Inhibitory Effects of 1α, 25-Dihydroxycholecalciferol on Parathyroid Hormone Secretion in Rats

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In an attempt to study the influence of vitamin D metabolites on PTH secretion, serum calcium and urinary excretion of cAMP were sequentially measured in conscious perfused rats, and the effects of a single iv injection of the metabolites on these paramenters were examined. Four hours after the administration of 0.25μg/kg (0.6 nmol/kg, probably a physiological dose) of 1α, 25-dihydroxycholecalciferol [1α, 25 (OH) 2D3], the urinary excretion of cAMP decreased to a level compatible with that of parathyroidectomized rats (-60% of the initial value; P<0.05) and this level was sustained for nearly 24h. Serum concentrations of calcium (total and ionized) did not change. In parathyroidectomized rats which were continuously infused with bovine PTH (1U/h), the vitamin D metabolite had no significant effect on the urinary excretion of cAMP. 24R, 25-dihydroxcholecalciferol (12.5μg/kg) had no significant effect either on the urinary excretion of cAMP or on serum calcium. These results suggest that in rats, a physiological dose of 1α, 25 (OH) 2D3 inhibits PTH secretion without causing a significant rise iu serum calcium, reflecting a feedback mechanism between active vitamin D metabolite, 1α, 25 (OH) 2D3 and the parathyroid glands.
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