H.F. De Luca

Intestinal calcium absorption: nature of defect in chronic renal disease.

When compared to that of normal animals, calcium-binding protein activity of duodenal mucosa obtained from uremic rats was decreased. There was no change in this activity after vitamin D3, therapy. In contrast, prior treatment with 25-hydroxycholecalcijerol resulted in increased transport of calcium-45 and calcium-binding protein activity in the intestine.

Synthesis, biological activity, and metabolism of 22, 23−3H-vitamin D4

Abstract The preparation of 22,23- 3 H-vitamin D 4 (specific activity = 100,000 dpm/0.025 μg has been described, its biopotency in rats established, and its metabolism in rats studied as compared with 1, 2- 3 H-vitamin D 3 . The highest concentrations of 3 H from a 0.25-μg intravenous dose of 3 H-vitamin D 4 were found in bone cells and intestinal mucosa. Liver initially (within 1 hour) accumulated much of the radioactivity but released it again quite rapidly. Other tissues attained a maximum of radioactivity at about 4 hours after dosage and declined slowly thereafter. The radioactivity from 22,23- 3 H-vitamin D 4 in the tissues declined more rapidly than that from 1,2- 3 H-vitamin D 3 but increased more rapidly in the intestinal contents and feces, which suggests that vitamin D 4 or a metabolite thereof is excreted more rapidly than vitamin D 3 and thereby renders it ineffective earlier.

25-Hydroxy-26,26,26,27,27,27-hexafluorovitamin D3: Biological activity in the rat☆

Abstract Vitamin D-deficient rats given 25-hydroxy-26,26,26,27,27,27-hexafluorovitamin D 3 have no detectable 24,25-dihydroxyvitamin D 3 , 1,25-dihydroxyvitamin D 3 , 25,26-dihydroxyvitamin D 3 , or 25-hydroxyvitamin D 3 -26,23-lactone in their blood while the same rats given 25-hydroxyvitamin D 3 have the expected levels of these compounds in blood. When assayed in the same rats for biological activity, 25-hydroxy-26,26,26,27,27,27-hexafluorovitamin D 3 was found to be equal to 25-hydroxyvitamin D 3 in stimulating intestinal calcium transport, bone calcium mobilization, bone mineralization, epiphyseal plate calcification, and elevation of serum inorganic phosphorus. Bilateral nephrectomy eliminates the response of intestine and bone to 25-hydroxy-26,26,26,27,27,27-hexafluorovitamin D 3 suggesting that this compound must be 1α-hydroxylated to be functional. These results provide evidence that the known functions of vitamin D do not require 26-hydroxylation or oxidation to 25-hydroxyvitamin D 3 -26,23-lactone.

Metabolism and binding properties of 24,24-difluoro-25-hydroxyvitamin D3☆

To evaluate possible functional roles for 24,25-dihydroxyvitamin D3, 24,24-difluoro-25-hydroxyvitamin D3 has been synthesized and shown to be equally as active as 25-hydroxyvitamin D3 in all known functions of vitamin D. The use of the difluoro compound for this purpose is based on the assumption that the C-F bonds are stable in vivo and that the fluorine atom does not act as hydroxyl in biological systems. No 24,25-dihydroxyvitamin D3 was detected in the serum obtained from vitamin D-deficient rats that had been given 24,24-difluoro-25-hydroxyvitamin D3, while large amounts were found when 25-hydroxyvitamin D3 was given. Incubation of the 24,24-difluoro compound with kidney homogenate prepared from vitamin D-replete chickens failed to produce 24,25-dihydroxyvitamin D3, while the same preparations produced large amounts of 24,25-dihydroxyvitamin D3 from 25-hydroxyvitamin D3. Kidney homogenate prepared from vitamin D-deficient chickens produced 24,24-difluoro-1,25-dihydroxyvitamin D3 from 24,24-difluoro-25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 from 25-hydroxyvitamin D3. In binding to the plasma transport protein for vitamin D compounds, 24,24-difluoro-25-hydroxyvitamin D3 is less active than 25-hydroxyvitamin D3 and 24R,25-dihydroxyvitamin D3. In binding to the chick intestinal cytosol receptor, 24,24-difluoro-25-hydroxyvitamin D3 is more active than 25-hydroxyvitamin D3 which is itself more active than 24R,25-dihydroxyvitamin D3. The 24,24-difluoro-1,25-dihydroxyvitamin D3 is equal to 1,25-dihydroxyvitamin D3, and both are 10 times more active than 1,24R,25-trihydroxyvitamin D3 in this system. These results provide strong evidence that the C-24 carbon of 24,24-difluoro-25-hydroxyvitamin D3 cannot be hydroxylated in vivo, and, further, the 24-F substitution acts similar to H and not to OH in discriminating binding systems for vitamin D compounds.

Thyroid hormone and vitamin D metabolism in the rat

Abstract The effect of induced hypothyroidism on vitamin D metabolism, intestinal calcium transport, and plasma levels of calcium and phosphorus by feeding an antithyroid drug, propylthiouracil, has been studied in rats. Hypothyroidism is associated with depressed growth and diminished circulating levels of calcium, phosphorus, and 1,25-dihydroxyvitamin D 3 . In contrast, plasma levels of 25-hydroxyvitamin D 3 and 24R,25-dihydroxyvitamin D 3 are unchanged by the hypothyroidism. The administration of thyroid hormone to hypophysectomized rats assumed to be hypothyroid increases plasma 1,25-dihydroxyvitamin D 3 suggesting that this effect of the thyroid hormone is indpendent of the pituitary. Intestinal calcium transport is increased by hypothyroidism in spite of low circulating levels of 1,25-dihydroxyvitamin D 3 . Since hypophysectomy also diminishes plasma 1,25-dihydroxyvitamin D and phosphorus levels but does not cause an increase in intestinal calcium transport, it is suggested that hypophosphatemic stimulation of intestinal calcium transport is mediated by a hypophyseal factor indpendent of vitamin D. However, the lowered plasma levels of calcium and phosphorus found in hypothyroidism may in part result from reduction in circulating 1,25-dihydroxyvitamin D 3 levels. The data also support the earlier observations that modulation of intestinal calcium transport by phosphorus is not entirely mediated by vitamin D.

The Distal Nephron in the Chick Embryo as a Target Tissue for 1-Alpha-25-DihydroxycholecaIciferoI

A histological and ultrastructural study as well as an autoradiographic analysis after injection of tritiated 1,25-dihydroxycholecalciferol (1,25-DHCC) were conducted on the distal convoluted tubules of chick embryos. Distal tubules in the embryo were shown to have the same spatial distribution as described for the adult kidney; they presented a convoluted portion located in the vicinity of the central intralobular veins and straight portions irradiating from this region towards the periphery. The epithelium in these tubules was well differentiated; its cells had numerous interdigitating folds in their lateral boundaries which were especially numerous at the basal ends. This device greatly increased the membrane surface available for interchange and was interpreted as an expression of active water and/or mineral transport. Nuclear concentration of radioactivity was found 2 h after injection of tritiated 1,25-DHCC in both the pars convoluta and the pars recta of the distal tubules. This concentration could be blocked by the previous administration of large amounts of nonradioactive 1,25-DHCC. These facts were interpreted as indicating that distal convoluted tubules in the chick embryo are functionally differentiated and contain target cells for 1,25-DHCC.

1α-hydroxycholecalciferol and 25-hydroxycholecalciferol in renal bone disease

SummaryFour μg of 1α-OH-D3 were given orally every other day to 10 uremic patients (8 on chronic hemodialysis) for 1–12 weeks and 200 μg of 25-OH-D3 to 3 patients on chronic hemodialysis for 4–8 weeks. Before and at the end of therapy a transilial bone biopsy was analysed. Serial evaluations of serum immunoreactive PTH (P.iPTH), calsium, phosphate and alkaline phosphatase were made.It is concluded that both 1α-OH-D3 and 25-OH-D3 (this latter, however, at a dose 50 times higher) are able to depress hyperparathyroidism in 2/3 of the cases and to improve consistently the defect of mineralisation. Neither PiPTH nor the bone histomorphometric parameters returned to normal in any case, so that long-term evaluation of these 2 drugs in warranted.

Decarboxylation of retinoic acid in tissue slices from rat kidney and liver

Abstract The oxidative decarboxylation of the isoprenoid side chain of retinoic acid has been studied in vitro in tissue slices of rat liver and kidney. The intestinal mucosa has significantly less activity than either the kidney or liver. The reaction occurs to the same degree in stock rats or in either retinol-deficient or stock rats which have been supplemented with retinoic acid, an indication that the decarboxylation enzymes are not inducible. In 2 hours, about 30% of the C-15-is released as CO 2 when either liver or kidney slices are used. The oxidation of the C-14 to CO 2 occurs to a greater extent in the kidney (4.5–6%) than in the liver slices (1.5 – 2.5%). Studies with inhibitors indicate that the production of 14-CO 2 requires the participation of the Krebs cycle enzymes, whereas the C-15-decarboxylation exhibits no requirement for these or for the mitochondrial electron transport system. A rate study demonstrates that the C-15-decarboxylation is very rapid and that the oxidation of the C-14 to CO 2 appears to result from an enzymic reaction on the product of the former.