Response of chick parathyroid glands to the vitamin D metabolites, 1,25-dihydroxycholecalciferol and 24,25-dihydroxycholecalciferol

The dynamic relationships between the active and catabolic vitamin D metabolites, their ratios, and associations with PTH

Vitamin D status, assessed by serum concentration of 25(OH)D, is the prime candidate marker for many disease-association studies, but the interplay between the subsequent 1,25-dihydroxyvitamin D (1,25(OH)₂D) and 24,25-dihydroxyvitamin D (24,25(OH)₂D) metabolites is unclear. In this study, we conducted an analysis from a large cohort of healthy, physically fit, young army recruits (n = 940). We found a significant, inverse relationship between serum 25(OH)D and 1,25(OH)₂D:24,25(OH)₂D vitamin D metabolite ratio (VMR) (r²Exp = 0.582, p < 0.0001), and demonstrated a significant association with increasing PTH concentration (p < 0.001). Circannual rhythms were evident for all vitamin D metabolites and VMRs except for 1,25(OH)₂D when fitted to Cosinor curves. We estimated 1,25(OH)₂D:24,25(OH)₂D VMR of ≥35 to be the threshold value for vitamin D insufficiency, and ≥51 to be predictive of vitamin D deficiency. Our three-dimensional model provides mechanistic insight into the vitamin D-PTH endocrine system, and further substantiates the role of 24,25(OH)₂D in human physiology. The model sets a new paradigm for vitamin D treatment strategy, and may help the establishment of vitamin D-adjusted PTH reference intervals. The study was approved by the UK Ministry of Defence research ethics committee (MODREC 165/Gen/10 and 692/MoDREC/15). ClinicalTrials.gov Identifier NCT02416895.

CYP24A1 loss of function: Clinical phenotype of monoallelic and biallelic mutations

CYP24A1, encoding the vitamin D-24-hydroxylase, is of major clinical and physiologic importance, serving to regulate the catabolism of 1,25-(OH)₂D, the physiologically active vitamin D metabolite. In addition to facilitating catabolism of 1,25-(OH)₂D, CYP24A1 also enhances the turnover and elimination of 25-OHD, the abundant precursor metabolite and storage form of the vitamin. CYP24A1 can be stimulated hormonally by 1,25-(OH)₂D and by FGF23, whereas CYP27B1, encoding the vitamin D-1α-hydroxylase, is stimulated hormonally by parathyroid hormone (PTH) and downregulated by FGF23. Thus CYP24A1 and CYP27B1, together, provide for alternate and regulated fates of 25-OHD, and control the availability of the active metabolite, 1,25-(OH)₂D, depending upon physiologic needs. These two enzymes, are therefore central to the homeostatic control of vitamin D metabolism, and as a result affect calcium metabolism in critical ways. Disruption of CYP24A1 in mice results in elevated circulating 1,25-(OH)₂D, substantiating the importance of the enzyme in the maintenance of vitamin D metabolism. The consequential skeletal phenotype in these mice further demonstrates the biologic sequelae of the disruption of the vitamin D pathway, and illustrates a specific developmental pathology mediated largely by oversupply of 1,25-(OH)₂D. More recent evidence has identified loss of function mutations in CYP24A1 in association with hypercalcemia, hypercalciuria and nephrolithiasis in humans. Initial reports described certain variant mutations in CYP24A1 as an unrecognized cause of "Idiopathic Infantile Hypercalcemia," and more recently older children and adults have been identified with a similar phenotype. Over 25 likely disease-causing variants are described. Homozygous and compound heterozygote mutations account for the overwhelming majority of cases, however the heterozygous loss-of-function mutations of CYP24A1 do not appear to consistently result in symptomatic hypercalcemia. Considerations ripe for exploration include the potential role for such mutations in the tolerance to challenges to the calcium homeostatic system, such as changes in dietary calcium intake, vitamin D supplementation, sunlight exposure or pregnancy.

Vitamin D and the kidney

The kidney is essential for the maintenance of normal calcium and phosphorus homeostasis. Calcium and inorganic phosphorus are filtered at the glomerulus, and are reabsorbed from tubular segments by transporters and channels which are regulated by 1α,25-dihydroxyvitamin (1α,25(OH)(2)D) and parathyroid hormone (PTH). The kidney is the major site of the synthesis of 1α,25(OH)(2)D under physiologic conditions, and is one of the sites of 24,25-dihydroxyvitamin D (24,25(OH)(2)D) synthesis. The activity of the 25(OH)D-1α-hydroxylase, the mixed function oxidase responsible for the synthesis of 1α,25(OH)(2)D, is regulated by PTH, 1α,25(OH)(2)D, fibroblast growth factor 23 (FGF23), inorganic phosphorus and other growth factors. Additionally, the vitamin D receptor which binds to, and mediates the activity of 1α,25(OH)(2)D, is widely distributed in the kidney. Thus, the kidney, by regulating multiple transport and synthetic processes is indispensible in the maintenance of mineral homeostasis in physiological states.

25-Hydroxyvitamin D(3) is an agonistic vitamin D receptor ligand

25-Hydroxyvitamin D(3) 1alpha-hydroxylase encoded by CYP27B1 converts 25-hydroxyvitamin D(3) into 1alpha,25-dihydroxyvitamin D(3), a vitamin D receptor ligand. 25-Hydroxyvitamin D(3) has been regarded as a prohormone. Using Cyp27b1 knockout cells and a 1alpha-hydroxylase-specific inhibitor we provide in four cellular systems, primary mouse kidney, skin, prostate cells and human MCF-7 breast cancer cells, evidence that 25-hydroxyvitamin D(3) has direct gene regulatory properties. The high expression of megalin, involved in 25-hydroxyvitamin D(3) internalisation, in Cyp27b1(-/-) cells explains their higher sensitivity to 25-hydroxyvitamin D(3). 25-Hydroxyvitamin D(3) action depends on the vitamin D receptor signalling supported by the unresponsiveness of the vitamin D receptor knockout cells. Molecular dynamics simulations show the identical binding mode for both 25-hydroxyvitamin D(3) and 1alpha,25-dihydroxyvitamin D(3) with the larger volume of the ligand-binding pocket for 25-hydroxyvitamin D(3). Furthermore, we demonstrate direct anti-proliferative effects of 25-hydroxyvitamin D(3) in human LNCaP prostate cancer cells. The synergistic effect of 25-hydroxyvitamin D(3) with 1alpha,25-dihydroxyvitamin D(3) in Cyp27b1(-/-) cells further demonstrates the agonistic action of 25-hydroxyvitamin D(3) and suggests that a synergism between 25-hydroxyvitamin D(3) and 1alpha,25-dihydroxyvitamin D(3) might be physiologically important. In conclusion, 25-hydroxyvitamin D(3) is an agonistic vitamin D receptor ligand with gene regulatory and anti-proliferative properties.

Influence of parathyroid mass on the regulation of PTH secretion

In advanced uremia, parathyroid hormone (PTH) levels should be controlled at a moderately elevated level in order to promote normal bone turnover. As such, a certain degree of parathyroid gland (PG) hyperplasia has to be accepted. No convincing evidence of apoptosis or of involution of PG hyperplasia exists. However, even considerable parathyroid hyperplasia can be controlled when the functional demand for increased PTH levels is abolished. When 20 isogenic PG were implanted into one parathyroidectomized (PTX) rat normalization of Ca(2+) and PTH levels and normal suppressibility of PTH secretion by high Ca(2+) was obtained. Similarly, normal levels of Ca(2+) and PTH and suppressibility of PTH secretion were obtained when Eight isogenic PG from uremic rats were implanted into normal rats or when long-term uremia and severe secondary hyperparathyroidism (sec. HPT) was reversed by an isogenic kidney transplantation. Normalization of PTH levels after experimental kidney transplantation took place despite a persistent decrease of vitamin D receptor (VDR) mRNA and calcium sensing receptor (CaR) mRNA in PG. Thus, in experimental models PTH levels are determined by the functional demand and not by parathyroid mass, per se. When non-suppressible sec. HPT is present in patients referred to PTX, nodular hyperplasia with differences in gene expression between different nodules has been observed in most cases. An altered expression of some autocrine/paracrine factors has been demonstrated in the nodules. Enhanced expression of PTH-related peptide (PTHrP) has been demonstrated in PG from patients with severe secondary HPT. PTHrP has been shown to stimulate PTH secretion in vivo and in vitro. PTH/PTHrP receptor was demonstrated in the parathyroids. The low Ca(2+) stimulated PTH secretion was enhanced by 300% by PTHrP 1-40. The altered quality of the parathyroid mass and not only the increased parathyroid mass, per se, might be responsible for non-controllable hyperparathyroidism in uremia and after kidney transplantation.

Basic Science and Dialysis: Parathyroid Growth and Suppression in Renal Failure

In advanced uremia, parathyroid hormone (PTH) levels should be controlled at a moderately elevated level in order to promote normal bone turnover. As such, a certain degree of parathyroid hyperplasia has to be accepted. Uremia is associated with parathyroid growth. In experimental studies, proliferation of the parathyroid cells is induced by uremia and further promoted by hypocalcemia, phosphorus retention, and vitamin D deficiency. On the other hand, parathyroid cell proliferation might be arrested by treatment with a low-phosphate diet, vitamin D analogs, or calcimimetics. When established, parathyroid hyperplasia is poorly reversible. There exists no convincing evidence of programmed parathyroid cell death or apoptosis in hyperplastic parathyroid tissue or of involution of parathyroid hyperplasia. However, even considerable parathyroid hyperplasia can be controlled when the functional demand for increased PTH levels is removed by normalization of kidney function. Today, secondary hyperparathyroidism can be controlled in patients with long-term uremia in whom considerable parathyroid hyperplasia is to be expected. PTH levels can be suppressed in most uremic patients and this suppression can be maintained by continuous treatment with phosphate binders, vitamin D analogs, or calcimimetics. Thus modern therapy permits controlled development of parathyroid growth. When nonsuppressible secondary hyperparathyroidism is present, nodular hyperplasia with suppressed expression of the calcium-sensing receptor (CaR) and vitamin D receptor (VDR) has been found in most cases. An altered expression of some autocrine/paracrine factors has been demonstrated in the nodules. The altered quality of the parathyroid mass, and not only the increased parathyroid mass per se, might be responsible for uncontrollable hyperparathyroidism in uremia and after kidney transplantation.

Parathyroid cell growth in patients with advanced secondary hyperparathyroidism: vitamin D receptor, calcium sensing receptor, and cell cycle regulating factors

The parathyroid gland (PTG) is a unique endocrine organ in which the quiescent glandular cells begin to proliferate in response to the demand for maintaining calcium (Ca) homeostasis in the progressive course of renal failure, leading to secondary hypereparathyroidism (SHPT). SHPT is characterized with continuous over-secretion of parathyroid hormone (PTH) and high turn-over bone disease, osteitis fibrosa, and the major factors include a deficiency of active vitamin D, hypocalcemia, and phosphate retention. With long-term end-stage renal failure, SHPT becomes resistant to conventional medical treatment such as phosphate binders and active vitamin D supplementation, and the growth of the PTG accelerates with the pattern of hyperplasia changing from diffuse to nodular type. In this process, the sigmoid curve between extracellular Ca concentration (exCa) and the plasma level of PTH shifts to the upper-rightward, indicating both an absolute increase in PTH secretion and the resistance of PT cells to exCa. Many experimental and human studies have revealed down-regulation of vitamin D receptor (VDR), calcium-sensing receptor (CaSR), and retinoid X receptor (RXR) in PT cells. The sustained proliferation of PT cells after obtaining autonomicity is another characteristic feature of SHPT. In this context, it has been demonstrated that the cell cycle is markedly progressed, where the expression of cyclin-dependent kinase inhibitor (CDKI), p21 and p27, is depressed in a VDR-dependent manner. These pathological features are most evident in nodular hyperplasia, in which monoclonal proliferation is obvious, indicating the phenotypic changes have occured in PT cells. It has been observed by Fukagawa and colleagues that pharmacologically high dose of active vitamin D administered orally can cause small-size PTG hyperplasia to regress in patients with advanced SHPT. Successful renal transplantation may also restore VDR and CaSR expressions in the diffuse type, in association with increasing TUNEL-positive cells. Thus, it is important to vigorously treat SHPT when the PT cell proliferation is in the reversible stage of diffuse hyperplasia.

Can hyperparathyroid bone disease be arrested or reversed?

Parathyroid hyperplasia, oversecretion of parathyroid hormone (PTH), and hyperparathyroid bone disease are characteristic features of chronic uremia; they develop early in the course of uremia and often in a progressive way. This review focuses on the potential for arrest or regression of hyperparathyroid-induced bone disease. For this purpose, the review addresses investigations that have used bone histology and not investigations that indirectly attempted to demonstrate changes in the skeleton by measurements of bone mineral density or laboratory indices of bone turnover, other than PTH. A prerequisite for inducing regression of the hyperparathyroid bone disease is a significant suppression of PTH secretion or reversal of hyperparathyroidism and uremia. It is concluded, on the basis of paired bone biopsy studies in patients with established hyperparathyroid bone disease, that bone histology can be improved or normalized after treatment that diminishes PTH levels. Oversuppression of PTH levels, however, might lead to adynamic bone disease.

Involution of the parathyroid glands after renal transplantation

PURPOSE OF REVIEW

The aim of this article is to review the most recent development on the reversibility of secondary hyperparathyroidism after kidney transplantation. A successful kidney transplantation is expected to correct the abnormalities of mineral metabolism that during uremia lead to secondary hyperparathyroidism. Kidney transplanted patients might, however, still present persistent hyperparathyroidism and hypercalcemia. In order to improve the understanding of the fate of secondary hyperparathyroidism after kidney transplantation an experimental model on reversal of uremia by an experimental isogenic kidney transplantation was established.

RECENT FINDINGS

In recent years clinical and experimental studies have suggested an important role of the calcium sensing receptor and vitamin D receptor in the parathyroid glands for the abnormal regulation of parathyroid hormone secretion and parathyroid cell proliferation in uremia. The expression of these receptors is diminished in the parathyroid glands of uremic patients with severe secondary hyperparathyroidism and in experimental models of uremic rats on a high phosphorus diet. Secondary hyperparathyroidism is reversed rapidly by reversal of uremia by an experimental kidney transplantation in the rat. Despite normalization of the circulating parathyroid hormone levels, diminished expression of parathyroid calcium sensing and vitamin D receptor messenger RNA persist. Implantation of several isogenic parathyroid glands into a single rat results in a transient, short lasting period of hypercalcemia followed by normalization of parathyroid hormone and plasma calcium levels, despite persistent increased parathyroid mass.

SUMMARY

Advances are clearly being made in the understanding of the molecular mechanisms of disturbed parathyroid function in uremia. How the hyperplastic uremic parathyroid glands are regulated after reversal of uremia by kidney transplantation remains, however, to be elucidated.

Update on biological actions of 1alpha,25(OH)2-vitamin D3 (rapid effects) and 24R,25(OH)2-vitamin D3

All biologic responses to vitamin D are now known to arise as a consequence of the metabolism of this seco-steroid into its two principal biologically active metabolites 1alpha,25(OH)(2)-vitamin D(3) (1ALPHA;,25(OH)(2)D(3)) and 24R,25(OH)(2)-vitamin D(3) (24R,25(OH)(2)D(3)). 1alpha,25(OH)(2)D(3) is the dominant metabolite and produces a wide array of biological responses via interacting both with the classical vitamin D nuclear receptor (VDR(nuc)) that regulates gene transcription in over 30 target organs and with a putative cell membrane receptor (VDR(mem1,25)) that mediates rapid (within seconds to minutes) biological responses. Ligand occupancy of VDR(mem1,25) is linked to signal transduction systems that can mediate the opening of Ca(2+) and chloride voltage gated channels as well as activation of MAP-kinase. MAP-kinase activation in some cells containing VDR(mem1,25)+VDR(nuc) then results in "cross-talk" from VDR(mem1,25) to VDR(nuc) which modulates transactivation of 1alpha,25(OH)(2)D(3) responsive gene promoters. The 24R,25(OH)(2)D(3) metabolite has been shown to be an essential hormone for the process of bone fracture healing. The activity of the enzyme responsible for the production of 24R,25(OH)(2)D(3), the renal 25(OH)D-24-hydroxylase, becomes elevated within 4-11 days after imposition of a tibial fracture, thereby increasing the blood concentrations of 24R,25(OH)(2)D(3) by threefold. The 24R,25(OH)(2)D(3) likely initiates its biological responses via binding to the ligand binding domain of a second cell membrane receptor, the VDR(mem24,25), which is stereospecific for 24R,25(OH)(2)D(3) in comparison with 24S,25(OH)(2)D(3) and 1alpha,25(OH)(2)D(3). This report summarizes the status of several current research frontiers in this arena of the vitamin D endocrine system.

Human parathyroid cell proliferation in response to calcium, NPS R-467, calcitriol and phosphate

It remains uncertain how calcium, phosphate and calcitriol regulate parathyroid cell growth. The present study was aimed at examining possible direct effects of these modulators and of the calcimimetic NPS R-467 on parathyroid cell growth in vitro. Cell proliferation was determined by [3H]thymidine incorporation and cell cycle antigen Ki 67 expression in a parathyroid cell culture model derived from uraemic patients. The effect of NPS R-467 on parathyroid hormone (PTH) secretion and intracellular [Ca2+]i response was also examined. Increasing the [Ca2+] in the medium from 0.5 to 1.7 mM increased DNA synthesis (P < 0.005) and the number of Ki 67-positive cells (P < 0.005). However, NPS R-467 (0.01-1 microM) inhibited 3[H]thymidine incorporation by 35% in the presence of 0.5 mM [Ca2+]e. Exposure of cells to Ca2+ or NPS R-467 led to a rapid increase of intracellular Ca2+, although the pattern of increase differed. Addition of calcitriol (10-10-10-7 M) to the culture medium suppressed [3H]thymidine incorporation dose-dependently. Finally, high levels of phosphate (3.5 mM) in the medium led to a significant (P < 0.05) increase in [3H]thymidine incorporation. The observed stimulatory effect of Ca2+ in the medium in vitro appears to be at variance with the inhibitory effect of calcimimetic NPS R-467 in vitro. In an attempt to solve these apparent discrepancies, and based on the notion of a reduced calcium-sensing receptor (CaR) expression in parathyroid tissues of uraemic patients, we hypothesize that Ca2+ may regulate parathyroid cell proliferation via two different pathways, with predominant growth inhibition in cases of high CaR expression or activation, but prevailing stimulation of proliferation in cases of low CaR expression.

24,25-Dihydroxyvitamin D(3) and bone metabolism

The 1alpha-hydroxylated metabolite of 25-hydroxyvitamin D(3), 1,25-dihydroxyvitamin D(3), is the biologically most active metabolite of vitamin D. The 24-hydroxylated metabolites were generally considered as degradation products of a catabolic pathway finally leading to excretion of calcitroic acid. Studies with analogues fluorinated at the C-24 position did not indicate a physiological function for 24R,25(OH)(2)D(3). Nevertheless throughout the years various studies showed biologic effects of other metabolites than 1alpha,25(OH)(2)D(3). In particular the metabolite 24R,25(OH)(2)D(3) has been functionally analyzed, e.g. with respect to a role in normal chicken egg hatchability and effects on chondrocytes in the resting zone of cartilage. Numerous studies have shown the presence of the vitamin D receptor in bone cells and effects of 1alpha,25(OH)(2)D(3) on bone and bone cells. Also for 24R,25(OH)(2)D(3) studies have been performed focusing on effects on bone and bone cells. The purpose of this review is to summarize the data regarding 24R,25(OH)(2)D(3) and bone and to evaluate its role in bone biology.

Failure of high doses of calcitriol and hypercalcaemia to induce apoptosis in hyperplastic parathyroid glands of azotaemic rats

BACKGROUND

Whether calcitriol administration, which is used to treat secondary hyperparathyroidism in dialysis patients, induces regression of parathyroid-gland hyperplasia remains a subject of interest and debate. If regression of the parathyroid gland were to occur, the presumed mechanism would be apoptosis. However, information on whether high doses of calcitriol can induce apoptosis of parathyroid cells in hyperplastic parathyroid glands is lacking. Consequently, high doses of calcitriol were given to azotaemic rats and the parathyroid glands were evaluated for apoptosis.

METHODS

Rats were either sham-operated (two groups) or underwent a two-stage 5/6 nephrectomy (three groups). For the first 4 weeks, all rats were given a high (1.2%) phosphorus (P) diet to stimulate parathyroid gland growth and then were changed to a normal (0.6%) P diet for 2 weeks. At week 7, three of the five groups were given high doses of calcitriol (500 pmol/100 g body weight) intraperitoneally every 24 h during 72 h before sacrifice. The five groups during week 7 were: (i) normal renal function (NRF)+0.6% P diet; (ii) NRF+0.6% P+calcitriol; (iii) renal failure (RF)+0.6% P; (iv) RF+1.2% P+calcitriol; and (v) RF+0.6% P+calcitriol. Parathyroid glands were removed at sacrifice and the TUNEL stain was performed to detect apoptosis.

RESULTS

At sacrifice, the respective serum calcium values in calcitriol-treated groups (groups 2, 4, and 5) were 15.52+/-0.26, 13.41+/-0.39 and 15.12+/-0.32 mg/dl. In group 3, PTH was 178+/-42 pg/ml, but in calcitriol-treated groups, PTH values were suppressed, 8+/-1 (group 2), 12+/-2 (group 4), and 7+/-1 pg/ml (group 5). Despite, the severe hypercalcaemia and marked PTH suppression in calcitriol-treated groups, the percentage of apoptotic cells in the parathyroid glands was very low (range 0.08+/-0.04 to 0.25+/-0.20%) and not different among the five groups.

CONCLUSIONS

We found no evidence in hyperplastic parathyroid glands that apoptosis could be induced in azotaemic rats by the combination of high doses of calcitriol and severe hypercalcaemia despite the marked reduction in PTH levels that was observed.

Normocalcemic hyperparathyroidism in Vietnamese immigrants living in southern California

OBJECTIVE

To characterize the normocalcemic hyperparathyroidism in Vietnamese immigrants living in southern California.

METHODS

Of 14 Vietnamese patients with primary hyperparathyroidism who were observed between 1991 and 1996, 50% (7 patients; 2 men and 5 women) had normal and/or fluctuating levels of serum total calcium. When the serum calcium was corrected for the albumin, the "corrected" calcium was lower than the measured serum total calcium. Their mean age was 56.4 +/- 11.4 years. All patients had normal serum levels of albumin and serum phosphate.

RESULTS

Women were affected more often than men by a ratio of 5:2. The serum-ionized calcium as well as intact PTH were increased in all patients. Five patients underwent surgery with confirmation of parathyroid adenomas. Two patients refused surgery. They did not have osteitis fibrosa cystica by radiological examination. One patient had low plasma levels of 25-hydroxyvitamin D. Five of 7 normocalcemic patients (70%) were born in the month of December compared with 2 of seven hypercalcemic patients (30%).

CONCLUSION

The blood ionized calcium and intact parathyroid hormone are necessary for confirmation of normocalcemic hyperparathyroidism. Most of our normocalcemic hyperparathyroid patients (70%) were born in the month of December. We postulate that a combination of exposure to solar ultraviolet light during the formation of the fetal parathyroid glands and stimulation from low vitamin D levels in the wintertime may be related to the development of hyperparathyroidism. However, it is difficult to prove a definite correlation between normocalcemic hyperparathyroidism and their month of birth (December), especially when these observations were seen in a small group of patients.

Effects of 24R,25-dihydroxyvitamin D3 on alkaline phosphatase activity in pig renal epithelial LLC-PK1 cells in culture

1. The effects of 24R,25-dihydroxyvitamin D3 [24,25(OH)2D3] on alkaline phosphatase activity (ALP) were evaluated in pig kidney LLC-PK1 cells in culture. 2. The vitamin D3 metabolite increased ALP activity in these cells, whereas no effect of the hormone was observed on gamma-glutamyltranspeptidase and acid phosphatase activities. 3. ALP activity was stimulated after 3- to 12-hr incubation in the presence of 10(-9) mol/l 24,25(OH)2D3 with a maximum after 6 hr. 4. The hormonal induction of ALP activity was prevented by pretreatment of cells by actinomycin D. 5. It is proposed that 24,25(OH)2D3 could increase ALP activity by de novo protein synthesis.

Reversibility of experimental secondary hyperparathyroidism

Chronic uremia is associated with secondary hyperparathyroidism (HPT). The purpose of the present investigation was to study the reversibility of secondary HPT after reversal of uremia by an isogenic kidney transplantation in the rat. Secondary HPT was induced in two models: Model A comprised 5/6 nephrectomized rats kept on a standard diet (N = 12; PTH 210 +/- 43 pg/ml; plasma urea 24 +/- 2 mmol/liter; and normal control rats, N = 12; PTH 45 +/- 5 pg/ml; plasma urea 6 +/- 0.2 mmol/liter); and Model B comprised 5/6 nephrectomized rats kept on a high phosphorus diet (N = 12; PTH 769 +/- 157 pg/ml; plasma urea 18 +/- 2 mmol/liter). The parathyroid function was examined by measuring the secretory response of PTH to an acute induction of hypo- and hypercalcemia. Acute hypocalcemia in the hyperphosphatemic uremic rats did not significantly increase serum PTH levels (N = 6, delta Ca2+ -0.56 mmol/liter; maximal PTH 1045 +/- 164 pg/ml; basal PTH 690 +/- 134 pg/ml; NS). During hypercalcemia the PTH levels were significantly higher than in the normal controls (N = 6; minimal PTH 24 +/- 5 pg/ml vs. normal controls 5 +/- 0.2 pg/ml, P < 0.05). After 20 weeks of uremia, the uremia was reversed by the isogenic kidney transplantation. One week after reversal of the uremia the PTH levels became normal in both models A and B (28 +/- 6 and 63 +/- 16 pg/ml, respectively) and the kidney transplanted rats from model B had a normal secretory response of PTH to both hypo- and hypercalcemia. To study whether both parathyroid cell hypertrophy and hyperplasia could be down-regulated, 8 uremic glands (N = 9) or 20 normal glands (N = 6) were implanted into one normal rat. Within two weeks the rats regained normocalcemia and PTH levels remained normal from the third day after the increase of glandular mass. The 20 gland rats all had normal PTH suppressibility in response to calcium (minimal PTH 5 +/- 0.3 pg/ml). In conclusion, experimental severe secondary hyperparathyroidism is reversible very quickly after the reversal of uremia. Hyperphosphatemia in uremia is important for the non-suppressibility of the parathyroid glands to calcium. In non-uremic rats even severe parathyroid hyperplasia can be controlled, resulting in normal plasma PTH and Ca2+ levels and in a normal response to hypercalcemia. Thus, the minimal PTH secretion obtained during the induction of hypercalcemia is not an expression of the parathyroid mass.

24R,25-dihydroxyvitamin D3: an essential vitamin D3 metabolite for both normal bone integrity and healing of tibial fracture in chicks

We tested the hypothesis that 24R,25-dihydroxyvitamin D3 [24R,25-(OH)2D3] is an essential vitamin D metabolite for the development of normal bone integrity and the healing of fractures. The natural 24R,25-(OH)2D3 and its synthetic epimer 24S,25-dihydroxyvitamin D3 [24S,25-(OH)2D3] were tested alone or in combination with 1alpha,25-dihydroxyvitamin D3 [1alpha,25-(OH)2D3], on normal bone development and other related variables of the Ca2+ homeostasis system [serum Ca2+, 25-hydroxyvitamin D3 (25OHD3), 24,25-(OH)2D3, and 1alpha,25-(OH)2D3 levels] in chicks. Mechanical testing of torsional strength was carried out on the femur. 24R,25-(OH)2D3 (80 nmol/kg diet) alone was sufficient for normal bone growth and integrity similar to that achieved by the vitamin D3-replete controls. Next, chicks were fed a 25OHD3-replete diet (75 nmol/kg diet) for 8 days after hatching, and then 25OHD3 was withdrawn to minimize any residual circulating metabolites before the imposition of standardized tibial fractures 14 days later. Vitamin D metabolites were administered for 2 weeks to determine their effects on the mechanical properties of healed tibia. 24S,25-(OH)2D3 combined with 1alpha,25-(OH)2D3 or 1alpha,25-(OH)2D3 alone resulted in poor healing [strength values of 0.158 +/- 0.011 and 0.123 +/- 0.009 Nm (Newton x meter), respectively] compared with that in the 25OHD3-treated control group (0.374 +/- 0.029 Nm). In contrast, the fractured tibia of the birds fed 24R,25-(OH)2D3 in combination with 1alpha,25-(OH)2D3 showed healing equivalent to that in the control group, with strength values of 0.296 +/- 0.043 Nm. These results suggest that when 24R,25-(OH)2D3 is present at normal physiological concentrations, it is an essential vitamin D3 metabolite for both normal bone integrity and healing of fracture in chicks.

Three-fold induction of renal 25-hydroxyvitamin D3-24-hydroxylase activity and increased serum 24,25-dihydroxyvitamin D3 levels are correlated with the healing process after chick tibial fracture

To investigate the possible biological actions of 24,25-dihydroxyvitamin D3 (24,25(OH)2D3), a tibial fracture-healing model was established in White Leghorn chicks. Three-week-old White Leghorn chicks fed a vitamin D3-replete diet were divided into four groups (control, anesthetized, sham, and fractured). On varying days after tibial fracture (F) or sham manipulation (S), renal 25(OH)D3-1 alpha-hydroxylase and 25(OH)D3-24-hydroxylase (24-hydroxylase) activities and serum Ca2+ concentrations were measured. Metofane anesthesia was found to have no effect on the activity of either of the hydroxylases; the activities of the hydroxylases in the control, anesthetized, and sham-operated birds were similar. By 10 days after tibial fracture, the renal 24-hydroxylase activity increased more than 3-fold in F (1.33 +/- 0.07 pmol/mg of protein) as compared with S (0.42 +/- 0.03 pmol/mg of protein) (p < 0.0001). A time-dependent study of the renal 24-hydroxylase activity during the fracture repair process revealed a slow increase from the first day after fracture, a higher activity at 8 days, which peaked at 10-11 days, which is consistent with the formation of the callus. The 24-hydroxylase activity then returned to the same level as the sham group 14 days after fracture. There was no significant difference in serum Ca2+ levels between the F and S groups over the 3-week postfracture period. Serum levels of vitamin D3 metabolites were also measured during the fracture healing process: a 3.4x increase of the 24,25(OH)2D3 level in the fractured group (3.64 +/- 1.16 nM) was observed as compared with the control groups (1.08 +/- 0.49 nM) at 10 days after fracture (p = 0.068). No significant differences were observed in the plasma levels of 25(OH)D3 or 1 alpha, 25(OH)2D3 between the group with a fracture and the controls. Exposure of primary chick kidney cells in culture to serum obtained from chicks with a tibial fracture for 20 h resulted in an approximately 40% increase in the activity of the 24-hydroxylase as compared with cells exposed to serum from control birds. These results suggest that 24,25(OH)2D3 is involved in the early process of fracture repair and that there is some form of physiological communication between the fractured bone and the kidney so as to increase the renal 24-hydroxylase and the circulating concentration of this metabolite.

Quantitative and three-dimensional aspects of the rat parathyroid gland in normo-, hypo-, and hypercalcemia

The ultrastructure of the rat parathyroid has been under study for more than 35 years, but controversies still exist, especially regarding structure-function relationships. The present review focuses on recent morphological parathyroid research on rats under normal conditions and in various states of disturbed calcium metabolism. To facilitate discussions on functional aspects, current biochemical data, particularly those dealing with the regulation of parathyroid hormone synthesis and release, are also considered. Our results from quantitative studies and from investigations employing serial sectioning form the basis for the discussions. A central issue is whether the parathyroid secretory cells undergo secretory cycles. Prompted by results obtained from improved fixation procedures and serial sectioning, we question the basis for the theory of secretory cycles. Since the rat parathyroid secretory cell is polar, a single section is not an appropriate sample for estimating functional activity and for comparing the structure and distribution of intracellular components of adjacent cells. The heterogeneity in ultrastructural appearance of intracellular vesicles calls for the use of specific markers in relating the structure of the vesicular compartment to intracellular processing of hormone. The importance of unbiased quantitative techniques is illustrated in discussions on cell number and size for estimating the response of the parathyroid gland to different functional states or disorders demanding changes in secretion of parathyroid hormone, e.g., hyper- and hypocalcemia.

Hypercalciuric response to dietary supplementation with DL-methionine and ammonium sulfate

Renal Ca and inorganic P (Pi) excretion were evaluated in Single Comb White Leghorn pullets reared on diets containing 1 or 3.5% Ca alone or supplemented with .6% DL-methionine or .53% ammonium sulfate. Plasma and urine samples were collected during a CONTROL period, and while 200 mM Ca was infused intravenously (Ca-LOADING). Excess Ca, whether supplied chronically in the feed or infused acutely into birds fed 1% Ca diets, significantly reduced glomerular filtration rates, effective renal plasma flow rates, and Pi excretion rates and significantly increased Ca excretion rates and urine pH. Birds fed diets supplemented with DL-methionine and ammonium sulfate maintained significantly lower plasma Ca concentrations during the CONTROL and Ca-LOADING periods than birds fed the respective 1 or 3.5% Ca basal diets. When compared with birds fed the respective 1 or 3.5% Ca basal diets, birds fed the 1% Ca diet supplemented with ammonium sulfate or the 3.5% Ca diet supplemented with DL-methionine had significantly higher absolute urinary Ca excretion rates during Ca-LOADING. Fractional Ca excretion during Ca-LOADING was significantly higher in birds fed 3.5% Ca supplemented with DL-methionine or ammonium sulfate than in birds fed the 3.5% Ca basal diet. These results indicate that DL-methionine and ammonium sulfate accelerated urinary Ca excretion and reduced Ca retention in the extracellular fluid. The hypercalciuric efficacies of DL-methionine and ammonium sulfate were revealed only when the filtered load of Ca was increased through intravenous Ca infusions.

Parathyroid cell number and size in hypocalcemic young rats

Weanling rats were fed diets with normal (1%) or low (0.08% or 0.02%, respectively) Ca content for 28 days prior to sacrifice. The total volume of the parathyroids was estimated from serial sections. Volume density of secretory cells was calculated according to conventional stereological techniques, whereas cell number and cell size were estimated by the dissector method. Compared with controls the animals of the experimental groups developed moderate and severe hypocalcemia and their parathyroids were enlarged with a proportional growth of parenchyma and interstitium. Related to the body weight, secretory cell volume was highest in animals with severe hypocalcemia. In the enlarged glands the size of parathyroid secretory cells was increased by 30-40%, whereas total cell number was unaltered. Thus, the increased parathyroid size was due to cell hypertrophy rather than hyperplasia.

Optimal dietary level of 1 alpha,25-dihydroxycholecalciferol for eggshell quality in laying hens

The optimal dietary level of 1 alpha,25-dihydroxycholecalciferol [1,25-(OH)2D3] for eggshell quality was established. White Leghorn hens, 59 wk of age, were fed one of eight diets that contained the same basal ingredients, including 3.1% calcium, but different levels (microgram/kg) or forms of calciferol supplements: no calciferol supplement of any form (56 hens); 27.5 (control) or 55.0 micrograms of cholecalciferol (56 hens each); 3, 5, or 7 micrograms of 1,25-(OH)2D3 (28 hens each); 5 micrograms of 24,25-dihydroxycholecalciferol [24,25-(OH)2D3] with 28 hens; 5 micrograms each of 1,25-(OH)2D3 and 24,25-(OH)2D3 (28 hens). All groups were fed the control diet prior to the 21-wk treatment. The group fed 5 micrograms 1,25-(OH)2D3/kg diet ranked first in specific gravity (SG), e.g., 1.081 versus 1.077 for the control group at Week 21 (P less than .05). The group fed 7 micrograms 1,25-(OH)2D3/kg consumed 30% less feed and laid 20% fewer eggs than the control, but shell quality was not affected. The groups receiving no calciferol supplement or receiving only 24,25-(OH)2D3 laid eggs with significantly lower SG than the control after 2 wk of treatment (1.072 or less versus 1.082 at Week 2). The rest of the treatment groups mentioned were comparable to the control in eggshell quality and egg production. Groups fed the combination of 1,25-(OH)2D3 and 24,25-(OH)2D3 per kilogram of feed, or 1,25-(OH)2D3 alone at 5 micrograms/kg, had significantly higher tibial weights relative to the control group. All groups receiving the diets without cholecalciferol supplementation had markedly reduced hatchability. It was concluded that the optimal dietary level of 1,25-(OH)2D3 for improving eggshell quality without affecting egg production was approximately 5 micrograms/kg and the toxic level was 7 micrograms/kg.

Parathyroid cell number and size in hypercalcemic rats: a stereologic study employing modern unbiased estimators

Recently developed stereologic methods for unbiased estimations of particle number and size were employed to study parathyroid growth in normal and hypercalcemic young rats. Thus, the parathyroid cell number and size of parathyroid secretory cells were estimated by both the disector method and the volume-weighted mean volume method. The glandular volume was calculated from serial sections, and the volume density of secretory cells was estimated by conventional stereologic techniques. Three groups of animals were studied: normal rats at 3 weeks of age, hypercalcemic rats at 7 weeks of age, and age-matched controls. Hypercalcemia was induced by feeding the animals a purified diet that was nutritionally adequate except for low amounts of phosphate (0.02%) from 3 weeks of age. During the period from 3 to 7 weeks of age, the number of parathyroid secretory cells increased by 100%, whereas the mean cell volume increased by 20%. However, when calculated per gram body weight the volume and number of cells were larger in the younger animals. The phosphate-depleted animals grew slowly and developed severe hypercalcemia. Their parathyroid secretory cells were smaller, and each gland contained fewer cells than in age-matched controls. The lower cell number and cell volume, however, were proportional to the reduced body weight. Data from the 3-week-old animals indicate that the reduced cell number and size in hypercalcemic rats reflected growth arrest rather than atrophy.

Characteristics of secondary hyperparathyroidism in vitamin D-deficient dogs

Characteristics of secondary hyperparathyroidism were evaluated in dogs with mild vitamin D deficiency. The animals were normocalcemic with reduced concentrations of 1,25-dihydroxy vitamin D3 [1,25(OH)2D3] and elevations in parathyroid hormone (PTH) concentrations, parathyroid mass, and prepro PTH mRNA levels. Dynamic testing revealed a sigmoidal relationship between plasma calcium and PTH, although PTH concentrations were increased relative to values in vitamin D-sufficient dogs. Infusions of chelator elicited lower plasma calcium levels and greater augmentations in biologically active PTH in vitamin D-deficient than in D-sufficient animals. Induced hypercalcemia lowered both immunoreactive and bioactive hormone to stable but detectable levels. The results demonstrate the decreased capacity of vitamin D-deficient animals to defend against acute hypocalcemia, despite the presence of abundant PTH, and indicate that increased circulating PTH levels in early vitamin D deficiency is due predominantly to an augmentation in the quantity of releasable hormone. The latter appears secondary to an increase in parathyroid mass and synthetic activity regulated by 1,25(OH)2D3 per se.

Bone turnover and 1,25-dihydroxycholecalciferol during treatment with phosphate binders

The effect of dietary phosphate restriction with high-dose aluminum hydroxide or calcium carbonate on bone disease assessed by histomorphometry and on the plasma levels of 1,25-dihydroxycholecalciferol was investigated in 12 children with chronic renal failure (GFR 8 to 45 ml/min/1.73 m2, age 5 to 15 years) over a one year period. Prior to treatment patients had biochemical and histological hyperparathyroidism with greatly increased bone formation rates. During treatment, plasma phosphate levels decreased from the upper to the lower limit of normal for age (pre, 1.69 +/- 0.06 mmol/liter; 6 months, 1.28 +/- 0.06 mmol/liter; 1 year, 1.34 +/- 0.06 mmol/liter; P less than 0.01). Circulating 1,25-dihydroxycholecalciferol rose to supranormal levels within three months and remained high throughout the period of study (pre, 96 +/- 32 pmol/liter; 6 months, 144 +/- 46 pmol/liter; 1 year, 169 +/- 53 pmol/liter; P less than 0.001). Significant falls in bone formation rate at tissue and cellular levels (P less than 0.005) and in total resorption surface (P less than 0.005) were observed. A mild mineralization defect present before treatment worsened, with a decrease in mineral appositional rate (P less than 0.01) and increase in mineralization lag time (P less than 0.01). Staining for aluminum in post-treatment biopsies was positive in 9 of 11 cases. Phosphate restriction produced suppression of biochemical and histological hyperparathyroidism and sustained elevation of circulating 1,25-dihydroxycholecalciferol. The adverse changes in bone mineralization may be related to aluminum hydroxide therapy; calcium carbonate is therefore recommended.

Opposite effects of 1,25(OH)2D3 on synthesis and release of PTH compared with secretory protein I

Aggregates of bovine parathyroid cells (organoids) were cultured with or without 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] and examined for effects on the synthesis, cellular content, and release of parathyroid hormone (PTH) and secretory protein I. Organoids cultured with 120 pM 1,25(OH)2D3 at 1.35 mM calcium contained 27 +/- 2% (mean +/- SE) less and released 26 +/- 7% less PTH and contained 21 +/- 2% more and released 49 +/- 16% more secretory protein I than untreated organoids. After 15-min incubations with radioactive leucine, treated organoids incorporated 25 +/- 2% less radioactivity into precipitable cellular proteins than did untreated organoids, indicating that the overall synthetic rate of proteins was decreased by 1,25(OH)2D3 at 1.35 mM calcium. After separation of cellular proteins by gel electrophoresis, analysis of individual protein bands indicated that the mean treated-control ratio for proparathyroid hormone radioactivity was 0.88 +/- 0.06 (P less than 0.10) while that for secretory protein I was 1.56 +/- 0.17 (P less than 0.01), respectively. Experiments were performed at 1.2, 12, 120, and 1,200 pM 1,25(OH)2D3 and both 1.4 and 1.8 mM calcium. In both cases, increasing levels of 1,25(OH)2D3 progressively decreased PTH synthesis and release and progressively increased that of secretory protein I, significant effects being observed at 1.2 pM 1,25(OH)2D3. We conclude that at physiological calcium levels, 1.2-1,200 pM 1,25(OH)2D3 suppresses the synthesis and secretion of PTH, increases that of secretory protein I, and at similar concentrations decreases the overall protein synthetic rate compared with untreated controls.

Histologic parathyroid abnormalities in an autopsy series

The parathyroid glands from 422 subjects without clinical, biochemical, or advanced histologic signs of renal disease were dissected at autopsy. The glands were evaluated histologically in relation to the subjects' age, sex, histologic renal features, other diseases, and medical therapy. Normal parathyroid glands generally had diffuse cellular arrangements, with variable fat cell content. Increased nodularity was observed, especially in enlarged parathyroid glands. Hyperplasia was present in 7 per cent and adenomas in 2.4 per cent. Hyperplastic glands were often nodular and asymmetric, and numbers of oxyphil cells were frequently increased. Some large nodules were histologically indistinguishable from adenomas. Serum calcium levels were elevated in subjects with adenomas or with hyperplastic glands containing large nodules. The findings seem to indicate that nodularity of the parathyroid tissue is a sign of abnormality and that adenomas may arise from such nodular hyperplasia. Hyperplasia and adenomas were more common in elderly subjects and in those with slight or moderate nephrosclerosis. Other diseases or medical therapy were not found to be correlated with parathyroid weight or histologic features.

The metabolism and functions of vitamin D

Vitamin D functions by stimulating intestinal calcium and phosphorus absorption, by stimulating bone calcium mobilization, and by increasing renal reabsorption of calcium in the distal tubule. These functions on bone and possibly kidney, but not intestine, require the parathyroid hormone. As a result of these functions, serum calcium and phosphorus concentrations are elevated to supersaturating levels required for the mineralization of bone to prevent rickets, osteomalacia, and hypocalcemic tetany. Recent experiments demonstrate that maintaining serum calcium and phosphorus levels in vitamin D-deficient rats in the normal range results in normal bone growth and mineralization. However, increased calcification results because bone resorption by osteoclasts is a vitamin D-dependent process. Thus, bone resorption, modeling and remodeling must be considered vitamin D-dependent processes. Vitamin D must be metabolized to 25-hydroxyvitamin D3 by the liver and subsequently by the kidney to 1,25-dihydroxyvitamin D3 before function. 1,25-Dihydroxyvitamin D3 is metabolized to a C-23 carboxylic acid (calcitroic acid) but the pathway is unknown. Although 25-hydroxyvitamin D3 is metabolized to 24R,25-dihydroxyvitamin D3, 25,26-dihydroxyvitamin D3 and 25-hydroxyvitamin D3-26,23-lactone, these pathways play no role in the function of vitamin D as shown by appropriate fluoro analogs of 25-hydroxyvitamin D3. 1,25-Dihydroxyvitamin D3 binds to a specific receptor in the intestinal nuclei to elicit a stimulation of calcium transport. 1,25-Dihydroxyvitamin D3 plus the receptor causes transcription of specific genes that code for calcium and phosphorus transport proteins. Only one protein, the calcium binding protein, has been identified as being vitamin D dependent. Two others have been described, but no clear description of the molecular mechanism of action of 1,25-dihydroxyvitamin D3 is yet available.

The effect of 1,25 dihydroxycholecalciferol on parathyroid hormone secretion by monolayer cultures of bovine parathyroid cells

Controversy exists over a direct effect of 1,25(OH)2D3 on PTH secretion. To investigate the possibility that the suppressive effect of 1,25(OH)2D3 on PTH secretion may be demonstrable in 1,25(OH)2D3-depleted tissue and/or after prolonged periods of exposure to 1,25(OH)2D3, primary monolayer cultures of bovine parathyroid cells were established in 1:1 DMEM/Ham's F-12 media supplemented with 2% calf serum but not 1,25(OH)2D3. Ionized calcium was maintained at 1.0 mM. Experiments were performed on 4-day-old culture cells. PTH concentration was measured using both a mid-region/carboxyl and an amino-terminal PTH antisera. 1,25(OH)2D3 at a concentration of 0.1 ng/ml suppressed PTH secretion by 32 +/- 7% after 48 hours. High calcium concentration (2.0 mM) suppressed PTH secretion by 37 +/- 10% and this effect was not additive over that of 1,25(OH)2D3. PTH secretion rate recovered fully 48 hours after normalization of the external calcium concentration but not after the removal of 1,25(OH)2D3. It is concluded that 1,25(OH)2D3 directly suppresses PTH secretion by monolayer culture of bovine parathyroid cells.

Vitamin D and skeletal tissues

It is now accepted that vitamin D is an integral part of a complex endocrine system, one with far-reaching implications in mineral metabolism. Reviews of the sources, functions and metabolism of vitamin D, as currently understood, are presented as a prelude to discussions of the role of vitamin D in calcium and phosphorous homeostatis and possible specific roles for vitamin D in mineralized tissues. Data describing a possible regulatory function for vitamin D in bone and bone protein metabolism are presented. Some of the controversy which presently exists regarding the biochemical mechanism of the action of this vitamin is discussed. Finally, the possible relationship of vitamin D and disorders of skeletal tissues is described.

Synergistic effects of 1,25(OH)2D3 and 24,25(OH)2D3 on duodenal CaBP in rachitic chicks and on eggshell weight in Japanese quails

The role of 24,25(OH)2D3 in calcium homeostasis is still controversial. In the present study the administration of low doses of 1,25(OH)2D3 and of higher doses of 24,25(OH)2D3 either alone or in conjunction with each other, were studied in rachitic chicks and in Japanese quails. Whereas 24,25(OH)2D3 alone had no significant effect on duodenal CaBP and on alkaline phosphatase in chick serum, it increased the influence of 1,25(OH)2D3 on these two parameters strongly. Also, when 1,25(OH)2D3 and 24,25(OH)2D3 were given simultaneously to Japanese quails, calcium excretion via the egg shell was clearly higher than when either metabolite had been administered alone. These results indicate that 1,25(OH)2D3 and 24,25(OH)2D3 exert a strong synergistic effect in rachitic animals.

Effect of 24,25(OH)2D3 on PTH levels and bone histology in dogs with chronic uremia

Controversy exists as to whether 24,25(OH)2D3 has a direct inhibitory effect on parathyroid hormone (PTH) secretion. Therefore, the present investigation examined the effect of long-term administration of 24,25(OH)2D3 on immunoassayable PTH levels (iPTH) and bone histology in dogs with chronic renal failure. Chronic renal failure was produced in 16 dogs, half of which served as controls whereas the other half received 2.5 micrograms/day of 24,25(OH)2D3, orally. Serum iPTH, serum total, ionized calcium, serum phosphorus, and creatinine were followed at weekly or biweekly intervals in both groups. Also, creatinine clearances, serum levels of 25(OH)D3, 24,25(OH)2D3, and 1,25(OD)2D3 and the intestinal absorption of calcium were measured. After 1 year of chronic renal failure the dogs were sacrificed and rib biopsy specimens were obtained for histological examination and measurement of mineral content. Serum iPTH increased equally in the two dog groups with no effect at any time of 24,25(OH)2D3 treatment, despite a significant increase in the serum levels of 24,25(OH)2D3 and a concomitant decrease of the 1,25(OH)2D3 levels. There was no difference in the levels of serum calcium or in the calcium content of bone. Furthermore, after 8 months of uremia three control dogs were switched to the group treated with 24,25(OH)2D3 and followed for another 7 months. No suppressive effect of administering 24,25(OH)2D3 on the iPTH levels could be demonstrated in these three dogs.(ABSTRACT TRUNCATED AT 250 WORDS)

Vitamin D3 and 1,25-dihydroxyvitamin D3 stimulate the skeletal muscle-calcium mobilization in rachitic chicks

The Ca content in skeletal muscle relative to vitamin D3 intake was studied in chicks. It was found that the Ca content in rachitic chick muscle was significantly higher than normal and it decreased with vitamin D3 treatment. In 4-week-old chicks fed a vitamin D-deficient diet, the Ca content in leg muscle reached 9.86 +/- 1.07 mg/100 g wet wt, although in chicks receiving vitamin D3 in doses of 100 and 500 IU/kg diet, it was 7.80 +/- 0.72 and 6.08 +/- 0.61 mg/100 g wet wt, respectively. A single i.m. dose of 0.50 micrograms of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) or vitamin D3 caused a dramatic decrease in the muscle Ca content by 3 to 6 h after the injection. A simultaneous rise in the Ca level in blood serum was observed. However, at this time the Ca binding protein content in duodenal mucosa and the stimulation of Ca absorption were negligible. These findings allow the conclusion that the vitamin D deficiency in chicks leads to a surplus Ca accumulation in skeletal muscle. The administration of vitamin D3 or its metabolites causes rapid Ca release during the first 6 h. This may be the source of the Ca level increase in blood serum. In this respect 1,25(OH)2D3 was much more effective than vitamin D3.

Extrarenal metabolism of 25-hydroxycholecalciferol in the rat: regulation by 1,25-dihydroxycholecalciferol

To determine the role of the kidney in regulation of 25-hydroxycholecalciferol (25OHD3, metabolism, the effects of 1,25-dihydroxycholecalciferol [1,25(OH)2D3] on 3H-25OHD3 were compared in intact and nephrectomized vitamin D-deficient rats. Sixteen hours after the intravenous administration of 3H-25OHD3, extracts of serum and pooled small intestinal mucosa were fractionated by Sephadex LH-20 column chromatography followed by high performance liquid chromatography. In intact rats, 1,25(OH)2D3 (50 ng/day i.p. for 7 days) increased mean serum 3H-24,25-dihydroxycholecalciferol [3H-24,25(OH)2D3] from 2 +/- 2-210 +/- 80 fmol/ml (mean +/- 1 SD), increased mean serum 3H-25,26-dihydroxycholecalciferol [3H-25,26(OH)2D3] from 2 +/- 2-12 +/- 6 fmol/ml and lowered mean serum 3H-1,25(OH)2D3 from 210 +/- 40-4 +/- 4 fmol/ml. Similarly, in nephrectomized animals, 1,25(OH)2D3 increased mean serum 3H-24,25-(OH)2D3 from 6 +/- 11-115 +/- 30 fmol/ml and increased mean serum 3H-25,26(OH)2D3 from 3 +/- 3-26 +/- 10 fmol/ml. Nephrectomy increased serum 3H-25(OH)D3 in untreated (from 1450 +/- 225-2675 +/- 225 fmol/ml serum) and 1,25(OH)2D3 treated rats (from 1600 +/- 175-3075 +/- 100 fmol/ml). 3H-1,25(OH)2D3 averaged 74 +/- 16% of total radioactivity in intestinal mucosa of untreated intact rats and was not detected in either the serum or intestinal mucosa of nephrectomized animals. The results suggest that in intact animals, extrarenal synthesis can account for substantial 24,25(OH)2D3 production and for most 25,26(OH)2D3 production.(ABSTRACT TRUNCATED AT 250 WORDS)

Lack of effects of 1,25- and 24,25-dihydroxy vitamin D on parathyroid hormone response to hypocalcemia in cattle

Effects of 1,25(OH)2D3 or 24,25(OH)2D3 on plasma PTH were examined following induced hypocalcemia with EGTA. EGTA infusions caused an elevation of plasma PTH within 10 min. Sixty min after the start of EGTA infusions, 1,25(OH)2D3 or 24,25(OH)2D3 were IV administered. Transient (within 5 min) elevations in plasma PTH were observed in two of five animals following the administration of 1,25(OH)2D3 or of 24,25(OH)2D3. Neither secosterol had an effect on the induced elevations in plasma PTH during the remaining 60 min of the EGTA infusions. Twenty-two hr following 24,25(OH)2D3 administration, plasma PTH, ionized and total calcium, inorganic phosphate, and magnesium were normal, while plasma 24,25(OH)2D was elevated. The plasma PTH response to EGTA-induced hypocalcemia was not significantly altered from that observed prior to the administration of 24,25(OH)2D3. Animals, which were IV injected with 1,25(OH)2D3 received the same amount IM 60 min later. Twenty-two h following IM 1,25(OH)2D3, plasma 1,25(OH)2D, ionized and total calcium, and plasma inorganic phosphate were elevated. Plasma PTH and magnesium were lowered. The PTH response to EGTA-induced hypocalcemia was significantly reduced in these animals. A similar reduction in the PTH response to induced hypocalcemia was observed in animals receiving 7 hr IV infusions of calcium chloride. The findings suggest that the blunted response was, in part, the consequence of the preceding hypercalcemia. These results indicate that 1,25(OH)2D3 does not directly regulate plasma PTH secretion and that 24,25(OH)2D3 has no effect on plasma PTH during induced hypocalcemia in the bovine species.