The site of 1 alpha,25-dihydroxyvitamin D3 production in pregnancy

Vitamin D, the placenta and pregnancy

Impaired vitamin D status is common to many populations around the world. However, data suggest that this is a particular problem for specific groups such as pregnant women. This has raised important questions concerning the physiological and clinical impact of low vitamin D levels during pregnancy, with implications for classical skeletal functions of vitamin D, as well as its diverse non-classical actions. The current review will discuss this with specific emphasis on the classical calciotropic effects of vitamin D as well as the less well established immunological functions of vitamin D that may influence pregnancy outcome. The review also describes the pathways that are required for metabolism and function of vitamin D, and the various clinical complications that have been linked to impaired vitamin D status during pregnancy.

Postnatal changes in plasma 1,25-dihydroxyvitamin D3 in sheep: role of altered clearance

The ontogeny of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] production and metabolic clearance rates (PR and MCR, respectively) was determined in chronically catheterized fetal (postconceptional age of 127 days; term = 145 days), neonatal (10 +/- 1 and again at 24 +/- 1 days postnatal), and adult (3 yr of age) sheep. Plasma concentrations of 1,25(OH)2D3 were more than twofold higher in fetuses than in adults and were increased by 78% during the 1st 24 days postnatally. In contrast, compared with values in fetuses, the MCR of 1,25(OH)2D3 was reduced by 40% in 10-day-old neonates and by 58% in 24-day neonates, although 24-day neonatal values were still 75% higher than adult values. In contrast, the PR of 1,25(OH)2D3 did not demonstrate an ontogenic change during development and remained at all times approximately ninefold the adult value. The finding, by regression analysis of an inverse relationship between the MCR and the plasma concentration of 1,25(OH)2D3, is interpreted to suggest that increased plasma concentrations are the result of a reduced MCR. We conclude that increases in plasma concentrations of 1,25(OH)2D3 in the perinatal period can occur by a mechanism that is independent of enhanced endogenous synthesis, namely a reduction in the MCR of the hormone.

Renal 25-hydroxyvitamin D-3 1 alpha-hydroxylase in guinea-pig: activity variations during development and pregnancy

The renal 25-hydroxyvitamin D-3-1 alpha-hydroxylase (1 alpha-hydroxylase) activity and circulating levels of 1,25-dihydroxyvitamin D (1,25(OH)2D) were measured in pregnant guinea-pigs and their offspring. Serum levels of 1,25(OH)2D were significantly elevated in pregnant guinea-pigs but the renal enzyme activity was not different from non-pregnant animals. The fetal renal 1 alpha-hydroxylase activity was about 6-fold higher than the maternal level, whereas circulating 1,25(OH)2D was low. Treatment with pharmacological doses of 1,25(OH)2D3 increased circulating 1,25(OH)2D and depressed the renal 1 alpha-hydroxylases both in the mother and the fetus. In newborn guinea-pigs the enzyme activity was up to 10-times that seen in adults. It declined over the first 3 weeks, showing no difference between the sexes. In sexually mature animals the males had a significantly higher 1 alpha-hydroxylase activity than the female. However, this higher enzyme activity was not correlated to serum testosterone. Around the time the animals reached sexual maturity serum 1,25(OH)2D increased in both sexes. In the males this rise was correlated to an increase in serum testosterone. It is concluded that the maternal renal 1 alpha-hydroxylase activity is unchange in late pregnancy, compared to non-pregnant females. The data indicate that the fetus produces 1,25(OH)2D, and may contribute to the maternal circulating 1,25(OH)2D. The sex difference in 1 alpha-hydroxylase activity previously demonstrated is manifest at about the time of puberty.

Vitamin D (soltriol), light, and reproduction

Evidence from autoradiographic studies with 1,25(OH)2-vitamin D3 (vitamin D, soltriol) labeled with tritium and from the literature indicates that the steroid hormone soltriol regulates and modulates reproductive processes in the female, as it does in the male. Nuclear receptors for soltriol have been discovered in the uterus, oviduct, ovary, mammary gland, placenta, and fetal membranes, as well as in the pituitary and hypothalamus. Soltriol is recognized as a transducer and hormonal messenger of sunlight, acting as a somatotropic activator and modulator of vital processes for the seasonal and estival adaptation of growth, development, and procreation. Its influence on calcium equilibrium is just one of its many functions to serve this goal. This article reviews experimental, clinical, and epidemiologic evidence that suggests the involvement of soltriol in the control of reproductive processes, noting its importance for the onset of puberty, fertility, pregnancy, lactation, and probably sexual behavior. Cooperative actions between soltriol and other steroid hormones, especially estradiol, are pointed out.

Evidence that calcitonin plays a role in the postnatal increase of serum 1 alpha,25-dihydroxyvitamin D

To investigate the changes in the 1 alpha,25-dihydroxyvitamin D [1,25(OH)2D] level and the role of parathyroid hormone (PTH) and calcitonin (CT) during the early neonatal periods, we measured 1,25(OH)2D, 25-hydroxyvitamin D [25(OH)D], PTH specific for mid-regions (mPTH) and urinary cAMP (UcAMP) to evaluate the renal tubular responsiveness to intrinsic PTH and CT, as well as serum Ca and P in 28 mothers at term delivery and in their babies at birth and 5 days of age. Cord serum 1,25(OH)2D levels were low (28.8 +/- 9.2 pg/ml, mean +/- SD), while maternal serum 1,25(OH)2D levels were high (62.2 +/- 22.6 pg/ml). The low 1,25(OH)2D value increased 2.5 times (62.2 +/- 22.6 pg/ml) in 5-day-old infants, reaching a high normal adult value, concomitant with the increases in mPTH and urinary cAMP/creatinine ratio (UcAMP/Cr). The correlations between 1,25(OH)2D and UcAMP/Cr, and 1,25(OH)2D and mPTH in all paired samples of babies at birth and at 5 days of age were r = 0.456, n = 50, P less than 0.01 and r = 0.341, n = 50, P less than 0.05, respectively. These data suggest that the parathyroid activation after birth is a major factor in the rapid 1,25(OH)2D increment at that time. CT levels were high in all paired samples and in 5-day-old infants. CT vs 1,25(OH)2D showed a significant correlation (r = 0.473, P less than 0.05, n = 24) as well as the relative increase of 1,25(OH)2D (delta pg/ml) after birth vs CT at age 5 days (r = 0.537, P less than 0.01, n = 24).(ABSTRACT TRUNCATED AT 250 WORDS)

Amniotic fluid 25-hydroxyvitamin D concentrations in normal and complicated pregnancy

The concentration of 25-hydroxyvitamin D (25(OH)D) was measured in 85 samples of amniotic fluid (AF) obtained near term from mothers with normal pregnancy, diabetes, pre-eclampsia or intrahepatic cholestasis of pregnancy. Significantly lower AF 25(OH)D levels were found in diabetic mothers than in the three other groups. Our results suggest that the vitamin D status of the fetoplacental unit may be somewhat impaired in diabetic pregnancy.

Vitamin D metabolism during pregnancy

Vitamin D metabolism is altered in the pregnant animal, presumably in response to fetal demands for calcium. Circulating levels of 1,25-dihydroxyvitamin D are elevated in the pregnant animal. The stimulus of this increase and the hydroxylase(s) (placental or renal) responsible are unknown. Maternal plasma 25-hydroxyvitamin D levels have been reported to be both unchanged and decreased during pregnancy but very much dependent upon exposure to ultraviolet light and vitamin D supplementation. The major vitamin D metabolites (25-hydroxyvitamin D, 24,25-dihydroxyvitamin D, and 1,25-dihydroxyvitamin D) circulate in fetal plasma but generally at lower concentrations than in the mother (exception is the sheep). All of these metabolites are able to cross the placenta. The fetal kidney and placenta both have 25-hydroxyvitamin D1 alpha- and 24-hydroxylase activity. However, the relative contribution of mother, fetus, and placenta to fetal vitamin D metabolism has yet to be fully determined.

Can the fetus regulate its calcium uptake?

To investigate the role of the fetus in vitamin D metabolism concentrations of vitamin D metabolites, 25(OH)D, 24,25(OH)2D and 1,25(OH)2D, were measured in human umbilical artery and vein. There were no differences between artery and vein in 25(OH)D and 24,25(OH)2D levels. 1,25(OH)2D concentrations were statistically significantly higher in the artery than in the vein. It has been shown in animal experiments that 1,25(OH)2D is an important factor in the maintenance of the placental calcium gradient. We suggest that the fetus actively produces 1,25(OH)2D and hence has the capacity to control its calcium influx.

Measurement of 25-hydroxyvitamin D-1 alpha-hydroxylase activity in mammalian kidney

Until recently measurement of 25-OH-D3-1 alpha-hydroxylase activity in mammalian kidney has not been possible due to the presence of a protein which inhibits the enzyme by reducing available substrate. However, utilization of sufficient unlabeled 25-OH-D3 (80 nmol/ml renal homogenate) to overcome the effect of the inhibitor while maintaining optimal concentration for 1-hydroxylation has made quantitation of enzyme activity possible. We have modified this existing technique in order to increase the sensitivity and to permit detailed study of 1 alpha-hydroxylate regulation in mouse kidney. The modifications that we have incorporated include (i) simplifying the purification scheme for obtaining measurable 1,25-(OH)2D3 by reducing to one the necessary number of high-performance liquid chromatography steps and (ii) quantifying 1,25-(OH)2D3 by radioligand assay. The sensitivity of the assay is 10 pg, which, corrected for fractionation and recovery (50-60%), allows the measurement of 0.5 fmol 1,25-(OH)2D3 produced per milligram kidney per minute. Moreover, reliability and precision of the assay have been confirmed by demonstrating that samples from carefully matched, identically treated mice have reproducible enzyme activity (interassay coefficient of variation = 9.1%, n = 5) and show appropriate dilution characteristics. We have also demonstrated appropriate modulation of enzyme activity by known effectors of 1-hydroxylation. Kidneys from D-deficient mice exhibit significantly higher enzyme activity (15.28 +/- 1.17, n = 21) than do normal mouse kidneys (5.14 +/- 0.26, n = 33). In contrast, enzyme activity is suppressed significantly in kidneys obtained from calcium-loaded (1.20 +/- 0.04, n = 5) and parathyroidectomized animals (2.94 +/- 0.29, n = 5). Our assay now permits the indepth study of 1 alpha-hydroxylase regulation in mammalian (mouse) kidneys.

Measurement of mammalian 25-hydroxyvitamin D3 24R-and 1 alpha-hydroxylase

An in vitro assay of mammalian 25-hydroxyvitamin D3 1 alpha- and 24R-hydroxylases in kidney has been developed. It had been suggested that 25-hydroxyvitamin D binding protein present in mammalian blood and tissues inhibits the enzyme activities in cell-free preparations by binding the substrate 25-hydroxyvitamin D3 more strongly than the hydroxylases bind it. This inhibitory effect is overcome by the addition of substantial amounts of unlabeled 25-hydroxyvitamin D3 to saturate the binding sites of this protein. The resulting metabolites produced in vitro by rat kidney homogenates were isolated and firmly identified by ultraviolet absorption spectrometry and mass spectrometry as 1,25-dihydroxyvitamin D3 and (24R)-24,25-dihydroxyvitamin D3. Maximal 1 alpha-hydroxylation of 25-hydroxyvitamin D3 could be demonstrated in kidney homogenates prepared from vitamin D-deficient rats. Thyroparathyroidectomy of these rats resulted in total suppression of the 1 alpha-hydroxylase. Homogenates of kidney from rats given vitamin D showed little or no 1 alpha-hydroxylase and substantial 24R-hydroxylase activity. Thyroparathyroidectomy of these rts markedly increased the 24R-hydroxylase activity.