Effect of vitamin D metabolites on bone metabolism in a rat model of postmenopausal osteoporosis

A rat model of postmenopausal osteoporosis was introduced, using ovariectomized rats on a low Ca diet. CT treatment of these animals for one month prevented the decrease in both mineral contents and physical properties of the femoral bone. Treatment of the animals with 1,25(OH)2D3 was effective in increasing bone mineral contents and maintaining positive mineral balance, but did not increase the physical tolerance of bones. In contrast, 24,25(OH)2D3 increased the breaking force of the femoral bone, with minimal effect on bone mineral contents and mineral balance. These results suggest that 1,25(OH)2D3 and 24,25(OH)2D3 act differently on the matrix phase and mineral phase of bones, but that they act together to maintain mineral balance and structural integrity of bones. The mechanism of how these vitamin D metabolites affect bone metabolism remain to be clarified.

End product inhibition of hepatic 25-hydroxyvitamin D production in the rat: specificity and kinetics

The role of vitamin D metabolites in the regulation of hepatic 25-hydroxyvitamin D production was investigated by examining the effects of 25-hydroxyvitamin D, 1,25-dihydroxyvitamin D, and 24,25-dihydroxyvitamin D on the synthesis of [25-3H]hydroxyvitamin D by rachitic rat liver homogenates. Production of [25-3H]hydroxyvitamin D was inhibited by 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D, but not by 24,25-dihydroxyvitamin D. 25-Hydroxyvitamin D increased the Km of the vitamin D-25-hydroxylase enzyme(s), while 1,25-dihydroxyvitamin D decreased the Vmax with a Ki of 88.7 ng/ml. Inhibition of hepatic 25-hydroxyvitamin D production by 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D may be another control mechanism to regulate circulating vitamin D levels.

24,25(OH)2D3 enhances the calcemic effect of 1,25(OH)2D3

The effect of 24,25(OH)2D3 on 1,25(OH)2D3-induced hypercalcemia was studied in normal rats. Serum (S) levels and urinary excretion of Ca2+ (UCaV) were measured in (a) control rats, (b) rats receiving a daily sc injection of 54 ng 1,25(OH)2D3, (c) rats receiving 24,25(OH)2D3 in the same dose and same manner, and (d) rats receiving 1,25(OH)2D3 + 24,25(OH)2D3. The animals were housed in metabolic cages and 24-hr urine specimens were collected. After 24 hr SCa2+ increased similarly with 1,25(OH)2D3 and with 1,25(OH)2D3 + 24,25(OH)2D3, while 24,25(OH)2D3 alone did not change SCa2+. UCaV after 24 hr increased significantly less (P less than 0.025) with 1,25(OH)2D3 + 24,25(OH)2D3 than with 1,25(OH)2D3 alone. After 5 days of 1,25(OH)2D3, SCa2+ rose from 5.1 +/- 0.15 to 6.29 +/- 0.08 whereas 1,25(OH)2D3 + 24,25(OH)2D3 effected a greater increase in SCa2+ up to 6.63 +/- 0.09 (P less than 0.01). 24,25(OH)2D3 alone did not change SCa2+. UCaV after 5 days of treatment rose similarly with 1,25(OH)2D3 and with 1,25(OH)2D3 + 24,25(OH)2D3. After 10 days of 1,25(OH)2D3 SCa2+ was 6.17 +/- 0.15 meq/liter while with the combination SCa2+ rose to 6.74 +/- 0.2 (P less than 0.025). 24,25(OH)2D3 alone did not change SCa2+. These results show that (a) 24,25(OH)2D3 alone does not alter SCa2+ in normal rats, (b) combined administration of 1,25(OH)2D3 + 24,25(OH)2D3 enhances the hypercalcemic response to 1,25(OH)2D3 without a parallel increase in UCaV, and (c) it is suggested that the effect of 24,25(OH)2D3 on serum Ca2+ level, at least partly, may result from its hypocalciuric effect.

Influence of vitamin D3 metabolites on the production of interleukins 1,2 and 3

We investigated the effect of vitamin D3 metabolites on the release of the three interleukins (IL) IL 1, IL 2 and IL 3 by mononuclear cells. Models for the production of these mediators were the release of IL 1 by the murine macrophage cell line P388D1, of IL2 by rat spleen cells, and of IL 3 by the murine WEHI-3 cell line. IL 1 production was significantly increased with 1 alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3) at 10(-10)M and above. 1,25(OH)2D3 did not stimulate cell proliferation as assessed with [methyl-3H]thymidine (3H-TdR) incorporation. 24,25-dihydroxyvitamin D3 (24,25(OH)2D3) and 25-hydroxyvitamin D3 (25(OH)D3) were about 1000 times less effective than 1,25(OH)2D3. IL 2 production, by cultured rat spleen cells stimulated with Concanavalin A, was decreased by increasing concentrations of 1,25(OH)2D3. The minimal effective dose varied between experiments and ranged from 10(-11) to 10(-8) M. Moreover, proliferation (3H-TdR incorporation) of mouse thymocytes treated with phytohemagglutinin and IL 1 was decreased in a dose-dependent fashion by 1,25(OH)2D3 starting at 10-11) M. This effect might be secondary to a decrease of endogenous IL 2 production. IL 3 release by WEHI-3 cells was significantly increased with 10(-11)-10(-9) M 1,25(OH)2D3, whereas higher concentrations were less effective or decreased IL 3 production. These results show that 1,25(OH)2D3 and, to a lesser extent, 24,25(OH)2D3 and 25(OH)D3 have selective effects on lymphokine production. It is tempting to speculate that the actions of 1,25(OH)2D3 on bone might in part be mediated by lymphokines. Moreover, we suggest that 1,25(OH)2D3 might not be an immunoregulator per se, but makes use of the immune system to exert its influence on one of its classical targets, namely the bone, and possibly on other connective tissues.

Inhibition of insulin receptor binding by A23187: synergy with phorbol esters

The ionophore A23187 inhibits the binding of insulin on U-937 monocyte-like cells which had been induced to differentiate by 1 alpha, 25-(OH)2 cholecalciferol, while it remains inactive on the undifferentiated cells. A 50% reduction of the specific binding of 125I-insulin is observed within 20 to 30 min at 37 degrees C. The effect is obtained at 10(-7)M to 10(-6)M A23187. It is reversible in 60 min at 37 degrees C. A suboptimal concentration of the ionophore potentiates the inhibitory action of phorbol esters on insulin binding.

Recombinant gamma-interferon and lipopolysaccharide enhance 1,25-dihydroxyvitamin D3-induced cell differentiation in human promyelocytic leukemia (HL-60) cells

The induction of cell differentiation by a combination of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3], recombinant gamma-interferon (rec gamma-IFN), and a lipopolysaccharide from E. coli (LPS) was studied in a clonal population (clone-9) of human promyelocytic HL-60 leukemia cells in vitro. Treatment of clone-9 cells with 10(-9) to 10(-7)M 1,25-(OH)2D3 yielded a macrophage cell differentiation. The addition of 10 or 100 U/ml of gamma-IFN and 2 or 10 micrograms/ml LPS caused a further increase in expression of the different differentiation markers. The most pronounced effects involved increases in cell attachment to the surface of tissue-culture Petri dishes and in lysozyme, nonspecific esterase, and cytolytic activities. The combined treatment with 1,25-(OH)2D3 and rec gamma-IFN and LPS also caused an increase in the percent of multinucleated giant cells. These results indicate the effectiveness of combining different agents in inducing cell differentiation in HL-60 cells. A similar approach may be useful in controlling myeloid leukemias in vivo.

1 alpha,25-Dihydroxyvitamin D3 (calcitriol) stimulates proliferation of human circulating monocytes in vitro

Previous studies demonstrated that human circulating monocytes can proliferate in vitro when incubated with lectin-induced factor(s) from lymphocytes [(1985) Biochem. Biophys. Res. Commun., in press]. This study shows that human monocytes were induced to proliferate when incubated with 1 alpha,25-dihydroxyvitamin D3 (calcitriol) at physiological concentrations. The optimal dose was about 10 nM. Proliferative activity was examined both by measuring the [3H]thymidine incorporation and by counting cell nuclei. Among other derivatives of vitamin D3, 1 alpha,24R-dihydroxyvitamin D3 and 1 alpha,24R,25-trihydroxyvitamin D3 stimulated mitotic activity of monocytes. Addition of both calcitriol and lectin-stimulated lymphocyte-conditioned medium to the monocyte culture had an additional effect on the mitotic activity of monocytes.

Vitamin D metabolites regulate osteocalcin synthesis and proliferation of human bone cells in vitro

The effects of six natural vitamin D metabolites of potential biological and therapeutic interest, 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3), 25-hydroxyvitamin D3 (25-OH-D3), 24R,25-dihydroxyvitamin D3 (24R,25-(OH)2D3), 1,24R,25-trihydroxyvitamin D3 (1,24R,25-(OH)3D3), 25S,26-dihydroxyvitamin D3 (25S,26-(OH)2D3) and 1,25S,26-trihydroxyvitamin D3 (1,25S,26-(OH)3D3) on cell replication and expression of the osteoblastic phenotype in terms of osteocalcin production were examined in cultured human bone cells. At a dose of 5 X 10(-12) mol/1, 1,25-(OH)2D3 stimulated cell proliferation, whereas at higher doses (5 X 10(-9)-5 X 10(-6) mol/1) cell growth was inhibited in a dose-dependent manner. The same pattern of effects was seen for the other metabolites in a rank order of potency: 1,25-(OH)2D3 greater than 1,25S,26-(OH)3D3 = 1,24R,25-(OH)3D3 greater than 25S,26-(OH)2D3 = 24R,25-(OH)2D3 = 25-OH-D3. Synthesis of osteocalcin was induced by 1,25-(OH)2D3 in doses similar to those required to inhibit cell proliferation. Biphasic responses were observed for some of the metabolites in terms of osteocalcin synthesis, inhibitory effects becoming apparent at 5 X 10(-6) mol/1. The cells did not secrete osteocalcin spontaneously. These results indicate that vitamin D metabolites may regulate growth and expression of differentiated functions of normal human osteoblasts.

1a,25-Dihydroxyvitamin D3 stimulates alkaline phosphatase activity and inhibits soft-tissue proliferation in implants of bone matrix

To test the importance of vitamin D metabolites on intramuscular implants of demineralized bone, four-month-old rats were given either 1a,25-(OH)2D3 or 24R,25-(OH)2D3, or a combination of both metabolites, and sacrificed at intervals ranging from five to 35 days after implantation. Histologically there was a reduced ingrowth of mesenchymal cells into the implanted matrix cylinders in the presence of 1a,25-(OH)2D3; the reduction was followed by decreased total DNA and protein values until the 16th experimental day. At 35 days postimplantation, the quantity of new bone was the same in all treated groups. However, 1a,25-(OH)2D3 increased the alkaline phosphatase activity 60%-110% (depending on the denominator used). The metabolite 24R,25-(OH)2R3 had no effect on cell growth or the alkaline phosphatase activity. These results provide evidence for the inhibitory effect of 1a,25-(OH)2D3 on mesenchymal cell growth and its stimulatory effect on osteoblasts, which are responsible for increased alkaline phosphatase activity and new bone formation in vivo.

[Effect of 1,25-dihydroxycholecalciferol and 24,25-dihydroxycholecalciferol on calcium homeostasis in rats treated with hydrocortisone]

Physiological dose of 1,25(OH)2D3 (0.03 microgram) normalized the phosphorus-calcium metabolism and improved the state of bone tissue in rats treated with hydrocortisone. An increased dose of 1,25(OH)2D3 (0.15 microgram) caused hyperphosphatemia and augmented osteoporotic alterations in the hydrocortisone-treated animals. 24,25(OH)2D3 at a dose of 0.3 mg did not exhibit any positive effect on phosphorus-calcium metabolism and on the state of bone tissue in rats with exogenous hypercorticoidism. At the same time, high doses of 24,25(OH)2D3 increased distinctly the bone tissue density as well as the content of calcium and phosphorus. The most favourable state of the calcium homeostasis and of bone tissue in exogenous hypercorticoidism was observed after simultaneous administration of 1,25(OH)2D3 and 24,25(OH)2D3. These data suggest that a set functionally active metabolites of vitamin D3 should be used in cases of long-term treatment with glucocorticoids.

1,25-Dihydroxyvitamin D3 and 1,24-dihydroxyvitamin D3 suppress in vitro antibody response to T cell-dependent antigen

1,25-Dihydroxyvitamin D3 and 1,24-dihydroxyvitamin D3 suppressed an antibody response to sheep red blood cells (SRBC, T cell-dependent antigen) by murine splenocytes, in concentrations ranging from 10(-10)-10(-7)M. These suppressive effects were markedly abrogated when T cell-depleted lymphocytes were cultured in the presence of a supernatant of concanavalin A-stimulated spleen cells. On the contrary, neither of them suppressed antibody response to trinitrophenyl-lipopolysaccharide (T cell-independent antigen). These results suggest that the suppressive effect of active vitamin D3 on anti-SRBC response was mediated by the inhibition of T cells.

[Effect of 1,25-dioxycholecalciferol and 24,25-dioxycholecalciferol on calcium homeostasis in rats given hydrocortisone in the diet with various phosphorus contents]

High content of phosphorus in the diet (1.8% of phosphorus in the diet, Ca:P ratio 1:3) accelerated the development of hypocalcemia and osteoporosis and increased their degree in rats which received hydrocortisone (3.5 mg/10 g bw a day for 4 weeks). Reduction of phosphorus consumption to 0.3% (Ca:P ration 1:0.5) essentially retarded the development of these disturbances and lowered their degree. The use of 1,25-dioxycholecalciferol and 24,25-dioxycholecalciferol in doses of 0.03 and 1.5 mg, respectively promoted the normalization of phosphorus-calcium metabolism and improvement of the status of the osseous tissue in rats given hydrocortisone coupled with both diets. The most beneficial affect on calcium homeostasis in exogenous hypercorticoidism was attained with the use of active metabolites of vitamin D3 coupled with the diet with a low phosphorus content (0.3%). In this case there was a complete normalization of the density of the osseous tissue and of the calcium and phosphorus content. In view of this fact it is advisable to combine active metabolites of vitamin D3 and the diet with a low phosphorus content.

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.

Effects of corticosteroid and 1,24R-dihydroxy-vitamin D3 administration on lymphoproliferation and autoimmune disease in MRL/MP-lpr/lpr mice

The pharmacological effects of prolonged administration of a corticosteroid, betamethasone, and active vitamin D3 [1,24R-(OH)2D3] on lymphoproliferation and autoimmune disease of MRL/MP-lpr/lpr (MRL/1) mice were examined. Relatively high doses of betamethasone (0.25 mg/kg/day) prevented lymphoproliferation, reduced serum levels of anti-dsDNA, anti-ssDNA, and anti-poly (ADP-ribose) antibodies, and brought about clinical improvement, such as reduced proteinuria and diminution of skin lesions. It is noteworthy that not only did prevention of lymphoproliferation occur, but also recovery of the Lyt-2+ T cell subset in the thymus and the spleen was observed. The administration of 1,24R-(OH)2D3 (0.1 microgram/kg/day) similarly prevented proteinuria, and produced recovery of a Lyt-2+ subset in the thymus.

Structural study on the effect of 24,25-dihydroxyvitamin D3 on epiphyseal growth plate in suckling mice

The in vivo effects of 24,25(OH)2D3 on cellular structure and organization, matrix metachromasia and mineralization were studied in epiphyseal growth plate of normal neonatal mice. A relatively low dose of the metabolite, 40 ng/kg body weight, significantly increased the overall size of humeral growth plate and the zone of cellular proliferation. By and large, the tissue's response to the metabolite did not change with the increase in dose administered except for a decrease in the number of chondroblasts. 24,25(OH)2D3 led to significant increases in the metachromatic reaction of the cartilaginous matrix, but appeared to depress the mineralization process. Qualitative structural changes were noted in chondroblasts and hypertrophic chondrocytes. 24,25(OH)2D3 affected the osteoblastic and osteocytic populations of cells in the metaphysis and diaphysis of the humerus. High doses of 24,25(OH)2D3 brought about distinct atrophic changes in the above cells. These findings indicate that excessive doses of 24,25(OH)2D3 in an intact animal may lead to retardative effects upon bone growth.

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)

Early and direct effect of 1,25-dihydroxycholecalciferol on calcium uptake by duodena of rachitic chicks

Injection of 1,25 dihydroxycholecalciferol (1,25(OH)2D3, 10 micrograms) directly into the in situ ligated duodenal loop of rachitic chicks significantly elevated the tissue accumulation of 47Ca within 20-30 min. The transfer of 47Ca from lumen to blood, during the same time period, was not increased nor was there any measurable intestinal calcium-binding protein synthesized. Lesser amounts of 1,25(OH)2D3 (1 or 5 micrograms) did not result in any statistically significant elevation of 47Ca tissue accumulation, nor did they have any effect on 47Ca transfer from lumen to blood (transmural). Ten micrograms of 1,24R,25(OH)3D3 was similarly effective in elevating tissue accumulation, whereas 24R,25(OH)2D3 and 25(OH)D3 were not. These results provide additional evidence for an early and direct action of 1,25(OH)2D3 in altering intestinal epithelial membrane transport prior to the induction of synthesis of specific transport proteins.

Hydroxylation of carbon-24 of 25-hydroxycholecalciferol is not necessary for normal embryonic development in chickens

Laying hens fed 25-hydroxycholecalciferol, 24,24-difluoro-25-hydroxycholecalciferol, 1,25-dihydroxycholecalciferol or 24,25-dihydroxycholecalciferol were used to investigate whether hydroxylation of C-24 of cholecalciferol is necessary for normal embryonic development in chickens. Laying hens were fed a rachitogenic diet and 25-hydroxycholecalciferol from hatching until normal egg production, fertility and hatchability were achieved. When the hens were 40 weeks old, 25-hydroxycholecalciferol was withdrawn and egg production ceased in 4 weeks. The hens were divided into 6 groups of 5 and dosed daily for 19 weeks with either 2.0 micrograms of 25-hydroxycholecalciferol, 2.0 micrograms of 24,24-difluoro-25-hydroxycholecalciferol, 0.4 microgram of 1,25-dihydroxycholecalciferol, 2.0 micrograms of 24,25-dihydroxycholecalciferol, both 1,25-dihydroxycholecalciferol and 24,25-dihydroxycholecalciferol or vehicle only. Egg production during this period was high for all hens fed the cholecalciferol compounds. Egg production of 3% occurred in hens given vehicle only. Fertility was over 90% for all groups of cholecalciferol compound-fed hens. Hatchability of over 90% was achieved with the eggs from hens given 25-hydroxycholecalciferol or 24,24-difluoro-25-hydroxycholecalciferol and 6% with eggs from hens fed both 1,25-dihydroxycholecalciferol and 24,25-dihydroxycholecalciferol. No eggs from hens fed 1,25-dihydroxycholecalciferol or 24,25-dihydroxycholecalciferol alone hatched (over 140 eggs in each group.

Chemical synthesis of (24R)-24,25-dihydroxy[26,27-3H]vitamin D3 of high specific activity

Chemical synthesis of (24R)-24,25-dihydroxy-[26,27-3H]vitamin D3, and its 24-epimer has been devised that allows introduction of 3H at the terminal step of the synthesis. The epimeric mixture is derivatized as the tris(trimethylsilyl) ethers and resolved by high-performance liquid chromatography. The product has a specific activity of 178 Ci/mmol and is fully active in binding to the rat plasma vitamin D binding protein and in the elevation of serum calcium levels of vitamin D deficient rats. The synthesis begins with the readily available 3 beta-hydroxy-5-cholenic acid methyl ester and involves a Pummerer rearrangement, introduction of the delta 7, irradiation, and isolation of the 26,27-dinor-25-carboxylic acid methyl ester of vitamin D3. This compound is then treated with a Grignard reagent containing 3H (80 +/- 10 Ci/mmol).

Absence of effect of 24,25-dihydroxycholecalciferol on serum immunoreactive PTH in patients with persistent hyperparathyroidism after renal transplantation

Three hypercalcemic renal transplant recipients with stable, excellent renal function (creatinine clearance 74 +/- 11.8 ml/min) were treated with 60 micrograms 24,25(OH)2D3 by mouth daily for three months. Immunoreactive c-terminal PTH, intact PTH, 1,25(OH)2D3, 25(OH)D3, 24,25(OH)2D3, and serum and 24 h urine calcium, phosphate, magnesium and creatinine were obtained before, at one week, one month and three months of treatment, and at six weeks post-treatment. Significant elevations in serum levels of 24,25(OH)2D3 were induced by therapy (1.32 +/- .16 ng/ml to 30.06 +/- 5.18 ng/ml at one month). Moderate elevations of c-terminal PTH and normal levels of intact PTH remained unchanged throughout the study. Serum calcium remained elevated, serum phosphate and magnesium remained depressed and creatinine clearance and urinary excretion of calcium, phosphate, and magnesium remained unchanged. Furthermore, 1,25(OH)2D3 and 25(OH)D3 remained in the normal range throughout the study. We conclude that 24,25(OH)2D3 did not have a suppressant effect on levels of iPTH in the clinical setting of persistent hyperparathyroidism after successful renal transplantation.

Effect of various vitamin D analogs on plasma calcium and phosphorus and intestinal calcium absorption in fed and unfed American eels, Anguilla rostrata

Injection of about 1 ng/g body wt per day of either vitamin D3 or 1,25-(OH)2-vitamin D3 for 7 days induces hypercalcemia and hyperphosphatemia in fed American eels, Anguilla rostrata, but only hyperphosphatemia in unfed eels. These same analogs also stimulated the uptake of 45Ca from intestinal sacs in situ. The vitamin D3 appeared to be relatively more effective than the 1,25-(OH)2D3 metabolite and chlorpromazine inhibited the effect of vitamin D3 on intestinal calcium uptake. 7-Dehydrocholesterol, vitamin D2, and 24,25-(OH)2D3 did not stimulate hypercalcemia, hyperphosphatemia, or intestinal calcium uptake.

1 alpha,25-dihydroxyvitamin D3 suppresses proliferation and immunoglobulin production by normal human peripheral blood mononuclear cells

Activated B and T lymphocytes from normal human subjects are known to have the specific high-affinity receptor for 1,25-dihydroxyvitamin D3 (1,25-(OH)2-D3). In an attempt to determine a functional role for the sterol in such cells, we studied the effect of 1,25-(OH)2-D3 on DNA synthesis and Ig production by normal human peripheral blood mononuclear (PBM) cells activated in vitro by the polyclonal lymphocyte activators pokeweed mitogen and phytohemagglutinin, and the specific antigen dermatophyton O. A dose-dependent inhibition of [3H]thymidine incorporation was observed in cells incubated with 1,25-(OH)2-D3 in concentrations ranging from 10(-10) to 10(-7) M. Production of IgG and IgM, determined by enzyme-linked immunosorbent assay, was similarly inhibited by increasing concentrations of 1,25-(OH)2-D3. Half-maximal inhibition of DNA and Ig synthesis was found at 10(-10) to 10(-9) M 1,25-(OH)2-D3. This suppressive effect was specific for 1,25-(OH)2-D3; of the other vitamin D metabolites examined, only 10(-7) M 24R,25 dihydroxyvitamin D3 (24,25-(OH)2-D3) had a similar inhibitory effect. 1,25-(OH)2-D3 was not cytotoxic and did not affect unactivated PBMs. These data demonstrate that 1,25-(OH)2-D3 is a potent inhibitor of human PBM Ig production in vitro and suggest that this action is mediated through the hormone's antiproliferative effect on Ig-producing B cells and/or helper T cells.

The effect of vitamin D3 metabolites on normal and leukemic bone marrow cells in vitro

We studied the effects of 1,25-dihydroxyvitamin D3 and other metabolites of vitamin D3 on the maturation in liquid culture and on colony formation in semisolid media of marrow and buffy coat cells from patients with myeloid leukemias and from normal individuals. After incubation with 1,25-dihydroxy-vitamin D3, a proportion of both normal and leukemic myeloid cells resembled cells of the monocyte-macrophage lineage; these cells expressed alpha-naphthylacetate esterase and were able to phagocytize and kill candida organisms. When granulocyte-macrophage progenitor cells (CFU-GM) were incubated with 1,25-dihydroxyvitamin D3, the number of monocyte-macrophage colonies was increased and the number of granulocyte colonies was reduced; megakaryocyte colony formation (CFU-Mk) was inhibited substantially; and there was no effect on erythroid (BFU-E) or multilineage (CFU-GEMM) progenitor cell colony formation. We propose that 1,25-dihydroxyvitamin D3 may induce cells that are normally committed to differentiate along the granulocytic pathways to differentiate instead along the monocyte-macrophage pathway. If these in vitro observations reflect the in vivo activity of 1,25-dihydroxyvitamin D3, it may be involved in the modulation of collagen deposits in the bone marrow.

[Effect of 1,25-dihydroxyvitamin D3 and 24,25-dihydroxyvitamin D3 on growth and remodeling of rat bones during hypokinesia (histomorphometric study)]

The tubular bones of the fore- and hindlimbs of rats immobilized for 5 weeks were examined morphometrically and histologically. The rats were regularly given per os 1,25(OH)2D3, 24,25(OH)2D3 or their combination. The uptake of 24,25(OH)2D3 at a dose of 1.25 micrograms or a combination of 1,25(OH)2D3 and 24,25(OH)2D3 at a dose of 0.03 + 0.25 micrograms led to the recovery of the linear and volume-weight rates of bone formation that changed during hypokinesia. However, these D3 metabolites did not restore the width of the epiphyseal growth plate, whereas the size of the primary and secondary spongiosa returned to normal or exceeded it in response to 24,25(OH)2D3 at a dose of 1.25 micrograms and 1,25(OH)2D3 at a dose of 0.15 micrograms, respectively (only these two doses were used); in other words, the D3 metabolites prevented osteoporosis which is typical of hypokinesia. It is assumed that hypokinesia may produce either disorders in D3 metabolism or changes in the sensitivity of bone cells to active D3 metabolites and other hormones that are directly or indirectly involved in osteogenesis regulation.

24,25-Dihydroxycholecalciferol induces the growth of chick cartilage in vitro

Recent studies have indicated that 24R,25-dihydroxycholecalciferol [24R,25(OH)2D3] induces development of endochondral bone. It binds specifically to cytoplasmic and nuclear receptors in epiphyseal cartilage cells. In the present investigation we report the effects of 24R,25(OH)2D3 in comparison to other active vitamin-D metabolites on cell growth. This study was performed on micro-mass cell cultures which were prepared from 4.5-day-old embryonic chick skeletal mesenchyme: this culture consists of a high proportion of chondrocytes. Twelve nM 24R,25(OH)2D3 induces a 2-fold increase in [3H] thymidine incorporation into DNA after 24 h of treatment. Other metabolites, either at this or higher concentrations, had no significant effect. [3H]-leucine incorporation into protein and ornithine decarboxylase (ODC) activity were also enhanced only by 24R,25(OH)2D3 at 12 nM (2.4- and 2.0-fold, respectively). These results present supporting evidence for the specific role of 24R,25 (OH)2D3 in the growth and differentiation of developing cartilage cells.

In vitro stimulation of articular chondrocyte differentiated function by 1,25-dihydroxycholecalciferol or 24R,25-dihydroxycholecalciferol

The effects of 1,25-dihydroxycholecalciferol (1,25-(OH)2D3) (10(-13)M-10(-8) M) and 24R ,25-dihydroxycholecalciferol ( 24R ,25-(OH)2D3) (10(-12)M-10(-7) M) on cell proliferation and proteoglycan deposition were examined in our newly developed multilayer culture system for rabbit and human articular chondrocytes. The cells are embedded in an extracellular matrix similar to that seen in vivo and maintain their in vivo phenotype. We extracted and purified native proteoglycans and degraded material from three culture compartments: the medium, intercellular matrix, and cells. Proteoglycan synthesis and deposition were analyzed by measuring 35SO4 incorporation, hexuronic acid, and galactose contents. In both rabbit and human chondrocyte cultures, chronic 1,25-(OH)2D3 treatment inhibited chondrocyte proliferation and stimulated proteoglycan synthesis and accumulation in the three compartments at 10(-12)-10(-8) M; maximal effect was at 10(-10)M. Cell proliferation was reduced by 55% and the content of hexuronic acid (or galactose) was increased to about three times that of controls in all compartments. 1,25-(OH)2D3 did not alter the proteoglycan composition. Chronic 24R ,25-(OH)2D3 treatment induced comparable effects with a maximum at 10(-8)M. When human dermal fibroblasts were treated as above both vitamin D metabolites increase mitosis. 1,25-(OH)2D3 mainly reduced the pericellular deposition of proteoglycans, while 24R ,25-(OH)2D3 appeared to reduce their synthesis and deposition in both medium and pericellular compartments. These results suggest that both 1,25-(OH)2D3 and 24R ,25-(OH)2D3 act specifically on articular chondrocytes to promote phenotype expression.

Vitamin D3 active metabolites as a countermeasure against disorders of calcium-phosphorus metabolism in hypokinetic rats

Male Wistar rats that were experimentally hypokinetic were fed 24,25(OH)2D3 or 1,25(OH)2D3 separately or in combination to determine the effect on bone growth and on bone formation and resorption. It was shown that these parameters of bone metabolism are influenced by these metabolites of vitamin D3 by their effect on bone sensitivity to their activity and perhaps in the regulation of bone histogenesis.

Stimulation of creatine kinase BB activity by 1 alpha,25-dihydroxycholecalciferol and 24R,25-dihydroxycholecalciferol in rat tissues

Vitamin D metabolites stimulate creatine kinase BB activity in organs of vitamin D-deficient rats. In epiphyses of long bones, creatine kinase BB activity increases 2.6-fold 24 h after injection of 24R,25-dihydroxycholecalciferol but not of 1 alpha,25-dihydroxycholecalciferol. Contrariwise, 1 alpha,25-dihydroxycholecalciferol, but not 24R,25-dihydroxycholecalciferol, increases creatine kinase BB activity in diaphyses and in kidney. Neither metabolite affects creatine kinase activity in duodenal mucosa.

Inhibitory effects of active vitamin D preparations on PTH secretion in rats

To study the effects of various vitamin D preparations on PTH secretion, serum calcium and urinary excretion of cAMP were monitored in conscious perfused rats, and the influences of a bolus iv injection of the preparations on these parameters were examined. Three hours after the administration of 0.25 microgram/kg (0.6 nmol/kg) of 1 alpha, 24(R)-dihydroxycholecalciferol [1 alpha, 24(OH)2D3], the urinary excretion of cAMP decreased to a level compatible with that of parathyroidectomized (PTX) rats (50% of initial value; p less than 0.05) with no change in the concentration of serum calcium (total and ionized). In PTX rats supplemented with bovine PTH (1 U/h), the vitamin D preparation showed no significant effects either on the urinary excretion of cAMP or on serum calcium. These effects were rather specific for active vitamin D preparations, i.e. 1 alpha, 25(OH)2D3 (0.25 micrograms/kg) and 1 alpha OHD3 (1.25-6.25 micrograms/kg). However, 24,25(OH)2D3 (up to 25 micrograms/kg) had no significant effect on these parameters. These results suggest that, in rats, active vitamin D preparations specifically inhibit PTH secretion without causing a significant increase in the serum calcium concentration, reflecting a direct feedback mechanism between active vitamin D metabolite and the parathyroid glands.

24,25(OH)2D3, bone formation, and bone resorption in vitamin D-deficient, azotemic rats

Bone formation, mineralization, and resorption were measured in vitamin D-deficient, azotemic rats given two different dosages of 24,25(OH)2D3 daily and in vehicle-treated controls (C). The intraperitoneal administration of 65 pmol over a 10 day period corrected the hypocalcemia observed in C, whereas 130 pmol produced mild hypercalcemia. Both dosages reduced osteoid width, osteoid area, and mineralization front width from control values. The rates of bone and matrix formation were unaffected by treatment. In C, matrix formation exceeded bone formation and resulted in osteoid accumulation; both dosages of 24,25(OH)2D3 reversed this relationship such that bone formation exceeded matrix formation in each treatment group. The rates of osteoid maturation and initial mineralization increased during repletion with 24,25(OH)2D3 at both dosage levels. However, the serum calcium concentration was correlated with both osteoid maturation rate (r = 0.68, P less than 0.01) and initial mineralization rate (r = 0.63, P less than 0.01) when all three experimental groups were considered. Bone resorption was unchanged from control values during treatment with 24,25(OH)2D3. The results suggest that 24,25(OH)2D3 promotes the maturation and mineralization of osteoid, and that this metabolite differs in its effects on bone formation and resorption. It is not clear, however, that the changes in bone dynamics observed are independent of the calcemic response induced by metabolite repletion under the conditions of this experiment.

Effects of 1,25- and 24,25-dihydroxycholecalciferol on parathyroid hormone release from human parathyroid cells in vitro

The effects of 1,25-dihydroxycholecalciferol (1,25-(OH)2D3) and 24,25-dihydroxycholecalciferol (24,25-(OH)2D3) on parathyroid hormone (PTH) release from human parathyroid cells were investigated using an in vitro system of dispersed cells. The cells were obtained from 7 patients with primary hyperparathyroidism (HPT) and adenoma, 4 patients with primary HPT due to hyperplasia and 2 patients with parathyroid hyperplasia secondary to chronic renal failure. The dispersed cells were incubated in tissue culture medium at low, normal and high external calcium concentrations for 2-16 h. There was a gradual suppression of PTH release (5-55%) when the calcium concentration in the medium was increased from 0.5 to 3.0 mM, thus indicating retained regulation of hormone release. The addition of 1,25-(OH)2D3 in concentrations of 0.1 and 1 ng/ml and of 24,25-(OH)2D3 in concentrations of 1.0 and 10 ng/ml during the incubations did not further affect the amount of PTH released by the cells. The concentrations of the different vitamin D metabolites tested closely correspond to levels observed under normal physiological conditions and during treatment with high doses of vitamin D in vivo. Thus, the findings contradict the idea of any direct short-term regulatory effect of either 1,25-(OH)2D3 or 24,25-(OH)2D3 on the secretion of PTH from hyperfunctioning human parathyroid tissue.

24R,25-dihydroxyvitamin D stimulates creatine kinase BB activity in chick cartilage cells in culture

In chick limb-bud cartilage cell cultures 24R,25-dihydroxycholecalciferol (24R,25(OH)2D3), but not 24S,25(OH)2D3, 1 alpha,25(OH)2D3 or 25(OH)D3, stimulates the activity of the brain type (BB) isozyme of creatine kinase (EC 2.7.3.2), the 'estrogen-induced protein' first identified in rat uterus. Cultures treated with bromodeoxyuridine, in which cartilage formation is inhibited, show no stimulation of creatine kinase BB by 24R,25(OH)2D3.

The effects of prostaglandin E2, parathyroid hormone, 1,25 dihydroxycholecalciferol, and cyclic nucleotide analogs on alkaline phosphatase activity in osteoblastic cells

Clone MC3T3-E1 cells isolated from newborn mouse calvaria had the same type of alkaline phosphatase (ALP) as that found in adult mouse calvaria (the liver-bone-kidney type), as judged by polyacrylamide gel electrophoresis as well as by heat lability and amino acid inactivation. The effects of prostaglandin E2 (PGE2), parathyroid hormone (PTH), 1,25 dihydroxycholecalciferol [1,25(OH)2D3], and adenosine-3',5'-cyclic monophosphate (cAMP) analogs on ALP were investigated. PGE2 and 1,25(OH)2D3 increased ALP activity in dose-related manner with a maximal effect at concentrations of 10 ng/ml and 40 pg/ml, respectively. N6,O2-dibutyryl adenosine-3',5'-cyclic monophosphate (DBcAMP) also induced an increase in ALP activity in a dose-related fashion with a maximal effect at a concentration of 0.5 mM which was 2.2-fold over that of the controls. Induced ALP was of the "liver-bone-kidney" type. Antinomycin D and cycloheximide inhibited the increase in ALP activity induced by DBcAMP. The level of ALP was elevated by 8-bromo-adenosine-3',5'-cyclic monophosphate and theophylline, but not by N6,O2-dibutyryl guanosine-3',5'-cyclic monophosphate and sodium butyrate. Moreover, PGE2 dramatically increased the level of cAMP in the cells with a maximal effect at a concentration of 10 ng/ml, indicating that PGE2 and DBcAMP induce an increase of ALP activity in clone MC3T3-E1 cells by increasing the cAMP level; PTH did not affect enzyme activity and cAMP level in the cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Production of osteocalcin by human bone cells in vitro. Effects of 1,25(OH)2D3, 24,25(OH)2D3, parathyroid hormone, and glucocorticoids

Cells have been cultured from human bone that possess several characteristics of osteoblasts, including the capacity to produce osteocalcin (bone Gla protein). In these cultures the production of osteocalcin is dependent on 1,25(OH)2D3 but is not affected by 24,25(OH)2D3 either alone or in combination with 1,25(OH)2D3. Two glucocorticoids, prednisolone and deflazacort, reverse the stimulation of osteocalcin synthesis by 1,25(OH)2D3 in a dose-dependent manner (10(-9) - 10(-6)M). Parathyroid hormone also inhibits osteocalcin production in a dose-dependent fashion (0.2-5 IU/ml). These results demonstrate that human bone cell cultures may be of considerable value in investigating the hormonal and pharmacologic regulation of the production of osteocalcin and other bone proteins in vitro.

The influence of dihydroxylated vitamin D metabolites on bone formation in the chick

The ability of 1,25(OH)2D3 and of 24,25(OH)2D3 to prevent or to heal rickets in chicks was evaluated by studies of plasma biochemistry, growth plate histology, bone morphometry and microradiography, and bone mineralization. 1,25(OH)2D3 at a dose of 100 ng/day produced fewest abnormalities compared with vitamin D3-treated control chicks. Bone growth was slightly greater than vitamin D3-treated controls in chicks given a lower dose of this metabolite; the reverse was observed in chicks given a higher dose. 24,25(OH)2D3 was less effective than 1,25(OH)2D3 in preventing rickets even at doses as high as 400 ng/day. Treatment of rachitic chicks with doses of 24,25(OH)2D3 up to 300 ng/day produced no healing effect on the bone lesions, in marked contrast to the beneficial effects observed with 1,25(OH)2D3.

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.

Normal egg hatchability requires the simultaneous administration to the hen of 1 alpha,25-dihydroxycholecalciferol and 24R,25-dihydroxycholecalciferol

A previous report [Science 201, 835-837 (1978)] presented evidence that the combined and simultaneous administration of the cholecalciferol (D3) metabolites 1 alpha,25-dihydroxycholecalciferol [1 alpha,25(OH)2D3] and 24R,25-dihydroxycholecalciferol [24R,25(OH)2D3] to White Leghorn hens was necessary for embryo development and normal egg hatchability; in the absence of 24R,25(OH)2D3 none of the fertile eggs hatched. The present study extends this fundamental observation to a second species, the Japanese quail, Coturnix coturnix japonica and compares the biological actions of the two stereoisomers of the 24,25(OH)2 metabolite, namely the naturally occurring 24R,25(OH)2D3 and its unnatural epimer 24S,25(OH)2D3. Groups of 12-14 vitamin D-depleted adult female Japanese quail were mated with normal male quail and eight consecutive batches of eggs (25-41 eggs from each group) were placed in an egg incubator, and egg hatchability for the fertile eggs monitored on days 21 and 22. The egg hatchability (in percent +/- SD) for each group was: D3 (56.5% +/- 12.8); 1 alpha,25(OH)2D3 (1.3% +/- 2.5); 24R,25(OH)2D3 (29.6% +/- 3.1); 24R25(OH)2D3 + 1 alpha,25-(OH)2D3 (32.8%); and 24S,25(OH)2D3 + 1 alpha,25(OH)2D3 (7.2%). Also for all treatment groups the blood level of the expected vitamin D metabolites were in the normal range, and there were no significant differences in the embryo weights and eggshell thickness (of both hatched and unhatched eggs). These results indicate that the Japanese quail have the inherent capability to discriminate between the stereoisomers of 24,25(OH)2D3 and therefore strongly support the concept that only the naturally occurring 24R,25(OH)2D3 has an identifiable, unique biological role which is different from that of 1 alpha,25(OH)2D3.

Support of embryonic chick survival by vitamin D metabolites

The provision of 1,25-dihydroxyvitamin D3 as the only source of dietary vitamin D3 to laying hens failed to support normal embryonic development in their fertile eggs. Significant (P less than .001) improvement in embryonic survival to hatching in these eggs resulted from injections of 1,25-dihydroxyvitamin D3, 24,25-dihydroxyvitamin D3, 25-hydroxyvitamin D3, or 24,24-difluoro-25-hydroxyvitamin D3 prior to incubation. Maximum embryonic survival with lowest embryonic mortality was observed when 0.20 micrograms/egg of 1,25-dihydroxyvitamin D3 or 0.60 micrograms/egg 25-hydroxyvitamin D3 was injected. These results indicate that several forms of vitamin D, two of which cannot be converted to 24,25-dihydroxyvitamin D3, can provide this activity; and of the vitamin D compounds tested, 1,25-dihydroxyvitamin D3 may be the most active in supporting embryonic survival in the chick when delivered directly by injection.

The synthesis of vitamin D metabolites by human melanoma cells

Two melanin-producing human melanoma cell lines originally established from fresh surgical specimens were incubated with 25 hydroxyvitamin D3 (25 OHD3). Both cell lines produced material comigrating with 1,25 dihydroxy-vitamin D3 (1,25(OH)2D3) and 24,25 dihydroxyvitamin D3 (24,25(OH)2D3) in straight and reverse phase high performance liquid chromatography systems and displacing the relevant labeled ligands in competitive binding assays. The material designated 1,25(OH)2D3 was found almost entirely within the cells, whereas 24,25(OH)2D3 was evenly distributed between cells and medium. The synthesis of dihydroxylated materials was time dependent and was not observed if the cells were boiled before incubation with 25 OHD3. Preincubation with 1,25(OH)2D3 caused an increase in the synthesis of 24,25(OH)2D3 and a decrease in the synthesis of 1,25(OH)2D3. Michaelis-Menten constant (Km) values were 1.4 X 10(-9) mol/liter 25 OHD3 for the 1-alpha-hydroxylase enzyme and 72 X 10(-9) mol/liter for 24-hydroxylase. These studies constitute further evidence for the extrarenal synthesis of 1,25(OH)2D3. The suppressibility of 1 alpha-hydroxylase by preincubation with 1,25(OH)2D3 suggests a regulatory function for this system in the skin.

[Comparative study of the effects of alpha-25-dihydroxycholecalciferol and 24, 25-dihydroxycholecalciferol on calcium-phosphorus metabolism and on bone tissue in experimental kidney insufficiency in rats]

Experimental chronic kidney insufficiency (CKI; within 2-6 months) in rats, kept on a diet containing 0.6% Ca2+ and 0.6% P was accompanied by distinct azotemia, hyperphosphatemia, by a decrease in specific weight, in content of Ca2+, P and hydroxyproline in diaphyses as well as by a decrease in epiphyseal Ca2+. Daily administration of 0.025 micrograms of 1 alpha, 25-dihydroxy-cholecalciferol (1,25 (OH)2D3) into the animals did not normalize any of the patterns studied. At the same time, 1,25 (OH)2D3 increased the rate of hypercalciemia and demineralization of epiphyses, causing a slight hypercalciemia and increasing distinctly calcinosis of aorta as well as of the remaining part of the kidney. After daily administration of 24, 25-dihydroxycholecalciferol (24, 25 (OH)2D3) at a dose of 0.25 micrograms most of the patterns studied were normalized; specific weight, content of Ca2+ and P were increased in diaphyses simultaneously with a decrease in blood phosphorus concentration and in the level of azotemia. 24, 25 (OH)2D3 increased also the collagen content in diaphyses and epiphyses. The higher dose of 24, 25 (OH)2D3 (1.25 micrograms) did not exhibit higher effectivity. No one of the 24, 25 (OH)2D3 doses used did cause hypercalciemia and calcinosis. Combination of 0.025 micrograms 1,25 (OH)2D3 with 1.25 micrograms of 24, 25 (OH)2D3 decreased slightly the hypercalciemic, hyperphosphatemic and calcinosis inducing effects of 1,25 (OH)2D3 preventing completely the osteoporotic alterations in diaphyses but increasing the epiphysis demineralization; these results indicate that the doses of these metabolites must be decreased if their combination is required. The data obtained suggest that 24, 25 (OH)2D3 is a more effective and safe drug in correction of Ca2+-P metabolism impairments as well as of bone destruction under kidney insufficiency conditions as compared with 1,25 (OH)2D3.

The effects of 24R,25-dihydroxycholecalciferol and of 1 alpha,25-dihydroxycholecalciferol on ornithine decarboxylase activity and on DNA synthesis in the epiphysis and diaphysis of rat bone and in the duodenum

The effect of cholecalciferol metabolites on ornithine decarboxylase activity and on DNA synthesis in developing long bones was investigated in vitamin D-depleted rats. In the epiphysis there was a 6.4-fold increase in ornithine decarboxylase activity 5 h after a single injection of 24R,25-dihydroxycholecalciferol but not of 24S,25-dihydroxycholecalciferol or other vitamin D metabolites. In comparison, in the diaphysis and duodenum, 1 alpha,25-dihydroxycholecalciferol, but not other vitamin D metabolites, caused a 3-3.5-fold increase in the enzyme activity. The enzyme activity in the tissues examined attained a maximal value at 5 h after the injection of the metabolites. The activity of ornithine decarboxylase in the epiphysial region increased dose-dependently as the result of a single injection of 24R,25-dihydroxycholecalciferol and attained a maximal value at a dose between 30 and 3000 ng. In addition, administration of 24R,25-dihydroxycholecalciferol, but not 24S,25-dihydroxycholecalciferol or other metabolites, caused within 24 h a 1.7-2.0-fold increase in [3H]thymidine incorporation into DNA of the epiphyses of tibial bones. In comparison, 1 alpha,25-dihydroxycholecalciferol caused a 1.5-fold increase in [3H]thymidine incorporation into DNA of the diaphyses and of the duodenum. The present data indicate that 24R,25-dihydroxycholecalciferol is involved in the regulation of epiphyseal growth, whereas 1 alpha,25,dihydroxycholecalciferol stimulates the proliferation of cells in the diaphysis of long bones and in the intestinal mucosa.

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.

Renal effect of vitamin D metabolites: evidence for the essential role of the 25(OH) group

The effects of 1 alpha (OH)vitamin D3 [1 alpha (OH)D3] and 24,25(OH)2vitamin D3 [24,25(OH)2D3] on the phosphaturic action of parathyroid hormone (PTH) were studied in two groups of parathyroidectomized (PTX) rats. In group 1, PTX PTH-infused rats received intravenous 1 alpha (OH)D3, and in group 2, PTX PTH-infused rats received intravenous 24,25(OH)2D3. PTX PTH-infused rats served as controls. The effects of both vitamin D metabolites on renal PTH-activated adenylate cyclase (AC) were studied in vitro. In group 1, PTH increased fractional excretion of phosphate (CP/CIn) from 0.045 +/- 0.012 (+/- SE) to 0.263 +/- 0.011 (P less than 0.005). 1 alpha (OH)D3 failed to influence this response. In group 2, PTH increased CP/CIn from 0.055 +/- 0.008 to 0.289 +/- 0.027 (P less than 0.005). 24,25(OH)2D3 reduced the PTH-induced rise in CP/CIn from 0.289 +/- 0.027 to 0.192 +/- 0.021 (P less than 0.01) and decreased the urinary excretion of adenosine 3',5'-cyclic monophosphate. In vitro, 24,25(OH)2D3 blunted the PTH-activated AC, whereas 1 alpha (OH)D3 had no effect. These results show that 24,25(OH)D3, similar to two other 25(OH) metabolites of vitamin D-25(OH)vitamin D3 and 1,25(OH)2vitamin D3-suppresses the phosphaturic action of PTH, whereas 1 alpha(OH)D3, which is devoid of a 25(OH) group, lacks this effect. This suggests that a 25(OH) group is a prerequisite for the antiphosphaturic effect of vitamin D, whereas the 1 alpha (OH) group is not essential for this action.

Effects of 1,25- and 24,25-dihydroxyvitamin D3 on bone formation in the cichlid teleost Sarotherodon mossambicus

1,25-Dihydroxyvitamin D3 and 24,25-dihydroxyvitamin D3 have antagonistic effects on the acellular bone of the tilapia Sarotherodon mossambicus. 1,25-Dihydroxyvitamin D3 inhibits the activity of the lining osteoblasts. Prolonged administration leads to demineralization of the bone matrix. Injection of 24,25-dihydroxyvitamin D3 induces structural signs of greatly enhanced appositional bone growth within three days. No effects were observed on the mineral content of pre-existing bone. Both 1,25- and 24,25-dihydroxyvitamin D3 may have distinct, but different physiological functions in fish.

1,25-dihydroxyvitamin D3 inhibits proliferation of human promyelocytic leukaemia (HL60) cells and induces monocyte-macrophage differentiation in HL60 and normal human bone marrow cells

1,25-dihydroxyvitamin D3 induces monocyte-macrophage differentiation and inhibits proliferation of cells from the human promyelocytic leukaemia cell line HL60. Similarly human bone marrow progenitor cells differentiate preferentially along the monocyte-macrophage pathway when incubated in the presence of 1,25-dihydroxyvitamin D3. We suggest that the inhibition of growth which occurs after addition of the vitamin to HL60 might be paralleled in vivo by inhibition of proliferation of leukaemic cells; also we speculate that the vitamin may be involved in the control of both monocyte-macrophage and osteoclast production in vivo.

[Effect of 1,25-dihydroxycholecalciferol and 24,25-dihydroxycholecalciferol on thyroid calcitonin content in the suckling or weanling rat]

We have examined the effects of vitamin D3 metabolites on plasma calcium, plasma calcitonin (CT) and thyroidal CT contents in Rats before and after weaning. A daily injection of 1,25-dihydroxycholecalciferol [1,25-(OH)2D3; approximately 25 pmoles/Rat/day during 4 days] induced a marked rise in plasma calcium (16.1 +/- 0.2 vs. 11.3 +/- 0.3 mg/dl in controls) and a great decrease in thyroidal CT content (-70% of control values) in 13-day-old suckling baby Rats, while no change occurred in response to 24,25-dihydroxycholecalciferol (24,25-(OH)2D3) administration. A negative correlation between plasma calcium level and thyroidal CT stores was found in suckling and in weaning Rats treated with different doses of 1,25-(OH)2D3. The significant difference between the slopes of the two regression lines demonstrated a greater mobilization of the CT stores in weaning than in suckling Rats in response to a given hypercalcemia. Our results could suggest that 24,25-(OH)2D3 at the dose used, does not regulate CT secretion while 1,25-(OH)2D3 has probably an indirect effect on CT release, but in vitro experiments will be performed to confirm this hypothesis.

[Effect of vitamin D3 and its metabolites 1,25-dihydroxycholecalciferol and 24,25-dihydroxycholecalciferol on callus mineralization in rats with femoral fracture]

Additional administration of vitamin D3 at a physiological dose of 0.25 microgram daily into rats with femur fracture within 4 weeks did not affect the specific weight and chemical composition (content of Ca2+, P) in diaphyses of intact and impaired femurs as well as the content of Ca2+. Pi and activity of alkaline phosphatase in blood serum of the animals. At a higher dose 2.5 microgram daily vitamin D3 increased concentration of Pi in blood serum but did not alter the other parameters studied. Physiological doses of 1,25-dihydroxycholecalciferol (1.25 (OH)2D3) and 24,25-dihydroxycholecalciferol (24,25 (OH)2D3) (00.3 microgram and 0.25 microgram daily, respectively) did not affect the specific weight and composition of the impaired diaphyses, content of Ca2+ and activity of alkaline phosphatase in blood but increased slightly the Pi concentration. After a 5-fold increase in the dose of 1,25(OH)2D3 (0.15 microgram daily) specific weight and content of Ca2+ were decreased in the impaired bones with simultaneous increase in concentration of Ca2+, Pi and activity of alkaline phosphatase in blood serum. These data suggest that reparation was impaired under the conditions of acceleration of the bone tissue resorption. Increased doses of 24, 25(OH)2D3 (1.25 micrograms daily) stimulated the increase in specific weight and mineralization of the impaired bones and normalized the increased alkaline phosphatase activity in blood serum. Clinical examination of 24,25(OH)2D3 could be recommended as a drug stimulating the reparation under conditions of bone fracture.