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Endocrine Unit, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02114
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Abstract |
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 Top Abstract I. IntroductionII. Maternal Physiology and...III. Fetal-Placental Physiology...IV. Maternal Physiology and...V. Neonatal Physiology and...VI. Discussion and ConclusionsReferences |
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I. Introduction |
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TopAbstractI. IntroductionII. Maternal Physiology and...III. Fetal-Placental Physiology...IV. Maternal Physiology and...V. Neonatal Physiology and...VI. Discussion and ConclusionsReferences |
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Although Albright and Reifenstein’s theory proved to be incorrect, it is now evident that mineralization of the fetal skeleton and continued skeletal growth in the infant both mandate a series of hormone-mediated adjustments in maternal calcium metabolism during pregnancy and lactation, respectively. These hormone-mediated adjustments normally satisfy the daily calcium needs of the fetus and infant without long-term consequences to the maternal skeleton. In addition, both fetal and neonatal calcium and bone metabolism are uniquely adapted to meet the specific needs of these developmental periods. The fetus must actively transport sufficient calcium across the placenta to meet the large demands of the rapidly mineralizing skeleton, whereas the neonate must quickly adjust to loss of placental calcium transport, while continuing to undergo rapid skeletal growth.
Here we review our present understanding of normal human calcium and bone metabolism during pregnancy, lactation, fetal development, and the neonatal period. We shall also discuss the relevant pathophysiology and management of clinical disorders of calcium and bone metabolism that can occur during these periods. Generally these conditions are due to preexisting disease (e.g., hyperparathyroidism) that is compounded by the alterations in calcium and bone metabolism naturally occurring during these reproductive periods.
Although the focus of this review is on human calcium physiology and pathophysiology, the animal literature will be closely considered as well. Many of our models for explaining human physiology are based on these animal studies, particularly since ethical constraints generally prevent all but observational studies from being performed during human pregnancy and fetal development. Where both human and animal data are available, we will point out several significant differences that have been found between the animal and human data. These differences illustrate the difficulty of extrapolating from the animal models in the absence of human data.
The literature reviewed in this paper was obtained from computerized searches of the MEDLINE database, manual searches of Index Medicus before 1966, and the bibliographies of individual articles and texts.
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II. Maternal Physiology and Pathophysiology During Pregnancy |
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TopAbstractI. IntroductionII. Maternal Physiology and...III. Fetal-Placental Physiology...IV. Maternal Physiology and...V. Neonatal Physiology and...VI. Discussion and ConclusionsReferences |
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The pregnant rat has typically been used as a model for studying
calcium metabolism during pregnancy, but the adaptive strategies of the
rat differ importantly from those of the human (Table 1
). These differences probably reflect
the large litter size (six to 12 fetuses) and the short gestational
period (22 days) of the rat; the rat must deliver 12 mg of calcium per
fetus between day 17 of gestation and term (6).
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In contrast, the serum total and ionized calcium have been reported to fall during the last several days of pregnancy in the rat (21). Maternal losses of calcium to a litter of rapidly growing fetuses may exceed the maternal capacity to maintain a normal serum calcium level. Indeed, larger litter sizes correlated with lower serum calcium in pregnant rats (22). In white-tailed deer, the corrected serum calcium falls in the last 1 to 2 weeks of gestation (23). Pregnant ewes have a mild decrease in total serum calcium over the last 6 weeks of pregnancy, likely due to the fall in serum albumin (24); moreover, in one study, about 13% of Awassi fat-tail ewes were found to develop signs and biochemical evidence of hypocalcemia in the last month of pregnancy (25). Therefore, data from several animal models suggest that maternal blood calcium regulation may be disrupted by fetal demands in late pregnancy.
2. Phosphate. Serum phosphate levels are normal throughout pregnancy in humans and animals, as is the renal tubular reabsorption of phosphate (14, 17, 26, 27, 28, 29).
3. PTH. The bulk of published human data on PTH levels in pregnancy was obtained from studies that used early-generation PTH RIAs (18, 26, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40); some of the more frequently cited studies reported high maternal serum levels of PTH in the latter half of pregnancy (18, 30, 31, 32, 33, 34, 39). These data must now be reinterpreted, because it is now known that these PTH RIAs were insensitive and heterogeneously measured multiple different fragments of PTH, most of which were biologically inactive (41, 42).
With the advent of sensitive two-site immunoradiometric (IRMA) PTH assays that accurately determine the level of intact PTH (42), PTH levels have been typically found to be low-normal in the serum of pregnant women in all three trimesters (11, 12, 17, 19, 20, 27, 43). Five prospective, longitudinal studies found that the mean PTH level was in the low-normal range (i.e., <50% of the mean nonpregnant value) during the first trimester but increased steadily to the mid-normal range by the end of pregnancy (14, 15, 16, 27, 44). These findings have been independently validated by reports of normal nephrogenous cAMP levels (12, 26, 44) and low to normal PTH-like bioactivity (11) throughout human pregnancy (although this may be confounded by synthesis of nephrogenous cAMP due to the effects of PTHrP). Studies in primates suggest that the parathyroid glands may have less secretory reserve as pregnancy progresses; the incremental PTH response to acute EDTA-induced hypocalcemia in rhesus monkeys decreased across the trimesters (39).
In contrast to humans, rats develop secondary hyperparathyroidism late
in pregnancy. Normally, in late pregnancy, both maternal levels of
intact immunoreactive (45) and bioactive (46) PTH rise to exceed the
normal range, and the maternal ionized and total calcium levels decline
slightly (21). The parathyroid gland volume has also been reported to
increase during normal rat pregnancy (47, 48). In vitro
studies in pregnant rats indicate that the parathyroids secrete more
PTH at a given extracellular calcium concentration, when compared with
parathyroid cells taken from nonpregnant rats (49). The PTH levels
begin to rise earlier in gestation, and peak at higher levels, in
pregnant rats fed a modestly calcium-restricted diet (21, 50). This
increase in PTH during late pregnancy is critical for normal maternal
calcium homeostasis; parathyroidectomized pregnant rats can exhibit
signs of tetany in the last 2–4 days of gestation and death during the
birthing process (51, 52, 53, 54). In parathyroidectomized pregnant rats,
dietary intake and weight gain decline, while serum
1,25-dihydroxyvitamin D and intestinal calbindin9K-D levels
fall (52, 53, 54). Maternal tetany coincides with the time onset of rapid
fetal accretion of calcium (6); therefore, the parathyroidectomized
pregnant rat has compromised dietary intake and intestinal calcium
absorption at the time of peak fetal demand for calcium. The calcium
abnormalities can be completely prevented when the rats are fed a
high-calcium, low-phosphorus diet. Taken together, these observations
indicate that rats (but not humans) normally develop a form of
secondary hyperparathyroidism during late pregnancy in response to the
fall in the maternal serum calcium level. Rats may be more dependent on
PTH-mediated bone resorption and PTH-induced 1
-hydroxylase
up-regulation during late pregnancy, at a time when the combined
calcium need of a litter of fetuses is at its peak.
In summary, immunoreactive and bioactive PTH levels are in the low-normal range during early human pregnancy and are in the mid-normal range at term; in contrast, immunoreactive and bioactive PTH levels in rats are normal in early pregnancy but exceed the normal range in late gestation.
4. 1,25-Dihydroxyvitamin D. Cross-sectional studies have found
that the serum level of 1,25-dihydroxyvitamin D more than doubles early
in the first trimester in human pregnancy (12, 27, 36, 37, 55, 56, 57, 58).
Longitudinal studies have found that the levels of both free and bound
1,25-dihydroxyvitamin D are doubled, and that this increase is
maintained until term (15, 20, 41, 57, 58, 59). Although clearance of
1,25-dihydroxyvitamin D has not been studied during human pregnancy, in
pregnant rats, sheep, and rabbits the increased 1,25-dihydroxyvitamin D
levels were due to increased production, and not decreased metabolic
clearance, of 1,25-dihydroxyvitamin D (60, 61, 62, 63). In vitro
measurements in homogenates of maternal kidney from rabbits and guinea
pigs show that the renal 1-hydroxylase may be up-regulated 2- to
5-fold (64, 65). The increase in the 1,25-dihydroxyvitamin D level
begins while the PTH level is in the low-normal range in humans
(Section II.B.3, above); this may indicate that PTH does
not mediate the up-regulation of the maternal renal
1-hydroxylase during early human pregnancy. Furthermore,
parathyroidectomy in pregnant sheep reduces, but does not eliminate,
the pregnancy-related increase in 1,25-dihydroxyvitamin D (66). Other
potential direct or indirect regulators of the 1-hydroxylase include
PTHrP (Section II.B.6, below), estradiol, PRL, and
placental lactogen. Estradiol (67), PRL (68, 69), and placental
lactogen (69) acutely stimulate the 1-hydroxylase in
vitro, and placental lactogen (but not PRL) raised the serum
1,25-dihydroxyvitamin D levels in hypophysectomized, nonpregnant rats
(70). The effect of estradiol on the 1-hydroxylase has been
confirmed in vivo by the observation that estrogen
replacement in postmenopausal women increases the free and total serum
1,25-dihydroxyvitamin D level (71). However, an effect of PRL in
vivo has not been confirmed, since hyperprolactinemic patients
showed no alteration in 1,25-dihydroxyvitamin D levels (72). Further,
in pregnant women, the high 1,25-dihydroxyvitamin D levels of pregnancy
did not correlate with serum PRL, estrogens, or human placental
lactogen (73).
In addition to the renal 1-hydroxylase, 1-hydroxylase activity
found in maternal decidua, placenta, and fetal kidneys may also add
1,25-dihydroxyvitamin D to the maternal circulation during pregnancy
(59, 74, 75, 76, 77). To test this hypothesis,
[3H]25-hydroxyvitamin D was administered to pregnant rats
after bilateral maternal nephrectomy (74, 78). Newly synthesized
(i.e., tritiated) 1,25-dihydroxyvitamin D appeared in
the maternal circulation of nephrectomized pregnant rats (but not in
nonpregnant nephrectomized rats). Although this study indicates that
extrarenally produced 1,25-dihydroxyvitamin D can reach the maternal
circulation (74), the specific extrarenal sites and the amounts of
1,25-dihydroxyvitamin D produced could not be ascertained. Data from
the Hannover pig model (autosomal recessive 1-hydroxylase
deficiency) indicate that the amounts contributed by these extrarenal
sites may be insignificant. In pregnant sows homozygous for absence of
1-hydroxylase activity, serum levels of 1,25-dihydroxyvitamin D were
very low, comparable to the nonpregnant values (79). The presence of
heterozygous fetuses did not increase the circulating level of
1,25-dihydroxyvitamin D in the homozygous sows (79). The same gene
controls renal and decidual 1-hydroxylase activity in this model
(77). A single case report of a human patient on chronic hemodialysis
found 1,25-dihydroxyvitamin D levels of 10–15 pg/ml during pregnancy;
these levels were higher than the nonpregnant level in the same
patient, but were far lower than in normal pregnancy (80). It is,
therefore, likely that increased maternal production of
1,25-dihydroxyvitamin D is mainly due to increased activity of
maternal, renal 1-hydroxylase and not to large contributions from
extrarenal sites.
Again, the pregnant rat model differs somewhat from the human, in that
the maternal rise in 1,25-dihydroxyvitamin D level does not occur in
rats until the time of fetal skeletal mineralization in late gestation
(22, 45, 45, 81, 82), at which time the serum PTH levels rise above
normal (22, 45) and serum ionized calcium levels fall (21, 22). Larger
litter sizes correlate with higher maternal 1,25-dihydroxyvitamin D
levels (22). These studies suggest that the effect of PTH on the renal
1-hydroxylase may dominate the production of 1,25-dihydroxyvitamin D
during late pregnancy in the rat.
Serum 25-hydroxyvitamin D levels are unchanged in human pregnancy, and 24,25-dihydroxyvitamin D levels are lower in pregnant women than in controls (35). Supplementation with 1000 IU of vitamin D3 daily after the first trimester in humans did not affect maternal calcium, phosphate, PTH, and 1,25-dihydroxyvitamin D levels; this suggests that the changes in calcitropic hormone levels observed in human pregnancy are not the result of occult vitamin D deficiency (83). Maternal vitamin D deficiency in the rat has been associated with reduced fertility and smaller litter sizes, and up to 20% of pregnant, vitamin D-deficient rats may die of hypocalcemia near term (84, 85).
In summary, free and total 1,25-dihydroxyvitamin D levels rise early in
human pregnancy to peak at twice the normal range, while in rats the
1,25-dihydroxyvitamin D level does not rise until late gestation. These
increases appear to be due to increased production of
1,25-dihydroxyvitamin D by the maternal kidneys, with possibly small
contributions from maternal decidua, placenta, and fetal kidneys. PTH
may be less important during pregnancy in humans compared with rats in
mediating this rise in 1-hydroxylase activity.
5. Calcitonin. Serum calcitonin levels in human pregnancy have generally been reported to be higher than nonpregnant values, with at least 20% of values exceeding the normal range (14, 34, 36, 37, 86, 87, 88, 89). Several human studies reported that calcitonin levels were not elevated in pregnancy (15, 18, 33, 35); however, these studies were flawed by the use of improper controls. For example, in some of these studies, postpartum measurements in the same women were used as the baseline, and it has since been shown that calcitonin is also elevated in the postpartum period (see Section IV.B.5, below). Similar data from monkeys (39), sheep (77, 90, 91), deer (23), goats (77), and rats (92) have confirmed that the maternal calcitonin level is elevated during pregnancy. No clearance data are available for humans or other animals, but the increased level of calcitonin is generally thought to reflect increased synthesis.
Thyroidal C cells, breast, and placenta are sites of calcitonin synthesis during pregnancy (93, 94). It is not surprising, therefore, that a rise in calcitonin is found in totally thyroidectomized women, most likely due to calcitonin synthesized by the placenta and breast (93, 94). In pregnant rhesus monkeys, acute calcium infusions led to a progressively greater calcitonin response across the trimesters, which may indicate greater secretory reserve of the thyroidal C cells and placenta (39).
It has been speculated that elevated calcitonin protects the maternal skeleton from excessive resorption of calcium, a hypothesis that has been difficult to prove. Indeed, the physiological role of calcitonin in human calcium and skeletal metabolism has not been established (95). No adequate model of experimental calcitonin deficiency has been created, partly because the extrathyroidal sites of calcitonin synthesis were not appreciated at the time. All models used total thyroidectomy with parathyroid gland autotransplantation and thyroid hormone replacement in pregnant goats or rats (53, 91, 96, 97, 98). In none of these models was the serum calcitonin or TSH measured to determine whether a calcitonin-deficient, euthyroid state had been attained. Thus, although these models suggested that an intact thyroid gland protected the maternal skeleton from loss of bone mineral during pregnancy, these findings remain to be confirmed by more rigorously controlled models.
In summary, calcitonin levels are increased during pregnancy in humans and animals, partly due to extrathyroidal synthesis in the placenta and breast. The possible role of calcitonin in protecting the maternal skeleton from increased resorption during pregnancy needs more study.
6. PTH-related protein (PTHrP). PTHrP was originally identified in 1987 as the cause of humoral hypercalcemia of malignancy (99). PTHrP has been postulated to be a prohormone, which is processed into several different circulating fragments or hormones, each of which, in turn, may have different functional roles and specific receptors (100). PTHrP has partial homology in its first 13 amino acids to PTH (101, 102, 103) and activates the common PTH/PTHrP receptor (42). Amino-terminal forms of PTHrP (PTHrP 1–34, 1–86, or 1–141) resemble PTH in their actions on kidney and bone (104) and can inhibit acetylcholine-induced uterine contractions in the rat (105). Levels of PTHrP decreased acutely in the amnion and myometrium at the time of onset of labor in humans (106). It has been suggested that amino-terminal forms of PTHrP may, therefore, have a role in regulating the onset of labor (106). A midmolecular form of PTHrP stimulates placental calcium transport in the fetus (Section III.C, below), although its possible role in the mother is unclear. The carboxyl-terminal portion of PTHrP, termed "osteostatin," is able to inhibit osteoclastic bone resorption in some in vitro assays (107, 108) and in rats in vivo (109); therefore, this fragment of PTHrP could have a role in protecting the maternal skeleton during pregnancy.
The development of RIAs for PTHrP has concentrated on detecting the PTH-like amino-terminal fragments of PTHrP and has thus far largely ignored the detection of other fragments that might be biologically active. Therefore, no data are available on the levels of midmolecular or carboxyl-terminal fragments of PTHrP during pregnancy compared with controls. An early RIA that used an antibody to PTHrP 1–34 found no elevation of PTHrP in pregnancy (110). Newer, more sensitive two-site immunoradiometric assays that measure forms of PTHrP that encompass amino acids 1 through 86 have found a significant increase in the maternal PTHrP level, beginning as early as weeks 3 to 13 of human pregnancy (44, 111). This increase is not due to any change in the clearance of PTHrP 1–34, 1–86, or 1–141 during pregnancy, as determined in sheep (112, 113). The increase in amino-terminal PTHrP, by activating the PTH/PTHrP receptor in kidney and bone, may well explain (at least in part) the increase in 1,25-dihydroxyvitamin D and ionized calcium, and the decrease in PTH levels, found during human pregnancy.
The source of PTHrP in the maternal circulation during pregnancy is not established, but several candidate sites are known. PTHrP is produced by the placenta (114), amnion (106), decidua (106), umbilical cord (115), and fetal parathyroid glands (116) and potentially might reach the maternal circulation. PTHrP produced by the breast tissue is detectable in human colostrum (117), and it is produced as early as day 14 of pregnancy by the mammary glands of the rat (118).
Overproduction of PTHrP by the breast might explain the development of hypercalcemia at 24 weeks of gestation in a woman with massive (4.5 kg) mammary hyperplasia of pregnancy, associated with hypercalciuria, hypophosphatemia, and undetectable PTH levels (119). Bilateral mastectomies in the second trimester of that same pregnancy corrected the hypercalcemia and the suppressed PTH level (119).
In summary, PTHrP may be made available to the maternal circulation by several different maternal and fetal sources. PTHrP fragments encompassing amino acids 1–86 are increased in the maternal circulation during pregnancy and may contribute to the elevations in 1,25-dihydroxyvitamin D and blood calcium, and suppression of PTH, noted during pregnancy. The true quantitative importance of PTHrP in maternal physiology needs to be established.
7. Other hormones. Pregnancy induces a dramatic rise in other hormones, including the sex steroids, PRL, and placental lactogen. The possibility that each of these, in turn, may have direct or indirect effects on calcium and bone metabolism during pregnancy has been largely unexplored. There is some evidence to suggest that PRL and placental lactogen may increase the intestinal transport of calcium (70, 120, 121), reduce urinary calcium excretion (122, 123), and stimulate synthesis of PTHrP (124) and 1,25-dihydroxyvitamin D (68, 69). This is discussed in more detail in the relevant sections.
C. Intestinal absorption of calcium
Calcium is absorbed throughout the small intestine, a small
portion by active transport in the duodenum and proximal jejunum, and
the major portion by passive mechanisms in the distal jejunum and ileum
(125). Mineral balance and calcium kinetic studies in humans using
stable isotopes of calcium (48Ca, 44Ca,
42Ca) have consistently found a positive calcium balance
and a doubling of the intestinal absorption of calcium during human
pregnancy from as early as 12 weeks of gestation (the earliest time
point studied) (27, 126, 127). By studying the effect of an oral
calcium load on serum calcium and urine calcium excretion, other
investigators indirectly confirmed that intestinal calcium absorption
must be increased in all trimesters (12, 128). The results of these
studies led to speculation that the increase was mediated by
1,25-dihydroxyvitamin D, and this appeared to be confirmed when
elevated levels of 1,25-dihydroxyvitamin D were found during human
pregnancy (Section II.B.4, above). 1,25-Dihydroxyvitamin D
probably stimulates intestinal calcium absorption by increasing the
synthesis of proteins, including the intestinal vitamin D-dependent
calcium-binding protein, calbindin9K-D. Protein and mRNA
levels of calbindin9K-D increase in the intestines of mice
and rats during pregnancy and plateau when both maternal
1,25-dihydroxyvitamin D levels and the efficiency of intestinal calcium
absorption are at peak levels (129, 130, 131). Maternal vitamin D deficiency
in rodents reduces the rise in the intestinal expression of
calbindin9K-D (132, 133), while 1,25-dihydroxyvitamin D
administration can restore it (133).
The rise in intestinal absorption of calcium occurs by midpregnancy in rats, before the onset of rapid skeletal mineralization in the fetus (45). The doubling of intestinal absorption persists in parathyroidectomized rats (134) and may, therefore, be independent of PTH regulation. The early increase in intestinal calcium absorption may allow the pregnant mother to accrete calcium (probably in the maternal skeleton), before the peak fetal demand for calcium in late pregnancy. Consistent with this hypothesis, it has been estimated from isotope studies in the pregnant rat that 92% of fetal skeletal calcium content was absorbed from the maternal diet at some point during pregnancy (135). Further, several investigators have found that pregnant rats normally store calcium during the first half of pregnancy (136), such that by the end of pregnancy, the calcium content of the femurs is unchanged (137). Inadequate accretion of calcium early in pregnancy may lead to a net loss of maternal skeletal calcium later in pregnancy. For example, under dietary calcium restriction, pregnant rats (138, 139) and goats (98) have reduced calcium content in their long bones by the end of gestation. Similarly, maternal vitamin D deficiency has been found to cause maternal skeletal demineralization by the end of pregnancy (140).
PRL treatment of pregnant, vitamin D-deficient rats resulted in an increase in the intestinal absorption of calcium; PRL might, therefore, have an effect on the intestine independent of 1,25-dihydroxyvitamin D (120). This is further supported by studies in everted gut sacs of nonpregnant, hypophysectomized rats, where PRL and placental lactogen stimulated the intestinal transport of calcium (70, 121). Also in rats, the increase in duodenal calcium absorption has been found to precede the rise in the 1,25-dihydroxyvitamin D level by 1 week, suggesting that the intestinal effect is not dependent solely on vitamin D (45, 141). Even in the absence of vitamin D, pregnancy in rats is associated with hypertrophy of the small intestine and a doubling of intestinal absorption of calcium (141, 142). Furthermore, rats hypocalcemic from vitamin D deficiency developed a progressive rise in serum calcium levels during pregnancy, despite unchanged serum PTH levels (143). However, an independent effect of PRL on intestinal calcium absorption could not be demonstrated in studies on humans. Hyperprolactinemic patients showed no alteration in the intestinal absorption of calcium (72).
In summary, intestinal calcium absorption is increased 2-fold early in human and rat pregnancy, probably through a 1,25-dihydroxyvitamin D-mediated increase in intestinal calbindin9K-D and other proteins. PRL and placental lactogen (or possibly other factors) may mediate part of the normal pregnancy-related increase in intestinal calcium absorption. The early rise in intestinal calcium absorption may allow the maternal skeleton to store calcium in advance of the peak fetal demands later in pregnancy. The increased intestinal calcium absorption appears to be a major maternal adaptation to meet the fetal need for calcium.
D. Renal handling of calcium
Pregnancy is associated with an increase in creatinine clearance
and glomerular filtration rate (144, 145). The 24-h urine calcium
excretion is increased as early as the 12th week of gestation (the
earliest time point studied), and averages 300 ± 61 mg in the
third trimester (levels in the hypercalciuric range are not uncommon)
(12, 14, 20, 27, 146, 147). Since fasting urine calcium values are
normal or low, the increase in 24-h urine calcium reflects the
increased intestinal absorption of calcium (absorptive hypercalciuria)
(12, 28, 44). A similar 2-fold increase in urinary calcium excretion
has been observed in the pregnant rat from the second week of gestation
(148). Although PRL and placental lactogen have been shown to reduce
urinary calcium excretion in nonpregnant rabbits in vivo
(122, 123), the effect (if any) of either hormone on the kidneys of
pregnant humans and rats must be very modest.
Interestingly, preeclampsia and pregnancy-induced hypertension (PIH) have been associated with hypocalciuria (147, 149, 150, 151, 152). Further studies have found the hypocalciuria to be associated with low 1,25-dihydroxyvitamin D levels (149, 150, 151, 152), but to be independent of PTH, calcitonin, or ionized calcium levels (147, 149, 150, 151). The finding of hypocalciuria prompted a large trial of calcium supplementation in pregnant women, which recently reported no benefit in preventing preeclampsia or PIH (153). These abnormalities in 1,25-dihydroxyvitamin D and urine calcium excretion are, therefore, probably secondary to a primary renal tubular defect occurring in preeclampsia and PIH and are likely not the primary cause of the hypertension (149).
E. Skeletal calcium metabolism
1. Bone formation and resorption. Histomorphometric parameters
of both bone formation and osteoclast-mediated resorption are increased
during pregnancy in rats (154). Pregnant beagle dogs also show
histomorphometric evidence of increased bone turnover in iliac
trabecular bone (155). Despite evidence of increased turnover, bone
mineral content during pregnancy in rats does not change (137, 140, 154, 156). In contrast, pregnant ewes have a 20% decrease in skeletal
calcium content during gestation (157).
Comparable histomorphometric data are not available for human pregnancy, but markers of bone formation and resorption have been assessed. Generally speaking, such indices are more reliable for measuring changes in bone resorption than bone formation (158, 159). Several markers of bone resorption (tartrate-resistant acid phosphatase, deoxypyridinoline/creatinine, pyridinoline/creatinine, and hydroxyproline/creatinine) are low in the first trimester but rise steadily to peak at values up to twice normal in the last trimester (27, 44, 160, 161). In contrast, osteocalcin, a marker of bone formation, is low or undetectable early in gestation and sometimes rises to normal levels by term (15, 27, 161, 162, 163). Other markers of bone formation (procollagen I carboxypeptides, bone-specific alkaline phosphatase) are low in the first trimester and have been found to remain low (44) or rise to normal or above in the last trimester (27, 160). Total alkaline phosphatase rises early in pregnancy due to contributions from the placental fraction, and, therefore, is not a useful marker of bone formation in pregnancy (14, 44).
Taken together, the histomorphometric data from animals, and the changes in the markers of bone formation and resorption in humans, indicate that bone turnover is probably low in the first half of pregnancy, but may be increased in the third trimester. The third trimester increase in bone turnover corresponds to time of the peak rate of calcium transfer to the fetus and may result from mobilization of skeletal calcium stores (which contain 99% of the body’s stores of calcium) to help supply the fetus.
2. Bone density. Concerns about fetal radiation exposure have resulted in few studies of changes in maternal bone mass during pregnancy; these studies used techniques that are far less precise or reproducible than the current standard, dual x-ray absorptiometry (DXA) (164, 165). Of the scant data available, an early study used x-ray spectrophotometry of the radius and femur to demonstrate a progressive decrease in trabecular bone density during pregnancy (166). Using more modern techniques, four prospective studies of bone density during pregnancy did not find a significant change in cortical or trabecular bone density, as respectively determined by single photon absorptiometry (SPA) and/or dual-photon absorptiometry (DPA) (28, 146, 167, 168). Another study found a significant decrease in bone mineral density of the femoral neck and radial shaft, but no change in lumbar bone density, by comparing preconception SPA and DPA measurements to those taken 6 weeks postpartum (169). Most recently, cross-sectional (170) and longitudinal studies (161, 171) have found a progressive decrease during pregnancy in indices thought to correlate with bone mineral density, as determined by ultrasonographic measurements of the os calcis in all three trimesters.
The majority of retrospective, epidemiological studies of pre- and postmenopausal women have found no association of parity with bone density or fracture risk (172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191). In contrast, several other studies found increased parity to be beneficial, as indicated by a slightly greater lumbar (192, 193), femoral (194), or radial bone density (194, 195, 196) and decreased hip fracture risk (197, 198). Four remaining studies linked parity to somewhat decreased lumbar bone density (199, 200, 201) or increased hip fracture risk (202). An epidemiological study of healthy women aged 21 to 95 found divergent effects of parity at different anatomical sites. Femoral neck bone mineral density was decreased in parous women by 1.5% per live birth, while lumbar spine bone density was not influenced by parity (203). Among the studies that found no significant association of parity, several reported that a first pregnancy as an adolescent was associated with decreased bone density (178, 190, 195), possibly because the fetal calcium demands of pregnancy reduce the peak bone mass that is eventually achieved in the adolescent. Overall, many of these epidemiological studies had significant methodological limitations, specifically the difficult problem of retrospectively separating out the effects of parity from those of lactation. Nevertheless, it may be reasonable to conclude from these studies that if parity has either a positive or a negative effect on bone density or fracture risk, it must be only a very modest effect.
Therefore, although changes in serum and urine markers of bone formation and resorption have indicated that bone turnover may be increased in the third trimester, it is impossible to determine from the available bone density data whether there is any acute change in bone mineral during human pregnancy. Further, it is also unknown whether any such acute change has a long-term effect on the calcium content or fragility of the maternal skeleton.
3. Osteoporosis in pregnancy. The rare presentation of idiopathic osteoporosis in a woman of child-bearing age has often been associated with a recent pregnancy, as noted by Albright and Reifenstein (1) and other early case reports (204, 205, 206). The exact prevalence of the condition is uncertain. The theory that pregnancy might cause osteoporosis (as proposed by Albright and Reifenstein) was disputed by an early observational study of five women with symptomatic, severe osteoporosis presenting in a first pregnancy (207). In subsequent pregnancies, these women were found to have no worsening of their condition, but the parameters used (new pain or fracture, worsening of osteopenia on plain roentgenograms) were crude and insensitive by methods available today (207). Despite better documentation of the absence of known causes of decreased bone density in more recent case reports (208, 209, 210, 211, 212), it has not been possible to exclude the possibility that low peak bone mass and/or an accelerated bone resorptive state preceded the pregnancy and simply became clinically obvious in pregnancy. In addition, some reported cases of osteoporosis in pregnancy have been clearly confounded by the presence of other recognizable causes of secondary osteoporosis, such as chronic heparin, anticonvulsant, or corticosteroid therapy (209, 212). In two documented cases of osteoporosis diagnosed in pregnancy, the female progeny were found at age 10 to also have low bone mineral density (213). This finding suggested that a shared genetic or environmental factor (and not pregnancy) was the cause of osteoporosis in the mothers and daughters. The limited data from bone biopsy typically show no evidence of osteomalacia, but only mild osteoporosis or normal architecture (208, 212). It remains intriguing to speculate that some of these rare cases of osteoporosis presenting in pregnancy may result from excessive resorption of calcium from the maternal skeleton, perhaps in the setting of inadequate intake of calcium, low stores of 25-hydroxyvitamin D, or an excessive rise in PTHrP in the maternal circulation (see also the discussion of osteoporosis in lactation, Section IV.E.3, below). Nevertheless, these rare cases may simply represent idiopathic osteoporosis occurring in pregnant women by mere chance.
A second (also rare) form of pregnancy-associated osteoporosis is a focal, transient osteoporosis of the hip (214, 215, 216, 217). Typically these patients present with unilateral or bilateral hip pain, limp, and/or hip fracture in the third trimester (214, 216, 218, 219). Radiolucency of the femoral head and neck was recognized on plain radiographs taken in early reports of this condition (215, 220); and more recently, DXA measurements have shown that the bone density of the symptomatic femoral head and neck is reduced (218). Magnetic resonance imaging (MRI) of the affected femoral head in one patient showed a joint effusion and images suggesting increased water content of the femoral head and marrow cavity (221). Routine serum chemistries are typically normal (222). Alkaline phosphatase and urine hydroxyproline have been reported to be elevated (215, 218, 223); however, the interpretation of these findings is uncertain, since control measurements from normal pregnant women were not compared. Intriguingly, the decreased bone mineral density of the femoral head and neck typically resolves within 2 to 6 months postpartum (214, 218, 219), including the MRI findings (221). Patients generally require only pain relief and continued mobilization for this self-limited condition. The fact that this rare condition is typically localized to one or both femoral heads, and not the rest of the skeleton, suggests that it is not the result of a generalized increase in skeletal resorption. Several theories have been proposed to explain this condition, including femoral venous stasis due to pressure from the pregnant uterus, a form of Sudeck’s atrophy, or reflex sympathetic dystrophy (causalgia), ischemia, trauma, viral infections, marrow hypertrophy, immobilization, and fetal pressure on the obturator nerve (214, 215, 216, 220). As yet, the etiology of transient osteoporosis of the hip in pregnancy remains unclear; its association with pregnancy may be not causal but incidental. In any case, it appears likely that this disorder is not a manifestation of altered calcitropic hormone levels or mineral balance during pregnancy.
F. Primary hyperparathyroidism
The presentation of primary hyperparathyroidism in pregnancy
raises important diagnostic and management considerations. Many cases
are asymptomatic, detected by routine prenatal biochemical tests or
after the presentation of hypocalcemia in the neonate. Several normal
pregnancy-related changes in calcium and PTH physiology (noted above)
may obscure the diagnosis of mild primary hyperparathyroidism. These
include the fall in total serum calcium, the rise in the corrected
serum calcium (111, 224), the fall in the intact PTH level (14, 15, 16, 44), and the rise in the 24-h urinary excretion of calcium, often into
the hypercalciuric range (14, 27) (see also Sections II.B and
II.D, above).
Although maternal primary hyperparathyroidism in pregnancy is probably a rare condition (there are no data available on its prevalence), it has been associated in the literature with an alarming rate of adverse outcomes in the fetus, including a 30% rate of spontaneous abortion or stillbirth (225, 226). In the neonatal period, a 50% rate of tetany and a 25% rate of neonatal death has been reported (225, 227). These adverse outcomes are thought to result from suppression of the fetal parathyroid glands; this suppression may occasionally be prolonged for months (228, 229). PTH cannot cross the placenta (230, 231, 232); therefore, the fetal parathyroid suppression is thought to result from increased net calcium flux across the placenta to the fetus, facilitated by maternal hypercalcemia. Evidence from animal models has confirmed that acute elevations in maternal serum calcium cause an increase in fetal serum calcium, and a fall in fetal PTH level (233). However, whether chronic maternal hypercalcemia has the same effect on fetal serum calcium, or placental calcium transport, has not been determined.
Surgical correction of primary hyperparathyroidism during the second trimester, to prevent fetal and neonatal complications, has been almost universally recommended (226, 234, 235, 236). Several case series have found elective surgery to be well tolerated and to dramatically reduce the rate of adverse events when compared with the earlier cases reported in the literature (234, 235, 237, 238). However, many of the women in those early cases were symptomatic and had nephrocalcinosis or renal insufficiency. Those early case reports may also have reflected reporting bias of adverse fetal and neonatal outcomes. Whether the milder, asymptomatic form of primary hyperparathyroidism commonly seen today has the same risk of adverse fetal or neonatal outcomes has not been determined. In several case reports, mild elevations in maternal serum calcium were followed without operative intervention, and no adverse fetal or neonatal outcome occurred (239, 240). However, in other cases the mild hypercalcemia of both asymptomatic primary hyperparathyroidism and familial hypocalciuric hypercalcemia has been reported to cause neonatal parathyroid suppression and tetany (241, 242, 243). Nevertheless, it is probably reasonable to follow cases of asymptomatic primary hyperparathyroidism with mild hypercalcemia conservatively and to reserve surgery in the second trimester for patients that are symptomatic or have more severe hypercalcemia. If surgery is deferred, the neonate must be monitored closely for the development of hypocalcemia.
G. Hypoparathyroidism and pseudohypoparathyroidism
As described earlier (Section II.B.4), free and bound
1,25-dihydroxyvitamin D levels normally double during human pregnancy
in the presence of low-to-normal intact PTH levels, and, therefore, it
is likely that PTH does not mediate the pregnancy-related rise in
1,25-dihydroxyvitamin D production. Other hormones of pregnancy, such
as estrogen, PTHrP, and perhaps placental lactogen and PRL, may
regulate the increased production of 1,25-dihydroxyvitamin D by
maternal kidney and decidua. Also, placenta and fetus may contribute to
the maternal increase in 1,25-dihydroxyvitamin D.
In multiple case reports, pregnant hypoparathyroid women have been found to have fewer hypocalcemic symptoms, a rise in the serum calcium, and decreased dependence on supplemental calcitriol to maintain a normal serum calcium (244, 245, 246, 247, 248, 249, 250, 251, 252). This finding is consistent with a limited role for PTH in the pregnant woman and suggests that an increase in 1,25-dihydroxyvitamin D and/or increased intestinal calcium absorption will occur in the absence of PTH. The literature on hypoparathyroidism in pregnancy is not entirely consistent on this point, since in other case reports the calcitriol dosage was increased for a variety of reasons (some incompletely documented) (253, 254, 255, 256, 257). Despite these contrasting views on the natural history of hypoparathyroidism in pregnancy, there is general agreement (244, 245, 248, 253, 254, 258) that in late pregnancy and the puerperium, hypercalcemia may result unless the calcitriol is discontinued, or the dosage is decreased below the prepartum requirement. Since this effect is even more pronounced in those who breast-feed, and since PTHrP is found at high concentrations in the breast during late pregnancy and lactation (further discussed in Section IV.B.6, below), the pregnancy-related rise in 1,25-dihydroxyvitamin D production may be regulated by PTHrP (secreted from the breast) in these hypoparathyroid women.
Calcitriol (rather than vitamin D or calcifediol) has typically been prescribed for hypoparathyroidism in pregnancy, and the dosage needed may range from 0.5–3.0 µg daily. Chronic maternal hypocalcemia must be avoided because it has been associated with the development of intrauterine hyperparathyroidism and death in the fetus (Section III.F, below).
Further illumination of the role of PTH in pregnancy has come from cases of pseudohypoparathyroidism in pregnancy. Pseudohypoparathyroidism is a heterogeneous group of genetic syndromes characterized by hypocalcemia due to PTH resistance (259). Although the data are limited, Breslau and Zerwekh (260) noted a normalization of serum calcium levels in two pregnant women with pseudohypoparathyroidism (probably type 1b). Before pregnancy the patients had hypocalcemia, markedly elevated PTH levels, and low 1,25-dihydroxyvitamin D levels. During four pregnancies (two for each patient), the serum calcium levels were normal, their PTH levels were halved, and the 1,25-dihydroxyvitamin D levels increased 2- to 3-fold. Contributions of 1,25-dihydroxyvitamin D from placental and fetal sources might have accounted for these findings; Zerwekh and Breslau (261) noted elsewhere that the placental production of 1,25-dihydroxyvitamin D was no different between placentas obtained from pseudohypoparathyroid women and controls. Alternatively, it is possible that the hormonal milieu of pregnancy lessened the renal resistance to PTH and PTHrP and thereby increased the formation of 1,25-dihydroxyvitamin D. It is apparent from Breslau’s observations that estrogens alone cannot be the explanation for such an improvement during pregnancy, because the same two pseudohypoparathyroid women were not improved by treatment with an oral contraceptive. In any case, calcitriol supplementation in these patients should be monitored carefully and adjusted during pregnancy. The progeny of these pregnancies are also at risk of intrauterine, fetal hyperparathyroidism (262, 263), perhaps because of relative maternal hypocalcemia during pregnancy.
H. Summary
The fetal demand for calcium, which largely occurs during the
third trimester, is met by a doubling of free and bound maternal
1,25-dihydroxyvitamin D levels, which, in turn, partly mediate a
doubling of the intestinal absorption of calcium. Some of the increased
intestinal calcium absorption may be mediated by PRL or other hormones
of pregnancy. Further, the increase in 1,25-dihydroxyvitamin D may be
largely independent of changes in PTH, since PTH levels are typically
low or normal at the time of the increase in 1,25-dihydroxyvitamin D.
The increased calcium intake and absorption leads to a marked increase
in renal calcium excretion (absorptive hypercalciuria). The serum
ionized calcium is normal, despite a fall in total serum calcium caused
by a reduction in the albumin-bound fraction. Calcitonin and PTHrP are
both elevated, particularly in the latter half of gestation, but the
physiological importance of these hormones in pregnancy is not known.
The typical changes in calcium and calcitropic hormone levels during
pregnancy are depicted schematically in Fig. 1
.
Bone resorption is increased during late pregnancy, as evidenced by a rise in the levels of serum and urine markers of bone resorption in the third trimester, and this may indicate that maternal skeletal calcium stores are mobilized during the time of rapid fetal accretion of calcium. As noted at the beginning of Section II.E.2, bone density studies during pregnancy have been of insufficient precision to determine whether this increased bone resorption results in significant loss of skeletal calcium during pregnancy or the third trimester. Retrospective epidemiological studies (although not definitive) have generally found no effect of parity on the risk of osteoporosis or fractures in later life. Uncommonly, pregnancy may be associated with osteoporosis and fractures, particularly if the woman enters pregnancy with a low peak bone mass. A distinct disorder, focal, transient osteoporosis of the hip in pregnancy, is not likely due to altered calcitropic hormone levels and calcium physiology.
Primary hyperparathyroidism in pregnancy has been classically associated with adverse fetal or neonatal outcomes, but the milder, asymptomatic form of primary hyperparathyroidism most often seen today may not share such outcomes. Maternal hypoparathyroidism may be improved in pregnancy by increased intestinal absorption of calcium, possibly mediated by increased production of 1,25-dihydroxyvitamin D caused by PTHrP or some other non-PTH factor. A similar improvement in biochemical indices has been seen in pregnant women with pseudohypoparathyroidism. In both hypoparathyroid and pseudohypoparathyroid women, maternal hypocalcemia may adversely affect the fetus and must be avoided.
The pregnant rat model differs from the human condition in several
important respects (Table 1). The rat normally develops a form of
secondary hyperparathyroidism in the last several days of pregnancy,
prompted by a fall in the maternal serum-ionized calcium at the time of
rapid fetal accretion of calcium. 1,25-Dihydroxyvitamin D increases
late in gestation in rats, approximately 1 week after the rise in
intestinal calcium absorption. This indicates that mechanisms
independent of 1,25-dihydroxyvitamin D may contribute to the increased
intestinal calcium absorption in rats.
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III. Fetal-Placental Physiology and Pathophysiology |
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TopAbstractI. IntroductionII. Maternal Physiology and...III. Fetal-Placental Physiology...IV. Maternal Physiology and...V. Neonatal Physiology and...VI. Discussion and ConclusionsReferences |
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Human handling of placental calcium transport must be largely inferred from data that have been obtained from studies in sheep, pigs, rats, and mice. Therefore, it must be emphasized that mice and rats have hemochorial placentas that are structurally very similar to those of humans (264, 265, 266, 267). In contrast, the epitheliochorial placentas of sheep and pigs differ significantly in structure from the human hemochorial placenta, and may, therefore, be functionally different as well (266).
B. Mineral ions and calcitropic hormones
1. Calcium. In humans, rodents, sheep, cattle, monkeys, and
other mammals, the fetal blood calcium (total and ionized) is
maintained at a higher level than in the maternal circulation
(268, 269, 270, 271, 272, 273, 274, 275). This elevation is mainly due to an increase in the ionized
calcium level (274). Ionized calcium is approximately 80% of the total
calcium in fetal rodents (276); only a small fraction is bound to
albumin.
In fetal rats, there is a progressive rise in total and ionized calcium over the last week of gestation, corresponding to the time of a progressive decline in fetal pH (277, 278, 279). Data are lacking on precisely how early in gestation the fetal blood calcium begins to exceed the maternal. In sheep, fetal hypercalcemia has been detected as early as the 35th day of gestation (280, 281). In humans, fetal hypercalcemia was documented at 15–20 weeks of gestation (by fetoscopy) (282) and at delivery of preterm singleton and twin pregnancies (mean gestational age 33 weeks) (283).
Two physiological models could explain fetal hypercalcemia: either the fetus maintains a fixed positive gradient of calcium with respect to the maternal level, or the fetus maintains a high, fixed level of calcium. Evidence from rat and mouse models indicates that the fetus sets its blood calcium at a higher level independently of the maternal calcium level. For example, in rats, the fetal blood calcium is unchanged in the presence of severe maternal hypocalcemia due to a calcium-restricted diet (284), vitamin D deficiency (29, 84, 285), or thyroparathyroidectomy (46, 134). The calcium gradient from mother to fetus is increased in these fetuses because the maternal blood calcium is lower. When both the pregnant rat and its fetus are thyroparathyroidectomized, the fetus still maintains a higher blood calcium level than the mother (286, 287). Also, in genetically engineered mice, maternal hypercalcemia due to heterozygous ablation of the calcium-sensing receptor (CaSR) gene does not affect the blood calcium level set by normal fetuses (288). Similarly, heterozygous calcium-sensing receptor knockout fetuses establish a constant, abnormally high fetal blood calcium level, regardless of whether the mother is heterozygous (and therefore hypercalcemic) or normal (288). The apparent "calcium gradient" is lower in offspring of these heterozygous mice, due to maternal hypercalcemia. Finally, acute alterations in the maternal blood calcium of rodents and primates (by calcium, 1,25-dihydroxyvitamin D, calcitonin, PTH, or EDTA infusions) are not reflected by much perturbation in the fetal blood calcium (232, 289, 290, 291, 292).
Others have reported a fall in the fetal blood calcium after maternal parathyroidectomy in rats (52, 53, 54). The fetal blood calcium was normal between the 12th and 17th day of gestation, but fell during the period of rapid fetal skeletal calcium accretion. Therefore, these data indicate that the ability of the fetal rat to set its blood calcium may break down during the time of rapid accretion of calcium by the skeleton, if the mother has been parathyroidectomized.
In summary, from early pregnancy, mammalian fetuses have higher levels of blood calcium than their mothers, mainly due to an increase in the ionized calcium level. The fetus does not establish a particular calcium gradient with respect to the maternal blood calcium; instead, it establishes a particular fetal blood calcium level, irrespective of the ambient maternal blood calcium level. This ability persists in the presence of significant maternal hypocalcemia of various causes, but may be impaired during the time of rapid accretion of calcium by the skeleton. The physiological importance of fetal hypercalcemia is not known.
2. Phosphate. Fetal phosphorus levels are higher than maternal in rats (279) and humans (32, 270, 273). This suggests that phosphate may be actively transported across the placenta, but the regulators of this active transport are unknown (293). PTHrP and PTH do not stimulate placental transport of phosphate in sheep (294); vitamin D may have a role (295).
3. PTH. Fetal parathyroid glands of rats and sheep contain PTH mRNA (114, 116), and PTH immunoreactivity is present in human fetal parathyroid glands as early as 10 weeks of gestation (296). These findings indicate that fetal parathyroid glands are capable of synthesizing PTH early in gestation. Furthermore, PTH detected in the fetal blood likely derives from fetal sources alone. Intact PTH does not cross the placenta of nonhuman primates, sheep, and rodents (230, 231, 232) and probably does not cross the human placenta.
The following evidence indicates that fetal parathyroid glands appear to contribute to calcium homeostasis, by secretion of PTH or PTHrP. Fetal thyroparathyroidectomy in sheep and fetal decapitation in rats caused hypocalcemia (52, 297, 298), and mice lacking the PTH/PTHrP receptor gene are hypocalcemic in utero (299). PTH can be regulated by the ambient fetal blood calcium, since EDTA-induced fetal hypocalcemia has been found to induce a rise in fetal PTH levels in rats (300), cattle (275), and rhesus monkeys (301), although another study in rhesus monkeys found no fetal PTH response (271). Removal of a maternal parathyroid adenoma was followed by a rise in amniotic fluid PTH levels and a decline in the amniotic fluid calcium level during a human pregnancy (302). Since maternal PTH cannot cross the placenta, the findings in this case have been interpreted to indicate that fetal PTH secretion can be influenced by the maternal blood calcium (302).
In fetal humans and other animals, immunoreactive PTH blood levels have been found to be undetectable or very low (i.e., <0.5 pmol/liter) with respect to maternal PTH level near the end of gestation (17, 32, 35, 38, 43, 110, 124, 268, 275, 303, 304, 305, 306, 307, 308, 309, 310). Little information is available on PTH levels earlier in gestation. One study in fetal rats found that the PTH level declined in the last several days of gestation as the serum ionized calcium rose (277), while two cross-sectional studies in preterm humans found that the fetal PTH level was not lower than the maternal PTH level (283, 308).
In summary, the available evidence suggests that the fetal parathyroids are capable of synthesizing PTH. Since blood levels of PTH have been typically found to be low in late gestation at a time when the fetal blood calcium is high, other factors must determine the fetal blood calcium level. The precise role of PTH in normal fetal calcium homeostasis will be clarified by ablating the PTH gene in mice.
4. 1,25-Dihydroxyvitamin D. Although maternal vitamin D deficiency reduces fertility and litter size in the rat (84, 85), evidence from several animal models indicates that 1,25-dihydroxyvitamin D is not necessary for normal fetal calcium and bone metabolism. In pregnant rats, sheep, and pigs that were hypocalcemic due to severe vitamin D deficiency, the fetuses maintained completely normal blood calcium and phosphate levels and had fully mineralized skeletons at term, as determined by total weight, ash weight, and calcium content of femurs (29, 84, 284, 285, 311). Each of these studies is limited by the possibility that low levels of vitamin D might have reached the fetus.
Further evidence that 1,25-dihydroxyvitamin D is not needed for normal
fetal calcium and bone homeostasis comes from the
1-hydroxylase-deficient Hannover pig model, in which the fetuses of
homozygous 1,25-dihydroxyvitamin D-deficient sows also maintained
completely normal blood calcium and phosphate levels and fully
mineralized their skeletons (79). Nephrectomy of fetal rats did not
affect fetal blood calcium or phosphate levels when measured 48 h
later, even though fetal 1,25-dihydroxyvitamin D levels fell (286).
Also in fetuses of vitamin D-deficient rats, normal levels of
calbindin28K-D and calbindin9K-D were found in
the placenta, intestine, and other tissues (132, 311, 312). In
addition, fetal mice that lack the gene encoding the receptor for
1,25-dihydroxyvitamin D are born with normal skeletons (313, 314).
Some data from humans lend support to the observation that 1,25-dihydroxyvitamin D is not needed for normal fetal calcium and bone metabolism. At term, the cord blood calcium and skeletal mineralization is completely normal in the offspring of vitamin D-deficient mothers (315, 316, 317). It is only in the first or second week after birth that hypocalcemia develops; skeletal demineralization and other rachitic changes are typically not detectable until 1 or 2 months of age (see Section V.E, below).
These observations of a minimal effect of vitamin D deficiency on fetal
calcium and skeletal metabolism do not mean that 1,25-dihydroxyvitamin
D is inactive or has no role in fetal life. In rats, the receptor for
1,25-dihydroxyvitamin D appears on day 13 of gestation in the
mesenchyme that will subsequently condense to form the skeletal
tissues, and by day 17 of gestation it is expressed in proliferating
and hypertrophic chondrocytes, and osteoblasts of limb buds and the
vertebral column (318). The widespread expression of the vitamin D
receptor early in fetal skeletal development suggests an important role
for its ligand in fetal bone development, but evidence for this
postulated role has not yet been found. Further studies manipulated the
1,25-dihydroxyvitamin D level in fetal animals to test the role of this
hormone. Infusion of antibody to 1,25-dihydroxyvitamin D decreased the
ovine fetal blood calcium level (66). 1,25-Dihydroxyvitamin D given to
pregnant guinea pigs increased the fetal calcium and phosphate levels
(319). Bilateral nephrectomy in fetal sheep resulted in reduced ionized
and total calcium and increased phosphate and PTH levels; these changes
could be reversed by administration of 1,25-dihydroxyvitamin D to the
fetus (295). Since these changes could be attributable to uremia and
not loss of the renal 1-hydroxylase enzyme, additional fetuses
underwent bilateral ureteral sectioning alone. This surgical procedure
allowed urine to drain into the fetal peritoneal cavity while retaining
functional kidneys in situ. In these fetuses, ureteral
sectioning had no effect on fetal calcium or calcitropic hormone
levels. Thus, at least in the absence of normal renal function,
1,25-dihydroxyvitamin D may have a substantial influence on fetal
mineral ion homeostasis.
1,25-Dihydroxyvitamin D does not readily cross the placenta in rats
(320); consequently, circulating levels of 1,25-dihydroxyvitamin D in
the fetus are largely derived from fetal sources. The fetal kidneys and
placenta possess the 1-hydroxylase enzyme and convert
25-hydroxyvitamin D to the active form (1, 25-dihydroxyvitamin D) (75, 76). The contribution of the fetal kidneys must be significant, since
fetal nephrectomy reduced the fetal 1,25-dihydroxyvitamin D levels in
sheep and rats (66, 286). Fetal blood levels of 1,25-dihydroxyvitamin D
are typically lower than maternal levels in humans (37, 56, 304, 321),
but umbilical artery levels of 1,25-dihydroxyvitamin D are slightly
higher than umbilical venous levels, confirming the contribution of the
fetal kidneys (37). 25-Hydroxyvitamin D levels have been found to be
roughly equal to maternal levels (37, 56); this is not surprising since
25-hydroxyvitamin D readily crosses the placenta in rats (322). Levels
of 24,25-dihydroxyvitamin D correlate with, but are typically lower
than, maternal levels at term in humans (283, 307, 321).
In summary, evidence from animal models indicates that deficiency of
1,25-dihydroxyvitamin D impairs neither fetal skeletal formation and
calcification nor the ability of the fetus to maintain a normal blood
calcium. Although these data suggest a limited role for
1,25-dihydroxyvitamin D in the fetus, fetal production of
1,25-dihydroxyvitamin D and the vitamin D receptor mandate a continued
search for fetal roles for 1,25-dihydroxyvitamin D. 25-Hydroxyvitamin D
readily crosses the placenta and can be 1-hydroxylated by the fetal
kidneys. However, 1,25-dihydroxyvitamin D does not cross the placenta,
and fetal blood levels of 1,25-dihydroxyvitamin D are low.
5. Calcitonin. Immunoreactive calcitonin can be detected in human fetal thyroid glands from as early as the 15th week of gestation (323), and fetal calcitonin levels are maintained at higher levels than maternal (35, 37, 86, 88, 89, 269, 273, 304, 324). Maternal calcitonin cannot cross the placenta (325). The increased fetal levels of calcitonin are thought to reflect increased synthesis, but the metabolism and clearance of calcitonin have not been studied in fetal animals.
Several acute experimental perturbations suggest a role for calcitonin in fetal calcium homeostasis. Infusion of calcitonin antiserum to fetal rats at day 21.5 of gestation slightly increased the fetal blood calcium 1 h later (326), while fetal injection of calcitonin caused hypocalcemia and hypophosphatemia (327). However, fetal thyroidectomy with subsequent T4 replacement did not affect the fetal blood calcium in sheep, indicating that fetal thyroidal C cells alone may not affect the regulation of the blood calcium level (298). Therefore, the precise role of calcitonin in fetal calcium homeostasis and skeletal metabolism has not yet been established.
6. PTHrP. Studies of PTH bioactivity in human umbilical cord blood (as determined by an in vitro cytochemical bioassay) found high PTH-like bioactivity, while immunoreactive PTH was simultaneously found to be undetectable or low (38, 303, 328). Subsequently, it has been recognized that human cord blood PTHrP levels are significantly higher than the simultaneous maternal levels at term (43, 310). When both PTH 1–84 and PTHrP 1–86 were simultaneously measured by two-site immunoradiometric assays [and expressed in equivalent units (picomoles/liter)], human cord blood PTHrP levels were 2–4 pmol/liter, up to 15-fold higher than the levels of PTH (0.2–0.5 pmol/liter) (43, 110, 124). It has yet to be confirmed that PTHrP accounts for the high PTH-like bioactivity in human cord blood; however, studies in fetal pigs (329) and sheep (116, 330) found that the levels of PTHrP and PTH-like bioactivity were tightly correlated in late gestation and the neonatal period.
As noted earlier (Section II.B.6), PTHrP may be a prohormone that is processed into separate circulating fragments, each of which may have different functional roles and receptors (100). Although the structures of these fragments have been deduced from studies of tumor cell lines transfected with the PTHrP gene, it has yet to be determined which of these fragments normally circulate in fetal life. Full-length PTHrP has twice the molecular weight of PTH; since PTH cannot cross the placenta, PTHrP probably does not either. PTHrP 1–86 did not cross the placentas of sheep and goats (113); however, the possibility that smaller, biologically active fragments of PTHrP might cross the placenta has not been evaluated.
PTHrP is produced in many sites throughout the developing embryo and fetus, including the fetal parathyroid glands (116, 331), skeletal growth plate (332, 333), trophoblast cells of the placenta (114, 331), amnion (106, 334), chorion (334), umbilical cord (115), and many other organs. All of these sites may contribute to the circulating level of PTHrP in the fetus and may thereby be relevant to fetal calcium and bone metabolism. Since venous umbilical PTHrP levels were higher than umbilical arterial levels in pigs, the placenta may be an important source of systemically circulating PTHrP in the fetus (329). Due to local production of PTHrP by the umbilical cord (115), the level of PTHrP in cord blood might not accurately reflect the systemic level of PTHrP, but this has not been tested.
PTHrP has multiple possible roles during embryonic and fetal development (335). PTHrP gene-ablated mice have abnormalities of chondrocyte differentiation (336) and aberrant breast development (337). PTHrP may also be an important regulator of the fetal blood calcium. PTHrP levels correlate with the fetal ionized calcium levels in pigs (329). In gene-tically engineered mice, homozygous ablation of the PTHrP gene results in a fetal blood calcium no higher than that of the mother (299). In sheep, fetal parathyroidectomy causes hypocalcemia that can be reversed by PTH or PTHrP infusion (297, 298, 338). Since PTH normally circulates at low or undetectable levels in the fetus near term (Section III.B.3, above), it is possible that the hypocalcemic effect of fetal parathyroidectomy is at least partly due to the loss of PTHrP produced by the parathyroids. In the next section (III.C), the unique role of PTHrP in stimulating placental calcium transport will be discussed.
In summary, PTHrP is produced by diverse fetal tissues and circulates in fetal blood at levels higher than adult levels. PTHrP appears to regulate the fetal blood calcium as well as fetal-placental calcium transport.
C. Fetal-placental calcium transport
Calcium is actively transported across the placenta (339, 340).
The site of active transport is likely at the fetus-facing basement
membrane of the syncytiotrophoblast cells in the human and at the
trophoblast cells and the basal surface of the endoderm of the
intraplacental yolk sac in rodents (341, 342). The active transport of
calcium may be mediated by a Ca2+-ATPase present at these
same sites (339, 342). This enzyme’s activity can be inhibited by
dinitrophenol, ouabain, quercetin, and antibody to the human
erythrocyte plasma membrane calcium pump (339, 342). Calcium-binding
proteins in the placenta and yolk sac are also thought to be involved
in active placental calcium transport. The placental
calbindin9K-D mRNA and protein levels increase over the
last week of gestation in rats (129, 343) and mice (130, 133) and are
unaffected by maternal vitamin D deficiency (132, 311). Transplacental
transport of calcium is generally found to be a one-way process,
i.e., fetal-to-maternal flow of calcium is typically less
than 1% of the forward (maternal-to-fetal) flow (344, 345). In rhesus
monkeys, backflux was reported to be 80% of the forward flow (345); it
is not certain whether this represented a true species difference or
methodological differences. It has not been determined when active
transport of calcium begins in gestation, due to technical difficulties
involved in studying placental physiology early in gestation. However,
active transport of calcium must be underway by the third trimester in
humans, which is the time of rapid skeletal mineralization and peak
fetal calcium requirement.
1. Maternal hormones. Maternal hormones might influence the fetal-placental calcium transport process by raising or lowering the ambient maternal calcium level, and by direct effects on the placenta. However, several lines of evidence from animal experiments indicate that fetal-placental calcium transport and net maternal-fetal calcium transfer are maintained relatively independently of maternal hypocalcemia or hormone deficiencies. In pregnant sheep, maternal hypocalcemia due to parathyroidectomy or dietary calcium restriction did not affect the rate of fetal-placental calcium transfer as directly assessed in placental perfusion experiments (297, 346); in addition, the fetal blood calcium, phosphate, PTH, and 1,25-dihydroxyvitamin D levels were all unchanged by maternal hypocalcemia (284, 347). The finding of a "normal" rate of calcium transfer across the placenta indicates that the fetal-placental unit must be working harder to extract the normal amount of calcium from a reduced amount of maternal calcium presented to the placenta. Indeed, the following observation from intact fetal rats confirmed that the rate of placental calcium transfer is up-regulated in response to parathyroidectomy-related chronic maternal hypocalcemia. In this experiment, a maternal calcium infusion caused a marked, acute rise in the blood calcium of fetuses from parathyroidectomized rats, but had no effect on the fetuses of normal rats (348). Such an up-regulation in placental calcium transfer may compensate for a low ambient maternal blood calcium and permit a normal amount of calcium to be transferred by the end of gestation.
Several additional studies have examined only indirectly the effect of other maternal hormone deficiencies on placental calcium transfer. In these studies, net fetal accumulation of calcium at term was used as an index of placental calcium transfer during pregnancy in vitamin D-deficient rats (311), thyroidectomized, T4-supplemented ("calcitonin-deficient") sheep (349, 350), and in sheep that received daily administration of PRL and/or bromocriptine (351). Since placental calcium transport was not directly assessed in these studies, conclusions cannot be drawn about the effect of maternal vitamin D, calcitonin, and PRL deficiency on placental calcium transport.
2. Fetal hormones. The role of 1,25-dihydroxyvitamin D in
fetal-placental calcium transport has not been thoroughly studied.
Receptors for 1,25-dihydroxyvitamin D are present in the placentas of
humans, rats, and sheep and might, therefore, have a role in placental
calcium physiology (352, 353, 354, 355). In placental perfusion models in rats,
guinea pigs, and sheep, pharmacological doses of 1,25-dihydroxyvitamin
D or 1-cholecalciferol increased the fetal blood calcium, transport
of calcium across the placenta, and mineral content of ashed fetuses
(319, 356, 357). Also, prior nephrectomy of fetal sheep reduced calcium
transfer in the placental perfusion model, and this effect could be
partly restored by administering 1,25-dihydroxyvitamin D (339). Thus,
the evidence indicates a possible role for 1,25-dihydroxyvitamin D in
fetal calcium homeostasis, but this role is not yet well defined.
The role of fetal calcitonin is even less well established. In intact fetal sheep, calcitonin has been found to reduce the PTHrP-mediated increases in the apparent rate of calcium transfer (349, 358). In contrast, fetal thyroidectomy with subsequent T4 replacement did not alter placental calcium transfer in sheep, indicating that loss of fetal calcitonin does not perturb the placental calcium pump (298).
The role of the parathyroid gland in fetal calcium metabolism has been extensively studied. Fetal thyroparathyroidectomy in sheep and fetal decapitation in rats resulted in a lower fetal blood calcium, such that the maternal-fetal calcium gradient was obliterated (297, 298, 359). In addition, when the thyroparathyroidectomized sheep or rat fetuses were removed so that the placentas could be artificially perfused in situ, active transport of calcium across these experimentally perfused placentas was found to be reduced (297, 298, 359, 360). These findings indicate that the parathyroid glands have a critical role in maintaining the fetal blood calcium and the active transport of calcium across the placenta. Infusion of autologous blood from fetuses with intact parathyroid glands restored calcium transport across the perfused placentas of thyroparathyroidectomized fetal sheep (298). However, PTH failed to restore the active transport of calcium in fetal sheep or rats under these conditions (338, 359, 361).
The placenta may be able to transport calcium actively to some extent even in the absence of fetal parathyroid glands. When rat dam and fetus were both thyroparathyroidectomized, the fetus maintained a higher blood calcium level than the dam (286, 287). Although calcium transport was not measured, the fact that relative fetal hypercalcemia was maintained may indicate that some capability for active transport of calcium persists after fetal parathyroidectomy.
In the studies of thyroparathyroidectomized fetal sheep, synthetic PTHrP molecules of amino acid lengths 1–141, 1–86, and 67–86 were found to stimulate placental calcium transport in the experimentally perfused placentas (338, 361, 362, 363). These results suggested that PTHrP, perhaps produced by the parathyroid glands, stimulates active transport of calcium across the placenta. In contrast, PTHrP 1–34, which contains only the PTH-like amino-terminus, failed (like PTH) to stimulate calcium transport in this model. Studies in genetically engineered mice support the hypothesis that PTHrP stimulates placental calcium transport. A reduction in blood calcium to the maternal level and reduced placental transfer of calcium has been found in homozygous PTHrP-gene knockout fetal mice (299). The placental transfer of calcium was acutely increased in the homozygous fetuses by treatment with PTHrP 1–86 or PTHrP 67–86, but not by PTHrP 1–34 or intact PTH (299). These data suggest that PTHrP increases placental calcium transport by acting through a receptor distinct from the PTH/PTHrP receptor, since the PTH/PTHrP receptor is stimulated equally by amino-terminal PTH and PTHrP. This hypothesis is further supported by the studies of fetal mice homozygous for deletion of the PTH/PTHrP receptor gene. These mice are hypocalcemic, but placental calcium transport in these fetuses is increased (299).
These data on the effects of PTHrP in parathyroidectomized fetal sheep and genetically engineered mice are not supported by the following observations in fetal rats. In the perfused placentas obtained from intact or decapitated fetal rats, active transport of calcium was found to be stimulated slightly by amino-terminal PTH or PTHrP, but not by fragments of PTHrP that do not contain the amino terminus (359, 360). This could reflect a true species difference or methodological differences.
Taken together, the studies in genetic knockout mice, and those in thyroparathyroidectomized fetal sheep, suggest that in fetal life, PTHrP is necessary to maintain the normal fetal hypercalcemia and at least part of the active transport of calcium across the placenta. This transport is regulated in part by a midmolecular portion of PTHrP acting on a novel (as yet uncloned) receptor distinct from the PTH/PTHrP receptor. Recent evidence suggests that the structure of this midmolecular fragment of PTHrP may encompass amino acids 38–94 (100, 364), but this has yet to be confirmed by RIAs of fetal blood. The studies in sheep suggested that the parathyroid glands secrete the PTHrP that controls placental calcium transport; however, direct evidence to support this hypothesis is not yet available. Finally, the physiological importance of the actions of fetal 1,25-dihydroxyvitamin D and calcitonin require further exploration.
D. Renal handling of calcium and the amniotic fluid
The fetal kidneys may play a role in regulating the fetal blood
calcium level, by adjusting the relative reabsorption and excretion of
calcium and phosphate by the renal tubules in response to the filtered
load and other factors, such as PTHrP and/or PTH. The fetal kidneys may
also participate by synthesizing 1,25-dihydroxyvitamin D. However,
little hard data are available on fetal kidney function and its
relative importance in regulating the fetal blood calcium. As noted
previously, nephrectomy in fetal lambs resulted in hypocalcemia,
hyperphosphatemia, and reduced placental calcium transfer; these
effects were attributed to loss of renal production of
1,25-dihydroxyvitamin D (295, 339). The fetal renal tubular function
may be under the control of PTHrP or PTH in fetal life, since
thyroparathyroidectomy in fetal sheep resulted in an increase in
fractional excretion of calcium by the fetal kidneys and reduced
phosphate excretion (365, 366). These effects were reversed by
treatment with amino-terminal fragments of either PTH or PTHrP (365, 366). Thyroparathyroidectomy also reduced placental calcium transport
in fetal sheep (Section III.C., above); therefore, the
hypocalcemia in thyroparathyroidectomized fetuses likely results from
the combined effects of reduced influx of calcium across the placenta,
increased excretion of calcium by the fetal kidneys, and possible
effects of the loss of PTHrP and/or PTH on the fetal skeleton and renal
1-hydroxylase.
In fetal life, renal excretion of calcium does not necessarily represent a permanent loss of calcium for the fetus. Fetal urine excretion is probably the major source of fluid and solute in the amniotic fluid, while fetal swallowing is likely the major pathway for clearance of amniotic fluid and is a pathway by which excreted calcium can be made available again to the fetus (367). The volume and composition of amniotic fluid have been used as indirect measures of fetal renal function. The amniotic fluid ionized calcium level has been found to be constant between 14–15 weeks of gestation and term in humans, while the total calcium and phosphate levels decline over the same interval (13). This may indicate that renal excretion of calcium is equally balanced by fetal swallowing and intestinal reabsorption of calcium. However, other sources may contribute to the amniotic fluid, including secretions from the respiratory tract, and exchange of fluid and/or solute across the fetal skin, fetal membranes (amnion, chorion and chorionic plate), and umbilical cord (367). Little is known about the relative contribution of these sites to the volume and composition of the amniotic fluid, and hence uncertainty remains about how accurately the amniotic fluid composition reflects renal function. Nevertheless, amniotic fluid represents a pathway by which excreted calcium may be recirculated to the fetus.
E. Skeletal calcium metabolism
The fetal skeleton may well have two interdependent roles —
substantial growth during late fetal life and participation in fetal
calcium homeostasis. Several lines of evidence indicate that the fetal
skeleton participates in fetal calcium homeostasis and that skeletal
calcium may be mobilized in response to reduced transfer of calcium
from mother to fetus. Maternal hypocalcemia due to
thyroparathyroidectomy or calcitonin infusion increased the basal level
of bone resorption in subsequently cultured fetal rat bones (368, 369).
These effects were blocked by prior fetal decapitation, which is
thought to mimic the effects of thyroparathyroidectomy (368, 369).
Further, the fetal parathyroid glands enlarge in response to maternal
hypocalcemia in the rat (47, 370), and fetal femur length and mineral
ash content are subsequently reduced (53). Additional, recent
observations in genetically engineered mice suggest a role for the
skeleton in fetal calcium homeostasis. The ionized calcium of fetal
mice that lack the PTH/PTHrP receptor gene is lower than that of fetal
mice that lack the PTHrP gene, despite the fact that placental calcium
transport is supranormal in PTH/PTHrP receptor knockout fetuses and
subnormal in PTHrP knockout fetuses (299). Lack of bone responsiveness
to amino-terminal PTH and PTHrP may well, therefore, contribute to the
hypocalcemia in mice without PTH/PTHrP receptors.
Intact fetal parathyroid glands are needed for normal skeletal development, since fetal thyroparathyroidectomy in sheep caused decreased ash content and rachitic changes in the fetal skeleton by term (371, 372). These effects could be partly reversed or prevented by fetal calcium and phosphate infusions; thus, much of the effect of fetal parathyroidectomy was caused by a decrease in blood levels of calcium and phosphate (372). However, although bone formation parameters were corrected by the calcium and phosphate infusion, bone resorption parameters remained abnormal (reduced resorption cavities, reduced osteoclast numbers). Therefore, functioning fetal parathyroids (and, therefore, parathyroid gland-produced PTH and/or PTHrP) are required for normal fetal bone resorption and mineralization.
PTHrP is clearly important for growth plate development, because absence of PTHrP (in the PTHrP gene-knockout mouse) results in a chondrodysplasia characterized by accelerated differentiation of growth cartilage and adjacent endochondral bone (336). This effect of PTHrP must be mediated by the PTH/PTHrP receptor, because mice homozygous for the absence of the PTH/PTHrP receptor have a similar skeletal phenotype (373). However, these gene knockout experiments could not determine whether systemically delivered PTHrP adds to the likely role of PTHrP produced in the growth plate itself.
In summary, normal mineralization of the fetal skeleton requires intact fetal parathyroid glands and adequate delivery of calcium to the fetal circulation. The fetal skeleton can participate in the regulation of fetal calcium homeostasis, probably through actions of PTHrP and/or PTH. In addition to its effects on regulating placental calcium transport, PTHrP is required for normal skeletal development. Evidence from experimental vitamin D deficiency and the vitamin D receptor knockout indicates that 1,25-dihydroxyvitamin D may not be required by the developing skeleton.
F. Fetal response to maternal hyper- or hypoparathyroidism
As discussed earlier (Section II.F), in humans,
maternal hypercalcemia due to primary hyperparathyroidism may suppress
the fetal parathyroid glands, since hypocalcemia can be present at
birth (225, 235). The fetal parathyroid glands may also be suppressed
when the mother has hypercalcemia due to familial hypocalciuric
hypercalcemia (241, 242, 243, 374).
On the other hand, maternal hypoparathyroidism in human pregnancy has been associated with the development of intrauterine, fetal hyperparathyroidism. This condition is characterized by fetal parathyroid gland hyperplasia, generalized skeletal demineralization, subperiosteal bone resorption, bowing of the long bones, osteitis fibrosa cystica, rib and limb fractures, and low birth weight (246, 375, 376, 377, 378). Spontaneous abortion, stillbirth, and neonatal death have also been associated with this condition (379, 380, 381). Similar skeletal findings have been reported in the fetuses and neonates of women with pseudohypoparathyroidism (262, 263), renal tubular acidosis (382), and chronic renal failure (383). Although these skeletal changes have been interpreted to indicate fetal hyperparathyroidism, no serum measurements of intact PTH (or PTHrP) have been reported for this condition, and the serum calcium level has been generally reported to be normal.
G. Integrated fetal calcium homeostasis
Previous sections have demonstrated that the fetus maintains a
blood calcium higher than the maternal level, and that the placenta and
bone, and perhaps fetal kidney and intestine, transport calcium into
and out of the bloodstream. However, the need for fetal hypercalcemia
and the mechanisms by which it is maintained are not fully understood.
The need for an increased fetal blood calcium level is uncertain. It may well not be necessary for normal accretion of calcium by the developing skeleton, since, despite obliteration of the normal fetal-maternal calcium gradient (299), homozygous PTHrP-null fetal mice do not have a deficit in skeletal mineral content, as assessed by alizarin red staining and ash mineral content (336, 384).
In adult life, the CaSRs on the parathyroid glands and kidneys set the ambient serum calcium level, mainly by controlling the synthesis and secretion of PTH by the parathyroid glands and regulating renal tubular handling of calcium (385, 386). Inactivating or loss-of-function mutations of this receptor raise the set point for PTH secretion and increase renal calcium retention; these actions lead to hypercalcemia (385, 386). If the parathyroid gland CaSR were responsible for the elevated fetal calcium level, the set point would have to be different in fetal life, and one would expect to find increased or inappropriately normal PTH levels in the presence of the increased fetal calcium level. Instead, PTH in humans is normally low or undetectable at term (Section V.B.3), and the fetal blood calcium and PTH level are negatively correlated (387). Therefore, it is likely that neither the parathyroid CaSR nor PTH is responsible for the high fetal blood calcium; instead, the normally functioning CaSR suppresses the parathyroids in response to a high fetal calcium that is maintained by other processes. Further evidence that factors other than PTH derived from the parathyroids have a role in regulating the fetal blood calcium come from studies of thyroparathyroidectomized rat fetuses, which maintain a higher blood calcium than their simultaneously thyroparathyroidectomized mothers (286, 287). Such factors might include PTHrP (derived from the placenta, parathyroids, and other tissues), among other factors that might still be undiscovered.
That PTHrP has a role in maintaining the fetal blood calcium is suggested by the finding of a reduction in the fetal calcium to a level equal to that of the mother, in the PTHrP gene knockout fetuses (299). PTH levels are sharply increased in the PTHrP-null fetus (388), indicating that, in the absence of PTHrP, the parathyroids respond to regulate the fetal calcium level. Indeed, other evidence indicates that eliminating the amino-terminal actions of PTH, in addition to those of PTHrP, has a greater effect on reducing the fetal blood calcium than removing the PTHrP gene. The ionized calcium level of PTH/PTHrP receptor-null fetuses is lower than the ambient maternal level, despite the presence of a supranormal rate of placental calcium transfer (299).
Since the ionized calcium level of PTHrP-null fetuses is not higher than the maternal calcium level, this might indicate that PTH cannot make up for lack of PTHrP (including the effect of PTHrP to stimulate placental calcium transport); alternatively, in the absence of PTHrP, the fetal parathyroid CaSR may then control the regulation of the blood calcium, by stimulating PTH and setting the ionized calcium at the normal adult level. Further work is needed to determine whether either of these proposed mechanisms is correct.
Thus, an integrated model of normal fetal calcium homeostasis proposes that the fetal blood calcium is set at a level higher than maternal through the actions of PTHrP (among other potential factors). The parathyroid CaSR responds appropriately to this increased level of calcium and suppresses the parathyroids. 1,25-Dihydroxyvitamin D synthesis and secretion are, in turn, suppressed due to the effects of low PTH and high blood calcium and phosphate. PTHrP may be autonomously produced by the placenta and other tissues, or its production may be regulated. This model further proposes that the fetal blood calcium is maintained not only by flux of calcium across the placenta from the mother, but by contributions from fetal skeleton and kidney.
This tentative model needs to be tested further, and the physiological benefit of fetal hypercalcemia requires clarification.
H. Summary
The fetal-placental unit has adapted to rapidly extract calcium
from the maternal blood stream in sufficient amounts to mineralize the
fetal skeleton in late gestation. Fetal blood calcium and active
transport of calcium across the placenta are regulated relatively
independently of the levels of maternal calcium and calcitropic
hormones. The fetus has a higher blood calcium than the ambient
maternal calcium level. PTH and 1,25-dihydroxyvitamin D, the
traditional calcitropic hormones, are present at low levels in the
fetal circulation and may have a limited role in fetal calcium
physiology. Calcitonin levels are elevated in the fetal circulation,
but the role of calcitonin in fetal calcium homeostasis is uncertain.
PTHrP is critical for maintaining normal fetal hypercalcemia and active
transport of calcium across the placenta, although it is likely that
other factors regulate placental calcium transport, as well. A
midmolecular form of PTHrP stimulates placental calcium transport,
through actions on a receptor that is distinct from the cloned
PTH/PTHrP re-ceptor.
The fetal-placental unit has a remarkable ability to meet its needs irrespective of maternal calcium or vitamin D levels. However, maternal hypercalcemia due to primary hyperparathyroidism or familial benign hypercalcemia can suppress the fetal parathyroid glands. In turn, maternal hypocalcemia due to hypoparathyroidism and pseudohypoparathyroidism (and other causes) can cause fetal parathyroid gland enlargement and increased resorption in the fetal skeleton.
The fetus does not set a calcium gradient against the maternal circulation; instead, the fetus sets its blood calcium at a particular level independently of the maternal value. The physiological role of this relative hypercalcemia is uncertain. The normal setting of this level requires the presence of a normal calcium-sensing receptor. The fetal blood calcium is not simply determined by the influx of calcium across the placenta. Rather, other important factors include fluxes of calcium in and out of the developing skeleton, and (probably to a lesser extent) renal tubular reabsorption and excretion of calcium, and reabsorption of calcium from swallowed amniotic fluid.
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IV. Maternal Physiology and Pathophysiology During Lactation |
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TopAbstractI. IntroductionII. Maternal Physiology and...III. Fetal-Placental Physiology...IV. Maternal Physiology and...V. Neonatal Physiology and...VI. Discussion and ConclusionsReferences |
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In a classic study in 1931, Donelson et al. (396) carefully measured dietary calcium intake and losses of calcium in feces, urine, and breast milk in a group of lactating women who exclusively pumped breast milk and did not permit their infants to suckle. Despite adequate vitamin D and calcium intake (in the form of cod liver oil and yeast), calcium balance was negative in these women throughout lactation (396). A similar study of three lactating women found a negative calcium balance (net loss of 1 g calcium) during the interval of greatest milk production, despite ingestion of supplemental calcium and phosphorus; only during the time of lessened milk production (weaning) did the calcium balance return to normal (391).
To compensate for the calcium requirements of lactation, the maternal adaptations could, in theory, include increased intestinal absorption of calcium, renal conservation of calcium, and increased resorption of calcium from the skeleton. In fact, the studies reviewed in this section indicate that a temporary demineralization of the skeleton is the main mechanism by which lactating humans and animals meet these calcium requirements. This demineralization does not appear to be mediated by PTH or 1,25-dihydroxyvitamin D, but may be mediated by PTHrP in the setting of a fall in estrogen levels.
B. Mineral ions and calcitropic hormones
The changes that occur in maternal calcium, phosphate, and
calcitropic hormone levels during lactation, weaning, and postweaning
are schematically depicted in Fig. 2.
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In contrast, the data from lactating rats are conflicting: two reports found lactating rats to be slightly hypocalcemic compared with nonlactating controls (50, 404); another found normal calcium levels that could be decreased by feeding a low calcium diet (405), while a fourth found mild hypercalcemia (406). More intensive lactation (as determined by the relative rate of weight gain in the pups of a litter or by the number of pups in a litter) correlated with a lower maternal serum calcium (22, 50, 407, 408). Deer have also been found to have higher corrected serum calcium levels during lactation than after weaning (23). With abrupt weaning, the serum calcium of lactating rats typically rebounds into the hypercalcemic range (81, 409).
2. Phosphate. Serum phosphate levels are typically higher during lactation in humans, and, in some cases, exceed the normal range (14, 28, 398, 401, 410, 411, 412); similar results have been reported for lactating rats (50) and deer (23). Reabsorption of phosphate by the kidneys may be increased, although few measurements of tubular reabsorption of phosphate have been made (27, 160, 410, 411), and the data are conflicting. The increased serum phosphate levels may, therefore, reflect the combined effects of increased flux of phosphate into the blood from diet and from skeletal resorption (Sections IV.E.1 and IV.E.2, below) in the setting of decreased renal phosphate excretion (Section IV.D, below).
3. PTH. Lactation had been described as a state of secondary hyperparathyroidism by Albright and Reifenstein (1) and, as with pregnancy, early PTH assay results appeared to confirm this hypothesis (413) [although some early assays reported normal levels (30, 55, 397, 414)]. However, intact PTH, as determined by a two-site IRMA, has been found to be reduced 50% or more in lactating women in both cross-sectional (399, 401) and longitudinal studies (27, 160, 398, 400, 411, 415, 416, 417). The intact PTH level remains low compared with that of nonlactating postpartum women, rising to normal (160), or above normal (27, 398, 410) after weaning. This postweaning increase in intact PTH level may be sustained for 2–3 months (27, 398). This elevation corresponds to the time when bone mineral is restored to the skeleton (see discussion on bone density during lactation, Section IV.E.2, below) (27, 410).
PTH, measured with a two-site IRMA, rises during lactation in rats (45, 49, 50). Under the stress of a low calcium diet, urine cAMP levels sharply increase (405), and PTH levels (N-terminal assay) rise even higher (21, 50). Similar effects have been seen in dams nursing larger litters (50, 408). However, a functioning parathyroid gland is not necessary to provide calcium to the milk of the lactating rat (418). After weaning in rats, PTH levels were found to fall within 6 h (as determined by an N-terminal assay) and were normal by 24–48 h (21).
These data suggest that PTH levels are likely subnormal or normal during lactation in humans; the original hypothesis of hyperparathyroidism in lactation has not been substantiated. The metabolism of lactating rats, on the other hand, may represent a functionally hyperparathyroid state, in response to a fall in maternal blood calcium during lactation.
4. 1,25-Dihydroxyvitamin D. In contrast to the high 1,25-dihydroxyvitamin D levels of pregnancy that contribute to the doubling of intestinal calcium absorption, within days of parturition, maternal free and bound 1,25-dihydroxyvitamin D levels fall to normal (20, 55, 58, 73, 160). The levels remain normal throughout lactation and postweaning (27, 58, 160, 397, 398, 410, 414, 417), although one study reported that the levels rose above the normal range when lactation continued beyond 6 months (414), and another found an elevation after weaning (398). Women nursing twins had higher 1,25-dihydroxyvitamin D levels than women nursing singletons (402). Serum 25-hydroxyvitamin D levels are typically unchanged by lactation (410).
In contrast to humans, lactating rats have elevated levels of 1,25-dihydroxyvitamin D (22, 81), and more intense lactation correlated with higher levels (22, 407). The lactational rise in 1,25-dihydroxyvitamin D seen in rats was prevented by a small increase in the calcium content of the diet and was exacerbated by a low-calcium diet (419). Parathyroidectomy in the lactating rat caused a 70% decrease in the level of 1,25-dihydroxyvitamin D (81). Therefore, 1,25-dihydroxyvitamin D levels are elevated in rats in response to a fall in blood ionized calcium and a rise in serum PTH.
5. Calcitonin. In lactating humans, high calcitonin levels that do not vary with suckling have been reported (14, 420), while normal levels 6 weeks to 6 months postpartum have been found by others (397, 402, 417). In these reports the serum calcium was not different between lactating women and controls. Nursing of twins resulted in higher calcitonin and corrected serum calcium levels compared with nursing of singletons, although the levels remained in the normal range (402). Calcitonin is secreted into breast milk at concentrations 45 times that of maternal plasma (93); its functional role within the breast is unknown. Since a rise in serum calcitonin during lactation persists in totally thyroidectomized women, the breast may be an important extrathyroidal source of calcitonin during lactation (93).
In sheep, despite constancy of serum calcium levels, calcitonin levels fall to nonpregnant levels at parturition, rise over the period of lactation, and fall again at weaning (324). A similar rise in serum calcitonin has been found in deer, which also manifest a mild increase in the corrected serum calcium during lactation (23). In contrast, the data from lactating rats are conflicting. Investigators have reported low calcitonin levels in the presence of mild lactational hypercalcemia (406), while others have found high calcitonin levels until weaning in the presence of a normal or low serum calcium (421).
In the "calcitonin-deficiency" model previously described (Section II.B.5), rats made calcitonin-deficient during pregnancy lost more mineral content of their femurs than normal after 3 weeks of lactation (96, 97). Similarly, calcitonin-deficient goats fed a calcium-deficient diet lost more bone mineral by day 60 of lactation than control animals (77). Lactating, thyroparathyroidectomized rats had a prompt (1.8 mg/dl) rise in blood calcium after eating, while lactating control (intact) rats had no change in blood calcium (422). These findings indicate that in some species, during lactation, calcitonin protects the maternal skeleton from excessive resorption and regulates the maternal blood calcium level, particularly in response to meals. The relevance of these findings to humans has not been determined.
6. PTHrP. PTHrP has been detected in breast milk of humans and other animals at concentrations exceeding 10,000 times the level in the blood of patients with hypercalcemia of malignancy or normal human controls (110, 117). The primary role of PTHrP in the mammary glands or milk is not clear. A paracrine action of PTHrP in mammary tissue has been suggested because PTHrP concentrations in the milk are positively correlated with total milk calcium content of the human (423) and cow (424) [although no such correlations were found in rats (118, 425)], and administration of bromocriptine reduces both the PTHrP and calcium level in the milk of goats (426). However, a direct effect of PTHrP on the transport of calcium into the breast and breast milk has not been established. PTHrP has been found to have vasodilatory effects on mammary vessels and, therefore, may regulate mammary blood flow (427, 428). PTHrP has also been shown to have an essential role in mammary development (337).
PTHrP immunoreactivity in milk and PTHrP mRNA levels in mammary tissue have been observed to rise over the first few days postpartum in rats (118). Suckling induces PTHrP (mRNA and protein) locally in rat mammary glands (425, 429), and this response appears to be mediated by PRL and not oxytocin (429) and is blocked by bromocriptine (429). Milking a mammary gland in goats caused a marked increase in the PTHrP concentration in milk from that gland, but not in the milk of the contralateral (unstimulated) mammary gland (430). This result suggests that the synthesis and/or secretion of PTHrP by the mammary glands is under the control of local factors rather than the systemic level of PRL alone (430). Further, in rats, PTHrP is higher in milk at day 21 of lactation, after the PRL level has fallen, indicating that factors other than PRL may stimulate continued production of PTHrP by the mammary glands (431).
Studies in lactating rats have found that suckling induces phosphaturia and an increase in nephrogenous cAMP, which persists in parathyroidectomized, lactating rats (432). This result suggests that PTHrP (but not PTH) is released into the maternal circulation in physiologically relevant amounts in response to suckling. A similar study in cows found a milking-induced phosphaturia that could be blocked by (Tyr)34-bovine PTH(7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34)-NH2, a PTH/PTHrP receptor antagonist (433). Further, a significant venous-arterial concentration gradient of PTHrP was found across the mammary gland of the goat (426), consistent with PTHrP reaching the maternal circulation from the mammary glands. However, chronic, passive immunization of the lactating mouse with antibodies to amino-terminal PTHrP, sufficient to eliminate PTHrP bioactivity in the breast tissue, failed to affect maternal calcium or phosphate levels (434). Thus, PTHrP is not the sole determinant of mineral homeostasis in lactating rats. Perhaps PTH, which is elevated in lactating rats (Section IV.B.3), can substitute for PTHrP in this circumstance.
Although one less sensitive single-site assay found normal levels of PTHrP in maternal blood during human lactation (110), studies that used a more sensitive RIA, or two-site IRMAs, have found PTHrP levels to be significantly higher than in nonpregnant controls (399, 400, 401, 411, 435). In addition, a small rise in the systemic level of PTHrP can be demonstrated after suckling (400, 436). PTHrP was not elevated in the first 3 days postpartum, at which time lactation is not fully established (437); PTHrP immunoreactivity rises steadily in the breast milk over the first few days postpartum and subsequently declines as lactation wanes (400, 431).
Amino-terminal fragments of PTHrP and PTH are equipotent in activating
the human PTH/PTHrP receptor (104), and it is possible that
fluctuations in secretion of PTHrP into the maternal circulation from
the lactating breast cause resorption of calcium from the maternal
skeleton, renal tubular resorption of calcium, and suppression of PTH.
In support of this hypothesis, PTHrP levels have been found to
correlate negatively with PTH levels and positively with the ionized
calcium levels of lactating women (400, 401). Also, PTHrP levels
correlate with loss of bone mineral density during lactation in humans
(435). Furthermore, aparathyroid or hypoparathyroid women have been
found, while lactating, to be able to activate the 1-hydroxylase
enzyme to synthesize low-to-normal levels of 1,25-dihydroxyvitamin D
and maintain normal serum calcium levels while not receiving
supplemental calcitriol (244, 245, 258). This is consistent with PTHrP
reaching the maternal circulation in amounts sufficient to allow
stimulation of 1,25-dihydroxyvitamin D synthesis and maintenance of
normal (or slightly increased) maternal serum calcium. This consistent
finding has resulted in the recommendation that calcitriol
supplementation be reduced and the ionized calcium carefully followed
in the postpartum period of hypoparathyroid women who plan to
breast-feed (254).
PTH-independent hypercalcemia can occur during lactation and may be PTHrP-mediated. A 21-yr-old woman was reported to develop hypercalcemia and hypercalciuria during lactation in the presence of low PTH, low 1,25-dihydroxyvitamin D, and high PTHrP levels (438). These abnormalities resolved within 2 weeks of weaning, except that the elevated PTHrP level declined more gradually (438). A second woman had lactational hypercalcemia associated with low PTH levels and bone biopsy evidence of active cellular resorption, consistent with hyperparathyroidism (439). Another case of PTHrP-mediated hypercalcemia was reported to occur in a lactating woman with mammary hyperplasia and fully resolved only after a reduction mammoplasty was performed (440).
In summary, PTHrP is present at very high concentrations in breast milk and at higher than normal concentrations in the maternal circulation during lactation. PTHrP may affect maternal calcium metabolism, particularly by increasing skeletal resorption of calcium and phosphate, and renal tubular reabsorption of calcium. It may, thereby, be at least partially responsible for the slight increase in ionized calcium that occurs normally in lactating women and may contribute to the occasional occurrences of hypercalcemia and osteoporosis in lactation (Section IV.E.3, below).
C. Intestinal absorption of calcium
Intestinal absorption of calcium in lactating humans falls from
the high levels of pregnancy to control levels (12, 28, 126, 127, 128, 412, 441), coinciding with the corresponding fall in 1,25-dihydroxyvitamin D
levels to normal (Section IV.B.4, above). Even when women
are assigned to different levels of dietary calcium intake, a
lactational increase in efficiency of intestinal absorption of calcium
cannot be demonstrated (412). Intestinal absorption of calcium does
increase slightly in lactating women whose menses have resumed (412).
In some women, dietary intake of calcium may be increased during
lactation, such that the total amount of calcium absorbed is increased,
even though the efficiency of intestinal absorption is not (412).
However, other studies have indicated that such dietary calcium
supplementation is of uncertain benefit, since it will increase the
urine calcium excretion without affecting the calcium content of breast
milk or maternal skeletal losses of calcium (160, 442, 443) (see
discussion in Section IV.E.2, below). After weaning, there
is an increase in intestinal absorption of calcium (412), which may
facilitate restoration of calcium to the maternal skeleton.
The adaptations in lactating rats differ from those in humans. These may reflect the effect of greater calcium demands due to the larger litter size and far shorter lactational period. Rats demonstrate a 2-fold increase in duodenal calcium absorption in everted gut sacs, similar to the levels found during pregnancy (141, 444); 1,25-dihydroxyvitamin D levels are also elevated (81). Other factors, in addition to vitamin D, may influence calcium absorption, since, even in the setting of vitamin D deficiency, lactating rats exhibited an increase in duodenal calcium absorption and raised their serum calcium (141, 444). This finding suggests that PRL or some other factor present during lactation might stimulate intestinal calcium absorption. The increased intestinal calcium absorption also persisted in the presence of ovariectomy on the postpartum day 2 (445). At weaning, rats normally experience a rebound hypercalcemia (81, 409), and this is still detected in the presence of severe vitamin D deficiency (143), consistent with an increase in intestinal absorption of calcium. However, this rebound hypercalcemia could also be explained by enhanced release of calcium from the skeleton due to active bone resorption. Despite the persistent increase in dietary absorption, rats appear to also mobilize calcium from the skeleton to meet the demands of lactation (Sections IV.E.1 and IV.E.2, below).
Therefore, in summary, intestinal calcium absorption is not increased during lactation in humans, despite calcium requirements similar to those in pregnancy. In contrast, intestinal absorption of calcium is increased in rodents throughout lactation by increases in 1,25-dihydroxyvitamin D and probably other unknown mechanisms.
D. Renal handling of calcium
In humans, the glomerular filtration rate falls from the elevated
level of pregnancy, and renal excretion of calcium is typically reduced
to levels as low as 50 mg/24 h (12, 14, 27, 28, 44, 128, 146, 410). The
low urine calcium in the setting of high calcium in blood suggests that
tubular reabsorption of calcium might be increased to account for the
reduction in calcium excretion. The reduction in calcium excretion
appears to persist after weaning during the period of restoration of
bone density to the skeleton (410). Renal calcium excretion has been
shown to be similarly reduced in the lactating rat (446).
E. Skeletal calcium metabolism
1. Bone formation and resorption. Bone turnover is increased
during lactation in rats, as indicated by changes in histomorphometric
parameters of bone (136, 156, 447). Beagle dogs also show
histomorphometric evidence of increased bone turnover in iliac
trabecular bone (155), proximal femur (448), and lumbar vertebrae (448)
during lactation.
In humans, definitive histomorphometric data are lacking, and, therefore, serum and urine markers of bone formation and resorption have been used to assess bone turnover. Markers of bone resorption (tartrate-resistant acid phosphatase, deoxypyridinoline/creatinine, hydroxyproline/creatinine) are elevated 2- to 3-fold during lactation and are higher than the levels attained in the third trimester (27, 28, 160, 161, 400, 410, 415, 449). Although markers of bone formation have been reported in a few studies to be generally normal (27, 162), most studies have found such markers to be high during lactation and increased over the levels attained during the third trimester (28, 160, 161, 400, 410, 415, 449). Total alkaline phosphatase falls immediately postpartum due to loss of the placental fraction, but may still remain above normal due to the elevation in the bone-specific fraction (161, 162).
Overall, the results of these studies in humans of markers of bone formation and resorption are consistent with the histomorphometric data from animal models, indicating that bone turnover is significantly increased during lactation.
2. Bone density. Within 21 days of parturition, lactating rats lose about 35% of bone mineral, primarily from trabecular sites (primarily femurs, tibias, and lumbar vertebrae), as determined by a variety of methods that determined total or trabecular ash weight, ash mineral content, and changes in bone mineral density (2, 50, 139, 140, 405, 447, 450). These losses can be increased to 43% or more of trabecular bone mineral by consumption of a low calcium diet beginning at parturition (50, 405, 408, 451, 452). These changes are sufficient to adversely affect the mechanical properties of bone (strength, stiffness, toughness, and ductility) (408).
The loss of skeletal calcium can be worsened by larger litter sizes (408). The loss is similar in vitamin D-deficient and vitamin D-replete rats (137), although another study found a 2-fold greater loss of calcified bone in vitamin D-deficient rats (154). Moreover, vitamin D-replete rats regain the lost mineral after weaning while vitamin D-deficient rats do not; thus vitamin D is needed to restore the lost mineral (137). The lactational loss of bone mineral also persists in the absence of the maternal parathyroid glands (453, 454) and is not affected by ovariectomy, adrenalectomy, or immediate pregnancy (455), unless the animals are simultaneously fed a low calcium (0.1%) diet (445). Therefore, estrogen (and perhaps estrogen deficiency), adrenal hormones, 1,25-dihydroxyvitamin D, and PTH are not needed to mobilize skeletal calcium during lactation. On the basis of these observations, Brommage and DeLuca (455) first proposed that the putative hypercalcemia of malignancy factor, later identified to be PTHrP, might be the mediator of skeletal bone resorption during lactation.
The earliest longitudinal study of bone mineral density during human lactation followed 10 women with serial measurements of the femoral shaft, using a 241Am source, and found a mean loss of 2.2% of bone mineral content over 100 days of lactation (456). More recent studies have followed women with serial measurements of bone density during lactation (by SPA, DPA, or DXA), and a fall of 3–8.0% in bone mineral content has been reported after 2 to 6 months of lactation at trabecular sites (lumbar spine, hip, femur, and distal radius), with smaller losses at cortical sites (28, 146, 160, 166, 169, 400, 410, 415, 417, 449, 456, 457, 458, 459, 460, 461, 462, 463); this subject has recently been reviewed in detail elsewhere (390). In contrast, bottle-feeding, postpartum controls do not lose bone density at the lumbar spine over the same interval and may show a net gain in bone density (415, 417, 458, 462, 464). A 15% decrease in cortical bone density determined by SPA of the distal radius was found in teenaged mothers who consumed calcium and phosphate at levels below the recommended daily allowance. This result suggests that poor maternal nutrition might worsen the skeletal changes during lactation (459, 460). This was confirmed in a follow-up study, in which lactating adolescents who consumed calcium in excess of 1600 mg daily had no change in bone mineral density, compared with lactating adolescents who consumed 900 mg calcium daily and lost 10% of bone mineral content over the same interval (465). These teenaged mothers had not yet reached their peak bone mass, which might have been a factor in the responsiveness of the skeleton to dietary calcium supplementation.
Other evidence suggests that skeletal calcium will be preferentially resorbed and that supplementing the diet with calcium will not prevent resorption. For example, consumption of a calcium supplement by lactating Gambian women caused a sharp increase in urinary calcium excretion but had no effect on lactational bone mineral loss when compared with women who consumed less than the recommended daily allowance of calcium (442, 443). In a randomized clinical intervention trial that studied the effect of consuming dietary calcium in excess of the recommended daily allowance (2.4 g daily), lactating women still lost 6.3% of bone mineral density at the lumbar spine and up to 8% from the radius and ulna, as determined by DXA (160). Furthermore, in a preliminary report, the lactational decrease in lumbar spine bone mineral density was not influenced by maternal calcium intake but was negatively and significantly correlated with the breast milk output (466). Therefore, loss of bone mineral from the maternal skeleton appears to be a normal consequence of lactation and may not be preventable by raising the calcium intake above the recommended dietary allowance.
It is not clear whether this loss of bone mineral content is simply due
to relative estrogen deficiency of lactation or a more complex,
possibly humorally mediated, mechanism. Several studies have suggested
that estrogen withdrawal and the intensity and duration of lactation
are independent factors in determining the rate and magnitude of bone
loss during lactation (218, 416, 462, 464, 467). For example, early
resumption of menses (464) or an oral contraceptive (462) can reduce
the skeletal losses, but bone density may continue to decrease during
extended lactation, even after menses have resumed (218, 467). No
published study has adequately addressed the relative role of estrogen
withdrawal during lactation in a definitive way, since no study has
thus far manipulated estrogen independently of lactation. However, it
is evident from studying the effects of acute estrogen deficiency
induced by GnRH agonist therapy in young women (given for such diverse
conditions as endometriosis, uterine leiomyomata, and premenstrual
syndrome) that estrogen deficiency alone is unlikely to account for all
of the changes in skeletal calcium metabolism seen during lactation
(Table 2). After 6 months of GnRH agonist
therapy, the bone mineral content of trabecular-rich sites in the axial
skeleton alone is affected and is typically reduced by only 2–4%, as
determined by DXA (468, 469, 470, 471, 472, 473, 474, 475, 476, 477, 478). At the same time, the serum calcium and
phosphate are increased (469, 470, 471, 473, 479), 24-h urine calcium
excretion and urinary calcium/creatinine ratio are increased (469, 470, 471, 472, 473, 478, 479), but the levels of intact PTH and 1,25-dihydroxyvitamin D are
low (469, 473, 479). Therefore, in lactation, the greater losses of
bone mineral density (at both trabecular and cortical sites), the
normal 1,25-dihydroxyvitamin D levels, and the reduced urinary calcium
excretion may be due to the effects of other factors (such as PTHrP) in
addition to the effects of estrogen withdrawal. In one recent study of
lactating women, higher PTHrP levels were found to correlate with loss
of bone mineral density at the lumbar spine and femoral neck, even
after accounting for the effects of estradiol levels, PTH, and
breast-feeding status (435). In this same study, the high PTHrP levels
correlated with breast-feeding status, high PRL levels, and lower
estradiol levels (435). Further support for a role of PTHrP in
mediating the skeletal changes that occur during lactation comes from
the finding that PTHrP is also elevated in the serum of patients with
hyperprolactinemia due to pituitary adenomas (401, 480). In these
patients, PTHrP correlated positively with the serum PRL and calcium
and was negatively correlated with the serum PTH and bone density of
the lumbar spine.
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In the long-term, the consequences of lactation-induced depletion of bone mineral content appear clinically unimportant. The vast majority of epidemiological studies of pre- and postmenopausal women have found that a history of lactation has no adverse effect (172, 174, 175, 176, 177, 178, 179, 180, 181, 183, 188, 189, 190, 191, 195, 197, 202, 482) or a protective effect (184, 187, 203, 483, 484, 485, 486, 487) on peak bone mass, bone density, or hip fracture risk. Only five studies suggest that a history of lactation correlates with reduced radial (196, 488, 489, 490) or lumbar (199) bone mineral content. Thus, there appears to be little or no long-term harm caused by the temporary demineralization of the skeleton during lactation.
A similar pattern of decreasing bone density of the lumbar spine during lactation with recovery at weaning has been observed in another primate species, the African green monkey (491). The bone mineral density of the lumbar spine in these monkeys has been observed to fall by about 12% from the baseline value at parturition during 20 weeks of lactation; during an additional 20 weeks of observation after weaning, the bone mineral density increased but did not fully recover to baseline. This model may more closely resemble the human condition, but has not yet been thoroughly studied.
In summary, bone mineral density decreases of 3–8% at trabecular sites occur in the first 6 months of lactation in humans. By comparison, lactating rats lose about 35%; less dramatic losses have been reported in African green monkeys. In all cases, the lost bone mineral content is largely restored after weaning, and there appears to be little or no long-term increased risk of fracture or decreased bone density in women who have lactated. The loss of bone mineral density during lactation may be due to the combined effects of relative estrogen deficiency and PTHrP-induced skeletal resorption; both of these factors may correlate with more intense and prolonged lactation.
3. Osteoporosis of lactation. Rarely, osteoporosis presents during lactation; like the osteoporosis of pregnancy, this may represent a coincidental, unrelated disease. Alternatively, it may represent a continuum of the condition that may present in pregnancy and an exacerbation of the normal degree of skeletal demineralization during lactation (208, 223, 438, 492). In some of these cases, low peak bone mass may have preceded lactation (and pregnancy). Typically these women present 1 to 6 months postpartum with vertebral crush fractures, bone pain, loss of height, and rarely hypercalcemia; the osteopenia may resolve subsequently (438, 492). Bone biopsies show features indistinguishable from normal (223) or may show evidence of increased resorption (492). When measured, PTH is typically normal or reduced, and 1,25-dihydroxyvitamin D is also normal (438, 492). As for the osteoporosis of pregnancy, the difficult diagnostic question remains: when did the condition start? Given the preceding discussion, the possibility arises that excessive PTHrP release from the lactating breast into the maternal circulation could cause excessive bone resorption, osteoporosis, and fractures in these cases (438, 493). PTHrP was high in one case of lactational osteoporosis and was found to remain elevated for months after weaning (438). However, the extent to which PTHrP contributes to the reduction of bone density during lactation has yet to be established.
F. Hypoparathyroidism and pseudohypoparathyroidism
Calcitriol requirements of hypoparathyroid women fall early in the
postpartum period, especially if the woman breast-feeds, and
hypercalcemia may occur if the calcitriol dosage is not substantially
reduced (244, 245, 248, 253, 254, 255, 258, 494). As noted earlier in the
discussion of hypoparathyroidism in pregnancy (Section
II.G), this may result from the activation of the renal
1-hydroxylase by PTHrP.
The management of pseudohypoparathyroidism has been less well documented. Since these patients are likely resistant to the renal actions of PTHrP, and the placental sources of 1,25-dihydroxyvitamin D are lost at parturition, the calcitriol requirements can be expected to return to prepregnancy levels and should be unchanged by lactation.
G. Summary
Even when dietary intake of calcium exceeds the recommended daily
intake, the calcium demands of lactation in humans are preferentially
met by increased skeletal resorption of calcium and probably increased
renal conservation of calcium, but not by increased intestinal
absorption of calcium. Serum calcium is slightly increased or normal,
while phosphate is high normal or frankly elevated. These increases
reflect calcium and phosphate entering the circulation from bone in
increased amounts and reabsorption of calcium and phosphate by the
kidney. PTHrP levels are extremely high in breast milk and increased in
the maternal circulation, while PTH levels are generally low during
lactation. 1,25-Dihydroxyvitamin D levels are typically normal
throughout lactation but may be higher in women nursing twins. Markers
of bone resorption and formation are increased, and bone density has
been found to reversibly decrease during lactation. These lactational
decreases in bone density may not adversely affect the skeleton in the
long-term, although occasionally the normal lactation-induced decrease
in bone density may be excessive, leading to fractures and a clinical
diagnosis of osteoporosis.
Increasing the dietary intake of calcium does not consistently prevent the loss of skeletal calcium from occurring. Relative estrogen deficiency and elevation in PTHrP levels may both play a role in the lactational loss of skeletal calcium. PTH and 1,25-dihydroxyvitamin D elevations occurring after weaning may indicate a role for these hormones in providing the mineral needed to restore the skeleton.
In contrast to the findings in humans, skeletal losses of calcium are
far greater in lactating rats, and hypocalcemia may ensue with more
intense lactation. In that setting, the dietary absorption of calcium
is increased, and both PTH and 1,25-dihydroxyvitamin D levels are
elevated. These striking differences may limit the usefulness of the
lactating rat as a model for understanding the human condition (Table 3).
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). Such actions would act
against the provision of calcium for milk production. In this context,
the production of PTHrP can be viewed as the provision of a PTH
surrogate, but one that is not under negative feedback control by
calcium. Thus, even with low levels of PTH, 1,25-dihydroxyvitamin D
levels are then maintained, urinary calcium is reabsorbed, and bone
resorption continues (Fig. 3).
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V. Neonatal Physiology and Pathophysiology |
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TopAbstractI. IntroductionII. Maternal Physiology and...III. Fetal-Placental Physiology...IV. Maternal Physiology and...V. Neonatal Physiology and...VI. Discussion and ConclusionsReferences |
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B. Mineral ions and calcitropic hormones
The changes that occur in neonatal calcium, phosphate, and
calcitropic hormone levels over the first 4 days after birth are
schematically depicted in Fig. 4.
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Although data from umbilical cord and neonatal blood levels are less complete in humans, the progression in ionized and total calcium values appears to be similar to that of rats (268, 270, 495). The ionized calcium in normal neonates has been reported to fall from the umbilical cord level of 1.45 mmol/liter to a mean of 1.20 mmol/liter by 24 h after birth (496). Babies delivered by elective cesarean section were found to have lower blood calcium and higher PTH levels at birth compared with babies delivered by spontaneous vaginal delivery (387). This finding suggests that the mode of delivery can affect early neonatal calcium homeostasis, a variable that has not been controlled in most studies of neonatal calcium physiology.
2. Phosphate. Phosphate initially rises over the first 24 h of postnatal life in humans (270) and then gradually declines. The rise in phosphate corresponds to the early postnatal period when the parathyroid glands are still suppressed; phosphate declines as PTH secretion increases (see Section, V.B.3).
3. PTH. As discussed earlier (Section III.B.3), in fetal life PTH is present at low levels in the circulation and is apparently synthesized at low levels by the parathyroid glands. Based on measurements taken only at birth and 24 h of age in humans, the intact PTH level has been found to rise briskly after birth to within or near the normal adult range (17, 387, 497, 498, 499, 500, 501). However, these studies did not ascertain how soon the PTH level begins to rise postnatally and whether the peak level is attained by 24 h or even later. A more detailed time course for PTH was obtained using earlier generation PTH assays; as discussed earlier, these assays were less reliable, and measured fragments that accumulated in the circulation much more slowly than intact PTH. With these caveats in mind, data from these older assays found that PTH levels remained low over the first 12–24 h in humans and did not reach peak levels until 48 h or later (268, 272, 305, 306, 307).
Regardless of the exact time course, it is apparent that the increase in PTH levels follows the early postnatal drop in the serum ionized calcium and precedes the subsequent rise in ionized calcium (268, 272, 306, 495) and 1,25-dihydroxyvitamin D (Section V.B.4, below). The fall in ionized calcium is probably a consequence of the parathyroid gland suppression seen in late gestation (Section III.B.3, above) combined with the sudden loss of placental calcium delivery; the subsequent rise in PTH and the ionized calcium represents the progressive recovery from this suppression. During the first 48 h, the parathyroid glands have been found to respond sluggishly to more severe falls in the ionized calcium, such as that caused by exchange transfusion with citrated blood (268, 272, 306, 502). The degree of responsiveness to acute hypocalcemia appears to increase with postnatal age (306, 502); however, the interpretation of these data are limited by the nature of the older PTH assays from which they were obtained.
4. 1,25-Dihydroxyvitamin D. In humans, 1,25-dihydroxyvitamin D rises to adult levels over the first 48 h of postnatal life, likely in response to the rise in PTH (11, 40). Phosphate levels are high at this time and would tend to inhibit a rise in 1,25-dihydroxyvitamin D. Although in humans, rats, sheep, and pigs the fetal blood calcium and skeletal mineralization are unaffected by vitamin D deficiency, in the neonatal period deficiency of vitamin D can become manifest (29, 79, 84, 284, 285). This is further discussed in Section V.E, below.
5. Calcitonin. Data from humans have found a rise in the serum calcitonin, 2- to 10-fold over cord blood levels, over the first 24–48 h of life (11, 272, 500). Infants that are premature, asphyxiated, or hypocalcemic have the highest postnatal calcitonin levels (272); consequently, it has been suggested that hypercalcitoninemia may cause neonatal hypocalcemia (86). Consistent with this hypothesis, hypocalcemic preterm infants had higher calcitonin levels than normocalcemic, preterm control infants (503). However, the same study found that the calcitonin levels of hypocalcemic term infants did not differ from those of normocalcemic term infants (503). Therefore, the influence of calcitonin on neonatal calcium homeostasis may vary with gestational age at birth; or, alternatively, have little role in calcium homeostasis. The physiological effect of calcitonin administration to neonates (human or animal) has not been directly tested.
Further evidence suggests that calcitonin levels are not significantly regulated by calcium, or responsive to the serum calcium level, in the neonatal period. In infants of normal and diabetic pregnancies, calcitonin levels increased after birth irrespective of the rate of fall in serum calcium, indicating that the postnatal surge in serum calcitonin may not be the main mechanism of postnatal hypocalcemia (500). Similarly, in very-low-birth-weight, preterm infants, spontaneous hypocalcemia provoked a rise in PTH but no change in calcitonin levels, and a calcium infusion to treat the hypocalcemia reduced the PTH level but did not affect the calcitonin level (504).
After the early surge in calcitonin levels over the first 48 h of life, calcitonin gradually falls to adult levels over the first month after birth in sheep (324) and humans (86).
6. PTHrP. PTHrP secretion from placenta, amnion, and umbilical cord is lost at birth; secretion from the parathyroid glands is also apparently lost, since PTHrP circulates at low to undetectable levels (with currently available assays) during normal adult life in humans and animals (104, 116, 335, 493, 505). The loss of placental sources of PTHrP, in addition to loss of the placental calcium pump, may be one of the reasons that the ionized and total calcium levels fall dramatically at birth. However, a preliminary report in neonatal (1- to 3 day-old) rat pups has found persistently high PTHrP levels, in the range of 20–40 pmol/liter (506). Studies in neonatal lambs have shown a progressive decline in PTHrP immunoreactivity in the parathyroid glands, and serum bioactivity attributable to PTHrP, over the first several months after birth, corresponding to the delayed fall in blood calcium that occurs in this species (116, 330). As noted earlier (Section III.C), indirect evidence from fetal sheep suggests that the fetal parathyroid gland may be an important source of PTHrP for normal regulation of the blood calcium and placental calcium transport. However, it is not known whether PTHrP synthesis occurs in the parathyroid glands of neonatal humans, or is turned off at some point in postnatal development.
Although immunoreactive PTHrP in milk can be absorbed into the bloodstream of suckling neonates (and might account for high levels of PTHrP in neonatal plasma) (113, 507), PTHrP may not have a dramatic effect on neonatal mineral homeostasis, since blocking its activity in milk by passive immunization of the mother had no dramatic effect on mouse pup blood calcium, weight gain, or femur mass (434). These negative results, however, might result from compensatory changes in neonatal PTH, calcitonin, and 1,25-dihydroxyvitamin D, which were not measured in this study. Passive immunization of the 2-day-old neonatal mouse with antibodies to amino-terminal PTHrP did not affect neonatal calcium homeostasis (508), but again, compensations may have obscured the role of PTHrP. These findings are consistent with the observations described above from humans and rats, that PTH dominates the regulation of calcium homeostasis by 48 h of postnatal life. However, since there are no human data, and limited animal data on PTHrP levels in the neonatal period, an important role for PTHrP in calcium homeostasis after birth has not been ruled out.
C. Intestinal absorption of calcium
Calcium is absorbed in the gastrointestinal tract by both active
and passive mechanisms (125, 509). Active intestinal transport occurs
mainly in the duodenum and is regulated by 1,25-dihydroxyvitamin D,
partly through its stimulation of calbindin9K-D. Passive
transport of calcium appears to be an unregulated, nonsaturable process
that occurs throughout the small intestine; the rate of passive
transfer of calcium into intestinal cells is directly proportional to
the intraluminal concentration of calcium.
In newborn rat pups, intestinal calcium absorption is largely a passive, nonsaturable process that is not dependent on vitamin D (510, 511, 512). The high lactose content of milk has been shown to specifically increase the efficiency of intestinal calcium absorption, and net bioavailability of dietary calcium, through effects on paracellular diffusion in the distal small bowel (513, 514, 515). As the pups mature, receptors for 1,25-dihydroxyvitamin D begin to appear in intestinal cells (516), and the mucosal levels of calbindin9K-D increase sharply (517). Around the same time, vitamin D-dependent active transport of calcium becomes noticeable (512), while passive transfer of calcium into the intestinal cells declines (510, 511). By the time of weaning, the intestine is less permeable to passive absorption of calcium, and active transport has become the dominant means by which calcium is transferred into the intestinal mucosa (510, 511, 512).
Data from newborn humans is less complete; therefore, the timing of the development of vitamin D-dependent calcium transport in humans is not known. However, normal term and preterm infants exhibit a similar postnatal increase in the efficiency of intestinal calcium absorption (518, 519, 520). Studies from preterm infants indicate that passive, non-vitamin D-dependent absorption of calcium may be the dominant route of calcium transfer (518, 521). In addition, the lactose content of breast milk has been shown to increase the efficiency of intestinal calcium absorption in human infants (522, 523). Postnatal supplementation of the preterm infant (mean gestational age, 32 weeks) with vitamin D markedly increased the absorption of calcium by 2–4 weeks after birth, as compared with similar preterm infants that were not supplemented (520). However, the intestinal absorption of vitamin D has been shown to undergo a postnatal age-dependent increase (524); this may limit the efficacy of vitamin D supplementation in the newborn.
In summary, data from newborn rats and humans indicate that intestinal calcium absorption is largely a passive, non-vitamin D-dependent process until near the time of weaning. The high lactose content of milk increases the efficiency of passive absorption of calcium. The normal postnatal maturation of the neonatal intestine may affect the ability of preterm humans to accrete sufficient calcium for skeletal mineralization and to regulate their blood calcium.
D. Renal handling of calcium
Little data are available from animal or human studies on the
renal handling of calcium in the first few days of postnatal life
(509). In humans, urinary excretion of calcium is low over the first
few days of life and rises over the succeeding 2 weeks (498, 525, 526).
This rise in renal calcium excretion probably reflects the concurrent
2-fold rise in glomerular filtration rate over the first 2 weeks after
birth (525, 527). It may also reflect a greater filtered load of
calcium as a consequence of the gradually rising serum calcium,
1,25-dihydroxyvitamin D, and PTH levels and gradually declining serum
calcitonin (Section V.B, above). The responsiveness of the
renal proximal tubules to PTH may increase over the same time period.
In addition to evidence from animal studies presented earlier
(Section III.D), data in preterm and term human infants
indicate that the renal tubules are responsive to exogenously
administered PTH, as evidenced by a rise in urinary cAMP (528, 529);
further, this response to exogenous PTH increases with postnatal age
(277, 528, 529).
E. Skeletal calcium metabolism
In humans that receive adequate dietary calcium and phosphate, the
neonatal skeleton continues to accrete calcium at a rate of about 150
mg/kg per day, similar to the rate of the late-term fetus (5, 530, 531, 532). Having abruptly lost the placental calcium pump, the neonate
becomes completely dependent on intestinal intake of calcium and
phosphate to continue to meet the demands of the developing skeleton.
In turn, the blood calcium will be maintained by calcium obtained from
skeletal stores, absorbed by the intestine, and reabsorbed by the
kidneys. The effects of vitamin D deficiency during gestation become
manifest during the neonatal period because intestinal calcium
transport becomes necessary for supply of calcium to the skeleton. As
noted earlier, from studies of maternal vitamin D deficiency in rats
(29, 79, 84), pigs (79, 285), and sheep (284), the blood calcium is
typically normal at birth and the skeleton is fully mineralized, but
after birth the neonate will begin to manifest the complications of
vitamin D deficiency. In vitamin D-deficient rats and pigs, a slight
decrease in blood calcium was detected by 3 days after birth, but it
was only after 14 days of neonatal life that absence of
1,25-dihydroxyvitamin D resulted in significant, but still modest,
hypocalcemia (84, 285, 311). Thereafter, rat pups failed to thrive and
manifested rachitic skeletal growth plates and decreased longitudinal
growth (84, 285). These effects became more pronounced after weaning
(84, 285). Further, in vitamin D receptor knockout mice, hypocalcemia
and rachitic changes in the skeletons developed only after the first
three postnatal weeks (313, 314).
The development of vitamin D dependence is similar for humans. In human vitamin D deficiency, hypocalcemia appears late in the first or second week of life, and rickets develops after 2–3 months (316, 317). Vitamin D supplementation during pregnancy is associated with higher neonatal serum calcium levels and a reduced incidence of neonatal hypocalcemia (83, 533, 534). Neonates with rachitic changes in the skull (craniotabes) have lower 25-hydroxyvitamin D levels than those without craniotabes (535), and fewer neonates develop craniotabes if the mothers received a vitamin D supplement during pregnancy (534). Infants in China with lower cord blood 25-hydroxyvitamin D levels were found to have fewer wrist ossification centers at 3–5 days of postnatal age (536); furthermore, the crown-heel lengths of infants born to vitamin D-insufficient Pakistani women were found to correlate positively with the maternal serum calcium and negatively with the maternal PTH level (537). These findings suggest that fetal vitamin D deficiency may be manifest earlier in postnatal life than previously believed (and perhaps even during fetal life) or may be a marker for more global nutritional deficiency.
Although parathyroidectomy of rats at birth results in hypocalcemia and hyperphosphatemia (538, 539), by the time of weaning the neonatal rats still have normal body weight, normal shape and trabecular content of developing long bones, and normal calcium and phosphorus content of bone ash (539). These observations are similar to the finding of normal skeletons in experimental models of vitamin D deficiency, and in the vitamin D receptor knockout mice. These findings suggest that factors other than PTH and vitamin D may be required for normal accretion of calcium by the skeleton in the first several weeks after birth, while the infant is nursing. The lactose content of milk (discussed in Section V.C) and the high levels of PTHrP in milk may be important.
Premature infants are prone to develop metabolic bone disease of prematurity, a form of rickets precipitated by loss of the placental calcium pump at a time when the skeleton is accreting calcium at a peak rate (315, 316). These premature infants are born with a bone mineral content appropriate for gestation age (as assessed by SPA of the radius), but if untreated, their radial bone mineral content fails to increase as appropriate for their gestational age (540, 541, 542, 543, 544). By 2–3 months of age, the physical and radiological signs of rickets of prematurity may develop. These include craniotabes, rachitic rosary, chest deformity, osteopenia, pathological fractures, metaphyseal stippling, and flaring and widening of the epiphyses of long bones (315).
Rickets of prematurity is not due to vitamin D deficiency. 25-Hydroxyvitamin D and 1,25-dihydroxyvitamin D levels are typically normal, and vitamin D supplementation of the mother during pregnancy, or the infant postnatally, does not prevent its development (315, 545, 546). Rickets of prematurity appears to be the consequence of inadequate calcium and phosphate intake to meet the demands of the mineralizing neonatal skeleton. Special oral or parenteral formulas that are high in calcium and phosphorus content will correct the demineralization process and allow normal skeletal accretion of these minerals (315, 316, 544, 545, 547).
In summary, continued mineralization of the neonatal skeleton is dependent on adequate vitamin D stores and intact intestinal calcium absorption. Preterm infants are prone to develop a form of metabolic bone disease if the immature intestine cannot absorb calcium efficiently enough to make up for the loss of the placental calcium pump.
F. Neonatal response to maternal hyper- or hypoparathyroidism
Although the fetal blood calcium is set independently of the
maternal level in utero, and PTH does not cross the
placenta, it is clear from more than 100 reported cases in humans that
maternal hyperparathyroidism adversely affects the neonate (11, 225, 227, 229, 235, 237). Premature labor and stillbirth may result from
unrecognized maternal hyperparathyroidism (225, 235). Typically, the
parathyroid glands remain suppressed after birth, and complications of
neonatal hypocalcemia, tetany, permanent childhood hypoparathyroidism,
and even death may result. The mechanism of the prolonged suppression
is not known, but is probably due to increased flux of calcium across
the placenta when the mother is hypercalcemic (233). However, this
hypothesis has not been tested experimentally. A similar suppression
has also been observed in normal infants of women with hypercalcemia
due to familial hypocalciuric hypercalcemia, who typically have similar
elevations in the serum calcium without an increase in serum PTH level
(241, 242, 243, 374).
Similarly, maternal hypoparathyroidism in humans has been associated in the neonate with parathyroid gland hyperplasia, generalized skeletal demineralization, subperiosteal bone resorption, bowing of the limbs, fractures of ribs and long bones, and low birth weight (246, 375, 376, 377, 378). Stillbirth and neonatal death have also been associated with this condition (379, 380, 381). The serum calcium level of the neonate has been reported to be normal in most cases, while the PTH level (older assays) has been found to be elevated (263, 376, 548). The skeletal findings generally resolve over the first several months after birth, but acute interventions may be required to raise or lower the blood calcium in the neonate. In addition, subtotal parathyroidectomy, with or without parathyroid autotransplantation and cryopreservation, may be required to control more severe, autonomous disease (549).
Maternal hypocalcemia of any cause may result in parathyroid gland hyperplasia and hyperparathyroidism in the fetus and neonate (376, 382, 383). In women with pseudohypoparathyroidism, children who do not inherit the genetic disorder are usually normal at birth, although transient neonatal hyperparathyroidism has been reported in some cases (262, 263). Furthermore, children who did inherit the condition may also be normal at birth but may gradually develop the full biochemical features of pseudohypoparathyroidism over the first several years of life (550).
G. Neonatal hypocalcemia
Neonatal hypocalcemia typically presents as seizures, starting
between 4–28 days of age (551, 552). The preterm infant is
particularly prone to hypocalcemia, having lost the placental calcium
pump at a time when the skeleton is rapidly accreting calcium, and the
intestinal calcium absorption mechanism is relatively immature. In
addition to prematurity, other causes of neonatal hypocalcemia include
congenital hypoparathyroidism, magnesium deficiency, maternal diabetes,
vitamin D deficiency or resistance, and hyperphosphatemia (Table 4) (552). In many cases the etiology of
the hypocalcemia is unknown. One study reported that three
hypocalcemic, otherwise unremarkable, neonates had high PTH and serum
phosphate levels and a subnormal phosphaturic response to PTH infusion;
this "neonatal pseudohypoparathyroidism" completely resolved by 6
months of age (551). In addition, the higher phosphate content of
infant formula has been associated with increased serum phosphate and
decreased serum ionized calcium levels in formula-fed infants, as
compared with breast-fed neonates (553).
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Neonatal hypocalcemia can occur as a complication of maternal diabetes in pregnancy in up to 50% of cases (555, 556), although tight control of the maternal glucose during pregnancy reduces the incidence of neonatal hypocalcemia (557). The cause of hypocalcemia in these infants is likely to be multifactorial. As noted above, parathyroid gland secretion may be blunted during the first few days of life in both normocalcemic and hypocalcemic infants of diabetic pregnancies (272, 554, 558). Neonatal hypomagnesemia, which results from maternal wasting of magnesium in association with glycosuria during pregnancy, correlates with the severity of neonatal hypocalcemia and may be the major factor contributing to the sluggish neonatal parathyroid function (557, 559, 560). However, this hypothesis has not been tested by correcting the hypocalcemia through magnesium replacement alone. Although occult vitamin D deficiency has been suggested as a cause of hypocalcemia in these infants (561), supplementation of infants of diabetic pregnancies with vitamin D at 2, 24, 48, and 120 h after birth did not reduce the magnitude or incidence of hypocalcemia as compared with infants that were not supplemented (562).
In addition to the blunted parathyroid function and hypomagnesemia, it is also possible that alterations in maternal calcium homeostasis due to diabetes might predispose the neonate to become hypocalcemic. Although two cross-sectional studies in humans found no effect of maternal diabetes on maternal levels of calcium, phosphate, PTH, and calcitonin (563, 564), a longitudinal study found that pregnant diabetic women had lower 1,25-dihydroxyvitamin D, total serum calcium, and ionized calcium levels in the third trimester, as compared with nondiabetic pregnant women (565). Furthermore, experimental models of diabetes in pregnant rats demonstrate marked maternal hypercalciuria (566), reduced maternal-fetal transfer of calcium and magnesium (567), a 12-fold reduction in placental calbindin9K-D levels (567), and reduced calcium content of fetal ash (567). Collectively, the human and animal data indicate that maternal diabetes can affect maternal calcium homeostasis and reduce the placental transfer of calcium. However, whether these effects predispose to the development of hypocalcemia in infants of diabetic pregnancies has not been determined.
H. Summary
In the early neonatal period, the neonate is challenged by the
loss of the placental calcium pump and manifests a quick transition,
from an environment in which PTHrP plays an important role to a PTH-
and 1,25-dihydroxyvitamin D-controlled neonatal milieu. This is
reflected in a rapid fall in total and ionized calcium over the first
6 h of life. The calcium level gradually corrects over the
following 48 h, after PTH secretion by the parathyroid gland
increases, and 1,25-dihydroxyvitamin D levels ascend to adult values.
Serum phosphate persists at high levels until the rising PTH levels and
increasing renal responsiveness to PTH permit a phosphaturia.
Calcitonin levels may remain elevated for several weeks; the
physiological importance of this elevation is uncertain. These changes
in calcium and calcitropic hormone levels are summarized in Fig. 4. In
contrast to the rapid changes in calcitropic hormone levels, intestinal
calcium absorption changes gradually from a passive to an active,
1,25-dihydroxyvitamin D-mediated process over the first weeks of life.
Although human and animal fetuses develop remarkably normally in the presence of maternal calcium, PTH, and vitamin D deficiency, the resulting neonates demonstrate abnormalities that are consequences of the prior abnormal maternal calcium homeostasis. Maternal hyperparathyroidism and hypoparathyroidism during pregnancy can be manifest as disturbances of neonatal calcium and bone metabolism. In addition, maternal diabetes during pregnancy can predispose to neonatal hypocalcemia, probably as a consequence of fetal hypomagnesemia induced by maternal renal wasting of magnesium.
When data from fetal and neonatal humans and animals are compared, it is apparent that PTHrP has a significant role in fetal calcium homeostasis and circulates at higher levels than PTH. In contrast, there is as yet no evidence of an important role for PTHrP in normal postnatal calcium homeostasis. PTHrP is normally found at low to undetectable levels in the adult, but the postnatal time point at which PTHrP is lost is not known.
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VI. Discussion and Conclusions |
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TopAbstractI. IntroductionII. Maternal Physiology and...III. Fetal-Placental Physiology...IV. Maternal Physiology and...V. Neonatal Physiology and...VI. Discussion and ConclusionsReferences |
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). The
pregnant woman manifests a 2-fold increase in intestinal calcium
absorption that is mediated partly by a 2-fold increase in free and
total 1,25-dihydroxyvitamin D levels and partly by mechanisms that are
independent of vitamin D. In addition, the increased dietary intake of
calcium is offset by increased renal losses of calcium. Skeletal stores
of calcium do not appear to be mobilized to any significant degree
during pregnancy. PTH levels are low or low-normal for much of
pregnancy, and thus the concept of "physiological hyperparathyroidism
of pregnancy" is invalid. The increased intestinal calcium absorption
results in a gradual, slight increase in the corrected serum calcium
and ionized calcium and a marked increase in renal calcium excretion
(absorptive hypercalciuria). Bone mineral stores may be increased early
in pregnancy in anticipation of the peak fetal demand of the third
trimester; by term, the maternal skeleton has no apparent deficit of
bone mineral. In contrast, the lactating woman does not increase
intestinal calcium absorption, but increases bone turnover and renal
tubular reabsorption of calcium to provide adequate calcium for the
breast milk. Again, the serum ionized calcium is increased slightly,
while the serum phosphate may be frankly elevated. These increases
reflect the increased skeletal resorption and decreased renal excretion
of these minerals. The lactation-associated fall in estrogen levels,
along with secretion of PTHrP, leads to bone resorption. This, combined
with PTHrP-mediated calcium reabsorption from the urine, leads to
suppression of PTH. Lactation causes a loss of 3–8% of bone mineral
content that is restored after weaning; this reversible loss of bone
mineral does not appear to adversely affect the skeleton in the long
term.
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The regulatory mechanisms that direct the adaptive processes that occur
in pregnancy and lactation are by no means fully elucidated. The
potential roles of reproductive hormones of pregnancy in calcium
homeostasis (estrogen, PRL, CG, placental lactogen, etc.) have not been
adequately explored, and it is also possible that other calcitropic
factors, perhaps specific to pregnancy and lactation, remain to be
identified. The results of studies from animal models of pregnancy and
lactation must be interpreted carefully, given that there are
significant differences between the adaptive strategies for calcium and
bone homeostasis seen in humans, rodents, and other animals (Tables 1 and 3). For example, the pregnant and lactating rat both develop
secondary hyperparathyroidism, and the lactating rat can lose up to
35% of skeletal calcium before restoring it after weaning. The
lactating rat also increases the intestinal absorption of calcium,
whereas lactating women do not.
The rare disorders of osteoporosis in pregnancy and lactation may represent chance occurrences of idiopathic osteoporosis, or they may represent a spectrum of one common condition whose time of presentation is determined by the prepregnancy bone mass and the rate of bone resorption subsequent to conception. It seems likely that estrogen deficiency combines with the actions of PTHrP to stimulate the loss of calcium from the skeleton during lactation. The mechanism by which calcium is restored to the skeleton after weaning is not known, but if understood, might be adapted to a bone-restorative therapy for osteoporosis. The postpartum restoration of normal estrogen levels (recognized by the resumption of normal menses) is clearly an important factor, but probably not the only one, since part of the restoration of calcium to the skeleton occurs only with weaning.
Fetal calcium homeostasis in late gestation appears to be largely regulated by PTHrP, which stimulates placental calcium transport, resorbs bone, and may stimulate renal calcium reabsorption. PTH can also resorb bone, stimulate renal calcium reabsorption, and stimulate synthesis of 1,25-dihydroxyvitamin D. However, since PTH and 1,25-dihydroxyvitamin D are found at very low levels in the fetal circulation during late gestation, their roles in fetal life are less well defined. The relative importance of PTH and PTHrP earlier in fetal life are unknown. The fetus sets its blood calcium level irrespective of the ambient maternal blood calcium level. The usefulness to the fetus of a blood calcium higher than the mother’s is not understood, as it does not appear to be necessary for full mineralization of the skeleton to occur. It is possible that the high fetal calcium levels act as a safety margin at birth, allowing the newborn to experience a postnatal fall in ionized calcium without tetany or convulsions. Alternatively, a higher ionized calcium may be useful for cellular functioning under fetal conditions (low pO2, low pH, for example) that differ from those in later life. In any case, the fetus is able to maintain a normal blood calcium and fully mineralize its skeleton in the setting of significant maternal calcium and vitamin D deficiencies.
A changeover to a PTH- and 1,25-dihydroxyvitamin D-driven environment occurs during the first 48 h of postnatal life, accompanied by loss of hypercalcemia and high PTHrP levels. Placental calcium transfer may suppress PTH synthesis by the fetal parathyroid glands, and loss of the placenta may, therefore, stimulate PTH synthesis in the neonatal parathyroid glands. Since PTHrP is normally undetectable in the adult circulation, secretion of PTHrP into the circulation by fetal tissues must be lost postnatally; the time of this occurrence has not been determined. The rising PTH and 1,25-dihydroxyvitamin D levels mobilize skeletal calcium to maintain the blood calcium level and increase the efficiency of intestinal calcium absorption to meet the continued demands of the mineralizing skeleton. Preterm infants are compromised by the loss of the placental calcium pump at a gestational age when rapid accretion of calcium by the skeleton normally occurs. They cannot increase their bone mineral content postnatally unless given high amounts of calcium and phosphate parenterally or in their diet. Infants of hypercalcemic mothers may have suppressed parathyroid function in the first month of life, while infants of hypocalcemic mothers may have enlarged, overactive (and occasionally autonomous) parathyroid glands that caused significant skeletal demineralization in utero. Although increased mortality has been associated with both conditions, in most instances the disturbance in neonatal calcium homeostasis is self-limited.
As stated, the adaptive strategies of the pregnant woman differ from the lactating woman, in the face of similar calcium demands. The fetal and neonatal adaptations differ, largely because the former utilizes a placental calcium pump, while the source of calcium for the latter is the intestine. If more fully understood, these adaptive mechanisms might be exploited further to treat disorders of calcium and bone metabolism in later life, such as osteoporosis.
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Footnotes |
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1 The writing of this review was supported in part by a Fellowship Award
from the Medical Research Council of Canada (to C.S.K.) and by NIH
Grant DK-47038 (to H.M.K.). 
2 Current address: Room H-5309, Health Sciences Centre, Memorial
University of Newfoundland, 300 Prince Philip Drive, St. John’s,
Newfoundland A1B 3V6 Canada.
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References |
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