title: Inhibition of 25-hydroxyvitamin D3 production by 1, 25-dihydroxyvitamin D3 in rats.
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.
authors.affiliation: Department of Biochemistry, University of Wisconsin-Madison, Madison, Wisconsin, 53706, USA.
authors.forename: G G
authors.initials: GG
authors.lastname: Reinholz
authors.forename: H F
authors.initials: HF
authors.lastname: DeLuca
mesh_terms: D015090:25-Hydroxyvitamin D3 1-alpha-Hydroxylase / Q000378:metabolism; D000818:Animals; D002112:Calcifediol / Q000096:biosynthesis* / Q000097:blood; D002117:Calcitriol / Q000494:pharmacology*; D002118:Calcium / Q000097:blood; D002851:Chromatography, High Pressure Liquid; D006900:Hydroxylation; D066298:In Vitro Techniques; D008099:Liver / Q000187:drug effects / Q000378:metabolism; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley; D015203:Reproducibility of Results
chemical_list: D015090:25-Hydroxyvitamin D3 1-alpha-Hydroxylase; D002117:Calcitriol; D002112:Calcifediol; D002118:Calcium