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

Impaired stimulation of 25-hydroxyvitamin D-24-hydroxylase in fibroblasts from a patient with vitamin D-dependent rickets, type II. A form of receptor-positive resistance to 1,25-dihydroxyvitamin D3

We describe studies of the molecular defect in 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] action in cultured skin fibroblasts from a patient previously reported to have vitamin D-dependent rickets, type II. Binding of [3H]1,25-(OH)2D3 in fibroblast cytosol was normal with a Bmax (amount of high affinity binding) of 26 fmol/mg protein and a half-maximal saturation of 0.2 nM. Nuclear binding of [3H]1,25-(OH)2D3 following whole cell uptake was 1.5 fmol/micrograms DNA in patient fibroblasts compared with a range of 0.5-2.9 fmol/micrograms DNA in five control strains. The size of the [3H]1,25-(OH)2D3-receptor complex on sucrose density gradients, 3.8 S, was the same as in normal cells. This patient, therefore, appeared to have a receptor-positive form of resistance to 1,25-(OH)2D3. To document resistance to 1,25-(OH)2D3 in the fibroblasts we developed a method for detection of 1,25-(OH)2D3 action in normal skin fibroblasts. Following treatment of normal cell monolayers with 1,25-(OH)2D3 there was more than a 20-fold increase of 25-hydroxy-vitamin D-24-hydroxylase (24-hydroxylase) activity. Treatment of 10 control cell strains with 1,25-(OH)2D3 for 8 h increased the formation of 24,25-dihydroxy-vitamin D3 from 25-hydroxyvitamin D3 in cell sonicates from less than 0.02 to 0.11-0.27 pmol/min per mg protein. When cells from the patient with vitamin D-dependent rickets, type II were treated with 1,25-(OH)2D3 in a similar manner, maximal 24-hydroxylase activity was only 0.02 pmol/min per mg protein, less than a fifth the lower limit of normal. 24-Hydroxylase activity in fibroblasts from the parents of the patient increased normally following treatment with 1,25-(OH)2D3. We conclude that impaired induction of 24-hydroxylase in the presence of normal receptor binding is evidence for postreceptor resistance to the action of 1,25-(OH)2D3.

Lack of influence of 24,25-dihydroxyvitamin D3 on parathyroid hormone secretion from normal or hyperplastic glands

The role of 24,25(OH)2D3 on parathyroid gland function remains controversial. The present studies were performed in vitro using (a) dispersed normal bovine parathyroid cells (bPTC) and (b) dispersed canine PTC (cPTC) prepared from glands of normal dogs, dogs with chronic renal failure (CRF), and dogs with CRF treated with 24,25(OH)2D3, 2.5 micrograms orally every day for more than 6 months. Bovine parathyroid cells were incubated for up to 180 min at 0.5, 1.0, and 3.0 mM external calcium in the presence or absence of 24,25(OH)2D3 (100 or 1000 nM). Similar experiments were conducted with cells incubated for 24 h in the presence of either the ethanol vehicle or 24,25(OH)2D3 (1000 nM). Parathyroid hormone secretion, measured in the supernatant by both C-terminal and N-terminal assays, did not show any differences between control and experimental groups at any time interval. Canine parathyroid cells obtained from uremic animals showed an average threefold increase in the total amount of PTH secreted, on a per cell basis over 180 min at 0.5 mM Ca2+, when compared with normal controls. However, there was no significant difference in PTH secretion at any level of calcium concentration between the cells obtained from parathyroid glands of CRF dogs and 24,25(OH)2D3-treated CRF dogs. Acute exposure to 24,25(OH)2D3 (1000 nM) in vitro of the cells obtained from the glands of CRF dogs also had no effect on PTH secretion. We conclude that 24,25(OH)2D3 has no direct effect on PTH secretion from dispersed parathyroid cells of either normal or uremic animals.

Early changes in the adaptation to a low calcium diet in the chick

Twelve hours after the diet of 3-week-old chicks was changed from a 1% to a 0.1% concentration of calcium (Ca), the growth rate and circulating levels of growth hormone had fallen while renal 25-hydroxecholecalciferol-24-hydroxylase had risen. The amount of 47Ca incorporated into bone from an injection given 18 h previously was lower than in the control birds. Over the following 2 1/2 days on the low Ca diet, the renal 1-hydroxylase activity rose and the plasma prolactin level fell, but the other parameters moved back toward the control level. It was concluded that early adjustments in hormonal and mineral metabolism counteract the acute effects of a dietary Ca shortage until longer-term adaptive changes begin to compensate for a continuing Ca deficiency. The renal hydroxylase activities were not directly influenced by the level of circulating growth hormone or prolactin.

24-hydroxylation of 25-hydroxyvitamin D3: is it required for embryonic development in chicks?

As shown previously, laying hens given 1,25-dihydroxyvitamin D3 as their sole source of vitamin D produce fertile eggs having normal shells, but only 35 to 55 percent of the embryos are normal. Giving these hens additional 25-hydroxyvitamin D3, 24,25-dihydroxyvitamin D3, or 24,24-difluoro-25-hydroxyvitamin D3 at 1.25 nanomoles per day resulted in 90 to 100 percent normal embryos, and hence, hatchability. Since 24,24-difluoro-25-hydroxyvitamin D3 cannot be 24-hydroxylated, 24-hydroxylation is not required for this function of 25-hydroxyvitamin D3.

Extraction of vitamin D metabolites by bones of normal adult dogs

Using the isolated perfused canine tibia we examined the extraction of [(3)H]25(OH)D(3), [(3)H]1,25(OH)(2)D(3) and [(3)H]24,25(OH)(2)D(3) by bone of normal adult dogs. The studies were performed with and without vitamin D binding protein (DBP) in the perfusate to examine the effect of protein binding on the extraction of the vitamin D metabolites. An average of 48+/-2% of [(3)H]25(OH)D(3) was extracted by bone, when no DBP was present. However, addition of only a small amount of DBP ( approximately 720 ng/ml of perfusate) nearly completely abolished the extraction of [(3)H]25(OH)D(3) by bone. No degradation and/or transformation of the labeled 25(OH)D(3) could be demonstrated during passage through the isolated perfused bone. The extraction of [(3)H]24,25(OH)(2)D(3) in a DBP-free medium averaged 33+/-5%. Addition of 720 ng of DBP/ml of perfusate completely inhibited the extraction of this metabolite. The extraction of [(3)H]1,25(OH)(2)D(3) averaged 30+/-3% in a DBP free medium and no inhibition of the extraction was demonstrated after addition of DBP (720 ng/ml of perfusate). However, addition of DBP in a concentration of 14.4 mug/ml of perfusate reduced the extraction of 1,25(OH)(2)D(3) to 8+/-2%, a value still significantly higher than that seen after addition of 20 times less DBP to perfusions with 25(OH)D(3) and 24,25(OH)(2)D(3). It is concluded that the isolated perfused bone of normal dogs can extract significant amounts of 25(OH)D(3), 1,25(OH)(2)D(3), and 24,25(OH)(2)D(3). Small concentrations of DBP (720 ng/ml) in the perfusate significantly inhibited the extraction of 25(OH)D(3) and 24,25(OH)(2)D(3). A carrier role for DBP is suggested and it is proposed that the levels of free vitamin D are important for extraction of the metabolites by bone. Therefore, due to the different affinities of DBP for the various metabolites of vitamin D, only 1,25(OH)(2)D(3) is extracted in vitro in significant amounts by bone of normal adult dogs, in the presence of DBP.

Autoradiographic studies with 3H 1,25 (OH)2 vitamin D2 and 3H 25 (OH) vitamin D3 in rat parathyroid glands

After injection of radiolabeled 1,35 (OH)s vitamin D3, nuclear concentration of radioactivity is observed in parenchymal cells o hte parathyroid gland in pregnant, adult male, and 10-day male neonatal rats. In competition studies with unlabeled 1,25 (OH)2 vitamin D3, but not with 25 (OH) vitamin D3, in contrast to 3H 1,25 (OH)2 vitamin D3, do not show nuclear concentration in cells of te parathyroid. The results of te autoradiographic studies suggest the presence of receptors for a direct effect of 1,35 (OH)2 vitamin D3 on the parathyroid gland for modulation of parathyroid hormone secretion.

The vitamin D system: a view from basic science to the clinic

Vitamin D produced in the skin and absorbed in the small intestine must be modified metabolically before it can function. It is ultimately converted to a hormone in the kidney that stimulates intestine, bone and kidney to mobilize calcium and phosphorus. This results in normal bone development and normal neuromuscular function. The vitamin D hormone appears to act by a nuclear mechanism to facilitate a target organ response. Finally the vitamin D hormone is produced in response to the need for calcium and phosphorus. The calcium need is interpreted by the parathyroid gland that in turn secretes parathyroid hormone. The parathyroid hormone stimulates production of the vitamin D hormone. This constitutes the vitamin D endocrine system that plays an important role not only in calcium homeostasis but in phosphate homeostasis and in calcium economy of the body. A number of disease states including hypoparathyroidism, pseudohypoparathyroidism, renal osteodystrophy, certain types of vitamin D-resistant rickets and osteoporosis can in part be related to disturbance in the vitamin D endocrine system. Thus measurement of the vitamin D hormone and its precursor will be of great value in diagnosis of metabolic bone disease and most importantly, the availability of new vitamin D compounds will play an important role in the treatment of these bone diseases.

Pathology of experimental vitamin D deficiency in chickens and effects of treatment with vitamin D metabolites

Structural changes in bone, parathyroid, and ultimobranchial body were examined in three groups of chicks fed a vitamin D-deficient diet; one group was treated with vitamin D3 and another with 1,25(OH)2D3. Diets were fed from day of hatching until 5 weeks old, when deficient chicks were near death due to hypocalcemic tetany, loss of fat and muscle, and marked bone deformities. In deficient chicks, parathyroid mass increased linearly to 7.5 times normal at 5 weeks. Parathyroid cells were irregular and vacuolated, with few granules. 1,25(OH)2D3 had normal parathyroids until the fifth week, when parathyroid mass increased greatly. There were few differences in length of growth cartilage, but marked changes in length of metaphyses. Deficient chicks had metaphyses nearly five times longer than vitamin D3-treated chicks. Metaphyses in chicks given 1,25(OH)2D3 were twice as long as those of vitamin D-treated chicks at 5 weeks. Both osteoblasts and osteoclasts were more numerous in deficient chicks. These studies suggest that vitamin D3 is more effective than 1,25(OH)2D3 in preventing parathyroid and bone lesions of vitamin D deficiency.

Interaction between 24R,25-dihydroxycholecalciferol and 1,25-dihydroxycholecalciferol on 45Ca release from bone in vitro

Interaction among vitamin D3 metabolites on bone receptor sites is not known. Therefore, interaction between the most potent vitamin D3 metabolite, 1,25(OH)2D3, and the most abundant dihydroxymetabolite, 24R,25(OH)2D3, was studied on isolated rat fetal bone by measuring 45Ca release from prelabeled bones. 24R,25(OH)2D3 at concentrations of 10-50 ng/ml caused marked inhibition of the bone-resorbing activity of 1,25(OH)2D3 at concentrations of 10-50 pg/ml. 24S,25(OH)2 (unnatural enantiometer), on the other hand, at a concentration of 100 ng/ml did not inhibit the bone-resorbing effect of 10 pg/ml 1,25(OH)2D3. 24R,25(OH)2D3 at a concentration of 20 ng/ml did not inhibit the 45Ca-releasing effect of a submaximal concentration of PTH (500 ng/ml). Therefore, the inhibitory effect of 24R,25(OH)2D3 on the bone response to 1,25(OH)2D3 appeared to be specific and probably due to a competitive inhibitory effect. In addition, the inhibitory effect of 24R,25(OH)2D3 was weak, since it could be partially overcome by increasing the concentration of 1,25(OH)2D3.

Subcellular mechanisms involving vitamin D

During the past 15 years a vitamin D endocrine system has been demonstrated in which vitamin D produced normally in the skin is activated first by conversion in the liver and subsequently in the kidney to a hormonal form, 1,25-(OH)2D3. The production of the hormonal form of vitamin D3, is regulated, and much has been learned regarding the molecular mechanism of the hydroxylations of vitamin D and regarding the physiologic regulators of the 25-OH-D-1-hydroxylase. Much remains to be learned regarding the mechanism whereby the 1-hydroxylase is modulated. 1,25-(OH)2D3 appears to function in the target organs of bone, intestine, kidney, and elsewhere by a nucleus-mediated process. Receptors for 1,25-(OH)2D3 have been clearly demonstrated and characterized in crude form. How the receptor and ligand interact with the nucleus is not clear, nor are the gene products that result from this interaction known. One product, a calcium binding protein, is known but its role in calcium transport is in debate. Although much has been learned in the last decade and a half, much remains to be learned regarding the molecular mechanisms whereby vitamin D brings about its remarkable changes in mineral metabolism.

Vitamin D metabolites and their relationship to azotaemic osteodystrophy

Bone biopsies were performed and serum concentrations of vitamin D metabolites were measured in twenty-four patients with chronic renal failure. The concentrations of three metabolites--25-dihydroxyvitamin D (25-OHD), 24,25-dihydroxyvitamin D (24,25-OH2D) and 1,25-dihydroxyvitamin D (1,25-OH2D)--were significantly lower in uraemic than in normal sera. There was a positive correlation between serum 24,25-OH2D and serum 25-OHD, whereas serum 1,25-OH2D values, which were independent of those of the other vitamin D metabolites, were negatively correlated with serum creatinine and serum inorganic phosphate. The two major skeletal lesions, osteomalacia and parathyroid osteopathy, while coexisting in many patients, varied independently in relation to their severity and were correlated with different vitamin D metabolites. The severity of osteomalacia was negatively correlated with the serum concentrations of 25-OHD and 24,25-OH2D while th severity of parathyroid osteopathy was negatively correlated with serum 1,25-OH2D values. To our knowledge these are the first systematic studies correlating skeletal histology with dihydroxylated vitamin D metabolites in renal failure in man.

Vitamin D deficiency inhibits pancreatic secretion of insulin

The effects of a vitamin D deficiency on insulin and glucagon release was determined in the isolated perfused rat pancreas by radioimmunoassay of the secreted proteins. During a 30-minute period of perfusion with glucose and arginine, pancreases from vitamin D-deficient rats exhibited a 48 percent reduction in insulin secretion compared to that for pancreases from vitamin D-deficient rats that had been replenished with vitamin D. Vitamin D status had no effect on pancreatic glucagon secretion. This result, along with the previously demonstrated presence in the pancreas of a vitamin D-dependent calcium-binding protein and cytosol receptor for the hormonal form of vitamin D, 1,25-dihydroxyvitamin D3, indicates an important role for vitamin D in the endocrine functioning of the pancreas.

Effects of vitamin D metabolites on bovine parathyroid hormone release in vitro

We evaluated the effects of 1 alpha,25-dihydroxycholecalciferol (1,25(OH)2D3), 24R,25-dihydroxycholecalciferol (24,25(OH)2D3), and 25-hydroxycholecalciferol (25(OH)D3) on the release of parathyroid hormone (PTH). Bovine parathyroid tissues were incubated in vitro for 4 h in low-calcium (1.0 mM) medium. 1,25(OH)2D3 ((10(-9)-10(-12)M), 24,25(OH)2D3 (10(-6)-10(-8)M), and 25(OH)D3 (5 X 10(-7)-5 X 10(-9)M) inhibited PTH release. Inhibition by all metabolites was concentration and time dependent. On a molar basis, 1,25(OH)2D3 was the most potent metabolite, being at least 100 times more potent than 24,25(OH)2D3 and 25(OH)D3; 24,25(OH)2D3 was about 5 times more potent than 25(OH)D3 at concentrations producing 65% inhibition. Inhibition by high concentrations of metabolites was evident by 1 h of incubation; inhibition was progressive throughout incubation, and maximal suppression to 30-40% of control occurred during the fourth and final hour of incubation. 1,25(OH)2D3 (10(-11) M), a low concentration that did not inhibit secretion, transiently stimulated release. In conclusion, under conditions of low-calcium-stimulated PTH release, 1,25(OH)2D3, 24,25(OH)2D3, and 25(OH)D3 inhibited PTH release, 1,25(OH)2D3 was the most potent inhibitor.

Serum calcium and phosphate concentrations and parathyroid morphology in rats treated with vitamin D metabolites

The serum concentrations of calcium and phosphate and parathyroid morphology were studied in rats treated with vitamin D metabolites. Twenty hours after a single injection of 1.25-dihydroxycholecalciferol (1.25-DHCC) or 25-hydroxycholecalciferol (25-HCC) the serum calcium and phosphate concentrations were not significantly altered in any group, but the 1.25-DHCC treated rats exhibited an increased number of dark chief cells and occurrence of a few atrophic chief cells. Four to eight weeks after daily injections of the vitamin D metabolites the 1.25-DHCC treated rats exhibited significantly increased serum calcium concentrations and parathyroid glands composed of atrophic and dark chief cells in solid and follicular arrangement, whereas the rats treated with 25-HCC showed unaffected serum calcium concentrations and parathyroid glands composed of solid sheets of light chief cells, often with vacuolated cytoplasm, a few dark chief cells, but no atrophic cells. The findings suggest a direct or indirect suppressive influence of 1.25-DHCC on parathyroid activity in rats.

Vitamin D: two dihydroxylated metabolites are required for normal chicken egg hatchability

When hens are raised to sexual maturity from hatching with 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] as their sole source of cholecalciferol (vitamin D3), fertile eggs appear to develop normally but fail to hatch. When hens receive a combination of 1,25(OH)2D3 and 24R,25-dihydroxyvitamin D3 [24,25(OH)2D3], hatchability equivalent to that with hens given vitamin D3 is obtained. These results suggest a biological role for 24,25(OH)2D3 not previously recognized.