), although more recent work by another group showed a small but significant stimulatory effect of somatostatin antibody perfusion on glucagon secretion in low-glucose conditions only (36). However, this approach does not exclude communication via the interstitium, given that the large size of antibodies probably precludes their entry into intercellular spaces (64).

More recent work in the perfused rat pancreas showed increased arginine-stimulated glucagon secretion in the presence of the peptide antagonist DC-41–33 against the type 2 somatostatin receptor (139), which supports the concept of interstitial interaction between neighboring cells. The new wave of receptor agonists and antagonists available may prove invaluable in revealing interstitial communication between islet cells, but care should be taken to confirm that these compounds do not have nonspecific effects on hormone secretion, especially in the absence of the native receptor agonist. For instance, we have observed that the somatostatin type 2 receptor antagonist DC-41–33 (139) strongly stimulated insulin secretion from isolated rat ß-cells, in the absence of contaminating somatostatin (Fig. 2).

View larger version (20K): [in this window] [in a new window]   FIG. 2. Effect of somatostatin receptor type 2 antagonist on hormone secretion from FACS-purified rat - and ß-cells. FACS-isolated rat - or ß-cells were seeded at a density of 20,000 cells per well in polyornithine-coated 24-well plates and cultured overnight. Static assays were performed to determine -cell glucagon secretion (A) or ß-cell insulin release (B) during a 30-min incubation in basal (2.5 mM glucose) or stimulatory (A, 5 mM pyruvate; B, 16 mM glucose) conditions. An antagonist against the type 2 somatostatin receptor (SstR antagonist, 2 µM) (139 ) or an antibody against somatostatin (Sst antibody) was included where indicated. Endogenous somatostatin (10 pM) was detected in the -cell but not ß-cell fraction. The addition of exogenous somatostatin (10 nM) inhibited glucose-stimulated insulin release, confirming that the ß-cell fraction was essentially free of contaminating -cells, as expected (432 ). Data are the mean ± SEM of three different experiments. *, P 0.05.

F. Ghrelin
Ghrelin, a recently described GH-releasing peptide, is produced primarily in rat or human stomach during fasting. Ghrelin also appears to be synthesized in human and rat islets (21, 154). Although, in the case of the rat, there is controversy over the presence of ghrelin-positive cells in the adult islet (22, 23) contrasting with (154, 155, 156), it seems that they are almost certainly present earlier in development, comprising a small subpopulation of cells located at the islet periphery. Whether some of these cells in the rat islet are in fact also glucagon positive remains to be clarified (compare Refs. 154, 156 , and 157 with21 and 22), although in human islets ghrelin-positive cells probably comprise a distinct peripheral subpopulation (21). Such cells have also been detected in the mouse islet (158).

The receptor to which ghrelin binds, GHS-R, is widely expressed in the adult rat islet (22, 154) and has been colocalized with glucagon-positive cells (22). Locally released ghrelin would be anticipated to act in an autocrine/paracrine fashion on -cell glucagon release, intuitively perhaps promoting glucagon release during hypoglycemia. However, studies in perfused rat pancreas indicate that concentrations of ghrelin within the physiological range (0.5–3 nM) had no effect on basal or arginine-stimulated glucagon release, despite mild inhibition of both arginine-induced somatostatin release and glucose-induced insulin secretion (159). Perhaps, under the conditions tested, locally released ghrelin was already exerting a maximal effect on -cell activity, masking any further stimulatory effect of exogenously added ghrelin. Contrasting findings have been reported in mouse, where exogenously added ghrelin increased glucagon release from islets in static assays at all glucose concentrations tested (160), although under such assay conditions indirect effects cannot be excluded. In the same study, however, ghrelin (10 nM per kg body weight) did not raise basal plasma glucagon and indeed impaired the glucagon secretory response to carbachol, 3-isobutyl-1-methylxanthine, and arginine, suggesting an inhibitory effect on secretion. GHS-R expression has not yet been reported in mouse islets. The physiological relevance of islet ghrelin needs more investigation.

Interestingly, another peptide encoded by the ghrelin gene has recently been identified. The peptide is named obestatin, and contrary to the appetite-stimulating effects of ghrelin, treatment of rats with obestatin suppressed food intake, inhibited jejunal contraction, and decreased body weight gain (161). These effects of obestatin are mediated via the orphan G protein-coupled receptor GPR39 (161). It will be interesting to investigate whether obestatin is produced and secreted from ghrelin-producing islet cells. The GPR39 receptor is expressed in pancreas (162), and future research should be directed to understanding the potential role of obestatin as well as GPR39 in islet function.

G. GLP-1
Glucagon-like-peptide (GLP)-1 is an incretin released from the L cells in the small intestine after meal ingestion. One of the primary actions of GLP-1 is the augmentation of glucose-induced insulin secretion, directly by activation of GLP-1 receptors expressed on ß-cells and indirectly via the autonomic nervous system (for review, see Ref. 163). GLP-1 is currently under intensive investigation for its potential application as an antidiabetogenic peptide. Evidence suggests that GLP-1 may also have suppressive effects on glucagon release from -cells (for review, see Ref. 164). Any inhibitory effect of GLP-1 on glucagon release may involve direct (via GLP-1 receptor expression in -cells) and/or indirect mechanisms. An indirect action could be mediated by the stimulatory effect of GLP-1 on neighboring ß- or -cells, causing intraislet paracrine inhibition of glucagon release.

Clinical studies have demonstrated that patients with type 1 diabetes have decreased plasma glucagon levels in response to iv application of GLP-1 after an overnight fast (165). However, a small increase in plasma C-peptide levels was also detected, so an indirect effect of GLP-1 via activation of residual ß-cell activity cannot be ruled out. In a different study, patients with type 1 diabetes infused acutely with insulin or insulin plus GLP-1, under euglycemic clamped conditions (5.3 mM glucose), did not demonstrate increased inhibition of plasma glucagon levels in the presence of GLP-1 (166). The possibility that the inhibitory effect of insulin alone was already maximal cannot, however, be excluded. Long-term (5 d) administration of GLP-1 to type 1 diabetic patients caused a small reduction in postprandial plasma glucagon levels (167). Studies in perfused rat pancreas (168) have shown that GLP-1 alone can inhibit glucagon secretion normally associated with a decrease in glucose concentration (11 to 3.2 mM glucose), without effecting insulin or somatostatin release, and thereby largely excluding the possibility of indirect GLP-1 action via a paracrine mechanism.

GLP-1 receptor expression in human -cells has not been investigated. Conflicting reports have been published for the rat. Neither GLP-1 receptor nor its transcript could be detected in purified rat -cells (14, 169). Direct GLP-1 application to these cells did not alter glucagon secretion (14) or cause an increase in cAMP levels (in contrast to glucose-dependent inhibitory peptide, another glucoincretin) (169). However, GLP-1 receptor expression was detected by immunocytochemistry in a subpopulation (20%) of glucagon-positive cells in dispersed rat islets (170), and GLP-1 caused an increase in the rate of exocytosis in single rat -cells (171, 172). GLP-1 was also recently reported to elicit an increase in the cAMP content of isolated rat -cells (172). On the other hand, spontaneous Ca²⁺ oscillations detected in mouse -cells of dispersed islets were abolished by the application of GLP-1 (173).

Evidence for a primarily paracrine mechanism in GLP-1 inhibition of glucagon release in the mouse has recently come to light. In static islet assays, GLP-1 had a strong suppressive effect on glucagon release (65), and plasma glucagon levels were inhibited by administration of GLP-1 (174). In both cases, a loss of ß-cell responsiveness to GLP-1, either by global knockout of the K_ATP-channel subunit SUR1 (65) or ß-cell specific knockout of the transcription factor Pdx-1 (174), resulted in a loss of -cell responsiveness to GLP-1. It should be noted that, in the former case, the requirement for functional -cell K_ATP-channels for a direct inhibitory effect of GLP-1 on glucagon release cannot be ruled out (65).

The third, and as yet unexplored possibility, is that GLP-1 inhibition of glucagon secretion is mediated by local neural networks. GLP-1 is rapidly degraded in the circulatory system. Indeed, reports indicate that probably a substantial component of the potentiating effect of GLP-1 on glucose-induced insulin secretion is mediated by the autonomic nervous system in rats (175) and mice (163, 176), possibly via GLP-1 activation of the vagal hepatic nerves (177, 178). Neuronal control of glucagon secretion is well documented (see below) and could provide an explanation for the disparity between in vivo or pancreatic perfusion data (local neural networks intact) and the apparent absence (or at least very weak expression) of the GLP-1 receptor on -cells. We refer the reader to a recent review by Dunning et al. (164) for effects of GLP-1 on -cell function and glucagon release in healthy subjects and in diabetic patients.

H. Glucagon
Glucagon stimulates insulin and somatostatin secretion from perfused rat and human pancreata (36, 148). The autocrine effect of secreted glucagon on rat and mouse -cell activity has only recently been examined (172). The authors demonstrated glucagon receptor expression and a stimulatory effect of glucagon on cAMP levels and exocytosis in isolated mouse and rat -cells, suggesting a positive feedback effect. Transgenic mice unable either to produce glucagon (pro-convertase 2 knock-out) (179) or to express the glucagon receptor (180) exhibit -cell hyperplasia, indicating that either the loss of autocrine action or perhaps the absence of glucagon action in target tissues leads to -cell hyperplasia. Interestingly, in antisense oligonucleotide studies where glucagon receptor expression was specifically reduced in mouse liver, an increase in glucagon release from -cells in vivo was detected in the absence of an increase in -cell mass (181). The signal that triggers -cell hyperplasia may therefore indeed be autocrine, rather than the result of reduced glucagon action on the liver per se. PP cells substitute for glucagon-positive cells in the islets of the ventral rat pancreas (182). However, no effect of exogenous PP on glucagon secretion from the perfused rat pancreas was observed in low- or high-glucose conditions, despite an inhibitory effect on insulin secretion (83, 183).

	IV. Autonomic Regulation of Glucagon Secretion

Top Abstract I. Introduction II. Islet Endocrine Cell... III. Paracrine, Autocrine, and... IV. Autonomic Regulation of... V. Transcriptional Control of... VI. The Glucagon Gene:... VII. {alpha}-Cell Stimulus... VIII. {alpha}-Cell... IX. Summary and Conclusions Note Added in Proof References

 
Increased glucagon release represents the most important glucose counterregulatory factor that is critical to prevent or rapidly correct hypoglycemia. Glucose is an essential metabolic fuel for the brain under physiological conditions. Because the brain cannot synthesize glucose or store more than a few minutes’ worth of glucose consumption as glycogen, it is critically dependent on a continuous supply of glucose from the circulating blood (184). Falling arterial glucose concentrations are sensed in the hepatic portal vein, the carotid body, and different areas of the brain. One brain region in particular, the VMH, has been shown to play an important role not only in sensing decreases in blood glucose levels but also in initiating counterregulatory responses to hypoglycemia (16, 185, 186, 187, 188). As arterial plasma glucose levels decline in the physiological range, insulin secretion decreases mainly due to the importance of glucokinase-mediated glucose sensing in the ß-cell. This will enhance hepatic glucose production. A fall in plasma glucose concentration below the physiological range stimulates the secretion of glucagon and epinephrine. Glucagon stimulates hepatic glycogenolysis as well as gluconeogenesis. Epinephrine enhances hepatic glucose output and decreases glucose clearance by tissues such as muscle and fat. Epinephrine also mobilizes gluconeogenic precursors including lactate, amino acids, and glycerol (see Fig. 1).

Two main mechanisms have been proposed to mediate the -cell response to hypoglycemia (Fig. 3). The first mechanism involves relief from inhibitory paracrine/endocrine influences by neighboring ß- and -cells as discussed above. The second mediator of the glucagon release to hypoglycemia is the autonomic input to the -cell (Fig. 3). There are three major autonomic influences on the -cell: sympathetic nerves, parasympathetic nerves, and circulating epinephrine. All three autonomic inputs are activated by hypoglycemia and are capable of stimulating glucagon secretion (for review, see Ref. 43). However, the relative contribution of the different autonomic inputs to enhance glucagon secretion during hypoglycemia is unknown. This is emphasized by the observations that the denervated dog (46) and human (47) pancreas release glucagon in response to hypoglycemia. The evidence for autonomic mediation of glucagon secretion during hypoglycemia has been reviewed extensively (43, 184, 189).

View larger version (24K): [in this window] [in a new window]   FIG. 3. Mechanisms for regulation of glucagon release in response to hypoglycemia. A, During euglycemia, glucagon secretion may be suppressed (thin arrow) as a result of: 1) the lack of autonomic stimulation, including adrenergic stimulation of the -cell; and 2) a marked paracrine inhibition by factors released from ß-cells (and -cells, not indicated in figure) within the islet. Green arrows indicate stimulatory pathways, whereas red arrows symbolize inhibitory pathways. B, During hypoglycemia, glucagon secretion is markedly increased (thick arrow). This may arise from a marked reduction of the paracrine inhibitory effects (including 2-adrenergic receptor activation in ß-cells) as well as a stimulatory action of the autonomic nervous system (involving ß-adrenergic receptor activation in the -cell) secondary to its activation by central hypoglycemia.

The involvement of K_ATP-channels in the glucagon counterregulatory response is supported by the observation that VMH neurons in Kir6.2-deficient mice, which lack functional neuroendocrine-type K_ATP-channels, have a persistently elevated firing rate and exhibit a severely impaired glucagon counterregulatory response (188). However, it is important to emphasize that the mouse is a whole-body Kir6.2 knockout and therefore the contribution of VMH neurons to the impaired glucagon counterregulatory response is unknown. Pharmacological activation of K_ATP-channels in the VMH amplified the counterregulatory glucagon and epinephrine responses to hypoglycemia (194), whereas K_ATP-channel closure by sulfonylureas suppressed the counterregulatory responses to hypoglycemia (16). This is consistent with increased activity of the autonomic nervous system and stimulation of glucagon secretion in glucose transporter 2-deficient mice in the fed state and suppressed glucagon secretory response to hypoglycemia (195, 196). This dysregulation was caused by inactivation of centrally located glucose sensors that require expression of glucose transporter 2 in glial cells and control the activation of the nucleus of the tractus solitarius and dorsal motor nucleus of the vagus nerve (197). Neurons in the nucleus of the tractus solitarius are sensitive to small variations in blood glucose concentrations (198) and send projections to the hypothalamic nuclei as well as the dorsal motor nucleus of the vagus and are connected to the pancreas (199). Gene deletion of the glial cell line-derived factor family receptor 2 causes islet parasympathetic denervation and results in marked blunting of the glucagon secretory response to neuroglucopenic stimulation after injection of 2-deoxy-glucose (49). In man, section of the abdominal vagus trunc (vagal truncotomy) blunts the glucagon secretory response to induced hypoglycemia. Surprisingly, selective vagotomy (section of the branches to the pancreas) did not alter the glucagon response (200). This suggests that the response is mediated at least in part by stimulation of the splanchnic sympathetic nerves. Glucose recognition in the CNS has been suggested to involve metabolic coupling between astrocytes and neurons (201, 202). This model suggests that glucose is initially taken up by astrocytes and metabolized to lactate, which is then transported into neurons for ATP generation and the control of nerve firing rate.

As outlined above, several paracrine/endocrine factors as well as glucose-sensing neurons in the VMH have the potential to modulate glucagon secretion. However, the physiological importance of some of these factors remain to be established, as well as whether they exert their actions via direct effects on the -cell or by intraislet paracrine/endocrine mechanisms. The main modulators of -cell secretion and their proposed signaling mechanisms are outlined in Fig. 4A. Binding of insulin to its receptor causes activation of K_ATP-channels resulting in membrane hyperpolarization and inhibition of glucagon secretion. Insulin may also cause translocation to and activation of GABA_A receptors in the -cell plasma membrane. Additionally, K_ATP-channels are activated by Zn²⁺, which is coreleased with insulin from the ß-cell. Although it is well established that GABA_A and somatostatin receptor activation results in -cell membrane hyperpolarization, the physiological significance of intraislet GABA and somatostatin for regulation of glucagon release remains to be fully established. Circulating epinephrine stimulates adenylate cyclase activity, an increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i), and glucagon release. Finally, activation of intraislet parasympathetic nerve endings stimulates glucagon secretion by release of a cocktail of different neurotransmitters (Fig. 4A). The effects of the main modulators on glucagon secretion from FACS-isolated rat -cells are depicted in Fig. 4B. The effects of glucose, pyruvate, arginine, tolbutamide, and diazoxide on glucagon release from rat -cells (Fig. 4B) are also shown for completeness.

View larger version (34K): [in this window] [in a new window]   FIG. 4. A schematic overview of main physiological regulators of -cell function and glucagon secretion. A, Model summarizing site of action and intracellular signaling mechanisms for main physiological modulators of -cell stimulus-secretion coupling. Green dotted lines depict activation, whereas red dotted lines represent inhibition of ion channel activity or intracellular signaling pathways. AC, Adenylate cyclase; Gs, stimulatory G protein; Gi, inhibitory G protein; PNS, parasympathetic nervous system. B, Glucagon secretion from FACS-isolated rat -cells exposed for 30 min to Krebs-Ringer bicarbonate HEPES buffer (15 ) containing either 1 mM or 20 mM glucose. Where indicated, the extracellular medium was supplemented with pyruvate (5 mM), arginine (10 mM), Zn2+ (30 µM), or epinephrine (5 µM). Insulin, GABA, and somatostatin were tested at a concentration of 100 nM, whereas tolbutamide and diazoxide were applied at 100 µM. Data are mean ± SE of five to nine different experiments. *, P < 0.05; **, P < 0.01.

	V. Transcriptional Control of Pancreatic -Cell Development

Top Abstract I. Introduction II. Islet Endocrine Cell... III. Paracrine, Autocrine, and... IV. Autonomic Regulation of... V. Transcriptional Control of... VI. The Glucagon Gene:... VII. {alpha}-Cell Stimulus... VIII. {alpha}-Cell... IX. Summary and Conclusions Note Added in Proof References

 
During embryogenesis, the pancreas develops by fusion of a dorsal and ventral epithelial bud generated by evagination of the foregut (for review, see Ref. 203). The induction of the pancreatic phenotype in the dorsal pancreatic bud has been suggested to be dependent on signals from the adjacent notochord (204, 205), although more recent evidence suggests that this signal originates from blood vessels (206, 207). In the mouse, the dorsal bud appears on embryonic day 9.5 (E9.5) concomitantly with the first differentiated glucagon-producing -cells (208, 209, 210). The following day (E10.5), insulin-producing cells are detected, often coexpressing glucagon (209). At E14, numerous fully differentiated ß-cells appear and the endocrine cells start to become organized in small aggregates, and within the next 24 h, the first somatostatin-producing -cells become visible. Finally, shortly before birth, PP-producing cells differentiate, and the endocrine cells begin to form well-organized islets of Langerhans.

Several transcription factors have been identified based on their temporally and spatially restricted expression during pancreatic development. Furthermore, lineage studies and generation of mice with targeted mutations of the genes that encode these factors have enhanced our understanding of islet and -cell differentiation (Table 1). Deletion of pTF1a/p48, a basic helix-loop-helix transcription factor expressed early in endoderm and later in the mature exocrine pancreas, results in pancreagenesis (211). Similarly, gene inactivation of Pdx-1, a homeobox gene expressed in pancreatic buds, or of the LIM homeodomain gene Isl-1 also results in complete absence of pancreas development (212, 213, 214). Mice lacking Neurogenin 3 (Ngn3), a basic helix-loop-helix (bHLH) transcription factor, fail to develop endocrine cells (215). The proendocrine function for Ngn3 is supported by lineage studies (216) and ectopic expression of Ngn3 under the control of the Pdx-1 promoter, which leads to premature differentiation of the pancreatic progenitor cells into endocrine cells, although predominantly glucagon-producing cells (217, 218). Thus, whereas Ngn3 activation in pancreatic progenitor cells promotes an endocrine commitment, the specification of the different islet cell types is controlled by other transcription factors (for review, see Refs. 219 and 220) (Fig. 5). Pax-6 is specifically involved in the differentiation of -cells because no or few glucagon-producing cells are observed in homozygous mutants (221). Brain-4 (Brn-4), a POU-homeodomain-containing protein, is expressed in the pancreatic anlage of the mouse foregut at E10 in glucagon-producing cells and seems to transactivate glucagon gene expression (222). However, loss-of-function mutant mice do not exhibit any defect in -cell formation or function (223), although recent in vivo and in vitro studies have shown that forced expression of Brn-4 is sufficient to activate the glucagon gene but not a complete -cell phenotype (224, 225). This suggests a role for Brn-4 in the islet to promote hormone expression (Fig. 5). The cell type-restricted bHLH transcription factor Beta2/NeuroD is likely to be involved in the proliferation rather than the differentiation of endocrine cells. Beta2/NeuroD is expressed in all endocrine cells of the pancreatic epithelium, and targeted gene disruption results in diabetic mice that die shortly after birth (226, 227). Although the proliferation of insulin-producing cells seems to be most strongly affected, all endocrine cell types are significantly reduced in number. Nkx2.2 belongs to the NK2 homeodomain family of transcription factors. Early expression of Nkx2.2 can be detected in all endocrine cell types. As development proceeds, Nkx2.2 becomes restricted to ß-cells, most -cells, and PP cells (Fig. 5). Nkx2.2 inactivation is associated with a replacement of endocrine cells with ghrelin-producing cells (158). This was mostly at the expense of ß-cells, but a severe reduction in glucagon-producing cells and a slight reduction in PP cells were also observed (228). The homeobox-containing gene, Arx, is required for proper endocrine pancreas development. Arx mutant mice reveal an early onset loss of mature -cells with a concomitant increase in ß- and -cell numbers (229). Thus, Arx is required for -cell fate acquisition and repressive action on ß- and -cell destiny, which is exactly the opposite of the action of Pax-4 in endocrine commitment (229) (Fig. 5). This is supported by an accumulation of Pax-4 and Arx transcripts in Arx and Pax-4 mutant mice, respectively (229). This suggests that the antagonistic functions of Pax-4 and Arx for proper islet cell specification are related to the pancreatic levels of the respective transcripts. Combined Arx/Pax-4-deficient mice display a -cell phenotype at the expense of both - and ß-cells (230). Arx mediates activin A-induced inhibition of -cell proliferation but not the ability of activin A to inhibit glucagon gene expression (231). These effects of activin A in -cells are opposite to those in ß-cells where this growth and differentiation factor increases insulin and Pax-4 gene expression (232, 233, 234), a finding that may have relevance during pancreatic endocrine lineage specification. MafB, which belongs to the large Maf family of basic leucine-zipper-containing transcription factors, is only expressed in -cells and contributes to cell type-specific expression of the glucagon gene (235). MafB is also expressed in developing - and ß-cells as well as in proliferation of hormone-negative cells during pancreatogenesis (235, 236). Finally, the winged-helix transcription factor, Foxa2 (formerly hepatocyte nuclear factor 3-ß), is required for the differentiation of -cells because Foxa2 mutant mice are hypoglucagonemic secondary to a 90% reduction of glucagon expression (237). However, although the number of mature glucagon-positive -cells was dramatically reduced, specification of -cell progenitors was not affected by the Foxa2 deficiency (237). Thus, Foxa2 is a critical regulator of -cell differentiation, acting after the initial specification of the -cell lineage (Fig. 5).

View this table: [in this window] [in a new window]   TABLE 1. Transcription factors important for pancreatic -cell development

View larger version (19K): [in this window] [in a new window]   FIG. 5. A simplified model for the role of islet transcription factors in endocrine differentiation in the developing pancreas. The proposed position of the different transcription factors is based on their timing of expression, timing of predominant functional role, or both. It is important to emphasize that some transcription factors appear at several steps, but a single step is shown for simplicity. Somatostatin-secreting -cells, ghrelin-producing -cells, and PP cells are not depicted in the model. PP and -cells likely diverge at the stage where Pax-4 and Arx are coexpressed and before the cells differentiate into - and ß-cells. -cells most likely differentiate from ß-/-cell precursors. Little is known about the transcription factors controlling -, -, and PP-cell specification.

	VI. The Glucagon Gene: Transcriptional Control and Proglucagon Processing

Top Abstract I. Introduction II. Islet Endocrine Cell... III. Paracrine, Autocrine, and... IV. Autonomic Regulation of... V. Transcriptional Control of... VI. The Glucagon Gene:... VII. {alpha}-Cell Stimulus... VIII. {alpha}-Cell... IX. Summary and Conclusions Note Added in Proof References

View larger version (25K): [in this window] [in a new window]   FIG. 6. Schematic representation of the structure and processing of proglucagon. Top, Structure of proglucagon. At the end of each peptide are pairs of dibasic amino acid residues (KR, RR, KK; K, lysine; R, arginine), which represent potential sites for cleavage by prohormone convertases. Middle, Proglucagon processing pattern in the pancreatic -cell. The peptides shown are GRPP (proglucagon 1–30), glucagon (proglucagon 33–61), IP-1 (proglucagon 64–69), and MPGF (proglucagon 72–158). MPGF is only partially processed to GLP-1 (1–36) (proglucagon 72–107). Bottom, Proglucagon processing in the intestinal L cell to generate glicentin (proglucagon 1–69), truncated GLP-1 (tGLP-1; proglucagon 78–107), IP-2 (proglucagon 111–122), and GLP-2 (proglucagon 126–158). Glicentin is partially processed to GRPP and oxyntomodulin (proglucagon 33–69). [Adapted with permission from M. Furuta et al.: J Biol Chem 276:27197–27202, 2001 (248 ).]

Several lines of evidence suggest that Pdx-1 plays a key role not only in the islet cell differentiation but also in maintaining the differentiated state and control of islet hormone gene expression. Pdx-1 is a major transactivator of the insulin and somatostatin genes through its synergistic interaction with E47/ß2 and Pax-6 or Pbx/Prep1, respectively (261, 262, 263, 264, 265). ß-Cell-specific inactivation of Pdx-1 in developing mice resulted in a decrease in the number of ß-cells with a concomitant 2.5-fold increase in glucagon-positive cells and coexpression of insulin and glucagon in 22% of cells, suggesting that -cell differentiation and glucagon gene expression are favored by the absence of Pdx-1 in vivo (266). Furthermore, transient inhibition of Pdx-1 in ß-cells of adult mice led to an increase in the number of glucagon gene-expressing cells (267). In addition, functional inactivation of Pdx-1 in insulinoma cells resulted in differentiation of insulin-containing cells into glucagon-producing cells (225, 268). Finally, ectopic Pdx-1 expression in -cells inhibits glucagon gene transcription (269). However, a recent study has demonstrated that Pdx-1 alone is not sufficient for repression of glucagon gene transcription (270).

Although many nutrients, hormones, and neurotransmitters have been shown to regulate glucagon secretion, much less is known about the factors and processes that control glucagon biosynthesis. Insulin has been shown to inhibit glucagon gene expression by changes in transcription rates. However, cAMP analogs and catacholamines, which act through the cAMP second messenger pathway, stimulate transcription. Glucose does not affect proglucagon mRNA levels in rat islets (271) and in rats infused for 48 h with glucose (272), but it stimulates proglucagon gene expression in InR1G9 cells (273) and TC1–6 cells (274). The molecular mechanisms of these changes in proglucagon gene expression have been reviewed elsewhere (238, 275).

The mechanisms regulating degradation and clearance of glucagon remain incompletely understood. The enzyme neutralendopeptidase 24.11 has been shown to regulate the levels of glucagon in pigs (276, 277). This is exemplified in studies using candoxatril, a selective neutral endopeptidase inhibitor, where the levels of both endogenous and exogenously infused glucagon are increased after candoxatril administration (276). Glucagon has also been shown to be a pharmacological substrate for dipeptidyl peptidase-4 (DPP-4) in vitro (278, 279), however whether DPP-4 regulates physiological levels of endogenous glucagon remains unclear. For example, levels of intact glucagon are not significantly changed in pigs subjected to treatment with a DPP-4 inhibitor, with the kidney functioning as a major determinant for glucagon elimination (280).

	VII. -Cell Stimulus Secretion Coupling

Top Abstract I. Introduction II. Islet Endocrine Cell... III. Paracrine, Autocrine, and... IV. Autonomic Regulation of... V. Transcriptional Control of... VI. The Glucagon Gene:... VII. {alpha}-Cell Stimulus... VIII. {alpha}-Cell... IX. Summary and Conclusions Note Added in Proof References

 
Patch-clamp experiments on isolated mouse, rat, and guinea pig -cells have shown that, unlike ß-cells, they produce spontaneous action potentials in the absence of glucose (Fig. 7A) (14, 65, 98, 129, 130, 281, 282, 283). Activation of voltage-gated Na⁺ and Ca²⁺ currents underlie the upstroke of the action potentials, whereas K⁺ currents are responsible for spike repolarization. Spontaneous electrical activity has also been recorded from superficial -cells in intact mouse islets (284).

View larger version (33K): [in this window] [in a new window]   FIG. 7. Spontaneous electrical activity and plasma membrane ion channels in the -cell. A, Action potentials generated by isolated rat -cell in the absence of glucose. Note that the action potentials exceed 0 mV (dotted line). B, Types of ion channels described in the -cell. Glucose uptake and metabolism lead to an increase in the ATP/ADP ratio (dotted line) and closure of ATP-sensitive K+-channels (KATP-channels). Activation of voltage-gated Ca2+-channels and Ca2+ influx result in an increase in the [Ca2+]i and stimulation of exocytosis of the glucagon-containing granules. Channels are indicated by closed symbols and plasma membrane receptors by open symbols. KI-channel, a G protein-gated K+-channel activated by somatostatin; G protein, GTP-activated protein.

A. Ion channels present in the -cell plasma membrane
1. The K_ATP-channel.
The K_ATP-channel plays a central role in many tissues by coupling cell metabolism to electrical activity. To date, the best studied example is the pancreatic ß-cell where the K_ATP-channels link changes in the blood glucose concentration to insulin secretion. Under resting conditions, the tonic activity of the K_ATP-channels maintains a negative membrane potential. Glucose metabolism produces a concentration-dependent inhibition of the channel, which is mediated by changes in the intracellular concentrations of ATP and ADP. K_ATP-channel closure results in membrane depolarization. This leads to the opening of voltage-dependent Ca²⁺-channels, Ca²⁺-influx, and stimulation of Ca²⁺-dependent exocytosis (91).

K_ATP-channels are present in mouse (285, 286) and rat -cells (15, 281) as well as TC-6 cells (287, 288) but appear to be absent from guinea pig -cells (283). The maximum K_ATP conductance is more than 5-fold higher in rat -cells than mouse ß-cells when expressed relative to cell surface area (10 nS/pF vs. 1.9 nS/pF) (289). Recent evidence suggests that the K_ATP-channel densities in mouse - and ß-cells are comparable (286).

The K_ATP-channel present in rat -cells is highly sensitive to ATP, with a half-maximal inhibition (K_i) of 17 µM and an almost complete block observed at 100 µM in excised membrane patches (281). It has long been recognized that the ATP sensitivity of the K_ATP-channel measured in excised patches does not reflect that in the intact cell, which is shifted toward higher ATP concentrations in the presence of Mg-ADP and Mg-GDP, negatively charged phosphatidylinositol phosphates (290, 291), and long-chain acyl-coenzyme A esters (292). Indeed, the ATP sensitivity of the K_ATP-channel in intact rat -cells dialyzed with different ATP concentrations is much lower (K_i = 0.94 mM; Fig. 8A) compared with that observed in excised patches (K_i = 17 µM) (281). Interestingly, the ATP sensitivity of the rat -cell K_ATP-channel in intact cells is much lower compared with that reported for the mouse -cell K_ATP-channel (K_i = 0.16 mM) (286) and comparable to that observed for the rat (K_i = 0.92 mM; Fig. 8A) and mouse (K_i = 0.86 mM) (286) ß-cell K_ATP-channel. Because the mouse -cell K_ATP-channel is more sensitive to ATP inhibition than the mouse ß-cell K_ATP-channel, very low ATP concentrations (0.05 mM) are required for maximally triggering K_ATP-channel activity. The high sensitivity for ATP could explain why Barg et al. (285) observed that the magnitude of the maximal whole-cell K_ATP conductance in mouse -cells dialyzed with 0.3 mM ATP (the maximal concentration triggering ß-cell K_ATP currents) was smaller compared with that observed in mouse ß-cells. The reason for the higher ATP sensitivity of the mouse -cell K_ATP-channel is unknown.

View larger version (24K): [in this window] [in a new window]   FIG. 8. Metabolic regulation of rat -cell KATP-channels. A, Representative traces (left) of standard whole-cell KATP currents recorded in single rat -cells in response to 10-mV depolarizing and repolarizing voltage pulses from a holding potential of –70 mV. The currents were measured after full equilibration of the pipette-filling solution with the cytoplasm in the presence of 0.01, 1, or 3 mM Mg-ATP. The ATP-sensitivity of KATP current in isolated rat -cells (solid line) and ß-cells (dotted line) is depicted to the right. Standard whole-cell KATP currents were recorded as described above and were obtained after full equilibration of the cytoplasm with the pipette-filling solution (5 min) in the absence and presence of increasing concentrations of Mg-ATP. The whole-cell KATP currents are normalized to the maximum increase in KATP-current density (I/Imax). Data are fitted with a Hill equation and are mean values ± SE of five to seven different experiments. B, Effects of sodium azide (NaN3) on single KATP-channel activity in isolated rat -cell. Single channel activity was measured in the cell-attached patch clamp configuration. The pipette potential was 0 mV, and channel activity was recorded before, during, and after application of 2 mM NaN3, as well as in the simultaneous presence of NaN3 and 200 µM tolbutamide. At the times indicated by the triangles (top), the current trace is displayed at a higher temporal resolution. These parts of the trace show biphasic current deflections resulting from action potentials and only brief channel openings. The current trace is also displayed on a higher temporal resolution during the application of NaN3 alone, showing distinct openings and closures of three to four KATP-channels. [Part B of the figure was obtained from H. L. Olsen et al.: Endocrinology 146:4861–4870, 2005 (15 ). © The Endocrine Society.]

2. Voltage-gated K⁺-channels.
The -cell is equipped with voltage-dependent K⁺-channels. Their functional role is to repolarize the action potential. Delayed rectifying K⁺ (K_Dr) currents have been observed in dispersed mouse (285, 286) and guinea pig -cells (283). K_Dr currents have also been observed in isolated rat -cells (J. Gromada, unpublished data). The current was sensitive to tetraethylammonium (TEA; 10 mM), which reduced the outward current component by more than 90%. Interestingly, application of TEA to isolated rat -cells increased glucose-induced glucagon release through enhanced membrane depolarization (15).

A TEA-resistant transient voltage-activated K⁺ current (A-current) has been described in mouse -cells (284). The A-current in mouse -cells is sensitive to 4-aminopuridine (4-AP) (284). 4-AP reduced glucagon secretion at 1 mM glucose in mouse islets to the same extent as increasing glucose concentration to 20 mM (65). A-currents have not been described in rat -cells. This is consistent with the observation that 4-AP was without effect on glucagon secretion from rat islets (15), suggesting that K_Dr-channels are responsible for action potential repolarization in this species.

3. Na⁺-channels.
Na⁺-channels play an important role for action potential generation in -cells. The Na⁺ currents are tetrodotoxin (TTX) sensitive and are activated at potentials positive to –50 mV in guinea pig -cells (283) and approximately –30 mV in mouse -cells (284, 285, 297). TTX inhibits glucagon release in mouse islets to an extent similar to that observed in the presence of a maximally inhibitory concentration of glucose (65, 284). Glucagon release from isolated rat -cells was also inhibited by TTX (15).

4. Ca²⁺-channels.
Voltage-gated Ca²⁺-channels are divided into three subfamilies: 1) the L-type high voltage-activated (HVA) Ca²⁺-channel; 2) P/Q-, N-, and R-type HVA Ca²⁺-channels; and 3) the low voltage-activated T-type Ca²⁺-channel. N- and L-type Ca²⁺-channels have been described in rat -cells (283), whereas T- and L-type Ca²⁺-channels have been reported in guinea pig -cells (298). There is general agreement that mouse -cells express L-, N-, R-, and T-type Ca²⁺-channels (284, 285, 297, 299, 300, 301). However, in one study T- and N-type Ca²⁺-channels were not detected (297). This discrepancy could be explained by differences in distinguishing - and ß-cells as well as cell isolation and culture conditions. Whereas openings of the T-type Ca²⁺-channel occur at potentials as negative as –65 mV, depolarization above –40 to –30 mV is required to elicit HVA Ca²⁺-channel openings. The differences in the voltage dependence of activation suggest that T-type Ca²⁺-channels and HVA Ca²⁺-channels may have different functional roles. The T-type Ca²⁺-channels open around the action potential threshold and have been proposed to play a role in action potential initiation. The HVA Ca²⁺-channels activate at membrane potentials corresponding to the rapidly rising phase of the action potential, are of larger amplitudes, and are therefore responsible for most of the Ca²⁺ entry required for glucagon secretion.

Glucagon release measurements have revealed a close relationship between N-type Ca²⁺-channels and glucose regulation of secretion in rat and mouse islets (15, 96, 131). This is supported by the observations that Ca²⁺ influx through N-type Ca²⁺-channels controls basal secretion in rat -cells (131, 282) and that plasma glucagon levels are reduced in N-type Ca²⁺-channel-deficient mice (302). L-type Ca²⁺-channel activity is potentiated by cAMP and PKA in a variety of cell types. In -cells, agents that elevate cAMP, e.g., epinephrine, reduce the rate of Ca²⁺-channel inactivation without substantially increasing the peak amplitude of the Ca²⁺ current (282, 298). Because the effect of cAMP is abolished by selective PKA inhibitors, it appears that PKA activation mediates the effect of the nucleotide. The R-type Ca²⁺-channel has recently been suggested to be involved in glucose regulation of glucagon secretion in mouse (297, 300) but not in rat -cells (15).

5. Pumps and transporters.
Arginine increases mouse and guinea pig -cell electrical activity. This effect is characterized by a rapid onset, indicating that metabolic degradation of this amino acid is not required, and it has been suggested that this positively charged amino acid depolarizes the -cell by its electrogenic entry (65, 282). This is supported by the observation that the nonmetabolizable amino acid -amino-isobutyric acid elevates the [Ca²⁺]_i in mouse -cells (303). Furthermore, arginine elevates [Ca²⁺]_i and stimulates insulin secretion as a consequence of its electrogenic transport into the mouse pancreatic ß-cell. The uptake of arginine is mediated by murine cationic amino acid transporter mCAT2A (304).

Inhibition of the Na⁺/K⁺-ATPase results in -cell membrane depolarization and stimulation of glucagon secretion (303, 305, 306). This is probably because the pump is electrogenic, transporting three Na⁺ out for every two K⁺ in, thereby providing a small constant outward current, which normally hyperpolarizes the -cell by a few millivolts. A second possibility is that because the Na⁺ pump is likely to consume a substantial portion of cellular ATP, its inhibition will lead to a local rise in ATP and decrease in ADP. Consequently, K_ATP-channels will be blocked, triggering membrane depolarization.

B. Regulation of electrical activity in rat -cells
The pattern of action potential firing in -cells differs from that of the ß-cell. Whereas action potentials originate from a fairly depolarized membrane potential in the ß-cell (–40 mV), the action potentials in rat, mouse, and guinea pig -cells start at voltages as negative as –60 mV (Fig. 7A) (281, 282, 283, 285). Recent data suggest that the stimulus-secretion coupling in rat -cells mirrors that of the ß-cell (Figs. 4A and 9A) (14, 15, 57, 58). At low glucose concentrations, the K_ATP-channels are active. However, due to the high ATP concentration and ATP-to-ADP ratio in rat -cells already at low glucose concentrations (307), the resulting K⁺ conductance is only sufficient to repolarize the membrane potential to around the threshold for action potential initiation (–60 mV). Glucose-induced closure of K_ATP-channels brings the membrane potential into a range where the Na⁺-channels begin to open (between –40 and –30 mV), leading to regenerative opening of these channels, thus accounting for the rapid and large (often exceeding 0 mV) upstroke of the action potential. The involvement of K_ATP-channels in the control of electrical activity is supported by the observations that tolbutamide and pyruvate depolarize the plasma membrane and stimulate glucagon secretion in isolated rat -cells (Figs. 4B and 9B) (14, 15, 281). Furthermore, the stimulatory effect of glucose on glucagon secretion in isolated rat -cells is abolished by the K_ATP-channel opener diazoxide as well as by mannoheptulose and sodium azide (14, 15).

View larger version (40K): [in this window] [in a new window]   FIG. 9. Model for glucose- and pyruvate-dependent regulation of glucagon secretion in the rat -cell. A, At low glucose or pyruvate concentrations, the KATP-channels are active and maintain the membrane potential at around –60 mV. The transport of glucose into rat -cells is mainly mediated via glucose transporter 1, whereas pyruvate uptake is mediated by MCT-1 as indicated by the light green and yellow circles, respectively. Stimulation with high glucose or pyruvate concentrations increases rat -cell metabolism and the production of ATP at the expense of ADP. The resulting increase in the ATP-to-ADP ratio leads to closure of KATP-channels and membrane depolarization (). This in turn activates voltage-dependent Na+- and N-type Ca2+-channels responsible for action potential generation, Ca2+ influx, elevation of the [Ca2+]i, and stimulation of glucagon secretion. Sulfonylureas bind to the SUR moiety of the KATP-channel. The sulfonylurea-induced closure of the KATP-channels initiates the same series of events in the rat -cell as glucose or pyruvate. B, Current-clamp recording of membrane potential in a single rat -cell in the presence of 20 mM glucose in the extracellular solution. Tolbutamide (100 µM) was applied during the period indicated by the horizontal line.

C. Regulation of electrical activity in mouse -cells
At low glucose concentrations, the K_ATP-channels maintain the membrane potential at around –60 mV (65, 131, 283, 285). Mouse (and guinea pig) -cells express T-type Ca²⁺-channels and by analogy to the situation in other cell types (303) they serve as pacemakers (283, 284, 308). The activation of the T-type Ca²⁺-channels brings the membrane potential of the mouse -cell from approximately –60 mV into the range where the Na⁺-channels and HVA Ca²⁺-channels start to open, leading to regenerative opening of these channels, thus accounting for action potential generation and stimulation of glucagon release. As in the rat, basal glucagon secretion depends principally on Ca²⁺-influx through N-type Ca²⁺-channels (96, 285). Repolarization of the action potential, which is caused by activation of the A-current, is necessary for channel reactivation.

Addition of glucose leads to closure of the K_ATP-channels and membrane depolarization (65). However, because the mouse -cell action potential is dependent on T-type Ca²⁺-channels and Na⁺-channels, which inactivate, membrane depolarization in the mouse -cell reduces rather than increases electrical excitability (65, 284). After inactivation of the T-type Ca²⁺-channels and Na⁺-channels, action potentials are not generated, and the membrane potential never reaches the level where HVA Ca²⁺-channels start to open. Membrane depolarization can thus be envisaged to result in a paradoxical decrease in Ca²⁺-entry with resultant suppression of glucagon secretion. This proposed sequence of events is supported by the observations that glucagon secretion is abolished by the Na⁺-channel blocker TTX, the N-type Ca²⁺-channel inhibitor -conotoxin-GVIA, and the A-current blocker 4-AP (65).

Although the stimulus-secretion coupling controlling glucagon secretion in response to glucose stimulation appears to differ between mouse and rat -cells, important questions remain to be investigated. For example, the inhibitory action of glucose on glucagon secretion has only been investigated in intact mouse islets where paracrine interactions from ß- and -cells cannot be excluded. The importance of such investigations were recently exemplified by the finding that glucose inhibits glucagon release from intact rat islets but stimulates glucagon release from isolated rat -cells (14, 15). Therefore, it will be necessary to examine glucagon release from FACS-isolated mouse -cells to confirm the inhibitory effect of glucose. Furthermore, the proposed model for glucose regulation of glucagon release is in part based on electrophysiological recordings obtained from superficial -cells in intact mouse islets. Again, caution should be exerted in interpretation of direct vs. paracrine effects of glucose on -cell function in these studies. However, it is important to emphasize that differences in islet architecture and blood flow have been reported between mouse and rat islets. Such differences in islet morphology and function might explain some of the controversies currently in the literature and also prompt caution in comparing and extrapolating data between the two species. It will be exciting to see whether the stimulus-secretion coupling in human -cells resembles more closely the situation in mouse or rat -cells.

D. Metabolism of the -cell
1. Glucose.
Evidence that increases in extracellular glucose concentration suppress glucagon secretion was first provided by studies in dog (309). In humans, both ingestion of glucose (310) and infusion of glucose (311), which result in hyperglycemia, normally cause a decrease in plasma glucagon concentrations. Conversely, decrements from both normoglycemic and hyperglycemic plasma glucose concentrations stimulate glucagon secretion in dogs (309) as well as in healthy humans (312, 313, 314). Increments in plasma glucagon concentration are significantly correlated with the magnitude and the rate of the decrease in plasma glucose concentration (312, 315, 316).

It is well established that in vitro glucose causes monophasic inhibition of glucagon release in preparations containing the other islet endocrine cell types. Dose-response studies have revealed that the -cell is more sensitive to glucose than the ß-cell (317, 318, 319). Thus, the threshold for suppression of glucagon release (2–3 mM) is lower than the threshold for stimulation of insulin secretion (4–5 mM), and half-maximal suppression of glucagon release occurs at 3–6 mM, whereas half-maximal stimulation of insulin secretion occurs at 8–10 mM. Moreover, glucose concentrations of 6–10 mM generally cause maximal inhibition of glucagon release, whereas glucose concentrations in excess of 20 mM are usually required for maximal stimulation of insulin secretion (317, 318, 319). The inhibitory effect of glucose on glucagon release is shared by other glucose metabolites and related sugars (317, 320). In general, the inhibitory capacity is determined by the ability of -cells to metabolize the sugar (320). This is consistent with the fact that glucose-induced inhibition of glucagon secretion is counteracted by inhibitors of cellular metabolism (316).

Rat -cells express glucokinase and the glucose transporter Glut1, a lower capacity isoform than Glut2, expressed in ß-cells (321, 322, 323). The absence of Glut2 and low expression of Glut1 result in glucose uptake rates 10-fold lower than in ß-cells, but they are still 10 times higher than overall metabolic flux, suggesting that transport in -cells is not rate-limiting for overall glucose metabolism (321). Rates of glucose metabolism in isolated rat -cells are only 20–40% of those observed in ß-cells (307, 324, 325). Accordingly, glucose-induced increases in cytosolic ATP are significantly smaller in -cells than in ß-cells (57, 60) and might explain the observation that glucose did not affect total ATP and ADP levels in populations of isolated rat -cells (307).

Pyruvate is released from muscle during exercise and probably contributes to raising plasma glucagon concentrations. In contrast, normally ß-cells do not secrete insulin in response to pyruvate. In patients with familial exercise-induced hyperinsulinemic hypoglycemia, pyruvate was shown to elicit pronounced stimulation of insulin secretion, explaining the pathological response to physical exercise in these patients (326). The tissue-specific expression of the plasma membrane MCT-1 (327, 328, 329, 330) accounts for the ability of pyruvate to stimulate glucagon release, a process that requires functioning mitochondria (57). The absence of MCT-1 in ß-cells ensures that close to 100% of glucose-derived pyruvate enters the tricarboxylic acid cycle and is either broken down to H₂O and CO₂ yielding ATP (327) or assimilated into newly synthesized proteins (325). Compared with -cells, ß-cells have a 7-fold higher expression of the anaplerotic enzyme pyruvate carboxylase, whereas lactate dehydrogenase expression is approximately eight times higher in -cells than in ß-cells (325, 327). The higher expression of MCT-1 and lactate dehydrogenase in -cells than in ß-cells is likely to account for the low glucose oxidation rates compared with total glucose utilization and consequently modest mitochondrial ATP generation.

2. Amino acids.
Unlike glucose, which inhibits glucagon secretion and stimulates insulin release in perfused pancreas and islets, amino acids stimulate release of both hormones and in general are more effective stimulators of glucagon than of insulin secretion (150, 331). Individual amino acids differ in their ability to initiate glucagon secretion, with arginine being the most potent (83). Other amino acids have either no or only slight stimulatory action on their own but are capable of potentiating secretion induced by another amino acid (83).

Under physiological conditions, amino acids play an important role in the control of glucagon release, whereas amino acids generally have little effect on insulin release. However, because the presence of glucose augments insulin responses to amino acids while diminishing those to glucagon (it is unknown whether this is a direct or paracrine effect), amino acids in conjunction with glucose have an important role in the differential release of the two hormones. For example, insulin response to a protein meal would cause hypoglycemia if it were not for concomitant increase in glucagon secretion. These findings suggest that circulating amino acids play an important role for the regulation of glucagon release in vivo.

3. Fatty acids.
Free fatty acids (FFAs) affect glucagon secretion, although their effects vary between different species and experimental conditions (147, 332, 333, 334). Short-term exposure to FFAs inhibits glucagon release from guinea pig islets (333, 335). On the contrary, acute stimulation of mouse and rat islets as well as TC1–6 cells with palmitate is associated with stimulated glucagon secretion (147, 336, 337). The ability of FFAs to stimulate glucagon secretion in mouse islets is influenced by their chain length. The longer the chain length of saturated FFAs, the higher are the glucagon responses (337). Furthermore, saturated FFAs were more effective than unsaturated fatty acids in stimulating glucagon secretion. Finally, at an equimolar concentration, trans-fatty acids were more potent than their cis isomers (337). The stimulatory action of palmitate on glucagon secretion depends on enhanced influx of Ca²⁺, elevation of [Ca²⁺]_i, as well as a direct effect on the secretory process (147). Interestingly, the stimulatory effect of palmitate on glucagon release was paralleled by an approximately 50% inhibition of somatostatin release (147). This suggests that relief of paracrine inhibition by somatostatin released by neighboring -cells may contribute to the ability of palmitate to enhance glucagon secretion. Palmitate remained stimulatory in mouse islets depolarized with high extracellular K⁺ concentration (amplifying pathway), an effect that was dependent on palmitate uptake and conversion by acyl-coenzyme A synthetase (147).

Exposure of rat islets for 48 h to FFAs decreased total glucagon content by 65%. At the same time, glucagon release was stimulated 13 times, whereas only a marginal 1.5-fold increase in insulin and somatostatin secretion occurred (333). The decrease in glucagon content is likely the result of increased secretion in the face of unaltered glucagon biosynthesis (271). Long-term palmitate exposure also stimulates glucagon secretion in TC1–6 cells in a dose-dependent manner without affecting glucagon content (338). The differential regulation of pancreatic hormone release is likely to result from a negative effect of elevated FFA levels on Pdx-1 expression in ß- and -cells and consequently decreased expression of genes transactivated by Pdx-1 such as Glut2, glucokinase, insulin, and somatostatin (333). This phenomenon may be important in the pathogenesis of type 2 diabetes usually associated with increased plasma FFA concentrations.

E. Intracellular Ca²⁺ homeostasis
Spontaneous [Ca²⁺]_i oscillations have been observed in isolated -cells at low glucose concentrations (339, 340, 341, 342). Whereas arginine, alanine, and glycine transform the oscillations in mouse -cells into a sustained elevation of [Ca²⁺]_i, the oscillatory activity is suppressed by elevation of glucose to 20 mM, by blockade of voltage-gated L-type Ca²⁺-channels, and after removal of extracellular Ca²⁺ with the Ca²⁺ chelator EGTA (65, 294, 343, 344). Rat -cells respond with depolarization and increase in [Ca²⁺]_i after glucose stimulation (14, 15). In mouse -cells, epinephrine causes an initial mobilization of intracellular Ca²⁺ stores followed by activation of store-operated Ca²⁺ influx through L-type Ca²⁺-channels (345). The latter effect of epinephrine is dependent on the ambient glucose concentration, because Ca²⁺ influx is inhibited in the presence of high glucose (345). Because epinephrine only induces an increase in [Ca²⁺]_i in -cells and not in ß- and -cells, this represents a convenient tool with which to identify this cell type.

Based on measurements of [Ca²⁺]_i in isolated mouse -cells, it has recently been suggested that a store-operated membrane conductance plays a pivotal role in glucose regulation of glucagon secretion (345). At low glucose, intracellular Ca²⁺ stores are empty due to low ATP content and pronounced sarcoplasmic-endoplasmic reticulum calcium ATPase activity. This leads to activation of the depolarizing store-operated conductance with resulting initiation of -cell electrical activity and stimulation of glucagon secretion. After an increase in glucose concentration, metabolism is accelerated and the intracellular Ca²⁺ stores are filled, leading to reduction of the conductance, membrane repolarization, and suppression of glucagon secretion (345). It is unclear, however, how much the store-operated membrane conductance contributes to regulation of glucagon secretion because application of thapsigargin, an inhibitor of sarcoplasmic-endoplasmic reticulum calcium ATPase (346), only partially (<30%) antagonized the inhibitory action of glucose on glucagon secretion in mouse islets (65) and was without effect in isolated rat -cells (15). The identity of the store-operated membrane conductance in mouse -cells remains to be established.

F. Regulation of exocytosis of glucagon-containing granules
Measurements of glucagon release from groups of islets as well as perfused pancreas and isolated -cells have enabled a characterization of how metabolic fuels, hormones, neurotransmitters, and pharmacological agents influence -cell secretion. However, understanding of the fundamental processes involved in glucagon release requires more sensitive measurements of exocytosis. The application of capacitance measurements to -cells has allowed exocytosis to be monitored in individual -cells with millisecond resolution. Initiation of exocytosis in -cells requires high [Ca²⁺]_i, which is normally only achieved in the immediate vicinity of the voltage-gated Ca²⁺-channels. This suggests that upon membrane depolarization and Ca²⁺-channel opening, Ca²⁺ in the active zones immediately (<30 msec) reaches a high concentration sufficient to initiate exocytosis (285). It has been demonstrated that Ca²⁺ entry through N-type Ca²⁺-channels is particularly important for exocytosis under basal conditions, despite the fact that the majority of the whole-cell current is carried through L-type Ca²⁺-channels (282). It is implicit that secretory granules mainly associate with N-type Ca²⁺-channels in the basal state and that the domains of elevated [Ca²⁺]_i resulting from Ca²⁺ influx through N- and L-type Ca²⁺-channels do not overlap (Fig. 10A). When the Ca²⁺-channels close, the local Ca²⁺-transient that controls exocytosis collapses, and exocytosis stops. The dissipation is likely to be fast due to the Ca²⁺ extrusion/uptake and the high Ca²⁺ buffering capacity of the cytoplasm. This is consistent with the observation that exocytosis in -cells stops immediately upon membrane repolarization (131, 282, 285). The nature of the Ca²⁺-sensor(s) for exocytosis in -cells remains to be established. It should be noted that the Ca²⁺- and phospholipid-binding protein synaptotagmin has been implicated in neurotransmitter and insulin exocytosis. Synaptotagmin V has been localized to glucagon-containing granules in mouse (347) and rat (59) -cells. It is reasonable to assume that this isoform is implicated in the regulation of glucagon exocytosis because its suppression causes inhibition of Ca²⁺-mediated hormone secretion in INS-1E cells (59).

View larger version (37K): [in this window] [in a new window]   FIG. 10. Model describing the role of N- and L-type Ca2+-channels on basal and PKA-dependent exocytosis in rat -cells. A, Under basal condition (control), most (65%) of the granules ready for release are localized in the vicinity of the N-type Ca2+-channels, and exocytosis of these granules is blocked by the N-type Ca2+-channel blocker -conotoxin. The shaded areas indicate the domains within which [Ca2+]i rises to concentrations sufficient to trigger exocytosis. Note that the majority of the granules reside in a reserve pool and are not exocytosed after stimulation of Ca2+ influx through either N- or L-type Ca2+-channels. B, In the presence of cAMP-elevating agents (+cAMP), exocytosis is enhanced because: 1) the L-type Ca2+-current is increased and the domains around the L-type Ca2+-channels extend further into the -cell; and 2) granules that were previously outside the domains have been mobilized and brought into closer proximity to the Ca2+-channels. By the combination of these effects, approximately 80% of the releasable granules are now accessed by influx through L-type Ca2+-channels. The extent of the domains under control conditions is indicated by the dotted lines.

The stimulatory effect of cAMP on glucagon secretion is mediated via two distinct pathways: PKA-dependent and PKA-independent (172). The latter pathway involves the cAMP-sensing protein cAMP-guanidine nucleotide exchange factor II (cAMP-GEFII, also termed EPACII; reviewed in Ref. 348). This protein is expressed in -cells (172). Its implication is supported by the finding that the cAMP-GEFII-selective agonist 8-(4-chloro-phenylthio)-2'-O-methyladenosine-3',5'-cyclic monophosphate mimics the effect of cAMP and enhances PKA-independent exocytosis in -cells (172). The PKA-independent and cAMP-GEFII-dependent component of secretion in -cells accounts for the rapid component of exocytosis completed within a few hundred milliseconds of depolarization (172). This suggests that physiological agonists mainly exert their action by stimulation of PKA-dependent granule mobilization. Interestingly, the granules mobilized by this mechanism are targeted toward the L-type Ca²⁺-channels, and the relative contribution of Ca²⁺-influx through these channels increases from 30% under basal conditions to almost 80% in the presence of cAMP-elevating agents (Fig. 10B) (282).

G. Pharmacology
The sulfonylureas, such as tolbutamide and glibenclamide, are of clinical importance and have been used for many years in the treatment of type 2 diabetes. These drugs act as potent blockers of -cell K_ATP-channel activity, which is associated with stimulation of electrical activity and glucagon secretion in rat -cells (14, 281). Sulfonylureas also have a direct stimulatory action on exocytosis of the glucagon-containing granules (349). Sulfonylureas also cause membrane depolarization in mouse -cells, which, in this species leads to inhibition of electrical activity (65, 284). The K_ATP-channel opener diazoxide inhibits electrical activity, increases K_ATP-channel activity, and inhibits glucagon secretion in single rat -cells (14, 281).

It is well established that sulfonylureas stimulate insulin and somatostatin secretion in the perfused rat pancreas. Glucagon release, on the other hand, is reported to be unaffected (350, 351), stimulated (352), inhibited (353, 354, 355, 356, 357, 358), or dually stimulated-inhibited (359, 360) by this group of drugs. These conflicting results probably originate from the use of different experimental conditions, the type and concentration of sulfonylureas, as well as on the concentration of ambient nutrients and Ca²⁺. Because these factors also affect the ability of sulfonylureas to increase the secretion of insulin and somatostatin, paracrine interactions between ß- and -cells on the one hand and -cells on the other hand are likely to contribute to the apparent conflicting data. Sulfonylureas inhibit glucagon release from mouse islets (65, 66, 284) as well as rat islets (353, 361). Diazoxide inhibits glucagon release from mouse islets (65, 284) and perfused rat pancreas (57, 362).

The effects of sulfonylureas on glucagon secretion in man are also controversial. Oral sulfonylurea therapy lowers plasma glucagon levels in normal and type 2 diabetic subjects (363, 364, 365, 366, 367, 368, 369, 370). However, tolbutamide increases circulating glucagon levels in patients with advanced type 1 diabetes (371). This result corroborates the observation of a direct stimulant action of sulfonylureas in rat -cells in the absence of functional ß-cells (14). It is important to emphasize that in addition to differences in experimental conditions, the apparent contradictory effects of sulfonylureas on glucagon secretion also reflect species differences as well as variations in paracrine/endocrine effects depending on the biological system.

Imidazoline-containing compounds have attracted considerable interest for more than a decade as possible therapeutic agents for the treatment of type 2 diabetes. This is based on their ability to act as potent stimulators of glucose-dependent insulin secretion. However, imidazoline compounds stimulate not only insulin release but also somatostatin release while inhibiting glucagon secretion (372, 373, 374, 375). The inhibitory action of imidazoline compounds on glucagon secretion does not involve modulation of plasma membrane ion channel activity and inhibition of electrical activity but rather involves activation of the serine/threonine protein phosphatase calcineurin and inhibition of Ca²⁺-dependent exocytosis of the glucagon-containing granules (373). Somatostatin also inhibits exocytosis in rat -cells by activation of calcineurin and depriming of secretory granules (131). It is tempting to speculate that a similar mechanism is responsible for inhibition of exocytosis by imidazoline compounds.

	VIII. -Cell Pathophysiology and the Treatment of Diabetes

Top Abstract I. Introduction II. Islet Endocrine Cell... III. Paracrine, Autocrine, and... IV. Autonomic Regulation of... V. Transcriptional Control of... VI. The Glucagon Gene:... VII. {alpha}-Cell Stimulus... VIII. {alpha}-Cell... IX. Summary and Conclusions Note Added in Proof References

 
Type 2 diabetes is characterized by disrupted coordination of the glucagon-insulin balance due to impaired and delayed insulin secretion and insulin resistance. Hepatic insulin resistance results in the blunting of the ability of insulin to suppress glucose output. This, combined with elevated fasting and postprandial glucagon levels relative to those of insulin, leads to increased rates of hepatic glucose output, a major factor in the elevation of the postabsorptive and postprandial blood glucose levels in type 2 diabetes (376, 377, 378). Several studies have demonstrated that fasting blood glucose levels are positively correlated with basal rates of hepatic glucose production in patients with type 2 diabetes, illustrating the importance of the contribution of increased hepatic glucose production in diabetes (379, 380, 381).

A large body of evidence implicates hyperglucagonemia in the maintenance of increased rates of hepatic glucose output in type 2 diabetes (382, 383). The potential of reducing glucagon secretion (e.g., using GLP-1) or inhibiting glucagon action has received much attention as therapeutic strategies for the treatment of excess glucose production in patients with diabetes. Development of structural and functional glucagon receptor antagonists represents a potential approach to decrease hepatic glucose production and lower blood glucose in patients with diabetes. The fungal bisanthroquinone skyrin, isolated from Talaromyces wortmannin, inhibits glucagon-stimulated cAMP formation and glucose output from rat and human hepatocytes (384). Glucagon receptor antagonists that blocked glucagon action in experiments employing cell lines or primary cultures in vitro, or rodent studies in vivo, have been described (385, 386, 387). The majority of the initial antagonists were peptide-based, whereas more recent efforts have been directed at identification of nonpeptide orally available agents (388, 389). The concept that glucagon receptor antagonism may be a useful approach to therapy of diabetic patients was tested in preliminary human studies using the Bayer antagonist Bay 27-9955. This compound appeared safe and blocked exogenous glucagon action in short-term human studies; however, clinical development of this particular compound was not pursued (390). This is likely due to the fact that Bay 27-9955 did not reduce fasting glucose production or plasma glucose levels in humans (390). Nevertheless, the pharmaceutical industry remains interested in developing ideal compounds that might block glucagon receptor action in diabetic subjects (for review, see Refs. 391, 392, 393).

In addition to attempts to develop small molecular weight competitive and noncompetitive glucagon receptor antagonists, discovery efforts have also been focused on identifying antiglucagon monoclonal antibodies (394, 395, 396) and glucagon receptor antisense oligonucleotides (181, 397). Treatment of ob/ob and db/db mice and Zucker Diabetic Fatty rats with antisense oligonucleotides to reduce glucagon receptor expression mainly in the liver produced striking and long-lasting improvements in glycemia, together with reduced levels of triglycerides, and a decrease in plasma insulin. Consistent with findings in the glucagon receptor-deficient mouse, rodents treated with glucagon receptor antisense oligonucleotides exhibited -cell hyperplasia, markedly increased levels of circulating glucagon, and significantly (10- to 15-fold) increased levels of circulating GLP-1, together with increased levels of islet GLP-1. Hence, targeting the glucagon receptor and disrupting normal glucagon receptor signaling unmasks a compensatory increase in -cell activity accompanied by a shift toward islet GLP-1 production, with therapeutic benefits for the treatment of experimental diabetes (397).

Important target-related effects may influence the successful development of therapeutic agents targeting glucagon and the glucagon receptor. Given that glucagon plays a key role in elevating blood glucose levels, it is possible that its inhibition may result in hypoglycemia. Here it is important to emphasize that glucagon receptor-deficient mice have somewhat lower glycemia but are not hypoglycemic (180, 398). These mice have extremely elevated plasma levels of glucagon and display -cell hypertrophy (180, 398). PC2-deficient mice and animals treated with glucagon receptor antisense oligonucleotides also have pancreatic -cell hypertrophy (247, 248, 397). These observations clearly indicate a compensatory mechanism. It therefore remains to be seen whether small molecule antagonists will trigger similar compensation, leading to hyperglucagonemia and eventually the loss of efficacy of glucagon receptor antagonists in the long-term treatment. It also remains to be seen whether glucagon receptor antagonists will result in unfavorable accumulation of lipids in the liver. However, although it is known that glucagon reduces lipogenesis, mice deficient in glucagon receptor show a normal plasma lipid profile (398). The glucagon receptor is also of importance for the fetal growth and maturation of the pancreatic islets as well as for the proportion of the different endocrine cell types and the number of islets per pancreas (399). It is currently unknown whether a glucagon receptor antagonist would influence these parameters. Finally, the net effect of losing glucagon signaling on glycogenolysis has been hypothesized to result in excessive hepatic glycogen stores and hepatomegaly. However, excessive glycogen accumulation has not been reported in glucagon receptor-deficient mice or observed in animals treated with glucagon receptor antisense oligonucleotides (397, 398). There is no doubt that successful development of effective glucagon receptor antagonists without adverse side effects will help reset glucagon-insulin bihormonal regulation of hepatic glucose homeostasis to increase insulin sensitivity of the liver, ultimately resulting in improved glucose control in diabetes.

Islet transplantation in accord with the Edmonton protocol has recently received a great deal of attention as a potential cure for type 1 diabetes (400). However, there are still unresolved questions concerning the pancreatic counterregulatory response to hypoglycemia after islet transplantation. For example, the glucagon response to insulin-induced hypoglycemia did not improve after islet allo-transplantation and was similar to that of nontransplanted type 1 diabetic subjects, in contrast to a robust rise in healthy controls (401, 402, 403). A more recent study has reported a glucagon response to hypoglycemia after islet transplantation (404). The response was subnormal, which could be, at least in part, the result of a small -cell mass. Additionally, the -cell response to insulin-induced hypoglycemia was blunted in humans (402) and dogs (405) that underwent pancreatectomy with islet autotransplantation to the liver but was normal in dogs that underwent autotransplantation to the spleen (406) or peritoneal cavity (405). However, islet autotransplantation in pancreatectomized dogs into the omental or splenic site resulted in normal ß-cell but abnormal -cell response to mild non-insulin-induced hypoglycemia (407). The human studies show that simply correcting the diabetic background (hyperglycemia and lack of endogenous insulin) by islet transplantation does not result in hypoglycemic responsiveness of either native or transplanted -cells (402). Furthermore, the dog studies demonstrate that neither a prior hyperglycemic background nor immunosuppressive therapy is required for transplanted -cell dysfunction (407). The reason(s) for the dysfunctional -cell secretion during hypoglycemia is unclear because glucagon secretion was significantly stimulated after arginine administration to splenic transplanted dogs (408), implying that the -cells are functional. Furthermore, normal ß-cell function suggests that generalized cell damage is unlikely to occur, especially because in humans and dogs -cells are interspersed throughout the islet with no structured ß-cell core (24, 409). It thus seems more probable that the -cell defect in the transplanted dogs during hypoglycemia may be stimulus-specific, similar to the glucagon defect in long-standing type 1 diabetes. Possible explanations for the apparently stimulus-specific -cell dysfunction during hypoglycemia could be repeated unrecognized episodes of hypoglycemia, improperly reinnervated islets (410, 411, 412, 413, 414) or defective islet revascularization (415, 416, 417, 418, 419, 420), increased anaerobic glucose metabolism that occurs in transplanted islets (421), likely due to low blood perfusion (422, 423) and decreased tissue oxygen tension (423, 424, 425), or finally a reduced ß-cell to -cell insulin signal. The failure of glucagon to rise in response to hypoglycemia after islet transplantation may put islet transplant recipients at risk for mild hypoglycemic episodes associated with food deprivation or exercise (although to date this has not been observed clinically).

Glycemic control improves the quality of life in diabetic patients. Reduction of mean glycemia prevents or delays microvascular complications (retinopathy, nephropathy, and neuropathy) in both type 1 and type 2 diabetes. It could also reduce macrovascular events. However, iatrogenic hypoglycemia, which is the result of the interplay of absolute or relative insulin excess and compromised glucose counterregulation in type 1 and advanced type 2 diabetes, is the limiting factor for the glycemic management of diabetes (426). Iatrogenic hypoglycemia frequently causes recurrent physical and psychosocial morbidity often with fatal outcome (19). Furthermore, it precludes true glycemic control (i.e., maintenance of euglycemia over time) in the vast majority of subjects with diabetes. As documented, e.g., in the United Kingdom Prospective Diabetes Study, complications develop and progress despite aggressive therapy (427). Thus, if we learn to prevent, correct, or compensate for compromised glucose counterregulation, we would be able to achieve near euglycemia safely and thus more fully realize the benefits of glycemic control in the diabetic patient.

	IX. Summary and Conclusions

Top Abstract I. Introduction II. Islet Endocrine Cell... III. Paracrine, Autocrine, and... IV. Autonomic Regulation of... V. Transcriptional Control of... VI. The Glucagon Gene:... VII. {alpha}-Cell Stimulus... VIII. {alpha}-Cell... IX. Summary and Conclusions Note Added in Proof References

 
Despite intense research over the last 35 yr, the molecular mechanisms whereby glucose and other major nutrients modulate the -cell stimulus secretion coupling remain an enigma. Advances have been made in understanding how plasma membrane ion channel activity modulates -cell electrical activity and consequently glucagon release. Furthermore, accumulating evidence suggests that a complex network of interacting paracrine, hormonal, and neuronal signaling pathways modulate glucagon secretion. Important species differences in the -cell stimulus secretion coupling as well as in the relative importance of the different components of the signaling networks have significantly hampered our ability to propose a unifying hypothesis for regulation of glucagon secretion. However, accumulating evidence suggests that intraislet insulin and zinc play prominent roles for regulating glucagon release in response to hypoglycemia. A direct inhibitory action of glucose on -cell secretion seems to be of little physiological significance and at least for the rat -cell glucose has a direct stimulatory action on glucagon secretion via mechanisms reminiscent of those described for the ß-cell. Emerging evidence suggests a central role of glucose-sensing neurons in the VMH for the integration and control of whole body glucose homeostasis. However, the relative importance of peripheral vs. central control of glucagon secretion in the regulation of glucose homeostasis remains elusive. With the increased availability of human islets, it will be thrilling to investigate the stimulus secretion events in human -cells. Special focus should be devoted to understanding how glucose inhibits glucagon secretion because evidence suggests that -cell dysfunction associated with diabetes progression is principally a defect in the ability of glucose to suppress glucagon secretion. Therefore, defective signaling rather than a change in -cell mass must be involved (428, 429, 430). This could result from alterations in the paracrine, hormonal, or nervous signaling networks involved in glucose sensing. It is likely that the -cell dysfunction results from a combination of multiple factors, genetic as well as environmental, which will clearly complicate the investigations.

Three decades ago Unger and Orci (431) proposed the "bihormonal hypothesis" to explain the origin of hyperglycemia. The model takes into account that in addition to relative or absolute hypoinsulinemia, hyperglucagonemia is essential in the pathogenesis of type 2 diabetes. Since then, additional and important evidence has accumulated implicating hyperglucagonemia in the development of type 2 diabetes. This has prompted major investments in the development of potential drug candidates antagonizing glucagon action. A better understanding of the -cell (patho)physiology in the disease progression will aid the identification of new and better drug targets and therapies. However, it is important to emphasize that although the therapeutic potential of antagonism of glucagon in the treatment of diabetes is likely to be highly beneficial, it will not be a cure.

	Note Added in Proof

Top Abstract I. Introduction II. Islet Endocrine Cell... III. Paracrine, Autocrine, and... IV. Autonomic Regulation of... V. Transcriptional Control of... VI. The Glucagon Gene:... VII. {alpha}-Cell Stimulus... VIII. {alpha}-Cell... IX. Summary and Conclusions Note Added in Proof References

 
The importance of zinc in the suppression of glucagon secretion was further emphasized in a recent study (433). The authors show that switching off pancreatic infusion of insulin or zinc, but not zinc-free insulin, increases glucagon secretion during hypoglycemia in streptozotocin-diabetic rats.

	Acknowledgments

 
We thank Krister Bokvist and Sten Theander for critical reading of the manuscript.

	Footnotes

 
Initial parts of this manuscript were written during J.G.’s employment at Eli Lilly & Co. (Hamburg, Germany). C.B.W. was supported by the Swiss National Science Foundation.

Disclosure Statement: I.F. has nothing to disclose. C.B.W. consults ad hoc for Eli Lilly & Co., Roche, and Takeda. J.G. holds stock in Novo Nordisk A/S, Eli Lilly & Co., and Novartis.

First Published Online January 16, 2007

Abbreviations: A-current, Transient K⁺-channel; 4-AP, 4-aminopuridine; bHLH, basic helix-loop-helix; Brn-4, brain-4; [Ca²⁺]_i, intracellular Ca²⁺ concentration; cAMP-GEFII, cAMP-guanidine nucleotide exchange factor II; CNS, central nervous system; E, embryonic day; FACS, fluorescence-activated cell sorting (or sorted); FFA, free fatty acid; GABA, -aminobutyric acid; GHS-R, ghrelin receptor; GLP, glucagon-like peptide; GRPP, glicentin-related polypeptide; HVA, high voltage-activated; IP, intervening peptide; K_ATP-channel, ATP-sensitive K⁺-channel; K_Dr-channel, delayed rectifying K⁺-channel; K_i, half-maximal inhibition; MCT-1, monocarboxylate transporter-1; MPGF, major proglucagon fragment; Ngn3, neurogenin 3; PC, prohormone convertase; PI3K, phosphatidylinositol 3 kinase; PKA, protein kinase A; PP, pancreatic polypeptide; SUR1, sulfonylurea receptor subunit 1; TEA, tetraethylammonium; TTX, tetrodotoxin; VMH, ventromedial hypothalamus.

	References

Top Abstract I. Introduction II. Islet Endocrine Cell... III. Paracrine, Autocrine, and... IV. Autonomic Regulation of... V. Transcriptional Control of... VI. The Glucagon Gene:... VII. {alpha}-Cell Stimulus... VIII. {alpha}-Cell... IX. Summary and Conclusions Note Added in Proof References

 

1. Best CH 1972 Letter to PP Foà cited in Changing concepts of glucagon: fifty years of research. In: Lefèbvre PJ, Unger RH, eds. Glucagon: molecular physiology, clinical and therapeutic implications. Oxford, UK: Pergamon; 1
2. Murlin JR, Clough HD, Gibbs CBF, Stokes AM 1923 Aqueous extracts of the pancreas. 1. Influence on the carbohydrate metabolism of depancreatized animals. J Biol Chem 56:253–296[Free Full Text]
3. Sutherland EW, de Duve C 1948 Origin and distribution of the hyperglycemic-glycogenolytic factor of pancreas. J Biol Chem 175:663–674[Free Full Text]
4. Foà PP, Weinstein HR, Smith JA 1949 Secretion of insulin and of a hyperglycemic substance studied by means of pancreatic-femoral cross-circulation experiments. Am J Physiol 157:197–204[Free Full Text]
5. Foà PP, Santamaria L, Weinstein H, Berger S, Smith JA 1952 Secretion of the hyperglycemic-glycogenolytic factor in normal dog. Am J Physiol 171:32–36[Free Full Text]
6. Foà PP, Galansino G, Pozza G 1957 Glucagon, a second pancreatic hormone. Recent Prog Horm Res 13:473–510[Medline]
7. Unger RH, Eisentraut AM, McCall MS, Keller S, Lanz HC, Madison LL 1959 Glucagon antibodies and their use for immunoassay for glucagon. Proc Soc Exp Biol Med 102:621–623[Medline]
8. Unger RH, Eisentraut AM, McCall MS, Madison LL, Sims KR, Timm L, Patman L 1961 Glucagon antibodies and an immunoassay for glucagon. J Clin Invest 40:1280–1289[Medline]
9. Unger RH, Eisentraut AM, McCall MS, Madison LL, Sims KR, Patman L 1962 Measurements of endogenous glucagon in plasma and the influence of blood glucose concentration upon its secretion. J Clin Invest 41:682–689[Medline]
10. Haymond MW, Schreiner B 2001 Mini-dose glucagon rescue for hypoglycemia in children with type 1 diabetes. Diabetes Care 24:643–645[Abstract/Free Full Text]
11. Lins P-E, Wajngot A, Adamson U, Vranic M, Efendic S 1983 Minimal increases in glucagon levels enhance glucose production in man with partial hypoinsulinemia. Diabetes 32:633–636[Abstract]
12. Myers SR, Diamond MP, Adkins-Marshall BA, Williams PE, Stinsen R, Cherrington AD 1991 Effects of small changes in glucagon and glucose production during a euglycemic, hyperinsulinemic clamp. Metabolism 40:66–71[CrossRef][Medline]
13. Cryer PE 2001 The prevention and correction of hypoglycemia. In: Jefferson LS, Charrington AD, eds. Handbook of physiology: a critical, comprehensive presentation of physiological knowledge and concepts. Section 7. The endocrine system. Vol 2. The endocrine pancreas and regulation of metabolism. New York: Oxford University Press; 1057–1092
14. Franklin I, Gromada J, Gjinovci A, Theander S, Wollheim CB 2005 ß-Cell secretory products activate -cell ATP-dependent potassium channels to inhibit glucagon release. Diabetes 54:1808–1815[Abstract/Free Full Text]
15. Olsen HL, Theander S, Bokvist K, Buschard K, Wollheim CB, Gromada J 2005 Glucose stimulates glucagon release in single rat -cells by mechanisms that mirror the stimulus-secretion coupling in ß-cells. Endocrinology 146:4861–4870[CrossRef][Medline]
16. Evans ML, McCrimmon RJ, Flanagan DE, Keshavarz T, Fan X, McNay EC, Jacob RJ, Sherwin RS 2004 Hypothalamic ATP-sensitive K⁺ channels play a key role in sensing hypoglycemia and triggering counterregulatory epinephrine and glucagon responses. Diabetes 53:2542–2551[Abstract/Free Full Text]
17. Song Z, Levin BE, McArdle JJ, Bakhos N, Routh VH 2001 Convergence of pre- and postsynaptic influences on glucosensing neurons in the ventromedial hypothalamic nucleus. Diabetes 50:2673–2681[Abstract/Free Full Text]
18. Song Z, Routh VH 2005 Differential effects of glucose and lactate on glucosensing neurons in the ventromedial hypothalamic nucleus. Diabetes 54:15–22[Abstract/Free Full Text]
19. Cryer PE 2004 Diverse causes of hypoglycemia-associated autonomic failure in diabetes. N Engl J Med 350:2272–2279[Free Full Text]
20. Lane MA 1907 The cytological characters of the areas of Langerhans. Am J Anat 7:409–422[CrossRef]
21. Wierup N, Svensson H, Mulder H, Sundler F 2002 The ghrelin cell: a novel developmentally regulated islet cell in the human pancreas. Regul Pept 107:63–69[CrossRef][Medline]
22. Wierup N, Yang S, McEvilly RJ, Mulder H, Sundler F 2004 Ghrelin is expressed in a novel endocrine cell type in developing rat islets and inhibits insulin secretion from INS-1 (832/13) cells. J Histochem Cytochem 52:301–310[Abstract/Free Full Text]
23. Lee HM, Wang G, Englander EW, Kojima M, Greeley Jr GH 2002 Ghrelin, a new gastrointestinal endocrine peptide that stimulates insulin secretion: enteric distribution, ontogeny, influence of endocrine, and dietary manipulations. Endocrinology 143:185–190[Abstract/Free Full Text]
24. Cabrera O, Berman DM, Kenyon NS, Ricordi C, Berggren PO, Caicedo A 2006 The unique cytoarchitecture of human pancreatic islets has implications for islet cell function. Proc Natl Acad Sci USA 103:2334–2339[Abstract/Free Full Text]
25. Bonner-Weir S, Orci L 1982 New perspectives on the microvasculature of the islets of Langerhans in the rat. Diabetes 31:883–889[Abstract]
26. Orci L, Baetens D, Rufener C, Amherdt M, Ravazzola M, Studer P, Malaisse-Lagae F, Unger RH 1976 Hypertrophy and hyperplasia of somatostatin-containing D-cells in diabetes. Proc Natl Acad Sci USA 73:1338–1342[Abstract/Free Full Text]
27. Rahier J, Goebbels RM, Henquin JC 1983 Cellular composition of the human diabetic pancreas. Diabetologia 24:366–371[Medline]
28. Butler AE, Janson J, Bonner-Weir S, Ritzel R, Rizza RA, Butler PC 2003 ß-Cell deficit and increased ß-cell apoptosis in humans with type 2 diabetes. Diabetes 52:102–110[Abstract/Free Full Text]
29. Ageev AK 1984 Age and changes in the proportions of cells in the pancreatic islets of the human pancreas. Arkh Anat Gistol Embriol 87:57–62[Medline]
30. Murakami T, Fujita T 1992 Microcirculation of the rat pancreas, with special reference to the insulo-acinar portal and insulo-venous drainage systems: a further scanning electron microscope study of corrosion casts. Arch Histol Cytol 55:453–476[Medline]
31. Moldovan S, Brunicardi FC 2001 Endocrine pancreas: summary of observations generated by surgical fellows. World J Surg 25:468–473[CrossRef][Medline]
32. Liu YM, Guth PH, Kaneko K, Livingston EH, Brunicardi FC 1993 Dynamic in vivo observation of rat islet microcirculation. Pancreas 8:15–21[Medline]
33. Samols E, Stagner JI 1988 Intra-islet regulation. Am J Med 85:31–35[Medline]
34. Samols E, Stagner JI, Ewart RB, Marks V 1988 The order of islet microvascular cellular perfusion is BAD in the perfused rat pancreas. J Clin Invest 82:350–353[Medline]
35. Stagner JI, Samols E 1992 The vascular order of islet cellular perfusion in the human pancreas. Diabetes 41:93–97[Abstract]
36. Brunicardi FC, Kleinman R, Moldovan S, Nguyen TH, Watt PC, Walsh J, Gingerich R 2001 Immunoneutralization of somatostatin, insulin, and glucagon causes alterations in islet cell secretion in the isolated perfused human pancreas. Pancreas 23:302–308[CrossRef][Medline]
37. Halban PA, Wollheim CB, Blondel B, Meda P, Niesor EN, Mintz DH 1982 The possible importance of contact between pancreatic islet cells for the control of insulin release. Endocrinology 111:86–94[Abstract/Free Full Text]
38. Meda P 2003 Cx36 involvement in insulin secretion: characteristics and mechanism. Cell Commun Adhes 10:431–435[Medline]
39. Serre-Beinier V, Le Gurun S, Belluardo N, Trovato-Salinaro A, Charolllais A, Haefliger J-A, Condorelli DF, Meda P 2000 Cx36 preferentially connects ß-cells within pancreatic islets. Diabetes 49:727–734[Abstract]
40. Theis M, Mas C, Döring B, Degen J, Brink C, Caille D, Charollais A, Krüger O, Plum A, Nepote V, Herrera P, Meda P, Willecke K 2004 Replacement by a lacZ reporter gene assigns mouse connexin 36, 45 and 43 to distinct cell types in pancreatic islets. Exp Cell Res 294:18–29[CrossRef][Medline]
41. Charollais A, Gjinovci A, Huarte J, Bauquis J, Nadal A, Andreu FM, Sanchez-Andres JV, Calabrese A, Bosco D, Soria B, Wollheim CB, Herrera PL, Meda P 2000 Junctional communication of pancreatic ß cells contributes to the control of insulin secretion and glucose tolerance. J Clin Invest 106:235–243[Medline]
42. Sorenson RL, Garry DG, Brelje TC 1991 Structural and functional considerations of GABA in islets of Langerhans. ß-Cells and nerves. Diabetes 40:1365–1374[Abstract]
43. Ahren B 2000 Autonomic regulation of islet hormone secretion - implications for health and disease. Diabetologia 43:393–410[CrossRef][Medline]
44. Schuit FC, Pipeleers DG 1986 Differences in adrenergic recognition by pancreatic A and B cells. Science 232:875–877[Abstract/Free Full Text]
45. Vieira E, Liu Y-J, Gylfe E 2004 Involvement of 1 and ß-adrenoceptors in adrenaline stimulation of the glucagon-secreting mouse -cell. Nauyn-Schmiedebergs Arch Pharmacol 369:179–183[CrossRef][Medline]
46. Sherck SM, Shiota M, Saccomando J, Cardin S, Allen EJ, Hastings JR, Neal DW, Williams PE, Cherrington AD 2001 Pancreatic response to mild non-insulin-induced hypoglycemia does not involve extrinsic neural input. Diabetes 50:2487–2496[Abstract/Free Full Text]
47. Diem P, Redmon JB, Abid M, Moran A, Sutherland DE, Halter JB, Robertson RP 1990 Glucagon, catecholamine and pancreatic polypeptide secretion in type 1 diabetic recipients of pancreas allografts. J Clin Invest 86:2008–2013[Medline]
48. Schuit FC, Huypens P, Heimberg H, Pipeleers DG 2001 Glucose sensing in pancreatic ß-cells: a model for the study of other glucose-regulated cells in gut, pancreas, and hypothalamus. Diabetes 50:1–11[Abstract/Free Full Text]
49. Rossi J, Santamaki P, Airaksinen MS, Herzig KH 2005 Parasympathetic innervation and function of endocrine pancreas requires the glial cell line-derived factor family receptor 2 (GFR2). Diabetes 54:1324–1330[Abstract/Free Full Text]
50. Kurose T, Tsuda K, Ishida H, Tsuji K, Okamoto Y, Tsuura Y, Kato S, Usami M, Imura H, Seino Y 1992 Glucagon, insulin and somatostatin secretion in response to sympathetic neural activation in streptozotocin-induced diabetic rats. A study with the isolated perfused rat pancreas in vitro. Diabetologia 35:1035–1041[CrossRef][Medline]
51. Bolli G, de Feo P, Compagnucci P, Cartechini MG, Angeletti G, Santeusanio F, Brunetti P, Gerich JE 1983 Abnormal glucose counterregulation in insulin-dependent diabetes mellitus. Interaction of anti-insulin antibodies and impaired glucagon and epinephrine secretion. Diabetes 32:134–141[Abstract]
52. Greenbaum CJ, Prigeon RL, D’Alessio DA 2002 Impaired ß-cell function, incretin effect, and glucagon suppression in patients with type 1 diabetes who have normal fasting glucose. Diabetes 51:951–957[Abstract/Free Full Text]
53. Braaten JT, Faloona GR, Unger RH 1974 The effect of insulin on the -cell response to hyperglycemia in long-standing alloxan diabetes. J Clin Invest 53:1017–1021[Medline]
54. Stagner JI, Samols E 1986 Retrograde perfusion as a model for testing the relative effects of glucose versus insulin on the A cell. J Clin Invest 77:1034–1037[Medline]
55. Salehi A, Vieira E, Gylfe E 2006 Paradoxical stimulation of glucagon secretion by high glucose concentrations. Diabetes 55:2318–2323[Abstract/Free Full Text]
56. Hussain K, Bryan J, Christesen HT, Brusgaard K, Aguilar-Bryan L 2005 Serum glucagon counterregulatory hormonal response to hypoglycemia is blunted in congenital hyperinsulinism. Diabetes 54:2946–2951[Abstract/Free Full Text]
57. Ishihara H, Maechler P, Gjinovci A, Herrera PL, Wollheim CB 2003 Islet ß-cell secretion determines glucagon release from neighboring -cells. Nat Cell Biol 5:330–335[CrossRef][Medline]
58. Takahashi R, Ishihara H, Tamura A, Yamaguchi S, Yamada F, Takei D, Katagiri H, Endou H, Oka Y 2006 Cell-type specific activation of metabolism reveals that ß-cell secretion suppresses glucagon release from -cells in rat pancreatic islets. Am J Physiol 290:E308–E316
59. Iezzi M, Kouri G, Fukuda M, Wollheim CB 2004 Synaptotagmin V and IX isoforms control Ca²⁺-dependent insulin exocytosis. J Cell Sci 117:3119–3127[Abstract/Free Full Text]
60. Ravier MA, Rutter GA 2005 Glucose or insulin, but not zinc ions, inhibit glucagon secretion from mouse pancreatic -cells. Diabetes 54:1789–1797[Abstract/Free Full Text]
61. Matschinsky FM, Rujanavech C, Pagliara A, Norfleet WT 1980 Adaptations of -2- and ß-cells of rat and mouse pancreatic islets to starvation, to refeeding after starvation, and to obesity. J Clin Invest 65:207–218[Medline]
62. Takahashi N, Kishimoto T, Nemoto T, Kadowaki T, Kasai H 2002 Fusion pore dynamics and insulin granule exocytosis in the pancreatic islet. Science 297:1349–1352[Abstract/Free Full Text]
63. Unger RH 1985 Glucagon physiology and pathophysiology in the light of new advances. Diabetologia 28:574–578[Medline]
64. Maruyama H, Hisatomi A, Orci L, Grodsky GM, Unger RH 1984 Insulin within islets is a physiologic glucagon release inhibitor. J Clin Invest 74:2296–2299[Medline]
65. Gromada J, Ma X, Hoy M, Bokvist K, Salehi A, Berggren PO, Rorsman P 2004 ATP-sensitive K⁺ channel-dependent regulation of glucagon release and electrical activity by glucose in wild-type and SUR1–/– mouse -cells. Diabetes 53(Suppl 3):S181–S189
66. Muñoz A, Hu M, Hussain K, Bryan J, Aguilar-Bryan L, Rajan AS 2005 Regulation of glucagon secretion at low glucose concentration: evidence for adenosine triphosphate-sensitive potassium channel involvement. Endocrinology 146:5514–5521[CrossRef][Medline]
67. Meier JJ, Kjems LL, Veldhuis JD, Lefebvre P, Butler PC 2006 Postprandial suppression of glucagon secretion depends on intact pulsatile insulin secretion. Diabetes 55:1051–1056[Abstract/Free Full Text]
68. Weir GC, Knowlton SD, Atkins RF, McKennan KX, Martin DB 1976 Glucagon secretion from the perfused pancreas of streptozotocin-treated rats. Diabetes 25:275–282[Abstract]
69. Greenbaum CJ, Havel PJ, Taborsky GJ, Klaff LJ 1991 Intra-islet insulin permits glucose to directly suppress pancreatic A cell function. J Clin Invest 88:767–773[Medline]
70. Dunbar JC, Brown A 1984 Glucagon secretion by dispersed -cell enriched islets from streptozotocin treated hamsters in perifusion. Horm Metab Res 16:221–225[Medline]
71. Östenson CG, Grebing C 1985 Evidence for metabolic regulation of pancreatic glucagon secretion by L-glutamine. Acta Endocrinol (Copenh) 108:386–391[Abstract/Free Full Text]
72. Banarer S, McGregor VP, Cryer PE 2002 Intraislet hyperinsulinemia prevents the glucagon response to hypoglycemia despite an intact autonomic response. Diabetes 51:958–965[Abstract/Free Full Text]
73. Raju B, Cryer PE 2005 Loss of the decrement in intraislet insulin plausibly explains loss of the glucagon response to hypoglycemia in insulin-deficient diabetes. Diabetes 54:757–764[Abstract/Free Full Text]
74. Israelian Z, Gosmanov NR, Szoke E, Schorr M, Bokhari S, Cryer PE, Gerich JE, Meyer C 2005 Increasing the decrement in insulin secretion improves glucagon responses to hypoglycemia in advanced type 2 diabetes. Diabetes Care 28:2691–2696[Abstract/Free Full Text]
75. Zhou H, Tran POT, Yang S, Zhang T, LeRoy E, Oseid E, Robertson RP 2004 Regulation of -cell function by the ß-cell during hypoglycemia in Wistar rats: the "switch-off" hypothesis. Diabetes 53:1482–1487[Abstract/Free Full Text]
76. Hope KM, Tran POT, Zhou H, Oseid E, Leroy E, Robertson RP 2004 Regulation of -cell function by the ß-cell in isolated human and rat islets deprived of glucose: the "switch-off" hypothesis. Diabetes 53:1488–1495[Abstract/Free Full Text]
77. Kisanuki K, Kishikawa H, Araki E, Shirotani T, Uehara M, Isami S, Ura S, Jinnouchi H, Miyamura N, Shichiri M 1995 Expression of insulin receptor on clonal pancreatic cells and its possible role for insulin-stimulated negative regulation of glucagon secretion. Diabetologia 38:422–429[Medline]
78. Leung YM, Ahmed I, Sheu L, Gao, X, Hara M, Tsushima RG, Diamant NE, Gaisano HY 2006 Insulin regulates islet -cell function by reducing K_ATP channel sensitivity to adenosine 5'-triphosphate inhibition. Endocrinology 147:2155–2162[CrossRef][Medline]
79. Khan FA, Goforth PB, Zhang M, Satin LS 2001 Insulin activates ATP-sensitive K⁺ channels in pancreatic ß-cells through a phosphatidylinositol 3-kinase-dependent pathway. Diabetes 50:2192–2198[Abstract/Free Full Text]
80. Spanswick D, Smith MA, Mirshamsi S, Routh VH, Ashford ML 2000 Insulin activates ATP-sensitive K⁺ channels in hypothalamic neurons of lean, but not obese rats. Nat Neurosci 3:757–758[CrossRef][Medline]
81. Mirshamsi S, Laidlaw HA, Ning K, Anderson E, Burgess LA, Gray A, Sutherland C, Ashford ML 2004 Leptin and insulin stimulation of signalling pathways in arcuate nucleus neurones: PI3K dependent actin reorganization and K_ATP channel activation. BMC Neurosci 5:54[CrossRef][Medline]
82. Xu E, Kumar M, Zhang Y, Ju W, Obata T, Zhang N, Liu S, Wendt A, Deng S, Ebina Y, Wheeler MB, Braun M, Wang Q 2006 Intra-islet insulin suppresses glucagon release via GABA-GABA_A receptor system. Cell Metab 3:47–58[CrossRef][Medline]
83. Pipeleers DG, Schuit FC, Van Schravendijk CF, Van De Winkel M 1985 Interplay of nutrients and hormones in the regulation of glucagon release. Endocrinology 117:817–823[Abstract/Free Full Text]
84. Blundell T, Dodson G, Hodgkin D, Mercola D 1971 Insulin: the structure in the crystal and its reflection in chemistry and biology. Adv Protein Chem 26:279–402
85. Dodson G, Steiner D 1998 The role of assembly in insulin’s biosynthesis. Curr Opin Struct Biol 8:189–194[CrossRef][Medline]
86. Kristiansen LH, Rungby J, Sondergaard LG, Stoltenberg M, Danscher G 2001 Autometallography allows ultrastructural monitoring of zinc in the endocrine pancreas. Histochem Cell Biol 115:125–129[CrossRef][Medline]
87. Gee KR, Zhou ZL, Qian WJ, Kennedy R 2002 Detection and imaging of zinc secretion from pancreatic ß-cells using a new fluorescent zinc indicator. J Am Chem Soc 124:776–778[CrossRef][Medline]
88. Bancila V, Nikonenko I, Dunant Y, Bloc A 2004 Zinc inhibits glutamate release via activation of pre-synaptic K channels and reduces ischaemic damage in rat hippocampus. J Neurochem 90:1243–1250[CrossRef][Medline]
89. Bloc A, Cens T, Cruz H, Dunant Y 2000 Zinc-induced changes in ionic currents of clonal rat pancreatic ß-cells: activation of ATP-sensitive K⁺ channels. J Physiol 529:723–734[Abstract/Free Full Text]
90. Bancila V, Cens T, Monnier D, Chanson F, Faure C, Dunant Y, Bloc A 2005 Two SUR1-specific histidine residues mandatory for zinc-induced activation of the rat K_ATP channel. J Biol Chem 280:8793–8799[Abstract/Free Full Text]
91. Ashcroft F, Rorsman P 2004 Type 2 diabetes mellitus: not quite exciting enough? Hum Mol Genet 13(Suppl 1):R21–R31
92. Cherubini E, Gaiarsa JL, Ben-Ari Y 1991 GABA: an excitatory transmitter in early postnatal life. Trends Neurosci 14:515–519[CrossRef][Medline]
93. Franklin IK, Wollheim CB 2004 GABA in the endocrine pancreas: its putative role as an islet cell paracrine-signalling molecule. J Gen Physiol 123:185–190[Free Full Text]
94. Braun M, Wendt A, Buschard K, Salehi A, Sewing S, Gromada J, Rorsman P 2004 GABA_B receptor activation inhibits exocytosis in rat pancreatic ß-cells by G-protein-dependent activation of calcineurin. J Physiol 559:395–409
95. MacDonald PE, Obermüller S, Vikman J, Galvanovskis J, Rorsman P, Eliasson L 2005 Regulated exocytosis and kiss-and-run of synaptic-like microvesicles in INS-1 and primary rat ß-cells. Diabetes 54:736–743[Abstract/Free Full Text]
96. Wendt A, Birnir B, Buschard K, Gromada J, Salehi A, Sewing S, Rorsman P, Braun M 2004 Glucose inhibition of glucagon secretion from rat -cells is mediated by GABA released from neighboring ß-cells. Diabetes 53:1038–1045[Abstract/Free Full Text]
97. Gilon P, Bertrand G, Loubatieres-Mariani MM, Remacle C, Henquin JC 1991 The influence of -aminobutyric acid on hormone release by the mouse and rat endocrine pancreas. Endocrinology 129:2521–2529[Abstract/Free Full Text]
98. Rorsman P, Berggren P-O, Bokvist K, Ericson H, Möhler H, Östenson C-G, Smith PA 1989 Glucose-inhibition of glucagon secretion involves activation of GABA_A-receptor chloride channels. Nature 341:233–236[CrossRef][Medline]
99. Gaskins HR, Baldeon ME, Selassie L, Beverly JL 1995 Glucose modulates -aminobutyric acid release from the pancreatic ßTC6 cell line. J Biol Chem 270:30286–30289[Abstract/Free Full Text]
100. Winnock F, Ling Z, De Proft R, Dejonghe S, Schuit F, Gorus F, Pipeleers D 2002 Correlation between GABA release from rat islet ß-cells and their metabolic state. Am J Physiol Endocrinol Metab 282:E937–E942
101. Wang C, Kerckhofs K, Van de Casteele M, Smolders I, Pipeleers D, Ling Z 2006 Glucose inhibits GABA release by pancreatic ß-cells through an increase in GABA shunt activity. Am J Physiol Endocrinol Metab 290:E494–E499
102. Braun M, Wendt AK, Birnir B, Broman J, Eliasson L, Galvanovskis J, Gromada J, Mulder H, Rorsman P 2004 Regulated exocytosis of GABA-contained synaptic-like microvesicles in pancreatic ß-cells. J Gen Physiol 123:191–204[Abstract/Free Full Text]
103. Moriyama Y, Hayashi M 2003 Glutamate-mediated signaling in the islets of Langerhans: a thread entangled. Trends Pharmacol Sci 24:511–517[CrossRef][Medline]
104. Maechler P, Wollheim CB 1999 Mitochondrial glutamate acts as a messenger in glucose-induced insulin exocytosis. Nature 402:685–689[CrossRef][Medline]
105. Tong Q, Ouedraogo R, Kirchgessner AL 2002 Localization and function of group III metabotropic glutamate receptors in rat pancreatic islets. Am J Physiol Endocrinol Metab 282:E1324–E1333
106. Hayashi M, Yamada H, Uehara S, Morimoto R, Muroyama A, Yatsushiro S, Takeda J, Yamamoto A, Moriyama Y 2003 Secretory granule-mediated co-secretion of L-glutamate and glucagon triggers glutamatergic signal transmission in islets of Langerhans. J Biol Chem 278:1966–1974[Abstract/Free Full Text]
107. Yamada H, Otsuka M, Hayashi M, Nakatsuka S, Hamaguchi K, Yamamoto A, Moriyama Y 2001 Ca²⁺-dependent exocytosis of L-glutamate by TC6, clonal mouse pancreatic -cells. Diabetes 50:1012–1020[Abstract/Free Full Text]
108. Bai L, Zhang X, Ghishan F 2003 Characterization of vesiclar glutamate transporters in pancreatic - and ß-cells and its regulation by glucose. Am J Physiol 284:G808–G814
109. Weaver CD, Gundersen V, Verdoorn TA 1998 A high affinity glutamate/aspartate transport system in pancreatic islets of Langerhans modulates glucose-stimulated insulin secretion. J Biol Chem 273:1647–1653[Abstract/Free Full Text]
110. Weaver CD, Yao TL, Powers AC, Verdoorn TA 1996 Differential expression of glutamate receptor subtypes in rat pancreatic islets. J Biol Chem 271:12977–12984[Abstract/Free Full Text]
111. Uehara S, Muroyama A, Echigo N, Morimoto R, Otsuka M, Yatsushiro S, Moriyama Y 2004 Metabotropic glutamate receptor type 4 is involved in autoinhibitory cascade for glucagon secretion by -cells of islet of Langerhans. Diabetes 53:998–1006[Abstract/Free Full Text]
112. Brice NL, Varadi A, Ashcroft SJ, Molnar E 2002 Metabotropic glutamate and GABA_B receptors contribute to the modulation of glucose-stimulated insulin secretion in pancreatic ß cells. Diabetologia 45:242–252[CrossRef][Medline]
113. Inagaki N, Kuromi H, Gonoi T, Okamoto Y, Ishida H, Seino Y, Kaneko T, Iwanaga T, Seino S 1995 Expression and role of ionotropic glutamate receptors in pancreatic islet cells. FASEB J 9:686–691[Abstract]
114. Gonoi T, Mizuno N, Inagaki N, Kuromi H, Seino Y, Miyazaki J, Seino S 1994 Functional neuronal ionotropic glutamate receptors are expressed in the non-neuronal cell line MIN6. J Biol Chem 269:16989–16992[Abstract/Free Full Text]
115. Morley P, MacLean S, Gendron TF, Small DL, Tremblay R, Durkin JP, Mealing G 2000 Pharmacological and molecular characterization of glutamate receptors in the MIN6 pancreatic ß-cell line. Neurol Res 22:379–385[Medline]
116. Molnar E, Varadi A, McIlhinney RA, Ashcroft SJ 1995 Identification of functional ionotropic glutamate receptor proteins in pancreatic ß-cells and in islets of Langerhans. FEBS Lett 371:253–257[CrossRef][Medline]
117. Muroyama A, Uehara S, Yatsushiro S, Echigo N, Morimoto R, Morita M, Hayashi M, Yamamoto A, Koh DS, Moriyama Y 2004 A novel variant of ionotropic glutamate receptor regulates somatostatin secretion from -cells of islets of Langerhans. Diabetes 53:1743–1753[Abstract/Free Full Text]
118. Bertrand G, Gross R, Puech R, Loubatieres-Mariani MM, Bockaert J 1993 Glutamate stimulates glucagon secretion via an excitatory amino acid receptor of the AMPA subtype in rat pancreas. Eur J Pharmacol 237:45–50[CrossRef][Medline]
119. Bertrand G, Puech R, Loubatieres-Mariani MM, Bockaert J 1995 Glutamate stimulates insulin secretion and improves glucose tolerance in rats. Am J Physiol 269:E551–E556
120. Bertrand G, Gross R, Puech R, Loubatieres-Mariani MM, Bockaert J 1992 Evidence for a glutamate receptor of the AMPA subtype which mediates insulin release from rat perfused pancreas. Br J Pharmacol 106:354–359[Medline]
121. Sakurai H, Dobbs R, Unger RH 1974 Somatostatin-induced changes in insulin and glucagon secretion in normal and diabetic dogs. J Clin Invest 54:1395–1402[Medline]
122. Luft R, Efendic S, Hokfelt T 1978 Somatostatin—both hormone and neurotransmitter? Diabetologia 14:1–13[CrossRef][Medline]
123. Patel YC 1999 Somatostatin and its receptor family. Front Neuroendocrinol 20:157–198[CrossRef][Medline]
124. Kumar U, Sasi R, Suresh S, Patel A, Thangaraju M, Metrakos P, Patel SC, Patel YC 1999 Subtype-selective expression of the five somatostatin receptors (hSSTR1–5) in human pancreatic islet cells: a quantitative double-label immunohistochemical analysis. Diabetes 48:77–85[Abstract]
125. Hunyady B, Hipkin RW, Schonbrunn A, Mezey E 1997 Immunohistochemical localization of somatostatin receptor SST2A in the rat pancreas. Endocrinology 138:2632–2635[CrossRef][Medline]
126. Portela-Gomes GM, Stridsberg M, Grimelius L, Oberg K, Janson ET 2000 Expression of the five different somatostatin receptor subtypes in endocrine cells of the pancreas. Appl Immunohistochem Mol Morphol 8:126–132[CrossRef][Medline]
127. Mitra SW, Mezey E, Hunyady B, Chamberlain L, Hayes E, Foor F, Wang Y, Schonbrunn A, Schaeffer JM 1999 Colocalization of somatostatin receptor sst5 and insulin in rat pancreatic ß-cells. Endocrinology 140:3790–3796[Abstract/Free Full Text]
128. Ludvigsen E, Olsson R, Stridsberg M, Janson ET, Sandler S 2004 Expression and distribution of somatostatin receptor subtypes in the pancreatic islets of mice and rats. J Histochem Cytochem 52:391–400[Abstract/Free Full Text]
129. Gromada J, Høy M, Olsen HL, Gotfredsen CF, Buschard K, Rorsman P, Bokvist K 2001 G_i2 proteins couple somatostatin receptors to low-conductance K⁺ channels in rat pancreatic -cells. Pflügers Arch 442:19–26[CrossRef][Medline]
130. Yoshimoto Y, Fukuyama Y, Horio Y, Inanobe A, Gotoh M, Kurachi Y 1999 Somatostatin induces hyperpolarization in pancreatic islet cells by activating a G protein-gated K⁺ channel. FEBS Lett 444:265–269[CrossRef][Medline]
131. Gromada J, Høy M, Buschard K, Salehi A, Rorsman P 2001 Somatostatin inhibits exocytosis in rat pancreatic -cells by G_i2-dependent activation of calcineurin and depriming of secretory granules. J Physiol 535:519–532[Abstract/Free Full Text]
132. Schuit FC, Derde MP, Pipeleers DG 1989 Sensitivity of rat pancreatic A and B cells to somatostatin. Diabetologia 32:207–212[CrossRef][Medline]
133. Fehmann HC, Strowski M, Goke B 1995 Functional characterization of somatostatin receptors expressed on hamster glucagonoma cells. Am J Physiol 268:E40–E47
134. Hahn HJ, Gottschling HD, Woltanski P 1978 Effect of somatostatin on insulin secretion and cAMP content of isolated pancreatic rat islets. Metabolism 27:1291–1294[CrossRef][Medline]
135. Strowski M, Casher DE, Birzin ET, Yang L, Singh V, Jacks TM, Nowak K, Rohrer SP, Patchett AA, Smith RG, Schaeffer JM 2006 Antidiabetic activity of a highly potent and selective non-peptide somatostatin receptor subtype-2 agonist. Endocrinology 146:4664–4673
136. Rossowski WJ, Coy DH 1994 Specific inhibition of rat pancreatic insulin or glucagon release by receptor-selective somatostatin analogs. Biochem Biophys Res Commun 205:341–346[CrossRef][Medline]
137. Strowski MZ, Parmar RM, Blake AD, Schaeffer JM 2000 Somatostatin inhibits insulin and glucagon secretion via two receptors subtypes: an in vitro study of pancreatic islets from somatostatin receptor 2 knockout mice. Endocrinology 141:111–117[Abstract/Free Full Text]
138. Ludvigsen E, Stridsberg M, Taylor JE, Culler MD, Öberg K, Janson ET, Sandler S, Regulation of insulin and glucagon secretion from rat pancreatic islets in vitro by somatostatin analogues. Regul Pept, in press
139. Cejvan K, Coy DH, Efendic S 2003 Intra-islet somatostatin regulates glucagon release via type 2 somatostatin receptors in rats. Diabetes 52:1176–1181[Abstract/Free Full Text]
140. Kawai K, Ipp E, Orci L, Perrelet A, Unger RH 1982 Circulating somatostatin acts on the islets of Langerhans by way of a somatostatin-poor compartment. Science 218:477–478[Abstract/Free Full Text]
141. Kleinman R, Gingerich R, Ohning G, Wong H, Olthoff K, Walsh J, Brunicardi FC 1995 The influence of somatostatin on glucagon and pancreatic polypeptide secretion in the isolated perfused human pancreas. Int J Pancreatol 18:51–57[Medline]
142. Dimitriadis G, Tessari P, Gerich J 1983 Effects of a long-acting somatostatin analogue on postprandial hyperglycemia in insulin-dependent diabetes mellitus. Metabolism 32:987–992[CrossRef][Medline]
143. Bolaffi JL, Rodd G, Grodsky GM 1990 Effect of glucagon or somatostatin on desensitized insulin secretion. Endocrinology 126:1750–1755[Abstract/Free Full Text]
144. Gerber PP, Trimble ER, Wollheim CB, Renold AE, Miller RE 1981 Glucose and cyclic AMP as stimulators of somatostatin and insulin secretion from the isolated, perfused rat pancreas: a quantitative study. Diabetes 30:40–44[Abstract]
145. Honey RN, Schwarz JA, Mathe CJ, Weir GC 1980 Insulin, glucagon, and somatostatin secretion from isolated perfused rat and chicken pancreas-duodenum. Am J Physiol 238:E150–E156
146. Murakami K, Taniguchi H, Tamagawa M, Ejiri K, Baba S 1982 Modulation of somatostatin release by endogenous glucagon and insulin: physiological relationship between A, B and D cells in rat pancreatic islets. Endocrinol Jpn 29:503–508[Medline]
147. Olofsson CS, Salehi A, Göpel SO, Holm C, Rorsman P 2004 Palmitate stimulation of glucagon secretion in mouse pancreatic -cells results from activation of L-type calcium channels and elevation of cytoplasmic calcium. Diabetes 53:2836–2843[Abstract/Free Full Text]
148. Stagner JI, Samols E, Marks V 1989 The anterograde and retrograde infusion of glucagon antibodies suggests that A cells are vascularly perfused before D cells within the rat islet. Diabetologia 32:203–206[CrossRef][Medline]
149. Brunicardi FC, Elahi D, Andersen DK 1994 Splanchnic neural regulation of somatostatin secretion in the isolated perfused human pancreas. Ann Surg 219:258–266[Medline]
150. Ipp E, Dobbs RE, Arimura A, Vale W, Harris V, Unger RH 1977 Release of immunoreactive somatostatin from the pancreas in response to glucose, amino acids, pancreozymin-cholecystokinin, and tolbutamide. J Clin Invest 60:760–765[Medline]
151. Gerich JE 1990 Role of somatostatin and its analogues in the pathogenesis and treatment of diabetes mellitus. Metabolism 39:52–54[CrossRef][Medline]
152. Göpel S, Zhang Q, Eliasson L, Ma XS, Galvanovskis J, Kanno T, Salehi A, Rorsman P 2004 Capacitance measurements of exocytosis in mouse pancreatic -, ß- and -cells within intact islets of Langerhans. J Physiol 556:711–726[Abstract/Free Full Text]
153. Ullrich S, Prentki M, Wollheim CB 1990 Somatostatin inhibition of Ca²⁺-induced insulin secretion in permeabilized HIT-T15 cells. Biochem J 270:273–276[Medline]
154. Date Y, Nakazato M, Hashiguchi S, Dezaki K, Mondal MS, Hosoda H, Kojima M, Kangawa K, Arima T, Matsuo H, Yada T, Matsukura S 2002 Ghrelin is present in pancreatic -cells of humans and rats and stimulates insulin secretion. Diabetes 51:124–129[Abstract/Free Full Text]
155. Adeghate E, Ponery AS 2002 Ghrelin stimulates insulin secretion from the pancreas of normal and diabetic rats. J Neuroendocrinol 14:555–560[CrossRef][Medline]
156. Kageyama H, Funahashi H, Hirayama M, Takenoya F, Kita T, Kato S, Sakurai J, Lee EY, Inoue S, Date Y, Nakazato M, Kangawa K, Shioda S 2005 Morphological analysis of ghrelin and its receptor distribution in the rat pancreas. Regul Pept 126:67–71[CrossRef][Medline]
157. Heller RS, Jenny M, Collombat P, Mansouri A, Tomasetto C, Madsen OD, Mellitzer G, Gradwohl G, Serup P 2005 Genetic determinants of pancreatic -cell development. Dev Biol 286:217–224[CrossRef][Medline]
158. Prado CL, Pugh-Bernard AE, Elghazi L, Sosa-Pineda B, Sussel L 2004 Ghrelin cells replace insulin-producing ß cells in two mouse models of pancreas development. Proc Natl Acad Sci USA 101:2924–2929[Abstract/Free Full Text]
159. Egido EM, Rodriguez-Gallardo J, Silvestre RA, Marco J 2002 Inhibitory effect of ghrelin on insulin and pancreatic somatostatin secretion. Eur J Endocrinol 146:241–244[Abstract]
160. Salehi A, Dornonville de la Cour C, Hakanson R, Lundquist I 2004 Effects of ghrelin on insulin and glucagon secretion: a study of isolated pancreatic islets and intact mice. Regul Pept 118:143–150[CrossRef][Medline]
161. Zhang JV, Ren P-G, Avsian-Kretchmer O, Luo C-W, Rauch R, Klein C, Hsueh AJW 2005 Obestatin, a peptide encoded by the ghrelin gene, opposes ghrelin’s effects on food intake. Science 310:996–999[Abstract/Free Full Text]
162. McKee KK, Tan CP, Palyha OC, Liu J, Feighner SD, Hreniuk DL, Smith RG, Howard AD, Van der Ploeg LHT 1997 Cloning and characterization of two human G protein-coupled receptor genes (GPR38 and GPR39) related to the growth hormone secretagogue and neurotensin receptors. Genomics 46:426–434[CrossRef][Medline]
163. Ahren B 2004 Sensory nerves contribute to insulin secretion by glucagon-like peptide-1 in mice. Am J Physiol Regul Integr Comp Physiol 286:R269–R272
164. Dunning BE, Foley JE, Ahren B 2005 -Cell function in health and disease: influence of glucagon-like peptide-1. Diabetologia 48:1700–1713[CrossRef][Medline]
165. Creutzfeldt WO, Kleine N, Willms B, Orskov C, Holst JJ, Nauck MA 1996 Glucagonostatic actions and reduction of fasting hyperglycemia by exogenous glucagon-like peptide I(7–36) amide in type I diabetic patients. Diabetes Care 19:580–586[Abstract]
166. Meneilly GS, McIntosh CH, Pederson RA, Habener JF, Ehlers MR, Egan JM, Elahi D 2003 Effect of glucagon-like peptide 1 (7–36 amide) on insulin-mediated glucose uptake in patients with type 1 diabetes. Diabetes Care 26:837–842[Abstract/Free Full Text]
167. Behme MT, Dupre J, McDonald TJ 2003 Glucagon-like peptide 1 improved glycemic control in type 1 diabetes. BMC Endocr Disord 3:3[CrossRef][Medline]
168. Silvestre RA, Rodriguez-Gallardo J, Egido EM, Marco J 2003 Interrelationship among insulin, glucagon and somatostatin secretory responses to exendin-4 in the perfused rat pancreas. Eur J Pharmacol 469:195–200[CrossRef][Medline]
169. Moens K, Heimberg H, Flamez D, Huypens P, Quartier E, Ling Z, Pipeleers D, Gremlich S, Thorens B, Schuit F 1996 Expression and functional activity of glucagon, glucagon-like peptide I, and glucose-dependent insulinotropic peptide receptors in rat pancreatic islet cells. Diabetes 45:257–261[Abstract]
170. Heller RS, Kieffer TJ, Habener JF 1997 Insulinotropic glucagon-like peptide I receptor expression in glucagon-producing -cells of the rat endocrine pancreas. Diabetes 46:785–791[Abstract]
171. Ding W-G, Renström E, Rorsman P, Buschard K, Gromada J 1997 Glucagon-like peptide 1 and glucose-dependent insulinotropic polypeptide stimulate Ca²⁺-induced secretion in rat -cells by a protein kinase A-mediated mechanism. Diabetes 46:792–800[Abstract]
172. Ma X, Zhang Y, Gromada J, Sewing S, Berggren P-O, Buschard K, Salehi A, Vikman J, Rorsman P, Eliasson L 2005 Glucagon stimulates exocytosis in mouse and rat pancreatic -cells by binding to glucagon receptors. Mol Endocrinol 19:198–212[Abstract/Free Full Text]
173. Gromada J, Rorsman P 2004 New insights into the regulation of glucagon secretion by glucagon-like peptide-1. Horm Metab Res 36:822–829[CrossRef][Medline]
174. Li Y, Cao X, Li LX, Brubaker PL, Edlund H, Drucker DJ 2005 ß-Cell Pdx1 expression is essential for the glucoregulatory, proliferative, and cytoprotective actions of glucagon-like peptide-1. Diabetes 54:482–491[Abstract/Free Full Text]
175. Balkan B, Li X 2000 Portal GLP-1 administration in rats augments the insulin response to glucose via neuronal mechanisms. Am J Physiol Regul Integr Comp Physiol 279:R1449–R1454
176. Preitner F, Ibberson M, Franklin I, Binnert C, Pende M, Gjinovci A, Hansotia T, Drucker DJ, Wollheim C, Burcelin R, Thorens B 2004 Gluco-incretins control insulin secretion at multiple levels as revealed in mice lacking GLP-1 and GIP receptors. J Clin Invest 113:635–645[CrossRef][Medline]
177. Nakabayashi H, Nishizawa M, Nakagawa A, Takeda R, Niijima A 1996 Vagal hepatopancreatic reflex effect evoked by intraportal appearance of tGLP-1. Am J Physiol 271:E808–E813
178. Nakagawa A, Satake H, Nakabayashi H, Nishizawa M, Furuya K, Nakano S, Kigoshi T, Nakayama K, Uchida K 2004 Receptor gene expression of glucagon-like peptide-1, but not glucose-dependent insulinotropic polypeptide, in rat nodose ganglion cells. Auton Neurosci 110:36–43[CrossRef][Medline]
179. Vincent M, Guz Y, Rozenberg M, Webb G, Furuta M, Steiner D, Teitelman G 2003 Abrogation of protein convertase 2 activity results in delayed islet cell differentiation and maturation, increased -cell proliferation, and islet neogenesis. Endocrinology 144:4061–4069[Abstract/Free Full Text]
180. Gelling RW, Du XQ, Dichmann DS, Romer J, Huang H, Cui L, Obici S, Tang B, Holst JJ, Fledelius C, Johansen PB, Rossetti L, Jelicks LA, Serup P, Nishimura E, Charron MJ 2003 Lower blood glucose, hyperglucagonemia, and pancreatic cell hyperplasia in glucagon receptor knockout mice. Proc Natl Acad Sci USA 100:1438–1443[Abstract/Free Full Text]
181. Liang Y, Osborne MC, Monia BP, Bhanot S, Gaarde WA, Reed C, She P, Jetton TL, Demarest KT 2004 Reduction in glucagon receptor expression by an antisense oligonucleotide ameliorates diabetic syndrome in db/db mice. Diabetes 53:410–417[Abstract/Free Full Text]
182. Trimble ER, Halban PA, Wollheim CB, Renold AE 1982 Functional differences between rat islets of ventral and dorsal pancreatic origin. J Clin Invest 69:405–413[Medline]
183. Degano P, Peiro E, Miralles P, Silvestre RA, Marco J 1992 Effects of rat pancreatic polypeptide on islet-cell secretion in the perfused rat pancreas. Metabolism 41:306–309[CrossRef][Medline]
184. Bolli GB, Fanelli CG 1999 Physiology of glucose counterregulation to hypoglycemia. Endocrinol Metab Clin North Am 28:467–493[CrossRef][Medline]
185. Borg MA, Sherwin RS, Borg WP, Tamborlane WV, Shulman GI 1997 Local ventromedial hypothalamus glucose perfusion blocks counterregulation during systemic hypoglycemia in awake rats. J Clin Invest 99:361–365[Medline]
186. Borg MA, Tamborlane WV, Shulman GI, Sherwin RS 2003 Local lactate perfusion of the ventromedial hypothalamus suppresses hypoglycemic counterregulation. Diabetes 52:663–666[Abstract/Free Full Text]
187. McCrimmon RJ, Fan X, Ding Y, Zhu W, Jacob RJ, Sherwin RS 2004 Potential role for AMP-activated protein kinase in hypoglycemia sensing in the ventromedial hypothalamus. Diabetes 53:1953–1958[Abstract/Free Full Text]
188. Miki T, Liss B, Minami K, Shiuchi T, Saraya A, Kashima Y, Horiuchi M, Ashcroft F, Minokoshi Y, Roeper J, Seino S 2001 ATP-sensitive K⁺ channels in the hypothalamus are essential for the maintenance of glucose homeostasis. Nat Neurosci 4:507–512[Medline]
189. Taborsky GJ, Ahren B, Havel PJ 1998 Autonomic mediation of glucagon secretion during hypoglycemia: implications for impaired -cell responses in type 1 diabetes. Diabetes 47:995–1005[Abstract]
190. Borg WP, During MJ, Sherwin RS, Borg MA, Brines ML, Shulman GI 1994 Ventromedial hypothalamic lesions in rats suppress counterregulatory responses to hypoglycemia. J Clin Invest 93:1677–1682[Medline]
191. Borg WP, Sherwin RS, During MJ, Borg MA, Shulman GI 1995 Local ventromedial hypothalamus glucopenia triggers counterregulatory hormone release. Diabetes 44:180–184[Abstract]
192. de Vries MG, Arseneau LM, Lawson ME, Beverly JL 2003 Extracellular glucose in rat ventromedial hypothalamus during acute and recurrent hypoglycemia. Diabetes 52:2767–2773[Abstract/Free Full Text]
193. Kang L, Routh VH, Kuzhikandathil EV, Gaspers LD, Levin BE 2004 Physiological and molecular characteristics of rat hypothalamic ventromedial nucleus glucosensing neurons. Diabetes 53:549–559[Abstract/Free Full Text]
194. McCrimmon RJ, Evans ML, Fan X, McNay EC, Chan O, Ding Y, Zhu W, Gram DX, Sherwin RS 2005 Activation of ATP-sensitive K⁺ channels in the ventromedial hypothalamus amplifies counterregulatory hormone responses to hypoglycemia in normal and recurrently hypoglycemic rats. Diabetes 54:3169–3174[Abstract/Free Full Text]
195. Burcelin R, Thorens B 2001 Evidence that extrapancreatic GLUT2-dependent glucose sensors control glucagon secretion. Diabetes 50:1282–1289[Abstract/Free Full Text]
196. Thorens B, Guillam M-T, Beermann F, Burcelin R, Jaquet M 2000 Transgenic reexpression of Glut1 or Glut2 in pancreatic ß cells rescues Glut2-null mice from early death and restores normal glucose-stimulated insulin secretion. J Biol Chem 275:23751–23758[Abstract/Free Full Text]
197. Marty N, Dallaporta M, Foretz M, Emery M, Tarussio D, Bady I, Binnert C, Beermann F, Thorens B 2005 Regulation of glucagon secretion by glucose transporter type 2 (glut2) and astrocyte-dependent glucose sensors. J Clin Invest 115:3545–3553[CrossRef][Medline]
198. Dallaporta M, Himmi T, Perrin J, Orsini J-C 1999 Solitary tract nucleus sensitivity to moderate changes in glucose levels. Neuroreport 10:2657–2660[Medline]
199. Jansen ASP, Hoffmann JL, Loewy AD 1997 CNS sites involved in sympathetic and parasympathetic control of the pancreas: a viral tracing study. Brain Res 766:29–38[CrossRef][Medline]
200. Bloom SR, Vaughan NJ, Russel RC 1974 Vagal control of glucagon release in man. Lancet 2:546–549[Medline]
201. Magistretti PJ, Pellerin L, Rothman DL, Shulman RG 1999 Energy on demand. Science 283:496–497[Free Full Text]
202. Lam TK, Gutierrez-Juarez R, Pocai A, Rossetti L 2005 Regulation of blood glucose by hypothalamic pyruvate metabolism. Science 309:943–947[Abstract/Free Full Text]
203. Edlund H 2002 Pancreatic organogenesis—developmental mechanisms and implications for therapy. Nat Rev Gen 3:524–532[CrossRef][Medline]
204. Kim SK, Hebrok M, Melton DA 1997 Notochord to endoderm signaling is required for pancreas development. Development 124:4243–4252[Abstract]
205. Slack JM 1995 Developmental biology of the pancreas. Development 121:1569–1580[Abstract]
206. Lammert E, Cleaver O, Melton D 2001 Induction of pancreatic differentiation by signals from blood vessels. Science 294:564–567[Abstract/Free Full Text]
207. Lammert E, Gu G, McLaughlin M, Brown D, Brekken R, Murtagh LC, Gerber H-P, Ferrara N, Melton DA 2003 Role of VEGF-A in vascularization of pancreatic islets. Curr Biol 13:1070–1074[CrossRef][Medline]
208. Herrera PL, Huarte J, Sanvito F, Meda P, Orci L, Vassalli JD 1991 Embryogenesis of the murine endocrine pancreas; early expression of pancreatic polypeptide gene. Development 113:1257–1265[Abstract]
209. Teitelman G, Alpert S, Polak JM, Martinez A, Hanahan D 1993 Precursor cells of mouse endocrine pancreas coexpress insulin, glucagon and the neuronal proteins tyrosine hydrolase and neuropeptide Y, but not pancreatic polypeptide. Development 118:1031–1039[Abstract]
210. Upchurch BH, Aponte GW, Leiter AB 1994 Expression of peptide YY in all four islet cell types in the developing mouse pancreas suggests a common peptide YY-producing progenitor. Development 120:245–252[Abstract]
211. Kawaguchi Y, Cooper B, Gannon M, Ray M, MacDonald RJ, Wright CV 2002 The role of the transcriptional regulator Ptf1a in converting intestinal to pancreatic progenitors. Nat Genet 32:128–134[CrossRef][Medline]
212. Jonsson J, Carlsson L, Edlund T, Edlund H 1994 Insulin-promoter-factor 1 is required for pancreas development in mice. Nature 371:606–609[CrossRef][Medline]
213. Offield MF, Jetton TL, Labosky PA, Ray M, Stein RW, Magnuson MA, Hogan BL, Wright CV 1996 PDX-1 is required for pancreatic outgrowth and differentiation of the rostral duodenum. Development 122:983–995[Abstract]
214. Ahlgren U, Pfaff S, Jessel TM, Edlund T, Edlund H 1997 Independent requirement for ISL1 in the formation of the pancreatic mesenchyme and islet cells. Nature 385:257–260[CrossRef][Medline]
215. Gradwohl G, Dierich A, LeMeur M, Guillemot F 2000 Neurogenin3 is required for the development of the four endocrine cell lineages of the pancreas. Proc Natl Acad Sci USA 97:1607–1611[Abstract/Free Full Text]
216. Gu G, Dubauskaite J, Melton DA 2002 Direct evidence for the pancreatic lineage: NGN3+ cells are islet progenitors and are distinct from duct progenitors. Development 129:2447–2457
217. Apelqvist A, Li H, Sommer L, Beatus P, Anderson DJ, Honjo T, Hrabe de Angelis M, Lendahl U, Edlund H 1999 Notch signalling controls pancreatic cell differentiation. Nature 400:877–881[CrossRef][Medline]
218. Schwitzgebel VM, Scheel DW, Conners JR, Kalamaras J, Lee JE, Anderson DJ, Sussel L, Johnson JD, German MS 2000 Expression of neurogenin3 reveals an islet cell precursor population in the pancreas. Development 127:3533–3542[Abstract]
219. Edlund H 2001 Factors controlling pancreatic cell differentiation and function. Diabetologia 44:1071–1079[CrossRef][Medline]
220. Collombat P, Hecksher-Sørensen J, Serup P, Mansouri A 2006 Specifying pancreatic endocrine cell fates. Mech Dev 123:501–512[CrossRef][Medline]
221. St-Onge L, Sosa-Pineda B, Chowdhury K, Mansouri A, Gruss P 1997 Pax6 is required for differentiation of glucagon-producing -cells in mouse pancreas. Nature 387:406–409[CrossRef][Medline]
222. Hussain MA, Lee J, Miller CP, Habener JF 1997 POU domain transcription factor brain 4 confers pancreatic -cell-specific expression of the proglucagon gene through interaction with a novel proximal promoter G1 element. Mol Cell Biol 17:7186–7194[Abstract]
223. Heller RS, Stoffers DA, Liu A, Schedl A, Crenshaw III EB, Madsen OD, Serup P 2004 The role of Brn4/Pou3f4 and Pax6 in forming the pancreatic glucagon cell identity. Dev Biol 268:123–134[CrossRef][Medline]
224. Hussain MA, Miller CP, Habener JF 2002 Brn-4 transcription factor expression targeted to the early developing mouse pancreas induces ectopic glucagon gene expression in insulin-producing ß cells. J Biol Chem 277:16028–16032[Abstract/Free Full Text]
225. Wang H, Maechler P, Ritz-Laser B, Hagenfeld KA, Ishihara H, Philippe J, Wollheim CB 2001 Pdx1 level defines pancreatic gene expression pattern and cell lineage differentiation. J Biol Chem 276:25279–25286[Abstract/Free Full Text]
226. Lee JE, Hollenberg SM, Snider L, Turner DL, Lipnick N, Weintraub H 1995 Conversion of Xenopus ectoderm into neurons by NeuroD, a basic helix-loop-helix protein. Science 268:836–844[Abstract/Free Full Text]
227. Naya FJ, Huang HP, Qiu Y, Mutoh H, DeMayo FJ, Leiter AB, Tsai MJ 1997 Diabetes, defective pancreatic morphogenesis, and abnormal entoroendocrine differentiation in BETA2/neuroD-deficient mice. Genes Dev 11:2323–2334[Abstract/Free Full Text]
228. Sussel L, Kalamaras J, Hartigan-O’Connor DJ, Meneses JJ, Pedersen RA, Rubenstein JL, German MS 1998 Mice lacking the homeodomain transcription factor Nkx2.2 have diabetes due to arrested differentiation of pancreatic ß cells. Development 125:2213–2221[Abstract]
229. Collombat P, Mansouri A, Hecksher-Sorensen J, Serup P, Krull J, Gradwohl G, Gruss P 2003 Opposing actions of Arx and Pax4 in endocrine pancreas development. Genes Dev 17:2591–2603[Abstract/Free Full Text]
230. Collombat P, Hecksher-Sorensen J, Broccoli V, Krull J, Ponte I, Mundiger T, Smith J, Gruss P, Serup P, Mansouri A 2005 The simultaneous loss of Arx and Pax4 genes promotes a somatostatin-producing cell fate specification at the expense of the - and ß-cell lineages in the mouse endocrine pancreas. Development 132:2969–2980[Abstract/Free Full Text]
231. Mamin A, Philippe J 2007 Activin A decreases glucagon and arx gene expression in cell lines. Mol Endocrinol 21:259–273[Abstract/Free Full Text]
232. Zhang YQ, Mashima H, Kojima I 2001 Changes in the expression of transcription factors in pancreatic AR42J cells during differentiation into insulin-producing cells. Diabetes 50(Suppl 1):S10–S14
233. Ueda Y 2000 Activin A increases Pax4 gene expression in pancreatic ß cell lines. FEBS Lett 480:101–105[CrossRef][Medline]
234. Demeterco C, Beattie GM, Dib SA, Lopez AD, Hayek A 2000 A role for activin A and betacellulin in human fetal pancreatic cell differentiation and growth. J Clin Endocrinol Metab 85:3892–3897[Abstract/Free Full Text]
235. Artner I, Lay JL, Hang Y, Elghazi L, Schisler JC, Henderson E, Sosa-Pineda B, Stein R 2006 MafB: an activator of the glucagon gene expressed in developing islet - and ß-cells. Diabetes 55:297–304[Abstract/Free Full Text]
236. Nishimura W, Kondo T, Salameh T, El Khattabi I, Dodge R, Bonner-Weir S, Sharma A 2006 A switch from MafB to MafA expression accompanies differentiation to pancreatic ß-cells. Dev Biol 293:526–539[CrossRef][Medline]
237. Lee CS, Sund NJ, Behr R, Herrera PL, Kaestner KH 2005 Foxa2 is required for the differentiation of pancreatic -cells. Dev Biol 278:484–495[CrossRef][Medline]
238. Kieffer TJ, Habener JF 1999 The glucagon-like peptides. Endocr Rev 20:876–913[Abstract/Free Full Text]
239. Bell GI, Santerre RF, Mullenbach GT 1983 Hamster preproglucagon contains the sequence of glucagon and two related peptides. Nature 302:716–718[CrossRef][Medline]
240. Bell GI, Sanchez-Pescador R, Laybourn PJ, Najarian RC 1983 Exon duplication and divergence in the human preproglucagon gene. Nature 304:368–371[CrossRef][Medline]
241. Heinrich G, Gros P, Lund PK, Bentley RC, Habener JF 1984 Pre-proglucagon messenger ribonucleic acid: nucleotide and encoded amino acid sequences of the rat pancreatic complementary deoxyribonucleic acid. Endocrinology 115:2176–2181[Abstract/Free Full Text]
242. Steiner DF, Smeekens SP, Ohagi S, Chan SJ 1992 The new enzymology of precursor processing endoproteases. J Biol Chem 267:23435–23438[Free Full Text]
243. Rouille Y, Westermark G, Martin SK, Steiner DF 1994 Proglucagon is processed to glucagon by prohormone convertase PC2 in TC1–6 cells. Proc Natl Acad Sci USA 91:3242–3246[Abstract/Free Full Text]
244. Steiner DF 1998 The proprotein convertases. Curr Opin Chem Biol 2:31–39[CrossRef][Medline]
245. Steiner DF, Rouille Y, Gong Q, Martin S, Carroll R, Chan SJ 1996 The role of prohormone convertases in insulin biosynthesis: evidence for inherited defects in their action in man and experimental animals. Diabetes Metab 22:94–104[Medline]
246. Scopsi L, Gullo M, Rilke F, Martin S, Steiner DF 1995 Proprotein convertases (PC1/PC3 and PC2) in normal and neoplastic human tissues: their use as markers of neuroendocrine differentiation. J Clin Endocrinol Metab 80:294–301[Abstract]
247. Furuta M, Yano H, Zhou A, Rouille Y, Holst JJ, Carroll R, Ravazzola M, Orci L, Furuta H, Steiner DF 1997 Defective prohormone processing and altered pancreatic islet morphology in mice lacking active SPC2. Proc Natl Acad Sci USA 94:6646–6651[Abstract/Free Full Text]
248. Furuta M, Zhou A, Webb G, Carroll R, Ravazzola M, Orci L, Steiner DF 2001 Severe defect in proglucagon processing in islet A-cell of prohormone convertase 2 null mice. J Biol Chem 276:27197–27202[Abstract/Free Full Text]
249. Webb GC, Akbar MS, Zhao C, Swift HH, Steiner DF 2002 Glucagon replacement via micro-osmotic pump corrects hypoglycemia and -cell hyperplasia in prohormone convertase 2 knockout mice. Diabetes 51:398–405[Abstract/Free Full Text]
250. Raju B, Cryer PE 2005 Maintenance of the postabsorptive plasma glucose concentration: insulin or insulin plus glucagon? Am J Physiol Endocrinol Metab 289:E181–E186
251. Ugleholdt R, Zhu X, Deacon CF, Ørskov C, Steiner DF, Holst JJ 2004 Impaired intestinal proglucagon processing in mice lacking prohormone convertase 1. Endocrinology 145:1349–1355[Abstract/Free Full Text]
252. Wilson ME, Kalamaras JA, German MS 2002 Expression pattern of IAPP and prohormone convertase 1/3 reveals a distinctive set of endocrine cells in the embryonic pancreas. Mech Dev 115:171–176[CrossRef][Medline]
253. Holst JJ 1997 Enteroglucagon. Annu Rev Physiol 59:257–271[CrossRef][Medline]
254. Philippe J 1991 Structure and pancreatic expression of the insulin and glucagon genes. Endocrinol Rev 12:252–271[Abstract/Free Full Text]
255. Philippe J, Drucker DJ, Knepel W, Jepeal L, Misulovin Z, Habener JF 1988 -Cell-specific expression of the glucagon gene is conferred to the glucagon promoter element by the interactions of DNA-binding proteins. Mol Cell Biol 8:4877–4888[Abstract/Free Full Text]
256. Cordier-Bussat M, Morel C, Philippe J 1995 Homologous DNA sequences and cellular factors are implicated in the control of glucagon and insulin gene expression. Mol Cell Biol 15:3904–3916[Abstract]
257. Morel C, Cordier-Bussat M, Philippe J 1995 The upstream promoter element of the glucagon gene G1 confers pancreatic cell-specific expression. J Biol Chem 270:3046–3055[Abstract/Free Full Text]
258. Ritz-Laser B, Estreicher A, Klages N, Saule S, Philippe J 1999 Pax-6 and Cdx-2/3 interact to activate glucagon gene expression on the G1 control element. J Biol Chem 274:4124–4132[Abstract/Free Full Text]
259. Trinh DK, Zhang K, Hossain M, Brubaker PL, Drucker DJ 2003 Pax-6 activates endogenous proglucagon gene expression in the rodent gastrointestinal epithelium. Diabetes 52:425–433[Abstract/Free Full Text]
260. Ritz-Laser B, Estreicher A, Gauthier BR, Mamin A, Edlund H, Philippe J 2002 The pancreatic ß-cell-specific transcription factor Pax-4 inhibits glucagon gene expression through Pax-6. Diabetologia 45:97–107[CrossRef][Medline]
261. Peers B, Leonard J, Sharma S, Teitelmann G, Montminy MR 1994 Insulin expression in pancreatic islet cells relies on cooperative interactions between the helix loop helix factor E47 and the homeobox factor STF-1. Mol Endocrinol 8:1798–1806[Abstract/Free Full Text]
262. Peers B, Sharma S, Johnson T, Kamps M, Montminy M 1995 The pancreatic islet factor STF-1 binds cooperatively with Pbx to a regulatory element in the somatostatin promoter: importance of the FPWMK motif and of the homeodomain. Mol Cell Biol 15:7091–7097[Abstract]
263. Glick E, Leshkowitz D, Walker MD 2000 Transcription factor BETA2 acts cooperativeley with E2A and PDX1 to activate the insulin gene promoter. J Biol Chem 275:2199–2204[Abstract/Free Full Text]
264. Ohneda K, Mirmira RG, Wang J, Johnson JD, German MS 2000 The homeodomain of PDX-1 mediates multiple protein-protein interactions in the formation of a transcriptional activation complex on the insulin promoter. Mol Cell Biol 20:900–911[Abstract/Free Full Text]
265. Schwartz PT, Perez-Villamil B, Rivera A, Moratalla R, Vallejo M 2000 Pancreatic homeodomain transcription factor IDX1/IPF1 expressed in developing brain regulates somatostatin gene transcription in embryonic neural cells. J Biol Chem 275:19106–19114[Abstract/Free Full Text]
266. Ahlgren U, Jonsson J, Jonsson L, Simu K, Edlund H 1998 ß-Cell-specific inactivation of the mouse Ipf1/Pdx1 gene results in loss of the ß-cell phenotype and maturity onset diabetes. Genes Dev 12:1763–1768[Abstract/Free Full Text]
267. Lottmann H, Vanselow J, Hessabi B, Walther R 2001 The Tet-On system in transgenic mice: inhibition of the mouse pdx-1 gene activity by antisense RNA expression in pancreatic ß-cells. J Mol Med 79:321–328[CrossRef][Medline]
268. Wang H, Iezzi M, Theander S, Antinozzi PA, Gauthier BR, Halban PA, Wolheim CB 2005 Suppression of Pdx-1 perturbs proinsulin processing, insulin secretion and GLP-1 signaling in INS-1 cells. Diabetologia 48:720–731[CrossRef][Medline]
269. Ritz-Laser B, Gauthier BR, Estreicher A, Mamin A, Brun T, Ris F, Salmon P, Halban PA, Trono D, Phillipe J 2003 Ectopic expression of the ß-cell specific transcription factor Pdx1 inhibits glucagon gene transcription. Diabetologia 46:810–821[CrossRef][Medline]
270. Flock G, Cao X, Drucker DJ 2005 Pdx-1 is not sufficient for repression of proglucagon gene transcription in islet or enteroendocrine cells. Endocrinology 146:441–449[Abstract/Free Full Text]
271. Dumonteil E, Magnan C, Ritz-Laser B, Ktorza A, Meda P, Philippe J 2000 Glucose regulates proinsulin and prosomatostatin but not proglucagon messenger ribonucleic acid levels in rat pancreatic islets. Endocrinology 141:174–180[Abstract/Free Full Text]
272. Magnan C, Philippe J, Kassis N, Laury MC, Penicaud L, Gilbert M, Ktorza A 1995 In vivo effects of glucose and insulin secretion and gene expression of glucagon in rats. Endocrinology 136:5370–5376[Abstract]
273. Dumonteil E, Ritz-Laser B, Magnan C, Grigorescu I, Ktorza A, Phillipe J 1999 Chronic exposure to high glucose concentrations increases proglucagon messenger ribonucleic acid levels and glucagon release from InR1G9 cells. Endocrinology 140:4644–4650[Abstract/Free Full Text]
274. McGirr R, Ejbick CE, Carter DE, Andrews JD, Nie Y, Friedman TC, Dhanvantari S 2005 Glucose dependence of the regulated secretory pathway in TC1–6 cells. Endocrinology 146:4514–4523[CrossRef][Medline]
275. Philippe J 1996 The glucagon gene and its expression. In: Lefèbvre PJ, ed. Glucagon III. Berlin, Heidelberg: Springer-Verlag; 11–30
276. Trebbien R, Klarskov L, Olesen M, Holst JJ, Carr RD, Deacon CF 2004 Neutral endopeptidase 24.11 is important for the degradation of both endogenous and exogenous glucagon in anesthetized pigs. Am J Physiol Endocrinol Metab 287:E431–E438
277. Hupe-Sodmann K, McGregor GP, Bridenbaugh R, Goke R, Goke B, Thole H, Zimmermann B, Voigt K 1995 Characterisation of the processing by human neutral endopeptidase 24.11 of GLP-1 (7–36) amide and comparison of the substrate specificity of the enzyme for other glucagon-like peptides. Regul Pept 22:149–156
278. Pospisilik JA, Hinke SA, Pederson RA, Hoffmann T, Rosche F, Schlenzig D, Glund K, Heiser U, McIntosh CH, Demuth H 2001 Metabolism of glucagon by dipeptidyl peptidase IV (CD26). Regul Pept 12:133–141
279. Hinke SA, Pospisilik JA, Demuth HU, Mannhart S, Kuhn-Wache K, Hoffmann T, Nishimura E, Pederson RA, McIntosh CH 2000 Dipeptidyl peptidase IV (DPIV/CD26) degradation of glucagon. Characterization of glucagon degradation products and DPIV-resistant analogs. J Biol Chem 275:3827–3834[Abstract/Free Full Text]
280. Deacon CF, Kelstrup M, Trebbien R, Klarskov L, Olesen M, Holst JJ 2003 Differential regional metabolism of glucagon in anesthetized pigs. Am J Physiol Endocrinol Metab 285:E552–E560
281. Bokvist K, Olsen HL, Høy M, Gotfredsen CF, Holmes WF, Buschard K, Rorsman P, Gromada J 1999 Characterisation of sulphonylurea and ATP-regulated K⁺ channels in rat pancreatic A-cells. Pflügers Arch 438:428–436[CrossRef][Medline]
282. Gromada J, Bokvist K, Ding W-G, Barg S, Buschard K, Renström E, Rorsman P 1997 Adrenaline stimulates glucagon secretion in pancreatic A-cells by increasing the Ca²⁺ current and the number of granules close to the L-type Ca²⁺ channels. J Gen Physiol 110:217–228[Abstract/Free Full Text]
283. Rorsman P, Hellman B 1988 Voltage-activated currents in guinea pig pancreatic ₂ cells. Evidence for Ca²⁺-dependent action potentials. J Gen Physiol 91:223–242[Abstract/Free Full Text]
284. Göpel SO, Kanno T, Barg S, Weng X-G, Gromada J, Rorsman P 2000 Regulation of glucagon release in mouse -cells by K_ATP channels and inactivation of TTX-sensitive Na⁺ channels. J Physiol 528:509–520[Abstract/Free Full Text]
285. Barg S, Galvanovskis J, Göpel SO, Rorsman P, Eliasson L 2000 Tight coupling between electrical activity and exocytosis in mouse glucagon-secreting -cells. Diabetes 49:1500–1510[Abstract]
286. Leung YM, Ahmed I, Sheu L, Tsushima RG, Diamant NE, Hara M, Gaisano HY 2005 Electrophysiological characterization of pancreatic islet cells in the MIP-GFP mouse. Endocrinology 146:4766–4775[CrossRef][Medline]
287. Rajan AS, Aguilar-Bryan L, Nelson DA, Nichols CG, Wechsler SW, Lechago J, Bryan J 1993 Sulfonylurea receptors and ATP-sensitive K⁺ channels in clonal pancreatic cells. J Biol Chem 268:15221–15228[Abstract/Free Full Text]
288. Ronner P, Matschinsky FM, Hang TL, Epstein AJ, Buettger C 1993 Sulfonylurea-binding sites and ATP-sensitive K⁺ channels in -TC glucagonoma and ß-TC insulinoma cells. Diabetes 42:1760–1772[Abstract]
289. Rorsman P, Trube G 1985 Glucose-dependent K⁺-channels in pancreatic ß-cells are regulated by intracellular ATP. Pflügers Arch 405:305–309[CrossRef][Medline]
290. Baukrowitz T, Schulte U, Oliver D, Herlitze S, Krauter T, Tucker SJ, Ruppersberg JP, Fakler B 1998 PIP₂ and PIP as determinants for ATP inhibition of K_ATP channels. Science 282:1141–1144[Abstract/Free Full Text]
291. Shyng SL, Nichols CG 1998 Membrane phospholipid control of nucleotide sensitivity of K_ATP channels. Science 282:1138–1141[Abstract/Free Full Text]
292. Larsson O, Deeney JT, Bränström R, Berggren P-O, Corkey BE 1996 Activation of the ATP-sensitive K⁺ channel by long chain acyl-CoA. A role in modulation of pancreatic ß-cell glucose sensitivity. J Biol Chem 271:10623–10626[Abstract/Free Full Text]
293. Doliba NM, Qin W, Vatamaniuk MZ, Buettger CW, Collins HW, Magnuson MA, Kaestner KH, Wilson DF, Carr RD, Matschinsky FM 2006 Cholinergic regulation of fuel induced hormone secretion and respiration of SUR1^–/– mouse islets. Am J Physiol 291:E525—E535
294. Shiota C, Rocheleau JV, Shiota M, Piston DW, Magnuson MA 2005 Impaired glucagon secretory responses in mice lacking the type 1 sulfonylurea receptor. Am J Physiol Endocrinol Metab 289:E570–E577
295. Tschritter O, Stumvoll M, Machicao F, Holzwarth M, Weisser M, Maerker E, Teigler A, Häring H, Fritsche A 2002 The prevalent Glu23Lys polymorphism in the potassium inward rectifier 6.2 (KIR6.2) gene is associated with impaired glucagon suppression in response to hyperglycemia. Diabetes 51:2854–2860[Abstract/Free Full Text]
296. Schwanstecher C, Meyer U, Schwanstecher M 2002 Kir6.2 polymorphism predisposes to type 2 diabetes by inducing overactivity of pancreatic ß-cell ATP-sensitive K⁺ channels. Diabetes 51:875–879[Abstract/Free Full Text]
297. Vignali S, Leiss V, Karl R, Hofmann F, Welling A 2006 Characterization of voltage-dependent sodium and calcium channels in mouse pancreatic A- and B-cells. J Physiol 572:691–706[Abstract/Free Full Text]
298. Rorsman P 1988 Two types of Ca²⁺ currents with different sensitivities to organic Ca²⁺ channel antagonists in guinea pig pancreatic ₂ cells. J Gen Physiol 91:243–254[Abstract/Free Full Text]
299. Grabsch H, Pereverzev A, Weiergräber M, Schramm M, Henry M, Vajna R, Beattie RE, Volsen SG, Klöckner U, Hescheler J, Schneider T 1999 Immunohistochemical detection of 1E voltage-gated Ca²⁺ channel isoforms in cerebellum, INS-1 cells, and neuroendocrine cells of the digestive system. J Histochem Cytochem 47:981–994[Abstract/Free Full Text]
300. Pereverzev A, Salehi A, Mikhna M, Renström E, Hescheler J, Weiergräber M, Smyth N, Schneider T 2005 The ablation of the Ca_v2.3/E-type voltage-gated Ca²⁺ channel causes a mild phenotype despite an altered glucose induced glucagon response in isolated islets of Langerhans. Eur J Pharmacol 511:65–72[CrossRef][Medline]
301. Leung YM, Ahmed I, Sheu L, Tsushima RG, Diamant NE, Gaisano HY 2006 Two populations of pancreatic islet -cells displaying distinct Ca²⁺ channel properties. Biochem Biophys Res Commun 345:340–344[CrossRef][Medline]
302. Takahashi E, Ito M, Miyamoto N, Nagasu T, Ino M, Tanaka I 2005 Increased glucose tolerance in N-type Ca²⁺ channel 1B-subunit gene-deficient mice. Int J Mol Med 15:937–944[Medline]
303. Berts A, Gylfe E, Hellman B 1997 Cytoplasmic Ca²⁺ in glucagon-producing pancreatic -cells exposed to carbachol and agents affecting Na⁺ fluxes. Endocrine 6:79–83[Medline]
304. Smith PA, Sakura H, Coles B, Gummerson N, Proks P, Ashcroft FM 1997 Electrogenic arginine transport mediates stimulus-secretion coupling in mouse pancreatic ß-cells. J Physiol 499:625–635[Abstract/Free Full Text]
305. Chesney TM, Schofield JG 1969 Studies on the secretion of pancreatic glucagon. Diabetes 18:627–632[Medline]
306. Leclercq-Meyer V, Marchand J, Malaisse WJ 1983 Role of glucose and insulin in the dynamic regulation of glucagon release by the perfused rat pancreas. Diabetologia 24:191–195[Medline]
307. Detimary P, Dejonghe S, Ling Z, Pipeleers D, Schuit F, Henquin JC 1998 The changes in adenine nucleotides measured in glucose-stimulated rodent islets occur in ß cells but not in cells and are also observed in human islets. J Biol Chem 273:33905–33908[Abstract/Free Full Text]
308. Catterall WA 2000 Structure and regulation of voltage-gated Ca²⁺ channels. Annu Rev Cell Dev Biol 16:521–555[CrossRef][Medline]
309. Ohneda A, Aguilar-Parada E, Eisentraut A, Unger RH 1969 Control of pancreatic glucagon secretion by glucose. Diabetes 18:1–10[Medline]
310. Heding L 1971 Radioimmunological determination of pancreatic and gut glucagon in plasma. Diabetologia 7:10–19[CrossRef][Medline]
311. Unger RH, Aguilar-Parada E, Müller W, Eisentraut AM 1970 Studies of pancreatic cell function in normal and diabetic subjects. J Clin Invest 49:837–848[Medline]
312. Ohneda A, Sato M, Matsuda K, Yanbe A, Maruhama Y, Yamagata S 1972 Plasma glucagon response to blood glucose fall, gastrointestinal hormones and arginine in man. Tohoku J Exp Med 107:241–251[Medline]
313. Gerich JE, Schneider V, Dippe SE, Langlois M, Noacco C, Karam J, Forsham P 1974 Characterization of the glucagon response to hypoglycemia in man. J Clin Endocrinol Metab 38:77–82[Abstract/Free Full Text]
314. Santiago JV, Clarke WL, Shah SD, Cryer PE 1980 Epinephrine, norepinephrine, glucagon, and growth hormone release in association with physiological decrements in the plasma glucose concentration in normal and diabetic man. J Clin Endocrinol Metab 51:877–883[Abstract/Free Full Text]
315. Rizza RA, Cryer PE, Gerich JE 1979 Role of glucagon catecholamines, and growth hormone in human glucose counterregulation: effects of somatostatin and combined - and ß-adrenergic blockade on plasma glucose recovery and glucose flux rates after insulin-induced hypogycemia. J Clin Invest 64:62–71[Medline]
316. Gerich JE 1983 Glucose in the control of glucagon secretion. In: Lefebvre PJ, ed. Handbook of experimental pharmacology. Vol 66/II. Berlin: Springer; 3–18
317. Hahn HJ, Ziegler M, Mohr E 1974 Inhibition of glucagon secretion by glucose and glyceraldehyde on isolated islets of Wistar rats. FEBS Lett 49:100–102[CrossRef][Medline]
318. Gerich JE, Charles MA, Grodsky GM 1974 Characterization of the effect of arginine and glucose on glucagon and insulin release from the perfused rat pancreas. J Clin Invest 54:833–841[Medline]
319. Marliss EB, Wollheim CB, Blondel B, Orci L, Lambert AE, Stauffacher W, Like AA, Renold AE 1973 Insulin and glucagon release from monolayer cell cultures of pancreas from newborn rats. Eur J Clin Invest 3:16–26[Medline]
320. Hermansen K 1981 Pancreatic D-cell recognition of D-glucose: studies with D-glucose, D-glyceraldehyde, dihydroxyacetone, D-mannoheptulose, D-fructose, D-galactose, and D-ribose. Diabetes 30:203–210[Abstract]
321. Heimberg H, De Vos A, Pipeleers D, Thorens B, Schuit F 1995 Diffrences in glucose transporter gene expression between rat pancreatic - and ß-cells are correlated to diffrences in glucose transport but not in glucose utilization. J Biol Chem 270:8971–8975[Abstract/Free Full Text]
322. Heimberg H, De Vos A, Moens K, Quartier E, Bouwens L, Pipeleers D, Van Schaftingen E, Madsen O, Schuit F 1996 The glucose sensor protein glucokinase is expressed in glucagon-producing -cells. Proc Natl Acad Sci USA 93:7036–7041[Abstract/Free Full Text]
323. Tu J, Tuch BE, Si Z 1999 Expression and regulation of glucokinase in rat islet ß- and -cells during development. Endocrinology 140:3762–3766[Abstract/Free Full Text]
324. Gorus FK, Malaisse WJ, Pipeleers DG 1984 Differences in glucose handling by pancreatic A- and B-cells. J Biol Chem 259:1196–1200[Abstract/Free Full Text]
325. Schuit F, De Vos A, Farfari, Moens K, Pipeleers D, Brun T, Prentki M 1997 Metabolic fate of glucose in purified islet cells. Glucose-regulated anaplerosis in ß-cells. J Biol Chem 272:18572–18579[Abstract/Free Full Text]
326. Otonkoski T, Kaminen N, Ustinov J, Lapatto R, Meissner T, Mayatepek E, Kere J, Sipila I 2003 Physical exercise-induced hyperinsulinemic hypoglycemia is an autosomal-dominant trait characterized by abnormal pyruvate-induced insulin release. Diabetes 52:199–204[Abstract/Free Full Text]
327. Sekine N, Cirulli V, Regazzi R, Brown LJ, Gine E, Tamarit-Rodriguez J, Girotti M, Marie S, MacDonald MJ, Wollheim CB, Rutter GA 1994 Low lactate dehydrogenase and high mitochondrial glycerol phosphate dehydrogenase in pancreatic ß-cell. Potential role in nutrient sensing. J Biol Chem 269:4895–4902[Abstract/Free Full Text]
328. Liang Y, Bai G, Doliba N, Buettger C, Wang L, Berner DK, Matschinsky FM 1996 Glucose metabolism and insulin release in mouse ßHC9 cells, as model for wild-type pancreatic ß cells. Am J Physiol 270:E846–E857
329. Zhao C, Wilson MC, Schuit F, Halestrap AP, Rutter GA 2001 Expression and distribution of lactate/monocarboxylate transporter (MCT) isoforms in pancreatic islets and the exocrine pancreas. Diabetes 50:361–366[Abstract/Free Full Text]
330. Ishihara H, Wang H, Drewes LR, Wollheim CB 1999 Overexpression of monocarboxylate transporter and lactate dehydrogenase alters insulin secretory responses to pyruvate and lactate in ß cells. J Clin Invest 104:1621–1629[Medline]
331. Gerich JE, Charles MA, Grodsky GM 1976 Regulation of pancreatic insulin and glucagon secretion. Annu Rev Physiol 38:353–388[CrossRef][Medline]
332. Edwards JC, Taylor KW 1970 Fatty acids and the release of glucagon from isolated guinea pig islets of Langerhans incubated in vitro. Biochim Biophys Acta 215:310–315[Medline]
333. Gremlich S, Bonny C, Waeber G, Thorens B 1997 Fatty acids decrease IDX-1 expression in rat pancreatic islets and reduce GLUT2, glucokinase, insulin, and somatostatin levels. J Biol Chem 272:30261–30269[Abstract/Free Full Text]
334. Gross R, Mialhe P 1986 Free fatty acids and pancreatic function in the duck. Acta Endocrinol 112:100–104
335. Edwards JC, Howell SL, Taylor KW 1969 Fatty acids as regulators of glucagon secretion. Nature 224:808–809[CrossRef][Medline]
336. Bollheimer LC, Landauer HC, Troll S, Schweimer J, Wrede CE, Schölmerich J, Buettner R 2004 Stimulatory short-term effects of free fatty acids on glucagon secretion at low to normal glucose concentrations. Metabolism 53:1443–1448[CrossRef][Medline]
337. Hong J, Abudula R, Chen J, Jeppesen PB, Dyrskog SEU, Xiao J, Colombo M, Hermansen K 2005 The short-term effect of fatty acids on glucagon secretion is influenced by their chain length, spatial configuration, and degree of unsaturation: studies in vitro. Metabolism 54:1329–1336[CrossRef][Medline]
338. Hong J, Chen L, Jeppesen PB, Nordentoft I, Hermansen K 2005 Stevioside counteracts the cell hypersecretion caused by long-term palmitate exposure. Am J Physiol Endocrinol Metab 290:E416–E422
339. Asada N, Shibuya I, Iwanaga T, Niwa K, Kanno T 1998 Identification of - and ß-cells in intact isolated islets of Langerhans by their characteristic cytoplasmic Ca²⁺ concentration dynamics and immunocytochemical staining. Diabetes 47:751–757[Abstract]
340. Nadal A, Quesada I, Soria B 1999 Homologous and heterologous asynchronicity between identified -, ß and -cells within intact islets of Langerhans in the mouse. J Physiol 517:85–93[Abstract/Free Full Text]
341. Quesada I, Nadal A, Soria B 1999 Different effects of tolbutamide and diazoxide in -, ß-, and -cells within intact islets of Langerhans. Diabetes 48:2390–2397[Abstract]
342. Quesada I, Todorova MG, Alonso-Magdalena P, Beltrá M, Carneiro EM, Martin F, Nadal A, Soria B 2006 Glucose induces opposite intracellular Ca²⁺ concentration oscillatory patterns in identified - and ß-cells within human islets of Langerhans. Diabetes 55:2463–2469[Abstract/Free Full Text]
343. Berts A, Ball A, Gylfe E, Hellman B 1996 Suppression of Ca²⁺ oscillations in glucagon-producing ₂-cells by insulin/glucose and amino acids. Biochim Biophys Acta 1310:212–216[Medline]
344. Johansson H, Gylfe E, Hellman B 1987 The actions of arginine and glucose on glucagon secretion are mediated by opposite effects on cytoplasmic Ca²⁺. Biochem Biophys Res Commun 147:309–314[CrossRef][Medline]
345. Liu Y-J, Vieira E, Gylfe E 2004 A store-operated mechanism determines the activity of the electrically excitable glucagon-secreting pancreatic -cell. Cell Calcium 35:357–365[CrossRef][Medline]
346. Thastrup O, Dawson AP, Scharff O, Foder B, Cullen PJ, Drobak BK, Bjerrum PJ, Christensen SB, Hanley MR 1989 Thapsigargin, a novel molecular probe for studying intracellular calcium release and storage. Agents Actions 27:17–23[CrossRef][Medline]
347. Saegusa C, Fukuda M, Mikoshiba K 2002 Synaptotagmin V is targeted to dense-core vesicles that undergo calcium-dependent exocytosis in PC12 cells. J Biol Chem 277:24499–24505[Abstract/Free Full Text]
348. Seino S, Shibasaki T 2005 PKA-dependent and PKA-independent pathways for cAMP-regulated exocytosis. Physiol Rev 85:1303–1342[Abstract/Free Full Text]
349. Høy M, Olsen HL, Bokvist K, Buschard K, Barg S, Rorsman P, Gromada J 2000 Tolbutamide stimulates exocytosis of glucagon by inhibition of a mitochondrial-like ATP-sensitive K⁺ (K_ATP) conductance in rat pancreatic A-cells. J Physiol 527:109–120[Abstract/Free Full Text]
350. Gregorio F, Ambrosi F, Cristallini S, Pedetti M, Filipponi P, Santeusanio F 1992 Therapeutical concentrations of tolbutamide, glibenclamide, gliclazide and gliquidone at different glucose levels: in vitro effects on pancreatic A- and B-cell function. Diabetes Res Clin Pract 18:197–206[CrossRef][Medline]
351. Gregorio F, Ambrosi F, Cristallini S, Filipponi P, Santeusanio F 1996 Effects of glimepiride on insulin and glucagon release from isolated rat pancreas at different glucose concentrations. Acta Diabetol 33:25–29[CrossRef][Medline]
352. Loubatières AL, Loubatières-Mariani MM, Alric R, Ribes G 1974 Tolbutamide and glucagon secretion. Diabetologia 10:271–276[Medline]
353. Cejvan K, Coy DH, Holst JJ, Cerasi E, Efendic S 2002 Gliclazide directly inhibits arginine-induced glucagon release. Diabetes 51:S381–S384
354. Efendi S, Enzmann F, Nylén A, Uvnäs-Wallensten K, Luft R 1980 Sulphonylurea (glibenclamide) enhances somatostatin and inhibits glucagon release induced by arginine. Acta Physiol Scand 108:231–233[Medline]
355. Kinukawa M, Ohnota H, Ajisawa Y 1996 Effect of a non-sulphonylurea hypoglycaemic agent, KAD-1229 on hormone secretion in the isolated perfused pancreas of the rat. Br J Pharmacol 117:1702–1706[Medline]
356. Laube H, Fussgänger R, Goberna R, Schröder K, Straub K, Sussman K, Pfeiffer EF 1971 Effects of tolbutamide on insulin and glucagon secretion of the isolated perfused rat pancreas. Horm Metab Res 3:238–242[Medline]
357. Östenson C-G, Nylén A, Grill V, Gutniak M, Efendi S 1986 Sulfonylurea-induced inhibition of glucagon secretion from the perfused rat pancreas: evidence for a direct, non-paracrine effect. Diabetologia 29:861–867[CrossRef][Medline]
358. Takahashi K, Yamatani K, Hara M, Sasaki H 1994 Gliclazide directly suppresses arginine-induced glucagon secretion. Diabetes Res Clin Pract 24:143–151[CrossRef][Medline]
359. Efendi S, Enzmann F, Nylén A, Uvnäs-Wallensten K, Luft R 1979 Effect of glucose/sulfonylurea interaction on release of insulin, glucagon, and somatostatin from isolated perfused rat pancreas. Proc Natl Acad Sci USA 76:5901–5904[Abstract/Free Full Text]
360. Grodsky GM, Epstein GH, Fanska R, Karam JH 1977 Pancreatic action of the sulfonylureas. Fed Proc 36:2714–2719[Medline]
361. Sako Y, Wasada T, Umeda F, Ibayashi H 1986 Effect of glibenclamide on pancreatic hormone release from isolated perifused islets of normal and cysteamine-treated rats. Metabolism 35:944–949[CrossRef][Medline]
362. Hirose H, Maruyama H, Ito K, Kido K, Koyama K, Saruta T 1994 Effects of diazoxide on - and ß-cell function in isolated perfused rat pancreas. Diabetes Res Clin Pract 25:77–82[CrossRef][Medline]
363. ter Braak EW, Appelman AM, van der Tweel I, Erkelens DW, van Haeften TW 2002 The sulfonylurea glyburide induces impairment of glucagon and growth hormone responses during mild insulin-induced hypoglycemia. Diabetes Care 25:107–112[Abstract/Free Full Text]
364. Landstedt-Hallin L, Adamson U, Lins P-E 1999 Oral glibenclamide suppresses glucagon secretion during insulin-induced hypoglycemia in patients with type 2 diabetes. J Clin Endocrinol Metab 84:3140–3145[Abstract/Free Full Text]
365. Loreti L, Sugase T, Foà PP 1974 Diurnal variations of serum insulin, total glucagon, cortisol, glucose and free fatty acids in normal and diabetic subjects before and after treatment with chlorpropamide. Horm Res 5:278–292[Medline]
366. Peacey SR, Rostami-Hodjegan A, George E, Tucker GT, Heller SR 1997 The use of tolbutamide-induced hypoglycemia to examine the intraislet role of insulin in mediating glucagon release in normal humans. J Clin Endocrinol Metab 82:1458–1461[Abstract/Free Full Text]
367. Pfeifer MA, Beard JC, Halter JB, Judzewitsch R, Best JD, Porte Jr D 1983 Suppression of glucagon secretion during a tolbutamide infusion in normal and noninsulin-dependent diabetic subjects. J Clin Endocrinol Metab 56:586–591[Abstract/Free Full Text]
368. Samols E, Tyler JM, Mialhe P 1969 Suppression of pancreatic glucagon release by the hypoglycaemic sulphonylureas. Lancet 293:174–176[CrossRef]
369. Telner AH, Taylor CI, Pek S, Crowther RL, Floyd Jr JD, Fajans SS 1977 Effects of chlorpropamide on basal and arginine-stimulated plasma levels of glucagon and insulin in diabetic patients. In: Foà PP, Bajaj JS, Foà NL, ed. Glucagon: its role in physiology and clinical medicine. New York: Springer; 723–733
370. Tsalikian E, Dunphy TW, Bohannon NV, Lorenzi M, Gerich JE, Forsham PH, Kane JP, Karam JH 1977 The effect of chronic oral antidiabetic therapy on insulin and glucagon responses to a meal. Diabetes 26:314–321[Abstract]
371. Bohannon NV, Lorenzi M, Grodksy GM, Karam JH 1982 Stimulatory effects of tolbutamide infusion on plasma glucagon in insulin-dependent diabetic subjects. J Clin Endocrinol Metab 54:459–462[Abstract/Free Full Text]
372. Efanova IB, Zaitsev SV, Efanov AM, Östenson C-G, Raap A, Mest H-J, Berggren P-O, Efendic S 1998 Effects of imidazoline derivative RX871024 on insulin, glucagon, and somatostatin secretion from isolated perfused rat pancreas. Biochem Biophys Res Commun 252:162–165[CrossRef][Medline]
373. Høy M, Bokvist K, Xiao-Gang W, Hansen J, Juhl K, Berggren P-O, Buschard K, Gromada J 2001 Phentolamine inhibits exocytosis of glucagon by G_i2 protein-dependent activation of calcineurin in rat pancreatic -cells. J Biol Chem 276:924–930[Abstract/Free Full Text]
374. Høy M, Olsen HL, Bokvist K, Petersen JS, Gromada J 2003 The imidazoline NNC77–0020 affects glucose-dependent insulin, glucagon and somatostatin secretion in mouse pancreatic islets. Naunyn Schmiedebergs Arch Pharmacol 368:284–293[CrossRef][Medline]
375. Høy M, Olsen HL, Andersen HS, Bokvist K, Buschard K, Hansen J, Jacobsen P, Petersen JS, Rorsman P, Gromada J 2003 Imidazoline NNC77–0074 stimulates insulin secretion and inhibits glucagon release by control of Ca²⁺-dependent exocytosis in pancreatic - and ß-cells. Eur J Pharmacol 466:213–221[CrossRef][Medline]
376. Dinneen S, Alzaid A, Turk D, Rizza R 1995 Failure of glucagon suppression contributes to postprandial hyperglycemia in IDDM. Diabetologia 38:337–343[Medline]
377. Shah P, Basu A, Basu R, Rizza RA 1999 Impact of lack of suppression of glucagon on glucose tolerance in humans. Am J Physiol 277:E283–E290
378. Shah P, Vella A, Basu A, Basu R, Schwenk WF, Rizza RA 2000 Lack of suppression of glucacon contributes to postprandial hyperglycemia in subjects with type 2 diabetes mellitus. J Clin Endocrinol Metab 85:4053–4059[Abstract/Free Full Text]
379. Defronzo RA, Ferraninini E, Simonson DC 1989 Fasting hyperglycemia in non-insulin-dependent diabetes mellitus: contributions of excessive hepatic glucose production and impaired tissue glucose uptake. Metabolism 38:387–395[CrossRef][Medline]
380. Consoli A, Nurjhan N, Capani F, Gerich J 1989 Predominant role of gluconeogenesis in increased hepatic glucose production in NIDDM. Diabetes 38:550–557[Abstract]
381. Butler PC, Rizza RA 1991 Contribution to postprandial hyperglycemia and effect on initial spanchnic glucose clearance of hepatic glucose cycling in glucose-intolerant or NIDDM patients. Diabetes 40:73–81[Abstract]
382. Reaven GM, Chen YD, Golay A, Swislocki AL, Jaspan JB 1987 Documentation of hyperglucagonemia throughout the day in nonobese and obese patients with noninsulin-dependent diabetes mellitus. J Clin Endocrinol Metab 64:106–110[Abstract/Free Full Text]
383. Baron AD, Schaeffer L, Shragg P, Kolterman OG 1987 Role of hyperglucagonemia in maintenance of increased rates of hepatic glucose output in type 2 diabetes. Diabetes 36:274–283[Abstract]
384. Parker JC, McPherson RK, Andrews KM, Levy CB, Dubins JS, Chin JE, Perry PV, Hulin B, Perry DA, Inagaki T, Dekker KA, Tachikawa K, Sugie Y, Treadway JL 2000 Effects of skyrin, a receptor-selective glucagon antagonist, in rat and human hepatocytes. Diabetes 49:2079–2086[Abstract/Free Full Text]
385. Gysin B, Trivedi D, Johnson DG, Hruby VJ 1986 Design and synthesis of glucagon partial agonists and antagonists. Biochemistry 16:8278–8284
386. Gysin B, Johnson DG, Trivedi D, Hruby VJ 1987 Synthesis of two glucagon antagonists: receptor binding, adenylate cyclase, and effects on blood plasma glucose levels. J Med Chem 30:1409–1415[CrossRef][Medline]
387. Unson CG, Gurzenda EM, Merrifield RB 1989 Biological activities of des-His1{Glu9}glucagon amide, a glucagon antagonist. Peptides 10:1171–1177[CrossRef][Medline]
388. Madsen, P, Knudsen LB, Wiberg FC, Carr RD 1998 Discovery and structure-activity relationship of the first non-peptide competitive human glucagon receptor antagonists. J Med Chem 17:5150–5157
389. Qureshi SA, Rios Candelore M, Xie D, Yang X, Tota LM, Ding VD, Li Z, Bansal A, Miller C, Cohen SM, Jiang G, Brady E, Saperstein R, Duffy JL, Tata JR, Chapman KT, Moller DE, Zhang BB 2004 A novel glucagon receptor antagonist inhibits glucagon-mediated biological effects. Diabetes 53:3267–3273[Abstract/Free Full Text]
390. Petersen KF, Sullivan JT 2001 Effects of a novel glucagon receptor antagonist (Bay 27–9955) on glucagon-stimulated glucose production in humans. Diabetologia 44:2018–2024[CrossRef][Medline]
391. Djuric SW, Grihalde N, Lin CW 2002 Glucagon receptor antagonists for the treatment of type II diabetes: current prospects. Curr Opin Investig Drugs 3:1617–1623[Medline]
392. Jiang G, Zhang BB 2003 Glucagon and regulation of glucose metabolism. Am J Physiol Endocrinol Metab 284:E671–E678
393. Sloop KW, Michael MD, Moyers JS 2005 Glucagon as a target for the treatment of type 2 diabetes. Expert Opin Ther Targets 9:593–600[CrossRef][Medline]
394. Brand CL, Rolin B, Jorgensen PN, Svendsen I, Kristensen JS, Holst JJ 1994 Immunoneutralization of endogenous glucagon with monoclonal glucagon antibody normalizes hyperglycemia in moderately streptozotocin-diabetic rats. Diabetologia 37:985–993[Medline]
395. Sørensen H, Brand CL, Neschen S, Holst JJ, Fosgerau K, Nishimura E, Shulman GI 2006 Immunoneutralization of endogenous glucagon reduces hepatic glucose output and improves long-term glycemic control in diabetic ob/ob mice. Diabetes 55:2843–2848[Abstract/Free Full Text]
396. Brand CL, Jørgensen PN, Knigge U, Warberg J, Svendsen I, Kristensen JS, Holst JJ 1995 Role of glucagon in maintenance of euglycemia in fed and fasted rats. Am J Physiol 269:E469–E477
397. Sloop KW, Cao JX-C, Siesky AM, Zhang HY, Bodenmiller DM, Cox AL, Jacobs SJ, Moyers JS, Owens RA, Showalter AD, Brenner MB, Raap A, Gromada J, Berridge BR, Monteith DKB, Porksen N, McKay RA, Monia BP, Bhanot S, Watts LM, Michael MD 2004 Hepatic and glucagon-like peptide-1-mediated reversal of diabetes by glucagon receptor antisense oligonucleotide inhibitors. J Clin Invest 113:1571–1581[CrossRef][Medline]
398. Parker JC, Andrews KM, Allen MR, Stock JL, McNeish JD 2002 Glycemic control in mice with targeted disruption of the glucagon receptor gene. Biochem Biophys Res Commun 290:839–843[CrossRef][Medline]
399. Vuguin PM, Kedees MH, Cui L, Guz Y, Gelling RW, Nejathaim M, Charron MJ, Teitelman G 2006 Ablation of the glucagon receptor gene increases fetal lethality and produces alterations in islet development and maturation. Endocrinology 147:3995–4006[Abstract/Free Full Text]
400. Shapiro AM, Lakey JR, Ryan EA, Korbutt GS, Toth E, Warnock GL, Kneteman GS, Rajotte RV 2000 Islet transplantation in seven patients with type 1 diabetes mellitus using a glucocorticoid-free immunosuppressive regimen. N Engl J Med 343:230–238[Abstract/Free Full Text]
401. Paty BW, Ryan EA, Shapiro AM, Lakey JR, Robertson RP 2002 Intrahepatic islet transplantation in type 1 diabetic patients does not restore hypoglycemic hormonal counterregulation or symptom recognition after insulin independence. Diabetes 51:3428–3434[Abstract/Free Full Text]
402. Kendall DM, Teuscher AU, Robertson RP 1997 Defective glucagon secretion during sustained hypoglycemia following successful islet allo- and autotransplantation in humans. Diabetes 46:23–27[Abstract]
403. Meyer C, Hering BJ, Grossmann R, Brandhorst H, Brandhorst D, Gerich J, Federlin K, Bretzer RG 1998 Improved glucose counterregulation and autonomic symptoms after intraportal islet transplants alone in patients with long-standing type I diabetes mellitus. Transplantation 66:233–240[Medline]
404. Rickels MR, Schutta MH, Mueller R, Markmann JF, Barker CF, Naji A, Teff KL 2005 Islet cell hormonal responses to hypoglycemia after human islet transplantation for type 1 diabetes. Diabetes 54:3205–3211[Abstract/Free Full Text]
405. Gupta V, Wahoff DC, Rooney DP, Poitout V, Sutherland DE, Kendall DM, Robertson RP 1997 The defective glucagon response from transplanted intrahepatic pancreatic islets during hypoglycemia is transplantation site- determined. Diabetes 46:28–33[Abstract]
406. Ansara MF, Saudek F, Newton M, Raynor AC, Cryer PE, Scharp DW 1994 Pancreatic islet transplantation prevents defective glucose counter-regulation in diabetic dogs. Transplant Proc 26:664–665[Medline]
407. Gustavson SM, Rajotte RV, Hunkeler D, Lakey JRT, Edgerton DS, Neal DW, Snead WL, Penaloza AR, Cherrington AD 2005 Islet auto-transplantation into an omental or splenic site results in a normal ß cell but abnormal cell response to mild non-insulin-induced hypoglycemia. Am J Transpl 5:2368–2377[CrossRef]
408. Robertson RP 2002 Transplanted intrahepatic islets: enigmatic absence of the -cell response to hypoglycaemia. Diabetes Nutr Metab 15:433–437[Medline]
409. Gustavson SM, Nishizawa M, Farmer B, Neal D, Brissova M, Powers AC, Cherrington AD 2003 A fall in portal vein insulin does not cause the -cell response to mild, non-insulin-induced hypoglycemia in conscious dogs. Metabolism 52:1418–1425[CrossRef][Medline]
410. Reimer MK, Mokshagundam SP, Wyler K, Sundler F, Ahren B, Stagner JI 2003 Local growth factors are beneficial for the autonomic reinnervation of transplanted islets in rats. Pancreas 26:392–397[CrossRef][Medline]
411. Gardemann A, Jungermann K, Grosse V, Cossel L, Wohlrab F, Hahn HJ, Blech W, Hildebrandt W 1994 Intraportal transplantation of pancreatic islets into livers of diabetic rats. Reinnervation of islets and regulation of insulin secretion by the hepatic sympathetic nerves. Diabetes 43:1345–1352[Abstract]
412. Gardemann A, Jungermann K, Grosse V, Cossel L, Wohlrab F, Hahn HJ, Blech W, Hildebrandt W 1995 Reinnervation of pancreatic islets and regulation of insulin secretion by hepatic sympathetic nerves after intraportal transplantation of islets into livers of diabetic rats. Exp Clin Endocrinol Diabetes 103(Suppl 2):107–111
413. Adeghate E 2002 Pancreatic tissue grafts are reinnervated by neuro-peptidergic and cholinergic nerves within five days of transplantation. Transpl Immunol 10:73–80[CrossRef][Medline]
414. Persson-Sjogren S, Forsgren S, Taljedal IB 2000 Peptides and other neuronal markers in transplanted pancreatic islets. Peptides 21:741–752[CrossRef][Medline]
415. Carlsson PO, Andersson A, Carlsson C, Hellerstrom C, Hoglund E, King A, Kallskog O, Liss P, Mattsson G, Olsson R, Palm F, Sandler S, Tyrberg B, Jansson L 2000 Engraftment and growth of transplanted pancreatic islets. Ups J Med Sci 105:107–123[Medline]
416. Carlsson PO, Palm F, Mattsson G 2002 Low revascularization of experimentally transplanted human pancreatic islets. J Clin Endocrinol Metab 87:5418–5423[Abstract/Free Full Text]
417. Carlsson PO, Mattsson G 2002 Oxygen tension and blood flow in relation to revascularization in transplanted adult and fetal rat pancreatic islets. Cell Transplant 11:813–820[Medline]
418. Mattsson G, Jansson L, Carlsson PO 2002 Decreased vascular density in mouse pancreatic islets after transplantation. Diabetes 51:1362–1366[Abstract/Free Full Text]
419. Mattsson G, Jansson L, Nordin A, Carlsson PO 2003 Impaired revascularization of transplanted mouse pancreatic islets is chronic and glucose-independent. Transplantation 75:736–739[CrossRef][Medline]
420. Menger MD, Yamauchi J, Vollmar B 2001 Revascularization and microcirculation of freely grafted islets of Langerhans. World J Surg 25:509–515[CrossRef][Medline]
421. Carlsson PO, Kiuru A, Nordin A, Olsson R, Lin JM, Bergsten P, Hillered L, Andersson A, Jansson L 2002 Microdialysis measurements demonstrate a shift to nonoxidative glucose metabolism in rat pancreatic islets transplanted beneath the renal capsule. Surgery 132:487–494[CrossRef][Medline]
422. Carlsson PO, Jansson L, Andersson A, Kallskog O 1998 Capillary blood pressure in syngeneic rat islets transplanted under the renal capsule is similar to that of the implantation organ. Diabetes 47:1586–1593[Abstract]
423. Carlsson PO, Liss P, Andersson A, Jansson L 1998 Measurements of oxygen tension in native and transplanted rat pancreatic islets. Diabetes 47:1027–1032[Abstract]
424. Carlsson PO, Palm F, Andersson A, Liss P 2001 Markedly decreased oxygen tension in transplanted rat pancreatic islets irrespective of the implantation site. Diabetes 50:489–495[Abstract/Free Full Text]
425. Carlsson PO, Palm F, Andersson A, Liss P 2000 Chronically decreased oxygen tension in rat pancreatic islets transplanted under the kidney capsule. Transplantation 69:761–766[CrossRef][Medline]
426. Cryer PE 2002 Hypoglycaemia: the limiting factor in the glycaemic management of type I and type II diabetes. Diabetologia 45:937–948[CrossRef][Medline]
427. UK Prospective Diabetes Study (UKPDS) Group 1998 Intensive blood-glucose control with sulphonylureas or insulin compared with conventional treatment and risk of complications in patients with type 2 diabetes (UKPDS 33). Lancet 352:837–853[CrossRef][Medline]
428. Gerich JE, Langlois M, Noacco C, Karam JH, Forsham PH 1973 Lack of glucagon response to hypoglycemia in diabetes: evidence for an intrinsic pancreatic -cell defect. Science 182:171–173[Abstract/Free Full Text]
429. Wiethop BV, Cryer PE 1993 Glycemic actions of alanine and terbutaline in IDDM. Diabetes Care 16:1124–1130[Abstract]
430. Caprio S, Tambolane WV, Zych K, Gerow K, Sherwin RS 1993 Loss of potentiating effect of hypoglycemia on the glucagon response to hyperaminoacidemia in IDDM. Diabetes 42:550–555[Abstract]
431. Unger RH, Orci L 1975 The essential role of glucagon in the pathogenesis of diabetes mellitus. Lancet 1:14–16[CrossRef][Medline]
432. Pipeleers DG, in't Veld PA, Van De Winkel M, Maes E, Schuit FC, Gepts W 1985 A new in vitro model for the study of pancreatic A and B cells. Endocrinology 117:806–816[Abstract/Free Full Text]
433. Zhou H, Zhang T, Harmon JS, Bryan J, Robertson RP, Zinc, not insulin, regulates the rat -cell response to hypoglycemia in vitro. Diabetes, in press

This article has been cited by other articles:

				S. C Lee, C. A Robson-Doucette, and M. B Wheeler Uncoupling protein 2 regulates reactive oxygen species formation in islets and influences susceptibility to diabetogenic action of streptozotocin J. Endocrinol., October 1, 2009; 203(1): 33 - 43. [Abstract] [Full Text] [PDF]

				H. Schrader, B. A. Menge, T. G. K. Breuer, P. R. Ritter, W. Uhl, W. E. Schmidt, J. J. Holst, and J. J. Meier Impaired Glucose-Induced Glucagon Suppression after Partial Pancreatectomy J. Clin. Endocrinol. Metab., August 1, 2009; 94(8): 2857 - 2863. [Abstract] [Full Text] [PDF]

				E. Tuduri, L. Marroqui, S. Soriano, A. B. Ropero, T. M. Batista, S. Piquer, M. A. Lopez-Boado, E. M. Carneiro, R. Gomis, A. Nadal, et al. Inhibitory Effects of Leptin on Pancreatic {alpha}-Cell Function Diabetes, July 1, 2009; 58(7): 1616 - 1624. [Abstract] [Full Text] [PDF]

				S. Z. Young and A. Bordey GABA's Control of Stem and Cancer Cell Proliferation in Adult Neural and Peripheral Niches Physiology, June 1, 2009; 24(3): 171 - 185. [Abstract] [Full Text] [PDF]

				L. S. Katz, Y. Gosmain, E. Marthinet, and J. Philippe Pax6 Regulates the Proglucagon Processing Enzyme PC2 and Its Chaperone 7B2 Mol. Cell. Biol., April 15, 2009; 29(8): 2322 - 2334. [Abstract] [Full Text] [PDF]

				M. N. Poy, J. Hausser, M. Trajkovski, M. Braun, S. Collins, P. Rorsman, M. Zavolan, and M. Stoffel miR-375 maintains normal pancreatic {alpha}- and {beta}-cell mass PNAS, April 7, 2009; 106(14): 5813 - 5818. [Abstract] [Full Text] [PDF]

				N. Gustavsson, S.-H. Wei, D. N. Hoang, Y. Lao, Q. Zhang, G. K. Radda, P. Rorsman, T. C. Sudhof, and W. Han Synaptotagmin-7 is a principal Ca2+ sensor for Ca2+-induced glucagon exocytosis in pancreas J. Physiol., March 15, 2009; 587(6): 1169 - 1178. [Abstract] [Full Text] [PDF]

				S. J. Ernst, L. Aguilar-Bryan, and J. L. Noebels Sodium Channel {beta}1 Regulatory Subunit Deficiency Reduces Pancreatic Islet Glucose-Stimulated Insulin and Glucagon Secretion Endocrinology, March 1, 2009; 150(3): 1132 - 1139. [Abstract] [Full Text] [PDF]

				G. A. Rutter Regulating Glucagon Secretion: Somatostatin in the Spotlight Diabetes, February 1, 2009; 58(2): 299 - 301. [Full Text] [PDF]

				D. A. Jacobson, B. L. Wicksteed, and L. H. Philipson The {alpha}-Cell Conundrum: ATP-Sensitive K+ Channels and Glucose Sensing Diabetes, February 1, 2009; 58(2): 304 - 306. [Full Text] [PDF]

				U. E. A. Martensson, S. A. Salehi, S. Windahl, M. F. Gomez, K. Sward, J. Daszkiewicz-Nilsson, A. Wendt, N. Andersson, P. Hellstrand, P.-O. Grande, et al. Deletion of the G Protein-Coupled Receptor 30 Impairs Glucose Tolerance, Reduces Bone Growth, Increases Blood Pressure, and Eliminates Estradiol-Stimulated Insulin Release in Female Mice Endocrinology, February 1, 2009; 150(2): 687 - 698. [Abstract] [Full Text] [PDF]

				N. Quoix, R. Cheng-Xue, L. Mattart, Z. Zeinoun, Y. Guiot, M. C. Beauvois, J.-C. Henquin, and P. Gilon Glucose and Pharmacological Modulators of ATP-Sensitive K+ Channels Control [Ca2+]c by Different Mechanisms in Isolated Mouse {alpha}-Cells Diabetes, February 1, 2009; 58(2): 412 - 421. [Abstract] [Full Text] [PDF]

				D. Islam, N. Zhang, P. Wang, H. Li, P. L. Brubaker, H. Y. Gaisano, Q. Wang, and T. Jin Epac is involved in cAMP-stimulated proglucagon expression and hormone production but not hormone secretion in pancreatic {alpha}- and intestinal L-cell lines Am J Physiol Endocrinol Metab, January 1, 2009; 296(1): E174 - E181. [Abstract] [Full Text] [PDF]

				B. R. Gauthier and C. B. Wollheim Synaptotagmins bind calcium to release insulin Am J Physiol Endocrinol Metab, December 1, 2008; 295(6): E1279 - E1286. [Abstract] [Full Text] [PDF]

				I. Quesada, E. Tuduri, C. Ripoll, and A. Nadal Physiology of the pancreatic {alpha}-cell and glucagon secretion: role in glucose homeostasis and diabetes J. Endocrinol., October 1, 2008; 199(1): 5 - 19. [Abstract] [Full Text] [PDF]

				P. Bansal and Q. Wang Insulin as a physiological modulator of glucagon secretion Am J Physiol Endocrinol Metab, October 1, 2008; 295(4): E751 - E761. [Abstract] [Full Text] [PDF]

				H. Ellingsgaard, J. A. Ehses, E. B. Hammar, L. Van Lommel, R. Quintens, G. Martens, J. Kerr-Conte, F. Pattou, T. Berney, D. Pipeleers, et al. Interleukin-6 regulates pancreatic {alpha}-cell mass expansion PNAS, September 2, 2008; 105(35): 13163 - 13168. [Abstract] [Full Text] [PDF]

				J. Diao, E. M. Allister, V. Koshkin, S. C. Lee, A. Bhattacharjee, C. Tang, A. Giacca, C. B. Chan, and M. B. Wheeler UCP2 is highly expressed in pancreatic {alpha}-cells and influences secretion and survival PNAS, August 19, 2008; 105(33): 12057 - 12062. [Abstract] [Full Text] [PDF]

				R. J. Brown, N. Sinaii, and K. I. Rother Too Much Glucagon, Too Little Insulin: Time course of pancreatic islet dysfunction in new-onset type 1 diabetes Diabetes Care, July 1, 2008; 31(7): 1403 - 1404. [Abstract] [Full Text] [PDF]

				J. A. Harney and R. L. Rodgers Insulin-like stimulation of cardiac fuel metabolism by physiological levels of glucagon: involvement of PI3K but not cAMP Am J Physiol Endocrinol Metab, July 1, 2008; 295(1): E155 - E161. [Abstract] [Full Text] [PDF]

				E. Tuduri, E. Filiputti, E. M. Carneiro, and I. Quesada Inhibition of Ca2+ signaling and glucagon secretion in mouse pancreatic {alpha}-cells by extracellular ATP and purinergic receptors Am J Physiol Endocrinol Metab, May 1, 2008; 294(5): E952 - E960. [Abstract] [Full Text] [PDF]

				A. V. Gyulkhandanyan, H. Lu, S. C. Lee, A. Bhattacharjee, N. Wijesekara, J. E. M. Fox, P. E. MacDonald, F. Chimienti, F. F. Dai, and M. B. Wheeler Investigation of Transport Mechanisms and Regulation of Intracellular Zn2+ in Pancreatic {alpha}-Cells J. Biol. Chem., April 11, 2008; 283(15): 10184 - 10197. [Abstract] [Full Text] [PDF]

				H. Chahal and T.A. Chowdhury Gliptins: a new class of oral hypoglycaemic agent QJM, November 1, 2007; 100(11): 671 - 677. [Abstract] [Full Text] [PDF]

This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Copyright Permission Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Gromada, J. Articles by Wollheim, C. B. Search for Related Content PubMed PubMed Citation Articles by Gromada, J. Articles by Wollheim, C. B.