Stimulation of 1,25-dihydroxycholecalciferol production by prolactin and related peptides in intact renal cell preparations in vitro

A Scoping Review of the Apparent Phenomenon of the Improvement in Hypoparathyroidism in Pregnant and Postpartum Females

Hypoparathyroidism requires management with both calcium supplementation and active vitamin D to avert a state of hypocalcemia. During late gestation and the postpartum period (specifically lactation), there is an under-recognized, yet intriguing occurrence of apparent 'pseudohyperparathyroidism', whereby supplementation dosages may need to either be reduced or discontinued, to prevent hypercalcemia. The explanation for this apparent phenomenon of improved parathyroid status ('remission' or 'resolution') is incompletely understood; the purpose of this review is to analyze the case reports of this enigma within the medical (and grey) literature, providing an overall pathophysiological explanation and recommendation for the management of such patients. A literature search was conducted through PubMed/Medline, CINAHL, Cochrane Library Database, Scopus, UpToDate, Google Scholar, and the grey literature without a time-restricted period, analyzing all available articles within the literature describing an apparent improvement in parathyroid status in late-gestation and postpartum (lactating) females. Non-hypoparathyroid case reports were also included to further analyze and synthesize an overall likely pathophysiological explanation. Through the literature search, 24 papers were identified covering such a phenomenon in patients with hypoparathyroidism, alongside multiple additional reports of a similar occurrence in patients without underlying hypoparathyroidism. The pathophysiology is believed to occur due to the placental production of parathyroid hormone-related peptide (PTHrP) during gestation, with further production from the lactating mammary glands during the postpartum period. A typical pattern is observed, with increased PTHrP and suppressed PTH throughout both gestation and lactation (present in both normal and hypoparathyroid subjects). The concept of PTHrP-induced hypercalcemia is further demonstrated in patients without hypoparathyroidism, including subjects with placental hypersecretion and mammary gland enlargement. It is evident that patients with hypoparathyroidism may require a dosage reduction during late gestation and lactation, due to the risk for hypercalcemia. In addition to patients with hypoparathyroidism, this pathophysiological phenomenon occurs in unsuspecting patients, demonstrating the need for all clinicians in contact with pregnant females to be aware of this uncommon - yet perilous - occurrence.

Uptake of Phosphate, Calcium, and Vitamin D by the Pregnant Uterus of Sheep in Late Gestation: Regulation by Chorionic Somatomammotropin Hormone

Minerals are required for the establishment and maintenance of pregnancy and regulation of fetal growth in mammals. Lentiviral-mediated RNA interference (RNAi) of chorionic somatomammotropin hormone (CSH) results in both an intrauterine growth restriction (IUGR) and a non-IUGR phenotype in sheep. This study determined the effects of CSH RNAi on the concentration and uptake of calcium, phosphate, and vitamin D, and the expression of candidate mRNAs known to mediate mineral signaling in caruncles (maternal component of placentome) and cotyledons (fetal component of placentome) on gestational day 132. CSH RNAi Non-IUGR pregnancies had a lower umbilical vein−umbilical artery calcium gradient (p < 0.05) and less cotyledonary calcium (p < 0.05) and phosphate (p < 0.05) compared to Control RNAi pregnancies. CSH RNAi IUGR pregnancies had less umbilical calcium uptake (p < 0.05), lower uterine arterial and venous concentrations of 25(OH)D (p < 0.05), and trends for lower umbilical 25(OH)D uptake (p = 0.059) compared to Control RNAi pregnancies. Furthermore, CSH RNAi IUGR pregnancies had decreased umbilical uptake of calcium (p < 0.05), less uterine venous 25(OH)D (vitamin D metabolite; p = 0.055), lower caruncular expression of SLC20A2 (sodium-dependent phosphate transporter; p < 0.05) mRNA, and lower cotyledonary expression of KL (klotho; p < 0.01), FGFR1 (fibroblast growth factor receptor 1; p < 0.05), FGFR2 (p < 0.05), and TRPV6 (transient receptor potential vanilloid member 6; p < 0.05) mRNAs compared to CSH RNAi Non-IUGR pregnancies. This study has provided novel insights into the regulatory role of CSH for calcium, phosphate, and vitamin D utilization in late gestation.

Calcium Metabolism and Breast Cancer: Echoes of Lactation?

Lactation requires a series of adaptations in maternal calcium and bone metabolism to ensure a steady supply of calcium to the lactating mammary gland. The alterations in systemic metabolism are accompanied by alterations in the expression of calcium receptors, channels, binding proteins, pumps and transporters in mammary epithelial cells to increase the uptake of calcium from the extracellular fluid and to transport it into milk. Intracellular calcium regulates signaling pathways that mediate changes in cell proliferation, differentiation and death and many of the molecules involved in supporting and coordinating calcium secretion into milk are re-expressed and redeployed to support malignant behavior in breast cancer cells. In this article, we review adaptations of systemic calcium homeostasis during lactation, as well as the mechanisms of milk calcium transport. We then discuss how reactivation of these pathways contributes to the pathophysiology of breast cancer.

Maternal vitamin D levels during pregnancy and neonatal health: evidence to date and clinical implications

Low maternal vitamin D levels during pregnancy have been associated with a plethora of adverse neonatal outcomes, including small for gestational age and preterm births, detrimental effect on offspring bone and teeth development, and risk of infectious diseases. Although most observational studies indicate a significant linear relationship between maternal 25-hydroxyvitamin D and the above outcomes, some randomized controlled trials to date are inconclusive, mostly due to differences in study design and supplementation regimen. The currently available results indicate that vitamin D supplementation during pregnancy reduces the risk of preterm birth, low birth weight, dental caries of infancy, and neonatal infectious diseases such as respiratory infections and sepsis. This narrative review aims to summarize available trial results regarding the effect of low maternal vitamin D levels during pregnancy, in conjunction with neonatal outcomes on the field, with a discourse on the appropriate clinical approach of this important issue.

Maternal Mineral and Bone Metabolism During Pregnancy, Lactation, and Post-Weaning Recovery

During pregnancy and lactation, female physiology adapts to meet the added nutritional demands of fetuses and neonates. An average full-term fetus contains ∼30 g calcium, 20 g phosphorus, and 0.8 g magnesium. About 80% of mineral is accreted during the third trimester; calcium transfers at 300-350 mg/day during the final 6 wk. The neonate requires 200 mg calcium daily from milk during the first 6 mo, and 120 mg calcium from milk during the second 6 mo (additional calcium comes from solid foods). Calcium transfers can be more than double and triple these values, respectively, in women who nurse twins and triplets. About 25% of dietary calcium is normally absorbed in healthy adults. Average maternal calcium intakes in American and Canadian women are insufficient to meet the fetal and neonatal calcium requirements if normal efficiency of intestinal calcium absorption is relied upon. However, several adaptations are invoked to meet the fetal and neonatal demands for mineral without requiring increased intakes by the mother. During pregnancy the efficiency of intestinal calcium absorption doubles, whereas during lactation the maternal skeleton is resorbed to provide calcium for milk. This review addresses our current knowledge regarding maternal adaptations in mineral and skeletal homeostasis that occur during pregnancy, lactation, and post-weaning recovery. Also considered are the impacts that these adaptations have on biochemical and hormonal parameters of mineral homeostasis, the consequences for long-term skeletal health, and the presentation and management of disorders of mineral and bone metabolism.

Extrarenal expression of the 25-hydroxyvitamin D-1-hydroxylase

Like the vitamin D receptor (VDR), the CYP27B1-hydroxylase is expressed widely in human tissues. This expression profile establishes the potential for interaction of the VDR with the product of the CYP27B1, 1,25-dihydroxyvitamin D (1,25-(OH)(2)D), in either an intracrine or paracrine mode. This expansive expression profile also suggests that the local production and action of 1,25-(OH)(2)D to regulate VDR-directed gene expression may be similarly wide-ranging and distinct from what occurs in the kidney; the proximal renal tubular epithelial cell is the richest source of the CYP27B1 and the site for production of 1,25-(OH)(2)D destined to function as a hormone. Existence of the CYP27B1 at extrarenal sites has been widely documented, although the functional impact of the enzyme in these tissues has yet to be fully demonstrated. Two notable exceptions are the disease-activated macrophage (e.g., in sarcoidosis or tuberculosis) and the placenta. These two tissues are capable of generating enough 1,25-(OH)(2)D so as to be detectable in the general circulation. As such, this review will focus on CYP27B1 expression only at these two sites, theorizing that 1,25-(OH)(2)D production at these sites is for the purpose of local immunoregulatory function, not for controlling calcium balance in the host or the fetus.

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.

VITAMIN D AND HUMAN PREGNANCY

At the end of 2007, Time magazine listed the “benefits of vitamin D” as one of its top 10 medical breakthroughs for that year. Since then there has been a remarkable upsurge of interest in vitamin D, with new research advances seemingly published on a weekly basis. In particular, there has been increasing awareness of the variability of vitamin D status in populations across the globe and, significantly, a growing debate about the need for revised parameters for vitamin D supplementation. Although sub-optimal vitamin D is likely to be a widespread problem for 21stcentury societies, it is also clear that some groups are at much greater risk of low vitamin D status. Prominent amongst these are pregnant women and the aim of the following review article will be to discuss this problem in further detail with specific emphasis on its potential physiological and clinical impact.

Evidence for a role of prolactin in calcium homeostasis: regulation of intestinal transient receptor potential vanilloid type 6, intestinal calcium absorption, and the 25-hydroxyvitamin D(3) 1alpha hydroxylase gene by prolactin

Increased calcium transport has been observed in vitamin D-deficient pregnant and lactating rats, indicating that another factor besides 1,25-Dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) is involved in intestinal calcium transport. To investigate prolactin as a hormone involved in calcium homeostasis, vitamin D-deficient male mice were injected with 1,25(OH)(2)D(3), prolactin, or prolactin + 1,25(OH)(2)D(3). Prolactin alone (1 microg/g body weight 48, 24, and 4 h before termination) significantly induced duodenal transient receptor potential vanilloid type 6 (TRPV6) mRNA (4-fold) but caused no change in calbindin-D(9k). Combined treatment with 1,25(OH)(2)D(3) and prolactin resulted in an enhancement of the 1,25(OH)(2)D(3) induction of duodenal TRPV6 mRNA, calbindin-D(9k) mRNA, and an induction of duodenal calcium transport [P < 0.05 compared with 1,25(OH)(2)D(3) alone]. Because lactation is associated with an increase in circulating 1,25(OH)(2)D(3), experiments were done to determine whether prolactin also has a direct effect on induction of 25-hydroxyvitamin D(3) 1alpha hydroxylase [1alpha(OH)ase]. Using AOK B-50 cells cotransfected with the prolactin receptor and the mouse 1alpha(OH)ase promoter -1651/+22 cooperative effects between prolactin and signal transducer and activator of transcription 5 were observed in the regulation of 1alpha(OH)ase. In addition, in prolactin receptor transfected AOK B-50 cells, prolactin treatment (400 ng/ml) and signal transducer and activator of transcription 5 significantly induced 1alpha(OH)ase protein as determined by Western blot analysis. Thus, prolactin, by multiple mechanisms, including regulation of vitamin D metabolism, induction of TRPV6 mRNA, and cooperation with 1,25(OH)(2)D(3) in induction of intestinal calcium transport genes and intestinal calcium transport, can act as an important modulator of vitamin D-regulated calcium homeostasis.

Pregnancy and Sarcoidosis

Hypercalciuria with or without hypercalcemia is a well-known complication of sarcoidosis, the pathogenesis of which is not fully understood. Pregnancy is associated with physiologic alterations in calcium metabolism. These changes can further alter the derangement of calcium metabolism that occurs in sarcoidosis, if the two conditions coexist. We had the opportunity to study prospectively the changes in serum and urine calcium along with all the hormonal changes that occur during pregnancy in a young woman with sarcoidosis, who had hypercalciuria at presentation. We believe that an increased level of calcitriol is central to the calcium abnormalities in our patient. In her case, the increased calcitriol is derived from sarcoid granulomas and renal sources enhanced by the effect of estradiol and prolactin on the conversion of 25(OH)D to 1,25(OH)(2) D. She acquired hypoparathyroidism, with normal serum calcium, which probably was due to the direct suppression of parathyroid hormone (PTH) secretion by calcitriol. Finally, hypercalciuria is the result of the combined effect of hyperabsorption of calcium from the gut (the result of increased calcitriol levels leading to increased filtration of calcium) and decreased tubular reabsorption of calcium, as a result of undetectable PTH.

Bone in the pregnant mother and newborn at birth

BACKGROUND

Changes in maternal bone during pregnancy may affect fetal bone mineralization.

ISSUES

The biphasic changes in maternal bone histology (temporary loss of cancellous bone in early pregnancy restored by term gestation) are consistent with corresponding blood biochemistry changes; increased bone resorption markers in the first trimester, while bone formation markers increased in the last trimester. Postpartum bone mineral density (BMD) by DEXA is increased at cortical bone and decreased at trabecular bone sites compared with prepregnancy values. The mean reduction of spine BMD is 3.5% from prepregnancy to immediate postpartum. Neonatal bone mineral content (BMC) is different by season of birth, low weight relative to gestation, and having a diabetic mother. Lower total body BMC and high bone resorption marker in winter vs. summer-born newborns was related to low vitamin D, indicating alterations of fetal bone metabolism by maternal D deficiency. Lower BMC and decreased bone formation marker in infants born small for gestational age than those born appropriate for gestation may relate to reduced transplacental mineral transfer. Low BMC in infants of diabetic mother was correlated inversely with poor control of maternal diabetes during early pregnancy.

CONCLUSIONS

During pregnancy, maternal bone mineral metabolism are changed, and influences on fetal bone mineralization occur in utero.

Calcium and vitamin D metabolism during lactation

Calcium transfer to the fetus in late pregnancy and the subsequent transfer of calcium to milk represent the greatest challenges to calcium homeostasis in adult animals. The adaptation of the maternal calcium homeostatic mechanisms is the result of a complex interplay between calciotropic hormones and the tissues, intestine, bone, and kidney, responsible for providing the large amounts of calcium needed to support fetal skeletal growth and lactation. In this review, we will discuss general calcium homeostasis followed by a review of the specific adaptations required by the human, rat, and cow to meet fetal and lactational demands for calcium. Finally, we will review what is known about the regulation of calcium transfer from the plasma to the milk.

Homeostatic control of plasma calcium concentration

Due to the importance of Ca2+ in the regulation of vital cellular and tissue functions, the concentration of Ca2+ in body fluids is closely guarded by an efficient feedback control system. This system includes Ca(2+)-transporting subsystems (bone, and kidney), Ca2+ sensing, possibly by a calcium-sensing receptor, and calcium-regulating hormones (parathyroid hormone [PTH], calcitonin [CT], and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]). In humans and birds, acute Ca2+ perturbations are handled mainly by modulation of kidney Ca2+ reabsorption and by bone Ca2+ flow under PTH and possibly CT regulation, respectively. Chronic perturbations are also handled by the more sluggish but economic regulatory action of 1,25(OH2)D3 on intestinal calcium absorption. Peptide hormone secretion is modulated by Ca2+ and several secretagogues. The hormones' signal is produced by interaction with their respective receptors, which evokes the cAMP and phospholipase C-IP3-Ca2+ signal transduction pathways. 1,25 (OH)2D3 operates through a cytoplasmic receptor in controlling transcription and through a membrane receptor that activates the Ca2+ and phospholipase C messenger system. The calciotropic hormones also influence processes not directly associated with Ca2+ regulation, such as cell differentiation, and may thus affect the calcium-regulating subsystems also indirectly.

Influence of exogenous porcine growth hormone on vitamin D metabolism and calcium and phosphorus absorption in intact pigs

The influence of growth hormone (GH) on vitamin D metabolism and calcium and phosphorus absorption in vivo is not clear. We, therefore, measured calcium and phosphorus balance, plasma 1,25-dihydroxyvitamin D (1,25(OH)2D), and intestinal vitamin D-dependent calcium-binding protein (CaBP 9k) in intact growing pigs given exogenous GH. Six 10-week-old pigs were given two daily subcutaneous injections of 50 micrograms porcine GH/kg body weight for 2 months; six control pigs were given vehicle. They were all fed a diet containing 1.1% Ca, 0.6% P, and 1000 IU vitamin D3/kg. Apparent Ca and P absorption and retention were measured in a 10-day balance trial at the end of the 2 months. The plasma levels of Ca, P, 1,25(OH)2D, IGF-I, and GH were determined, and the duodenal and jejunal mucosal CaBP 9k content was measured at slaughter. The plasma Ca and P of GH-treated pigs were unchanged, but all aspects of mineral metabolism, including the plasma 1,25(OH)2D concentration (40%), Ca absorption and retention (70%), P absorption (33%) and retention (45%), and jejunal CaBP 9k (40%), were stimulated, in addition to an increase in the circulating IGF-I concentration.

Renal effects of growth hormone. II. Electrolyte homeostasis and body composition

Growth hormone (GH), either directly or through insulin-like growth factor-1 (IGF-1), has a wide spectrum of physiological and renal effects. This review concentrates on the effects of GH (derived from either pituitary or recombinant technology) and IGF-1 in three main areas: (1) sodium and water homeostasis; (2) calcium and phosphate balance, bone density and interactions with mineral regulating hormones; (3) fat and lean body mass. Observations of physiological changes in states of GH deficiency and excess in humans and animal models are presented. The lack of long-term toxicological data indicates that GH treatment for short stature in non-GH deficient children, with or without renal disease, should proceed with caution.

Effect of biosynthetic chicken growth hormone on egg production in White Leghorn hens

Two studies were conducted to determine if injection of recombinant chicken growth hormone (cGH) influences egg production parameters in older laying hens. In both experiments, hens were subjected to consecutive 3-wk periods consisting of 1) a preinjection period, 2) an injection period, and 3) a postinjection period. During the 3-wk injection period, hens were injected once daily with either 5, 50 or 500 micrograms/kg BW of cGH or physiological saline (vehicle) alone (control group). In both experiments, injection of cGH did not affect (P greater than .05) percentage of daily egg production per hen, Haugh units, or BW during the injection period compared with preinjection hens. In Experiment 1, but not Experiment 2, control hens had reduced shell thickness and feed consumption during the injection and postinjection periods when compared with the preinjection controls. These reductions were not observed in hens receiving injections of cGH (all doses).

Effects of estrone and estradiol-17 beta on 25-hydroxycholecalciferol hydroxylase activities in female Japanese quail

Equimolar amounts of estrone and estradiol-17 beta injected into sexually immature female Japanese quail caused comparable increases and decreases in the renal activity of 25-hydroxycholecalciferol (25HCC) 1 alpha-hydroxylase and 25-hydroxycholecalciferol 24-hydroxylase, respectively. Peak activity of the former enzyme was induced by both estrogens within 6 hr and had declined by 24 hr. Plasma concentrations of estrone and estradiol following the injection were maximal within 3-6 hr of injecting the steroid i.m. Following [3H]estradiol injection, 25% of the radioactivity was located with estrone in the plasma following separation by TLC. Conversely, following [3H]estrone injection 30% of the radioactivity in plasma was located with estradiol. Thus, both estrone and estradiol may have physiological effects in quail, which in part may be due to their in vivo interconversion. Evidence that prolactin may be mediating the estrogenic stimulation of the 25HCC 1 alpha-hydroxylase is also presented.

Remission of hypoparathyroidism during lactation: evidence for a physiological role for prolactin in the regulation of vitamin D metabolism

We studied a young woman with surgical hypoparathyroidism who, on her usual maintenance dose of calcitriol, developed hypercalcaemia 9 d postpartum when lactation was established. Serum values of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) values were very high (127 pg/ml). The patient remained without exogenous calcitriol treatment for 40 d, during which time serum 1,25(OH)2D3 levels remained within the normal range and serum calcium fell with a half-time of 27 d. The requirements for calcitriol increased to antepartum levels when lactation had ceased. There was a close negative correlation between requirements for calcitriol and serum PRL values. After weaning, an episode of hypercalcaemia was induced by increasing the dose of calcitriol. On stopping calcitriol the serum 1,25(OH)2D3 fell to low values (4 pg/ml) within 2 d and serum calcium fell with a half-time of 3 d, necessitating the early reintroduction of calcitriol. We conclude that in hypoparathyroidism exogenous vitamin D requirements fall during lactation because of enhanced endogenous production of 1,25(OH)2D3. The lactation-associated increase in circulating 1,25(OH)2D3 concentrations thus results from a parathyroid hormone-independent mechanism, possibly by an effect of PRL on the 1 alpha-hydroxylase.

Pregnancy as state of physiologic absorptive hypercalciuria

An increase in circulating, 1,25-dihydroxyvitamin D level and net intestinal calcium absorption have been previously demonstrated in pregnant women and have been widely regarded as compensatory mechanisms whereby fetal mineral demands are satisfied. The alternate possibility, that these adjustments might anticipate such demands, has not previously been considered. To examine the effects of pregnancy on the intestinal absorption and renal excretion of calcium, oral calcium tolerance tests were performed and urinary calcium excretion was measured in 16 healthy women receiving a moderate calcium intake during and after pregnancy. Circulating 1,25-dihydroxyvitamin D levels and indexes of parathyroid function were also measured. As expected, 1,25-dihydroxyvitamin D levels were significantly (p less than 0.05) elevated throughout pregnancy (94 +/- 11, 118 +/- 9, and 117 +/- 11 pg/ml in the first, second, and third trimesters, respectively, versus 51 +/- 5 pg/ml after delivery). Twenty-four-hour calcium excretion also increased sharply (247 +/- 54, 316 +/- 42, 300 +/- 61 mg versus 91 +/- 18 mg), often to the point of hypercalciuria. Calcium tolerance test results included significant increases in the calciuric and calcemic responses during each trimester, whereas fasting calcium excretion and parathyroid function remained normal. These findings portray normal pregnancy as a state of physiologic absorptive hypercalciuria and call into question the widespread practice of supplementing calcium intake in otherwise well-nourished women during pregnancy.

Hypercalciuria in hyperprolactinemic rats: effects of benzthiazide

There is evidence that prolactin (PRL) excess plays a role in the etiology of osteoporosis associated with human prolactinoma. Calcium balance in human hyperprolactinemia has not been thoroughly investigated. In the present study, rats with excess circulating PRL levels (male anterior pituitary-grafted Fischer 344 rats) had urinary calcium excretion twice that of control rats (4.16 +/- 0.43 v 2.25 +/- 0.30 mg/24h X 100 g BW). Calcium excretion expressed per mg of calcium intake was also high in pituitary-grafted rats. The excess calcium excretion in hyperprolactinemic rats was not accompanied by a concomitant rise in sodium excretion. This dissociation suggests that PRL has an effect on the renal handling of calcium. Since thiazide diuretics have a well-described hypocalciuric action, their effect was tested in these rats. In normal rats, benzthiazide, a long-acting agent, significantly reduced urinary calcium excretion in a dose-dependent fashion. Hyperprolactinemic rats responded to benzthiazide in a manner similar to control rats. In pituitary-grafted rats, benzthiazide also decreased the calcium excretion to intake ratio and normalized the calcium to sodium excretion ratio. Since the hypercalciuria of experimental hyperprolactinemia can be corrected by thiazide diuretics, these agents may have therapeutic potential in human PRL excess.

Specific binding of 1,25 dihydroxyvitamin D3 in lymphocytes

We examined ten cellular or tissue sources of lymphocytes for specific binding of 1,25(OH)2D3, the hormonally active form of vitamin D3. A specific-binding protein was found in three of these sources. Scatchard analysis of cytosol from a follicular lymphoma cell line revealed binding sites with a Kd of 7.0 X 10(-11) and a receptor concentration of 6.6 fmol/mg protein. Sucrose density centrifugation of 3H-1,25(OH)2D3 labeled cytosol showed a 3.75 peak which was absent in cytosols incubated with excess nonradioactive 1,25(OH)2D3. The relative amounts of vitamin D3 metabolites required to displace 50% of the specifically bound 3H-1,25(OH)2D3 were 1,25(OH)2D3: 1,24,25(OH)3D3: 25(OH)D3: 24,25(OH)2D3 = 1: 180: 1000: 2700. Excess vitamin D3, cortisol, and estradiol failed to displace 3H-1,25(OH)2D3. Scatchard analysis of spleen cytosol from a patient with prolymphocytic transformation of chronic lymphocytic leukemia demonstrated a binding protein with a Kd of 1.2 X 10(-10) and a receptor concentration of 0.2 fmol/mg protein. DNA cellulose binding confirmed the presence of the specific-binding protein in this cytosol. Specific binding of 3H-1,25(OH)2D3 was also quantitated in a cell line from a patient with Burkitt's lymphoma with a Kd of 0.3 X 10(-10) and a receptor concentration of 29.6 fmol/mg protein. No specific binding of 3H-1,25(OH)2D3 was observed in lymphocytes from seven other malignant and nonmalignant sources. These results are the first to demonstrate a specific-binding protein for 1,25(OH)2D3 in lymphocytes from tissue and from these specific cell lines. The presence of this protein in some lymphocytes but not others may reflect the state of activation of the lymphocytes.

Effect of 1,25-dihydroxyvitamin D3 on plasma prolactin in patients with renal failure on regular dialysis treatment

Basal plasma prolactin (PRL) has been measured in 6 patients with chronic renal failure 3 months before and at monthly intervals during a 3 months 1,25-dihydroxyvitamin D3 [1,25 (OH)2-D] treatment. Plasma PRL decreased rapidly during, 1,25 (OH)2-D treatment, while it remained unchanged during the 3-months control period. There was no correlation between PRL and parathyroid hormone (PTH; measured either with carboxy- or amino-terminal assays) and several other parameters. A direct effect of 1,25 (OH)2-D on PRL secretion may exist.

Specific binding of 1,25-dihydroxycholecalciferol in human medullary thyroid carcinoma

A specific 1,25-dihydroxycholecalciferol-binding protein has been detected in high-salt cytosols prepared from human medullary thyroid carcinomas. The binding protein had the same equilibrium dissociation constant (Kd = 0.17 +/- 0.05 nM; n = 4) and sedimentation coefficient on sucrose gradients (3.7S) as than seen in established vitamin D target tissues. This protein was not detected in normal thyroid cytosols, which may reflect the low proportion of C-cells within the gland.

Stimulation of intestinal calcium transport and bone calcium mobilization by prolactin in vitamin D-deficient rats

In vitamin D-deficient rats intestinal calcium transport increased significantly 4 hours after an injection of prolactin, reached a maximum after 8 hours, and declined to preinjection levels after 24 hours. Similarly, in vitamin D-deficient rats fed a diet low in calcium or phosphorus prolactin stimulated an increase in serum calcium in both groups and an increase in serum phosphorus in the rats fed the diet low in phosphorus. Thus it appears that prolactin affects organs involved in calcium regulation in a manner that is independent of the vitamin D endocrine system.

Interrelations of calcium-regulating hormones during normal pregnancy

Profound changes in calcium metabolism occur during pregnancy. The mother has to make available extra calcium for fetal requirements while ensuring that her plasma and bone calcium concentrations are satisfactorily maintained. In a cross-sectional study plasma concentrations of the major calcium-regulating hormones--namely, calcitonin, parathyroid hormone, 25-hydroxyvitamin D (25-OHD), and 1,25-dihydroxyvitamin D (1,25-(OH)2D)--were measured to establish their interrelations during normal pregnancy. The major changes observed were increases in the circulating concentrations of 1,25-(OH)2D and calcitonin. Concentrations of parathyroid hormone and 25-OHD remained within the normal range. The increased concentrations of 1,25-(OH)2D enable the increased physiological need for calcium to be met by enhancing intestinal absorption of this element. The simultaneous rise in calcitonin opposes the bone-resorbing activities of 1,25-(OH)2D, thereby protecting the integrity of the maternal skeleton. Maternal calcium homeostasis is thus maintained yet the requirements of the fetus are fulfilled.