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The metabolism of 25-hydroxycholecalciferol (25-(OH)D3) was investigated in rats fed a diet low in calcium and without vitamin D for 4 weeks after hypophysectomy. Compared with intact rats on the same diet these animals had a low serum phosphorus concentration, a less marked degree of hypocalcaemia and their parathyroid gland was not hypertrophied. Eighteen hours after i.p. injection of a single dose of tritiated 25-(OH)D3, chromatography of serum extracts on Sephadex LH-20 showed that the percentage of radioactivity corresponding to 1,25-dihydroxycholecalciferol (1,25-(OH)2D3) was lower in hypophysectomized rats than in control rats. High-pressure liquid chromatography demonstrated that 81% of this material had the same elution profile as synthetic 1,25-(OH)2D3. The percentage of 1,25-(OH)2D3 in the serum of hypophysectomized rats could be increased to the level seen in the controls by chronic treatment with bovine growth hormone. This action of growth hormone was most probably independent of the parathyroid glands since the injection of parathyroid extract did not alter 25-(OH)D3 metabolism in hypophysectomized animals. These results suggest that the decrease in the conversion of 25-(OH)D3 to 1,25-(OH)2D3 after hypophysectomy may be related to the lack of growth hormone.
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The metabolism of 25-hydroxycholecalciferol (25-(OH)D3), plasma concentration of 1,25-dihydroxycholecalciferol (1,25-(OH)2D3) and the amount of calcium-binding protein (CaBP) in duodenal mucosa were determined in ovariectomized rats and were compared with data observed in normal age-matched cyclic rats. Sephadex LH-20 and high-pressure liquid chromatography were used for the study of the metabolism of 25-(OH)D3. The concentration of 1,25-(OH)2D3 in plasma and prolactin in serum were measured by radioimmunoassay. Calcium-binding protein in duodenal mucosa was determined immunologically using electroimmunodiffusion. The results showed that the lack of ovarian hormones and low prolactin levels observed in ovariectomized rats did not promote a significant change in the metabolism of 25-(OH)D3, in the levels of 1,25-(OH)2D3 in the circulation or in the amount of CaBP in duodenal mucosa. It is possible that the regulation of 25-(OH)D3 by sex hormones is restricted to the state of calcium stress such as during egg-laying in birds or pregnancy and lactation in mammals.
