Biological activity of 1-alpha-hydroxy vitamin D3 in vitamin D dependency

The biological activity of 1(OH) vitamin D3 was evaluated in 2 cases of vitamin D dependency. For the improvement of serum chemistry and increment of urinary excretion of calcium 1(OH) vitamin D3 was 500--1000 times more active than vitamin D2. The administration of 0.05 microg/kg/day and 0.1 microg/kg/day of 1(OH) vitamin D3 were equally effective to keep the serum chemistry within normal limits. However, urinary excretion of calcium increased to an abnormal height on 0.1 microg/kg/day of 1(OH) vitamin D3. On the other hand, administration of 0.05 microg/kg/day of 1(OH) vitamin D3 kept both serum level and urinary excretion of calcium normal. It is suggested that 0.05 microg/kg/day is close to the optimum requirement of 1(OH) vitamin D3 in vitamin D dependency and this should correspond to the essential requirement of 1(OH) vitamin D3 in man.

In vivo 24-hydroxylation of 25-hydroxyvitamin D3 in enterocolectomized rats

To determine if the renal 25-hydroxyvitamin D3 24-hydroxylase is active in addition to the intestinal 25-hydroxyvitamin D3 24-hydroxylase in vivo in the rat, radiolabeled 25-hydroxyvitamin D3 was administrated to normocalcemic, vitamin D-replete, enterocolectomized rats. Substantial amounts of 24,25-dihydroxyvitamin D3 were detected and the identity of the isolated plasma 24,25-hydroxyvitamin D3 region in these animals were confirmed by coelution on high pressure liquid chromatography and periodate cleavage. This suggests that both intestinal and renal 25-hydroxyvitamin D3 24-hydroxylases are active in vivo in the rat.

25-Hydroxycholecalciferol receptors in human breast cancer

Cytosol receptors for 25-hydroxycholecalciferol, oestradiol and progesterone were measured in human mammary carcinomas. Significant positive correlations were found between the concentrations of all three receptors.

Intestinal absorption of 25-hydroxyvitamin D3 in unanesthetized rat

We investigated the mechanism and characteristics of 25-hydroxyvitamin D3 (25-OH-D3) absorption in the unanesthetized rat by using a single-pass intestinal perfusion technique. The rate of 25-OH-D3 absorption remained linear for a wide range of concentrations (2-900 nM). Absorption rate of 25-OH-D3 increased as the pH, the bile acid concentration, and thickness of the unstirred water layer were decreased. Absorption did not change after the additions of fatty acids of varied chain lengths and degrees of saturation. In rats with lymph and bile fistulas, 18.5% and 16.3% of the infused radio-activity appeared in the lymph and bile drainage, respectively. These experiments indicate that 25-OH-D3 is absorbed by a passive diffusion mechanism that is influenced by the intestinal pH, bile acid concentration, and thickness of the unstirred water layer, but not by the presence of fatty acids. Approximately equal fractions of the infused hydroxylated vitamin are recovered from the lymphatic and biliary fluids.

Effect of phenytoin on bone and vitamin D metabolism

Calcium and vitamin D metabolism were evaluated in 5 adult epileptic patients before and during treatment with phenytoin. Significant decreases occurred in serum concentrations of calcium, albumin, and 25-hydroxy-cholecalciferol. The decreases in serum calcium paralleled those in serum albumin. Significant increases occurred in serum alkaline phosphatase and 1 alpha, 25-dihydroxycholecalciferol, in urinary hydroxyproline, and in the fractional gastrointestinal absorption of calcium. Urinary cyclic adenosine monophosphate and serum parathyroid hormone did not change. The results suggest that the bone disease resulting from phenytoin therapy may be associated with a deficiency of 25-hydroxycholecalciferol and not of 1 alpha, 25-dihydroxycholecalciferol, and that reduced gastrointestinal absorption of calcium or changes in parathyroid function may not be necessary for the development of bone disease.

Specific cytosol receptors for 1,25-dihydroxyvitamin D3 in human intestine

Human intestine is shown to contain a specific, high affinity cytosol receptor or binding protein for 1,25-(OH)2-vitamin D3 [1,25 (OH)2D3]. This receptor is a protein which sediments at 3.5S on sucrose gradients containing 0.3 M KCL. The receptor binds 1,25 (OH)2D3 with a Kd of approximately 2 x 10(-10) M at 4 degrees C. Competition binding studies using structural analogs of 1,25 (OH)2D3 indicate that the relative specificity of the receptor is 1,25 (OH)2D3 greater than 1,24R,25(OH)3D3 greater than greater than 25OHD3 1 OHD3 greater than 24R,25(OH)2D3.

Serum concentrations of 25-hydroxyvitamin D in rickets of extremely premature infants

Nine premature infants developed radiographic and biochemical rickets at a mean +/- SD of 12.6 +/- 2.8 weeks of age. Serum 25-hydroxyvitamin D concentrations were all low, with a mean of less than 3.6 +/- 2.1 ng/ml. The mean average daily intake of vitamin D since birth had been 300 +/- 181 IU, and the mean average daily intake during the week of diagnosis was 587 +/- 313 IU. All of the infants were extremely premature (mean weight 948 +/- 153 gm, mean gestation 27.7 +/- 1.1 weeks), and were being fed either a low-calcium "human milk-like" formula or a soy formula. It is postulated that low-calcium intake may have increased 25-OHD utilization in the face of a decreased ability of the extremely premature infant to produce 25-OHD. Because of multiple factors leading to both decreased production and possible increased utilization of 25-OHD, such infants have an increased requirement for vitamin D to maintain normal serum 25-OHD concentrations, and daily intakes of at least 400 IU vitamin D orally must be assured. Serum 25-OHD measurements and radiographs may be important in following infants at risk.

Plasma binding capacity for 25-hydroxy-vitamin D in the elderly

Plasma 25-hydroxy-vitamin D (25-OHD) concentrations and 25-OHD binding capacities were measured in 14 elderly patients and in 14 young controls. Both 25-OHD concentrations and 25-OHD binding capacities were reduced in the elderly. In neither group, however, was there a significant correlation between 25-OHD binding capacities and 25-OHD concentrations. Thus, although 25-OHD binding capacities are reduced in sick old people this does not account for the low 25-OHD concentrations often found in this group.

Highly specific binding of 1,25-dihydroxycholecalciferol in bone cytosol

A method developed initially for the detection of high-affinity binding of glucocorticoids in the cytosol from foetal rat calvaria has been adapted for metabolites of vitamin D. Consistent displacement of [3H]1,25-dihydroxycholecalciferol ([3H]1,25(OH)2D3) by unlabelled 1,25(OH)2D3 was obtained with an apparent dissociation constant (Kd) of 2.3 x 10(-9) mol/l. The specificity of this binding was examined by competition experiments. Displacement of labelled 1,25(OH)2D3 by a 100-fold excess of unlabelled metabolites, expressing the fall with unlabelled 1,25(OH)2D3 as 100%, was as follows: 25-hydroxycholecalciferol (25(OH)D3), 61%; 24,25-dihydroxycholecalciferol, 29%; cholecalciferol, 3%. These are similar to results for the chick mucosa nuclear 1,25(OH)2D3 receptor. No displacement was obtained with corticosterone, testosterone, oestradiol or progesterone. When [3H]25(OH)D3 was used as ligand, a displacement curve with unlabelled 25(OH)D3 indicated only binding with a greater Kd (approximately 10(-7) mol/l). These data suggest a direct action of 1,25(OH)2D3 on bone which is similar to that of steroid hormones on their target tissues.

Interaction of 1 alpha, 24-dihydroxyvitamin D3 with the chick intestinal 1 alpha, 25-dihydroxyvitamin D3 receptor system

The interaction of 1 alpha, 24-dihydroxyvitamin D3 with the 1 alpha, 25-dihydroxyvitamin D3 receptor system was studied in chick intestinal mucosa. The competitive receptor binding assay indicated that 1 alpha, 24-dihydroxyvitamin D3 bound to a cytosol 1 alpha, 25-dihydroxyvitamin D3 receptor with a relatively high affinity compared with other vitamin D3 analogs. The R-isomer of 1 alpha, 24-dihydroxyvitamin D3 revealed higher affinity for the receptor than the S-isomer. In the reconstituted cytosol-chromatin system, bith 1 alpha, 24(R)-dihydroxy[3H]-vitamin D3 and 1 alpha, 24(S)-dihydroxy-[3H]-vitamin D3 specifically associated with chromatin via a temperature-dependent process. The association of 1 alpha, 24-dihydroxy-[3H]-vitamin D3 with chromatin was reduced in the presence of competing unlabeled 1 alpha, 25-dihydroxyvitamin D3. Furthermore, the chromatin-associated 1 alpha, 24-dihydroxy-[3H]-vitamin D3 was dissociated by a high concentration of KCl, likewise 1 alpha, 25-dihydroxy-[3H]-vitamin D3. From these results, it is strongly indicated that 1 alpha, 24-dihydroxyvitaim D3 is recognized by cytosol 1 alpha, 25-dihydroxyvitamin D3 receptor as an analog of 1 alpha, 25-dihydroxyvitamin D3 and associates with chromatin by the same mechanism as 1 alpha, 25-dihydroxyvitamin D3.

Interaction of the parathyroid and 1,25-dihydroxyvitamin D3 in the control of renal 25-hydroxyvitamin D3 metabolism

Parathyroid extract and 1,25 dihydroxyvitamin D3 (1,25-(OH)2D3) were found to antagonize each other's action to regulate the kidney hydroxylation of 25-hydroxyvitamin D3. Parathyroid extract prompted an increase in serum 1,25-(OH)2D3 in thyroparathyroidectomized rats whereas the administration of exogenous 1,25 (OH)2D3 resulted in stimulated 24,25-dihydroxyvitamin D3 and suppressed 1,25-(OH)2D3 serum levels. The actions of parathyroid and 1,25-(OH)2D3 to modulate kidney 25-hydroxyvitamin D3 hydroxylase enzyme activities were not additive; rather, the hormone and metabolite acted to suppress each other's in vivo regulatory action. The specificity and rate of 25-hydroxyvitamin D3 hydroxylation was determined by the interdependent modulatory actions of parathyroid extract and 1,25-(OH)2D3. Such results suggest that the kidney 25-hydroxyvitamin D3-1- and 24-hydroxylase enzyme systems are regulated in response to the relative modulatory activities of parathyroid hormone and 1,25-(OH)2D3.

Osteomalacia: current concepts

Recently acquired knowledge about vitamin D metabolism has improved our understanding in different varieties of osteomalacia. Many new causes of osteomalacia continue to be found. Radiologic and biochemical changes are not always characteristic and may occasionally be misleading. Bone biopsy after a double tetracycline label is helpful in differentiating osteomalacia from high bone turnover conditions and is recommended in most patients with a generalized rarefying skeletal disorder. Even if the underlying disease state cannot be corrected, effective therapy is available in most varieties of osteomalacia. The newer metabolites of vitamin D should soon be generally available to the medical profession. Their use will make treatment of osteomalacia more individualized and specific.