1,25-Dihydroxycholecalciferol increases bone resorption in thyroparathyroidectomised mice

Mice, 1 week old, prelabelled with 45Ca, were either thyroparathyroidectomised or sham-operated; 1 day later half of the mice of each group were injected with 1,25-dihydroxycholecalciferol (5 ng/g), and 20 h later all the mice were killed. Bone resorption in explants was then measured by an in vivo/in vitro technique previously published; compared with untreated mice it was found that 1,25-dihydroxycholecalciferol had increased resorption irrespective of whether the mice had been thyroparathyroidectomised or not. These data suggest that 1,25-dihydroxycholecalciferol is able to increase bone resorption independently of parathyroid hormone.

The biological activity of synthetic 25,26-dihydroxycholecalciferol and 24,25-dihydroxycholecalciferol in vitamin D-deficient rats

The biological activity of synthetic 24,25 and 25,26 diOHD3 was studied in vitamin D-deficient rats. The purpose of this study was to investigate the influence of small doses of both metabolites (0.125-0.250 mug) upon intestinal calcium transport and bone calcium mobilization. Both metabolites were able to increase calcium absorption in rats maintained on a calcium-deficient diet, but failed to do it in rats on a normal calcium diet. Bilateral nephrectomy suppressed this effect. The "bone calcium mobilization" of both derivatives was measured in vitamin D and calcium- or phosphorus-deprived rats after one intravenous dose. When serum calcium was initially low, 24,25 and 25,26 diOHD3 increased serum calcium moderately, but the increment was only significant with 24,25 diOHD3. When serum calcium was normal before the injection, both metabolites decreased serum calcium significantly, and the decrease was greater with 24,25 diOHD3. Intraperitoneal administration of the metabolites for 5 consecutive days produced a significant increase of calcium in serum and bone ash.

Effects of vitamin D and its metabolites on calcium transport in the diabetic rat

We studied diabetic rats, 5 days after streptozotocin injection, and matched controls to determine whether depressed duodenal calcium absorption associated with uncontrolled diabetes in the rat would respond to vitamin D or its metabolites. At the appropriate time following the intravenous injection of 0.25 mug of either vitamin D3, 25-hydroxycholecalciferol (25-OHD3), 1,25-dihydroxycholecalciferol (1,25-OH)2D3), or 1alpha-hydroxycholecalciferol (1alpha-OHD3) to half of each diabetic and control group, calcium transport was evaluated using everted duodenal sacs with 0.4 mM40Ca and tracer 45Ca on both mucosal and serosal surfaces. All agents stimulated duodenal calcium absorption in controls. Diabetics responded only to 1,25-(OH)2D3, the metabolite that acts directly on the duodenum, and to its synthetic analog, 1alpha-OHD3. 1alpha-OHD3 is activated to 1,25-(OH)2D3 by 25-hydroxylation in the liver; 25-OHD3 must be 1alpha-hydroxylated in the kidney to be active. The stimulation of duodenal calcium absorption in diabetic rats by 1alpha-OHD3, but not by either vitamin D3 or 25-OHD3, is most consistent with a defect in vitamin D metabolism at the 1alpha-hydroxylation step in the kidney.

Side chain metabolism of 25-hydroxy-[26,27-14C] vitamin D3 and 1,25-dihydroxy-[26,27-14C] vitamin D3 in vivo

Radioactive CO2 was detected in expired air after the administration of 25-hydroxy-[26,27-14C] vitamin d3 to vitamin D-deficient hypocalcemic rats; 14co2 was also detected after the administration of 1,25-dihydroxy-[26,27-14C] vitamin D3 to rats raised on the same diet. Nephretcomy totally abolished 14CO2 formation after administration of 25-hydroxy-[26,27-14C] vitamin D3 but not after the administration of 1,25-dihydroxy-[26,27-14C] vitamin D3. The production of 14CO2 commenced within 4 hours after injection of 1,25-dihydroxy-[26,27-14C] vitamin D3, suggesting a possible relevance of this reaction to the function of 1,25-dihydroxyvitamin D3. These results at least demonstrate a new metabolic pathway of vitamin D3 metabolism involving the oxidation of a portion of the side chain of 1,25-dihydroxyvitamin D3 to CO2.

Comparison of effects of 1 alpha-hydroxy-vitamin D3 and 1,25-dihydroxy-vitamin D3 in man

The effects of short-term treatment with 1,25-dihydroxy-vitamin D3 [1,25(0H)2D3] or 1 alpha-hydroxy-vitamin D3 [1 alpha(OH)D3] on intestinal absorption of 47Ca were compared in 41 experiments in normals and 72 experiments in patients with chronic renal failure. 11 patients were studied a second time after treatment for 2-5 mo. Doses varied from 0.14 to 5.4 mug/day to establish dose-response relationships. Urinary calcium was monitored in normal subjects, nine of whom received a constant calcium intake on a metabolic unit. There was an increase in intestinal absorption of 47Ca and urinary calcium in normals receiving 1,25 (OH)2D3, 0.14 mug/day or greater, and 0.28 mug/day or greater augmented intestinal absorption of 47Ca in chronic renal failure. In contrast, 2.6 mug/day of 1 alpha (OH) D3 was required to increase intestinal absorption of 47Ca in both groups. The increase in urinary calcium to maximal levels was delayed during treatment with 1 alpha (OH) D3, 5-10 days vs. 2-5 days with 1,25 (OH)2D3. Moreover, half times for urinary calcium to decrease to pretreatment levels after stopping treatment were greater after 1 alpha-(OH) D3 (1.5-2.7 days) than 1,25(OH)2D3 (1.1-2.0 days). With long-term administration there was a progressive increase in intestinal absorption of 47Ca in the patients receiving 1 alpha (OH)D3; this was not observed with 1,25(OH)2D3. The pharmacologic differences between 1 alpha(OH) D3 and 1,25(OH)2D3 may be explained by the requirement for 25-hydroxylation of 1alpha(OH) D3 before biologic effects occur; at low doses (less than 1 mug/day), 1 alpha(OH) D3 competes with vitamin D3 for 25-hydroxylation. With prolonged treatment or larger doses (greater than 2 mug/day),, 1alpha(OH) D3 could accumulate and then be hydroxylated resulting in production of higher levels of 1,25(OH)2D3.

Biological activity of 24,25-dihydroxycholecalciferol in chicks and rats

The ability of 24R, 25- and 24S, 25-dihydroxycholecalciferol to stimulate intestinal calcium transport and bone calcium mobilization in chicks was measured. Enhancement of intestinal calcium transport by 325 or 130 nmoles of either compound was maximal by 24 hours. The effects of these compounds on bone calcium mobilization were also maximal by 24 to 36 hours. When lower doses were tested, 2 nmoles of the 24R, 25-dihydroxycholecalciferol significantly stimulated intestinal calcium transport, whereas 130 nmoles of the S isomer were required for a significant response. Neither steroid had a significant effect on bone calcium mobilization when doses of less than 130 nmoles were given. When chicks received orally 32.5, 325 or 1,625 pmoles of 24R, 25-dihydroxycholecalciferol daily from hatching for 4 weeks, several parameters showed a dose-related response. These included growth, serum calcium, bone ash, renal 25-hydroxycholecalciferol-1-hydroxylase and intestinal vitamin D-dependent calcium binding protein. Rats given 32.5 to 1,625 pmoles of 24R, 25-dihydroxycholecalciferol for 3 or 6 weeks were equivalent to vitamin D-treated controls in terms of growth and serum calcium levels. It is concluded that within the lower dose ranges (2 to 30 pmoles) the R isomer of 24,25-dihydroxycholecalciferol is more active in stimulating intestinal calcium transport than the S isomer but that neither compound increases bone calcium mobilization at these dose levels. Also, the rat is more responsive in terms of growth and serum calcium, to small dialy doses of 24R, 25-dihydroxycholecalciferol than is the chick.

Duodenal active transport of calcium and phosphate in vitamin D-deficient rats: effects of nephrectomy, Cestrum diurnum, and 1alpha,25-dihydroxyvitamin D3

Both the methanol:chloroform extractable material from the leaves of the Solanaceous plant, Cestrum diurnum (C.d.), and a 270 ng dose of 1alpha, 25-dihydroxyvitamin D3 (1alpha,25-(OH)2D3) increased the active absorption of calcium and phosphate across the proximal duodenum, studied in vitro, from sham-operated and nephrectomized (NPX) vitamin D-deficient rats. In these studies, conducted 24 h after surgery, the uremic state in the NPX animals markedly diminished the intestinal transport response to 1alpha,25-(OH)2D3 and also lowered baseline transport values across duodenum from the NPX vitamin D-deficient controls. Both C.d. and 1alpha, 25-(OH)2D3 elevated plasma Ca levels equally well in the sham-operated and NPX groups. The stimulation of intestinal Ca absorption in NPX animals indicates that, like the leaves of the South American plant, Solanum glaucophyllum, C.d. contains materials which can function in an analogous manner to compounds in the vitamin D group that have either a 1alpha hydroxyl group or its steric equivalent.

Increased intestinal chromatin template activity. Influence of 1alpha,25-dihydroxyvitamin D3 and hormone-receptor complexes

1alpha,25-Dihydroxyvitamin D3 administration to rachitic chicks results in an increase in the chromatin template activity of intestinal target tissue assayed in vitro using Escherichia coli RNA polymerase. The maximum stimulation of template capacity was 12 to 20% over control values and occurred 2 hours after administration of the sterol. This rapid effect preceded the biologic response to 1alpha,25-dihydroxyvitamin D3 in the intestine and was not observed in other tissues such as liver or kidney. The in vivo enhancement of intestinal chromatin template activity was specific for the 1alpha,25-dihydroxyvitamin D3 hormone in that equivalent doses of 25-hydroxyvitamin D3 or vitamin D3 did not elicit a response in 2 to 3 hours. Only 1alpha-hydroxyvitamin D3, a synthetic sterol which is very rapidly metabolized to the 1alpha,25-dihydroxyvitamin D3 form, was able to minic the natural hormone in vivo. To further elucidate the nuclear mechanism of action of 1alpha,25-dihydroxyvitamin D3, the hormone was preincubated at 0 degrees with intestinal cytosol to form hormone-receptor complexes. After addition of the hormone-receptor complexes to purified intestinal mucosa nuclei and incubation for 1 hour at 25 degrees, chromatin isolated from this reconstituted system displayed a significant increase in template activity as compared to chromatin prepared from similar in vitro incubations not containing hormone. This stimulation was 12 to 24% over control values and exhibited an absolute requirement for intestinal cell cytosol. The response was specific for physiologic levels of 1alpha,25-dihydroxyvitamin D3, but occurred with pharmacologic doses of 25-hydroxyvitamin D3. It is concluded that a stimulation of the chromatin template activity of intestinal target tissue by 1alpha,25-dihydroxyvitamin D3 may be an integral part of the ultimate physiologic response of enhanced calcium transport.

The effects of long term feeding of Solanum glaucophyllum to growing rats on Ca, Mg, P and bone metabolism

The effects of the long term ingestion of Solanum Glaucophyllum leaves (SG) by the rat was investigated in two series of experiments; the animals were fed a normal (for 8 weeks) or a low Ca diet (for 5 weeks). With both diets, the intestinal absorption of Ca was increased and the endogenous fecal Ca excretion was decreased by SG treatment. Added to a normal Ca diet, SG increased the urinary excretion of Ca, Mg and P and reduced the excretion of hydroxyproline and pyrophosphate. At the histological level, SG induced a higher rate of bone tissue synthesis on trabecular and endosteal surfaces. The bone content of hydroxyproline and citrate increased significantly. The total alkaline phosphatase activity of plasma decreased as a function of SG intake due to a decreased in the activity of the intestinal isoenzyme, which was not compensated by the increase in the bone isoenzyme activity. The Mg absorption was decreased by SG inducing lower Mg balances and lower plasma Mg levels. Added to a low Ca diet, SG increased the severity of the secondary hyperparathyroidism induced by the diet. The urinary excretion of hydroxyproline and the plasma alkaline phosphatase activity (both isoenzymes) were significantly increased. The Na and K content of bone decreased as a function of SG intake. 45Ca kinetic experiments revealed that SG increased the rate of Ca resorption and the rate constant of the fast exchangeable Ca pool, in both diets.

Studies on the 1alpha, 25-dihydroxycholecalciferol-like activity in a calcinogenic plant. Cestrum diurnum, in the chick

Cestrum diurnum (day-blooming jessamine) has been proposed to cause calcinosis in horses and cattle in Florida. The present studies investigated some physiological properties of the plant, using the chick as the experimental animal. The inclusion of dried leaf powder in a rachitogenic diet restored intestinal calcium-binding protein synthesis (CaBP) and increased calcium absorption in the cholecalciferol-deficient chick. The estimated level of cholecalciferol-equivalents in the dried leaf was about 30,000 to 35,000 IU/kg. Most of the activity was extractable with methanol:chloroform (2:1), indicating that the major cholecalciferol-like component in C. diurnum was different from the water soluble factor(s) in Solanum malacoxylon. The time course of effect of C. diurnum extract in rachitic chicks was similar to that ot 1,25-dihydroxycholecalciferol but the former had a longer lag time. The strontium fed chick, in which the kidney 25-hydroxycholecalciferol-1alpha-hydroxylase is inhibited, responded to C. diurnum extract, confirming the 1alpha,25-dihydroxycholecalciferol-like character of the Cestrum factor. The extract also appeared to interact with the intestinal 1 alpha,25-dihydroxycholecalciferol cytosol receptor although this observation is preliminary. These findings indicate that the l alpha,25-dihydroxycholecalciferol-like principle in C. diurnum many cause excessive calcium and phosphate absorption leading to calcinosis.

1,25-Dihydroxycholecalciferol effect on serum phosphorus homeostasis in rats

It has recently been shown that 1,25-dihydroxycholecalciferol (1,25-(OH)2D3) increases the serum phosphorus concentration of rats on a low-phosphorus diet. While studying the biological activity of 1,25(OH)2D3, we observed that under certain circumstances 1,25-(OH)2D3 would decrease the serum phosphorus concentration. The analysis of all data obtained in rat experiments during the past 3 years revealed highly significant linear correlations (P less than 0.001) between changes of serum phosphorus concentrations after the administration of 1,25-(OH)2-D3 (130 pmol/d for 1 or 5 days) and serum phosphorus or calcium levels in the animals before injection. Similar correlations could only be found with the higher dose of 25-hydroxycholecalciferol (130 pmol/d for 5 days). Another vitamin D3 metabolite, 24,25-dihydroxycholecalciferol, had no effect on serum phosphorus concentrations under our experimental conditions. The 1,25-(OH)2D3 effect on serum phosphorus concentration does not require the presence of circulating parathormone and/or calcitonin. We suggest that 1,25-(OH)2D3 might be an important factor in serum phosphorus homeostasis.

Metabolism and biologica

24R,24,25-Dihydroxyvitamin D3 is capable of inducing a minimal intestinal calcium transport response in chicks when compared to an equal amount of 25-hydroxyvitamin D3. 1,24,25-Trihydroxyvitamin D3 is also less active than 1,25-dihydroxyvitamin D3, and its activity is much shorter lived than that of 1,25-dihydroxyvitamin D3. A comparison of the metabolism of 25-hydroxy[26,27-3H]vitamin D3 and 24,25-dihydroxy[26,27-3H]vitamin D3 in the rat and chick shows that 24,25-dihydroxyvitamin D3 and 1,24,25-trihydroxyvitamin D3 disappear at least 10 times more rapidly from the blood and intestine of chicks. Furthermore, examination of the excretory products from both of these species demonstrates that chicks receiving a single dose of 24,25-dihydroxy[26,27-3H]vitamin D3 excrete 66% of the total radioactivity by 48 hours, whereas rats receiving the same dose excrete less than one-half that amount. These results demonstrate that 24,25-dihydroxyvitamin D3 is considerably less biologically active in the chick than in the rat, probably due to more rapid metabolism and excretion.

Elevation of cyclic AMP levels and adenylate cyclase activity in duodenal mucosa from vitamin D-deficient rats by 1alpha,25-dihydroxycholecalciferol (1alpha,25-(OH)2D3)

A single 270 ng dose of 1alpha,25-(OH2D3 rpoduced elevations in cyclic AMP content and adenylate cyclase activity in duodenal mucosa from previously vitamin D-deficient rats. No changes in jejunal or ileal cyclic AMP levels or duodenal cyclic GMP levels were observed. Since 1alpha,25-(OH)2D3 increased both baseline and NaF-stimulated adenylate cyclase activity, it is possible that the vitamin leads to enhanced enzyme synthesis. While parallel changes in duodenal cyclic AMP levels and active calcium absorption in response to 1alpha,25-(OH)2D3 were observed at 6,12,24 and 48 hr after treatment, increases in calcium absorption were observed at 3 hr in duodenum and at 48 hr in ileum in the absence of changes in cyclic AMP levels. Further studies will be required to determine whether or not the changes in duodenal cyclic AMP levels are direct or indirect effects of 1alpha,25-(OH)2D3 administration, and to determine the role, if any, of this nucleotide in the hormones' effect on intestinal calcium absorption.

Failure of 1, 25 dihydroxycholecalciferol (1, 25-(OH) 2-D3) to modify cyclic AMP levels in parathyroid hormone-treated and untreated bone cells

Treatment with 1, 25-(OH) 2-D3 (1 ng/ml-25 ng/ml) for periods ranging from 2.5 min. to 60 min. did not alter cyclic AMP levels in bone cells isolated from periosteum-free rat calvaria, or in cells isolated from rat periosteal tissues. 1, 25-(OH) 2-D3 failed to modify the acute increases in cyclic AMP elicited by PTH (10 ng/ml-1 ug/ml). Two separate 1, 25-(OH) 2-D3 preparations, biologically active in other systems, were ineffective under a wide variety of experimental conditions. These results suggest that 1, 25-(OH) 2-D3 is not an acute modulator of cyclic AMP metabolism in PTH-treated and untreated bone cells.

Stimulation in vitro by 1,25-dihydroxy-vitamin D3 of intestinal cell calcium uptake and calcium-binding protein

Treatment of duodenal tissue from rats deficient in vitamin D with 1,25-dihydroxy-vitamin D3 [1,25-(OH)2-D3] led to more than a doubling of calcium uptake by the isolated cells and the appearacne in those cells of previously undetectable calcium-binding protein (CaBP). Treatment with the precursor, 25-hydroxy-vitamin D3, was without effect on calcium uptake or CaBP. Cells from vitamin D-replete animals took up three and a half times more calcium than cells from deficient animals. This rapid (90-minute) effect of in vitro treatment with a physiological dose (4.7 X 10(-8)M) of 1,25-(OH)2-D is the first such report and is in accord with the regulatory role of the hormone-like sterol.

Effects of 1alpha-hydroxy-vitamin D3 and 1,25-dihydroxy-vitamin D3 on calcium and phosphorus metabolism in hypoparathyroidism

The effects of 1alpha-OH D3 or 1,25-(OH)2D3 on calcium and phosphorus metabolism have been evaluated in five hypoparathyroid patients to establish the direct effects of these compounds in adult humans, uncomplicated by compensatory changes in parathyroid hormone secretion. Doses of 1-2.5 mug/day in four patients (5 mug/day in a fifth patient on diphenylhydantoin and phenobarbital) caused a marked increase in serum calcium concentration and urinary calcium excretion, without significant changes in renal calcium clearance or urinary hydroxyproline excretion. These results suggest that the correction of hypocalcemia involved primarily a stimulation of intestinal calcium absorption rather than a stimulation of skeletal calcium resorption. Simultaneously, there were increases in urinary phosphorus excretion and variable changes in serum inorganic phosphate concentration. These effects were produced by doses of 1alpha-OH D3 and 1,25-(OH)2D3 which approach the dose needed to prevent rickets, in contrast to the very large doses of vitamin D or 25-OH D3 required for comparable effects in hypoparathyroid patients. The increased relative effectiveness of these one-hydroxylated forms of vitamin D reveals a deficiency of vitamin D one-hydroxylation in hypoparathyroidism. The rapidity of action of 1alpha-OH D3 and 1,25-(OH)2D3 was also striking. Apart from its physiologic implications, the potency of the one-hydroxylated forms of vitamin D offers significant therapeutic advantages in some patients whose hypoparathyroidism is difficult to control with vitamin D itself.

Effects of 1alpha,25-dihydroxyvitamin D3 and Solanum glaucophyllum on intestinal calcium and phosphate transport and on plasma Ca, Mg and P levels in the rat

The responses elicited by a single dose of either S. glaucophyllum (SG) or 1alpha,25-dihydroxyvitamin D3 (1alpha, 25-(OH)2D3) in vitamin D-deficient rats were qualitatively identical. Both compounds stimulated duodenal absorption of calcium and phosphate 3 h after administration, and the effects persisted for 96 h. The SG effects on calcium transport were maximal at 6 h and exceeded the greatest response to 1alpha,25-(OH)2D3 which occurred at 48 h. While the early SG effects on phosphate absorption exceeded those of 1alpha,25-(OH)2D3, the latter compound had a much greater effect at 48 h. Both SG and 1alpha, 25-(OH)2D3 produced marked hyperphosphatemia that was accompanied by hypocalcemia and transient hypermagnesemia at 3-6 h. The phosphatemia increased for 48 h in the 1alpha,25-(OH)2D3 group and fell toward baseline between 48 and 96 h, while this parameter was maximal in the SG group at 3-12 h and returned to baseline values by 96 h. Plasma calcium levels tended to vary inversely with phosphorus concentrations and did not exceed baseline values until 48 h for the SG group and 96 h for the 1alpha,25-(OH)2D3 group. We suggest that the hypocalcemia is due to calcium phosphate precipitation. The quantitative differences in the effects on calcium and phosphorus metabolism may be dose-related. The responses to SG suggest that a gram of dried leaf has approximately the same activity as 1 mug of 1alpha,25-(OH)2D3. It appears that the water-soluble SG factor has the same biological effects on Ca and P metabolism as does 1alpha,25-(OH)2D3.

Structural analogs of 1alpha,25-dihydroxycholecalciferol: preparation and biological assay of 1alpha-hydroxypregnacalciferol

The synthesis of 1alpha-hydroxypregnacalciferol, a side chain analog of 1alpha,25-dihydroxycholecalciferol (1alpha,25-dihydroxyvitamin D3), is described. Pregnenolone acetate was converted in five steps to 5-pregnen-1alpha,3beta-diol. Conversion of the diol to pregna-5,7-diene-1alpha,3beta diol diacetate followed by ultraviolet irradiation gave the corresponding previtamin derivative. Thermal isomerization, hydrolysis and chromatography then furnished the desired analog, 1alpha-hydroxypregnacalciferol. The compound was tested in vivo for its effect on intestinal calcium transport, serum calcium and phosphate levels and bone calcification, and in vitro for its effect on bone resorption. When given to intact rats, either as a single dose or in repeated daily doses, the analog even at high dose levels, exhibited no biological activity. The compound stimulated bone resorption in vitro, but only at high concentrations.

25-hydroxycholecalciferol-enhanced bone maturation in the parathyroprivic state

In vitro evidence presently favors a direct osteolytic effect of biologically active vitamin D metabolites. Studies were designed to evaluate the effect of 25-hydroxycholecalciferol (25OHD3) on bone collagen and mineral maturation in vivo and its dependence on parathyroid hormone (PTH). After treatment of sham-operated control and parathyroidectomized (PTX) mature rats with either 25OHD3 or an oil vehicle for 2 wk, tibial bone mineral-collagen maturation was quantitated by bromoform-toluene density gradient fractionation techniques. Intestinal calcium absorption was measured by in vivo 45Ca transport procedures. In contrast to the control group, the response to 25OHD3 of PTX rats was dramatic. Bone mineral and matrix maturation were both accelerated by 25OHD3 treatment without concomitant reduction in total bone mineral or collagen content or changes in the intestinal calcium absorption. These observations support the premise that biologically active vitamin D metabolites stimulate bone tissue maturation, and that PTH is not required in this regard.

Decrease in serum immunoreactive parathyroid hormone in rats and in parathyroid hormone secretion in vitro by 1,25-dihydroxycholecalciferol

The present study determined the effects of 1,25-dihydroxycholecalciferol on serum immunoactive parathyroid hormone and on parathyroid hormone secretion in vitro. Rats injected i.p. with 1,25-dihydroxycholecalciferol, 130 pmol (2 U)/140 g body wt, which is probably a physiologic dose, had a significant 43% decrease in serum immunoreactive parathyroid hormone at 4 h. In addition, this dose of 1,25-dihydroxycholecalciferol inhibited the serum immunoreactive parathyroid hormone response to hypocalcemia induced by phosphate injection. Because the increment in serum immunoreactive parathyroid hormone was less but the decrement in serum calcium more in phosphate plus 1,25-dihydroxycholecalciferol-treated than in phosphate plus vehicle-treated rats, the impaired serum immunoreactive parathyroid hormone response to 1,25-dihydroxycholecalciferol could not be attributed to the change in serum calcium. In studies of parathyroid hormone secretion from bovine parathyroid tissue in vitro, the concentration of 1,25-dihydroxycholecalciferol used for most experiments was 1nM, which is in the range found in rat serum. 1,25-Dihydroxycholecalciferol at 1 or 100 nM significantly inhibited parathyroid hormone secretion when medium calcium concentration was normal (1.5 mM), high (3.0 mM), and low (1.0 mM). Maximum inhibition ranged from 19 to 74%; inhibition was generally seen after 2 h of incubation; and inhibition was sustained or progressive thereafter. Vitamin A, 0.1 muM, caused a marked stimulation of parathyroid hormone secretion. 1,25-Dihydroxycholecalciferol at 1 nM markedly reduced (44%) the effect of vitamin A to stimulate parathyroid hormone secretion. This effect of 1,25-dihydroxycholecalciferol was maximal at 1 h and persisted thereafter. Another steroid, hydrocortisone, 10 muM, did not inhibit parathyroid hormone secretion, suggesting that the 1,25-dihydroxycholecalciferol effect was not a nonspecific inhibitory effect on parathyroid cells. Because other workers have shown that parathyroid hormone directly stimulates 1,25-dihydroxycholecalciferol secretion, our results are consistent with the concept that there is a feedback loop where parathyroid hormone directly stimulates secretion of 1,25-dihydroxycholecalciferol, which in turn directly inhibits secretion of parathyroid hormone.

Phosphatemic action of 1,25-dihydroxyvitamin D3

The action of a single intraperitoneal injection of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) was investigated in thyroparathyroidectomized (TPTX) vitamin D-deficient phosphate-depleted rats. After 14 h, plasma inorganic phosphorus (Pi) was significantly greater in animals receiving 1,25(OH)2D3 than in D-deficient controls, but urinary Pi excretion was very low in both groups and not significantly different in the rats given 1,25(OH)2D3. Clearance studies indicated that the D-deficient controls reabsorbed more than 99% of their filtered Pi. Avid Pi reabsorption continued even after the infusion of sufficient phosphate to raise the plasma and filtered Pi to approximately 3 times normal. Fractional calcium excretion (FECa) exceeded fractional sodium excretion (FENa) by severalfold, but FECa decreased strikingly during phosphate infusion. In animals that manifested a substantial elevation of plasma Pi after 1,25(OH)2D3, FECa was significantly less than in D-deficient controls. Therefore, the increase in plasma Pi following 1,25(OH)2D3 administration occurs independently of any effect on renal Pi reabsorption and may be responsible, at least in part, for the amelioration of hypercalciuria after 1,25(OH)2D3 treatment.

Action of 1,25-dihydroxyvitamin D3 and a diphosphonate on calcium metabolism in rats

The effect of 1,25-dihydroxycholecalciferol (1,25-(OH)2D3) on Ca balance, 45Ca kinetics, and bone morphology has been studied in control rats and rats given disodium ethane-1-hydroxy-1,1-diphosphonate (EHDP), 10 mg P/kg sc per day. This large dose of EHDP is known to inhibit bone mineralization and intestinal calcium absorption and to depress the endogenous production of 1,25-(OH)2D3. In conctrol rats, 1,25-(OH)2D3 increased intestinal calcium absorption. However, in contrast to the enhanced calcium absorption that results from an augmentation of dietary calcium, the 1,25(OH)2D3-induced augmentation of calcium absorption does not lead to a rise in calcium retention, the intestinal effect being matched by an increased excretion of urinary calcium. The EHDP-induced decrease of intestinal calcium absorption could be completely prevented by the concomitant administration of 1,25-(OH)2D3 but not the inhibition of bone mineralization. Therefore, in contrast to the impairment of calcium absorption, that of bone mineralization brought about by large doses of EHDP cannot be merely attributed to a decreased production of 1,25-(OH)2D3.

Effect of 1,25-dihydroxycholecalciferol on ethanol mediated suppression of calcium absorption

Rats raised on laboratory diet and given 20% ethanol in water as a sole source of fluid for 12 days were given 0.2 mug of 1,25-dihydroxycholecalciferol 24 h before measuring duodenal calcium transport by an in vitro everted gut sac technique. This vitamin D metabolite did not reverse the ethanol-induced inhibitiom of transport capacity. Ultrastructural studies of intestinal epithelial cells from animals ingesting ethanol demonstrated changes in mitochondria and other subcellular organelles as well as accumulation of electron-dense, calcium-containing granules in microvilli and a paucity of granules in mitochondria. The results suggest that the inhibition is mediated at the intestinal level, affecting mechanisms which are probably independent of Vitamin D, and in part by an interference in transport of calcium from microvilli to other intracellular sites.

The effect of 1,25-dihydroxycholecalciferol on renal tubular reabsorption of phosphate, intestinal absorption of calcium and bone histology in hypophosphataemic renal tubular rickets

1. 1,25-Dihydroxycholecalciferol (1,25-(OH)2-D3) was given at a dose of 5-0 nmol (2-1 mug) daily by mouth for 4-12 days to three patients with hypophosphataemic (type I), vitamin D-resistant rickets. 2. 1,25-(OH)2-D3 increased intestinal absorption and urinary excretion of calcium without significant effect on the renal handling of phosphate or its plasma concentration. 3. It is concluded that in this type of vitamin D-resistant rickets the renal phosphate abnormality is unlikely to be due to diminished endogenous production of 1,25-(OH)2-D3. 4. The difference between this condition and other hypophosphataemic states is discussed.

Synthesis and biological activity of 25xi,26-dihydroxycholecalciferol

25xi,26-Dihydroxycholecalciferol (25xi,26-dihydroxyvitamin D3), a metabolite of vitamin D3 preferentially active on intestine has been synthesized. This compound was prepared by converting 3beta-hydroxy-27-norcholest-5-en-25-one to 25xi,26-epoxy-5-cholesten--3beta-ol and base-catalyzed hydrolysis of the latter to 5-cholestene-3beta,25xi,26-triol; allylic bromination of the corresponding triacetate, and dehydrobromination gave the required 5,7-diene which yielded the vitamin derivative upon photolysis (Figure 3). The synthetic product shows the same activity pattern as the natural metabolite: at dose levels of 0.25 mug, the compound stimulates intestinal calcium transport, but has no effect on bone calcium mobilization in rats maintained on a vitamin D-deficient, low calcium diet. Higher doses (2.5 mug) elicit a more pronounced intestinal calcium transport response, but also have no significant effect on the bone mobilization system. The compound exhibits no biologial activity in nephrectomized animals.

Demonstration of the rapid action of pure crystalline 1 alpha-hydroxy vitamin D3 and 1 alpha,25-dihydroxy vitamin D3 on intestinal calcium uptake

The biological effects of crystalline 1alpha-hydroxyvitamin D3 and crystalline 1alpha,25-dihydroxyvitamin D3 have been compared on the intestinal uptake of calcium-45 by everted duodenal gut sacs from rachitic rats. Peak calcium-45 uptake was observed 1 hr after intravenous administration and both crystalline vitamin D2 analogs were of comparable potency. The rapid onset of calcium-45 uptake and the rapid attainment of maximal calcium-45 transport suggests a direct effect of these crystalline analogs on the mucosal membranes of the intestinal cell.