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  • Title: Inhibition of 25-hydroxyvitamin D3 production by 1, 25-dihydroxyvitamin D3 in rats.
    Author: Reinholz GG, DeLuca HF.
    Journal: Arch Biochem Biophys; 1998 Jul 01; 355(1):77-83. PubMed ID: 9647669.
    Abstract:
    The regulation of the hepatic vitamin D3 25-hydroxylase enzyme system by 1,25-dihydroxyvitamin D3 was examined using male rats. Circulating 25-hydroxyvitamin D3 levels decreased in response to increasing doses of 1,25-dihydroxyvitamin D3 as determined by HPLC and serum protein binding assay. A maximum reduction in serum 25-hydroxyvitamin D3 of 57.7% was achieved by the administration of 1,25-dihydroxyvitamin D3 and higher doses produced no further reduction. The in vitro rate of vitamin D 25-hydroxylation by liver homogenates from rats given 1,25-dihydroxyvitamin D3 was also reduced by 60.4%. Whole rat extracts from animals given [3H]vitamin D3 contained 50.4% less 25-[3H]hydroxyvitamin D3 in treated rats given 1,25-dihydroxyvitamin D3 than in those given only the vehicle. Further, 1,25-dihydroxyvitamin D3 treatment had no effect on in vivo disappearance of 25-[3H]hydroxyvitamin D3. Taken together, these results demonstrate that the decrease in 25-hydroxyvitamin D3 levels observed in 1,25-dihydroxyvitamin D3-treated rats results from a decrease in production and not an increase in the metabolic clearance of 25-[3H]hydroxyvitamin D3.
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