The effect of Prednisolone upon the metabolism and action of 25-hydroxy-and 1,25-dihydroxyvitamin D3

Treatment of vitamin D-deficient rats with Prednisolone(R) does not alter the rate of conversion of [(3)H]25-hydroxyvitamin D(3) to [(3)H]1,25-dihydroxyvitamin D(3), but the further conversion of [(3)H]1,25-dihydroxyvitamin D(3) to a more polar metabolite is more rapid in the Prednisolone(R)-treated animals. This more polar metabolite is biologically inactive, periodate-insensitive, and persists in the intestine as long as 1,25-dihydroxyvitamin D(3). Also, the time course of action of 1,25-dihydroxyvitamin D(3) upon intestinal calcium transport is altered by Prednisolone(R) treatment. Treatment with Prednisolone(R) did not change the magnitude of the initial response to 1,25-dihydroxyvitamin D(3) at 7 hr, but did decrease the response at 24 and 48 hr after a single dose of 1,25-dihydroxyvitamin D(3). The present results show that one of the means by which large doses of adrenal corticoids alter intestinal calcium transport is by stimulating the further metabolism of 1,25-dihydroxyvitamin D(3) to a more polar, biologically inactive intestinal metabolite.

Rapid enhancement of chick intestinal DNA-dependent RNA polymerase II activity by 1 alpha, 25-dihydroxyvitamin D3, in vivo

1alpha,25-dihydroxyvitamin D(3) was examined for its ability to affect the DNA-dependent RNA polymerases (nucleosidetriphosphate:RNA nucleotidyltransferase, EC 2.7.7.6) of rachitic chick intestinal cell nuclei in vivo. Nucleoplasmic (form II) RNA polymerase activity was stimulated 2-fold (P < 0.05) within 2-3 hr after an oral dose of 0.27 mug (0.65 nmol) of 1alpha,25-dihydroxyvitamin D(3) to rachitic chicks. The form II polymerase activity returned to control values by 5-9 hr after dosing with the sterol. In contrast, the nucleolar (form I) RNA polymerase was not increased within this period. Solubilization of nuclear protein and resolution of the two RNA polymerases on DEAE-Sephadex also revealed that there was an increase in polymerase II activity when assayed on exogenous DNA template. This evidence suggests that 1alpha,25-dihydroxyvitamin D(3) acts at the level of the enzymology of intestinal cell transcription and that increased mRNA synthesis after administration of this hormone cannot be due solely to a change in chromatin template activity.

Effects of nucleotides, hormones, ions, and 1,25-dihydroxycholecalciferon on 1,25-dihydroxycholecalciferol production in isolated chick renal tubules

1. The production of 1,25-dihydroxycholecalciferol (1,25-DHCC) from tritiated 25-hydroxycholecalciferol (25-HCC) was assessed in renal tubules prepared from vitamin D-deficient chicks.

2. Cyclic AMP and dibutyryl cyclic AMP enhanced the production of 1,25-DHCC.

3. Bovine parathyroid hormone (BPTH) and its synthetic analogue either had no effect or inhibited the production of 1,25-DHCC when calcium was present in the extracellular medium, but enhanced the conversion when calcium was absent. BPTH increased adenylate cyclase activity and cyclic AMP content of the renal tubule cells.

4. Synthetic salmon calcitonin enhanced the production of 1,25-DHCC, whereas human calcitonin had no effect at the concentrations tested. Salmon calcitonin did not increase chick renal adenylate cyclase activity or cyclic AMP levels.

5. A short period of incubation in media containing strontium gluconate (5 mmol/l) significantly increased production of 1,25-DHCC, whereas no effect was seen after more prolonged exposure to strontium. Calcium depletion decreased production of 1,25-DHCC, but when some calcium was present, variation in extracellular concentration of calcium did not affect conversion. Variation in extracellular phosphate concentration did not affect conversion.

6. A small quantity of 1,25-DHCC added to the medium inhibited the conversion of 25-HCC into 1,25-DHCC.

7. The production of 1,25-DHCC is subject to complex control, and intracellular concentrations of calcium, cyclic AMP and 1,25-DHCC may all be important regulatory influences.

Role of 1,25-dihydroxyvitamin D3 in maintaining serum phosphorus and curing rickets

The intravenous injection of a single dose of 650 pmoles of 1,25-dihydroxyvitamin D(3) to rats fed a vitamin D-deficient, low-phosphorus diet caused an elevation of serum phosphorus within 5 hours which reached a maximum in about 10-12 hours. This elevated serum phosphorus returned to deficiency levels 2-3 days later. On the other hand, a single injection of 650 pmoles of 25-hydroxyvitamin D(3) produced a significant rise at 12 hours, reached a maximum in 24-36 hours, and was maintained for at least 7 days. The single dose of 1,25-dihydroxyvitamin D(3) supported little calcification of bone, whereas the 25-hydroxyvitamin D(3) produced marked calcification. Six-hundred and fifty pmoles of 24,25-dihydroxyvitamin D(3) increased serum phosphorus only slightly and induced no calcification. When 1,25-dihydroxyvitamin D(3) was given each day, a sustained increase in serum phosphorus and marked bone calcification resulted. In contrast to the serum phosphorus responses, intestinal calcium transport remained high 5 days after administration of a single dose of 1,25-dihydroxyvitamin D(3). Serum calcium was not changed appreciably by any of the metabolites. Thyroparathyroidectomized rats or rats fed a diet extremely deficient in phosphate still exhibited a marked elevation of serum phosphorus in response to 1,25-dihydroxyvitamin D(3). The effect of 1,25-dihydroxyvitamin D(3) on serum phosphorus was greatly reduced in nephrectomized rats, suggesting that the serum phosphorus response to 1,25-dihydroxyvitamin D(3) may arise from an enhancement of phosphate reabsorption in the renal tubules. It is suggested that 1,25-dihydroxyvitamin D(3) cures rickets in rats by increasing the concentration of serum phosphorus rather than by increasing serum calcium concentration and calcium absorption.

Calcium absorption and calcium-binding protein synthesis: solanum malacoxylon reverses strontium inhibition

The ingestion of diets containing high concentrations of stable strontium inhibits calcium absorption and intestinal calcium-binding protein synthesis and, as shown by others, does so by inhibiting the conversion of 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol, the active form of vitamin D. The addition of the South American plant Solanum malacoxylon to strontium-containing diets counteracts the inhibitory action of dietary strontium, thereby indicating that the plant contains a factor which can mimic the action of 1,25-dihydroxycholecalciferol and representing the first such factor identified in a botanical source.

Biological activity of 1alpha-hydroxycholecalciferol, a synthetic analog of the hormonal form of vitamin D3

1,25-Dihydroxycholecalciferol, the apparent active hormonal form of cholecalciferol (vitamin D(2)), is formed from cholecalciferol by specific and sequential hydroxylations of the sterol at carbons 25 and 1. Recently, 1alpha-hydroxycholecalciferol was synthesized and we report on its biological activity in rachitic chicks. 1alpha-Hydroxycholecalciferol is identical in potency to 1,25-dihydroxycholecalciferol in stimulation of intestinal calcium absorption; either sterol elicits a near maximal effect at a dose of 0.3-0.6 nmol. The time-course of action of 1alpha-hydroxycholecalciferol also parallels that of the active metabolite 1,25-dihydroxycholecalciferol with a maximal increase in calcium transport occurring 5-10 hr after administration of sterol to vitamin D-deficient chicks. 6.5 nmol of 1alpha-hydroxycholecalciferol causes a doubling in calcium absorption in only 2-3 hr, which is the most rapid physiologic response yet detected for a vitamin D-sterol. 1alpha-Hydroxycholecalciferol is active also in enhancing bone calcium resorption and, like 1,25-dihydroxycholecalciferol, is at least 10 times as active as cholecalciferol in mobilizing bone calcium and raising plasma calcium concentration. It is concluded that 1alpha-hydroxycholecalciferol represents a synthetic analog of 1,25-dihydroxycholecalciferol that can be used both to study the mechanism of action of this hormone and as a therapeutic agent in the treatment of patients with certain metabolic bone diseases.

Embryonic chick intestine in organ culture. A unique system for the study of the intestinal calcium absorptive mechanism

Duodena from 20-day-old chick embryos can be maintained in large scale organ culture on specially designed stainless-steel grids in contact with serum-free medium for 48 h with excellent preservation of mucosal structure at both the light and electron microscope levels. Although mitotic rate was subnormal, several other factors attest to the essential viability of the cultured intestine: L-leucine incorporation into protein, as well as the synthesis of a specific vitamin D(3)-induced calcium-binding protein (CaBP), increased over a 48-h culture period, and the electropotential gradient across the intestine was maintained throughout the culture period as was a concentration gradient for calcium. The tissue responded to vitamin D(3) in the medium by synthesizing the calcium-binding protein within 6 h and by exhibiting enhanced (45)Ca uptake within 12-24 h. Concentrations of vitamin D(3), or its 25-hydroxylated derivative, higher than necessary for CaBP induction, also increased the activity of alkaline phosphatase. The 1,25-dihydroxylated derivative of vitamin D(3), at a level extremely potent in CaBP induction, did not stimulate alkaline phosphatase. Mucosal to serosal transport of (45)Ca could also be measured in everted duodenal sacs, subsequent to culture under similar conditions, and was also increased by vitamin D(3) in the medium. Other embryonic organs, esophagus, stomach, liver, pancreas, lung, skin, and muscle, did not produce CaBP in response to vitamin D(3) in the culture medium. However, CaBP-synthesizing capacity was present in the entire intestinal tract, exclusive of the rectum. (59)Fe and (32)P uptake by cultured duodenum were also stimulated by vitamin D(3). The system has proven quite useful in the study of the vitamin D-mediated calcium absorptive mechanism but should be applicable to the study of the absorption of other nutrients, drugs, hormones, etc., as well as other studies of intestinal function.

Effects of 1,25-dihydroxycholecalciferol on intestinal calcium transport in cortisone-treated rats

The administration of glucocorticoids may decrease intestinal calcium absorption in vivo and the active transport of calcium in rat duodenum in vitro. It has been suggested that this apparent "anti-vitamin D-like" effect of steroid hormones may be related to alterations in vitamin D metabolism. In order to test this hypothesis, vitamin D-deficient control and cortisone-treated rats were given an intraperitoneal injection of 5.5 IU of 1,25-dihydroxycholecalciferol (1,25-DHCC), the probable end-organ active vitamin D metabolite in the intestine, and 16 h later studies of duodenal calcium transport were performed in modified Ussing chambers. In the vitamin D-deficient state, cortisone administration was associated with a diminution in J(MS), J(Net), and the flux ratio (J(MS)/J(SM)). While the magnitude of the increases in J(MS) and J(Net) that resulted from 1,25-DHCC treatment were approximately the same in control and cortisone-treated animals, 1,25-DHCC failed to restore these parameters to "normal levels" in the steroid-treated rats. Furthermore, contrary to the results obtained in the saline-treated controls, 1,25-DHCC failed to reduce J(SM) in the duodenum from cortisone-treated rats. The cortisone-related defect in calcium transport was due to alterations in both unidirectional calcium fluxes (decrease in J(MS) and increase in J(SM)), such that the J(Net) and the flux ratio (J(MS)/J(SM)) were only approximately 50% of the levels achieved in vitamin D-deficient control animals repleted with the same dose of 1,25-DHCC. The administration of 1,25-DHCC was accompanied by a marked increase in the serum calcium levels of control rats, but there was no such response in the cortisone-treated group. The results support the concept that under the conditions of these experiments in the rat the apparent antagonism between glucocorticoids and vitamin D may be due to steroid hormone-related alterations in end organ function that are independent of any direct interaction between the hormone and the vitamin and that cannot be reversed by the vitamin.

Isotachysterol-3 and 25-hydroxyisotachysterol-3: analogs of 1,25-dihydrox vitamin D3

Isotachysterol(3), 25-hydroxyisotachysterol(3), and isovitamin D(3) have been synthesized and tested for biological activity. Like 1,25-dihydroxyvitamin D(3), isotachysterol(3) stimulates intestinal calcium transport and bone calcium mobilization in anephric rats, whereas 25-hydroxyvitamin D(3) does not. Although isovitamin D(3) is biologically active in normal rats it is inactive in anephric rats.