title: Metabolism of 25-hydroxyvitamin D3 in rats: low-calcium diet vs. calcitriol infusion.
abstract: It has been proposed that the decreased serum level of 25-hydroxyvitamin D [25(OH)D] observed with dietary Ca restriction is mediated by an increase in circulating 1,25-dihydroxyvitamin D [1,25(OH)2D]. We compared the effects of endogenous and exogenous elevations in serum 1,25(OH)2D on the production rate (PR), metabolic clearance rate (MCR), and excretory pathways of [3H]25(OH)D3 in rats, with the use of steady-state techniques. Low-Ca diet and 1,25(OH)2D3 infusion caused comparable reductions in serum 25(OH)D and elevations in 1,25(OH)2D. Low-Ca diet lowered serum 25(OH)D by increasing MCR from 21.8 +/- 3.2 to 29.1 +/- 5.4 (SD) microliters.min-1.kg-1 (P less than or equal to 0.005) and decreasing PR from 944 +/- 161 to 663 +/- 163 pg.Ain-1.kg-1 (P less than or equal to 0.001). In contrast, 1,25(OH)2D3 infusion produced a dramatic rise in the MCR of 25(OH)D from 23.4 +/- 4.5 to 62.8 +/- 13.7 microliters.min-1.kg-1 (P less than or equal to 0.001) and also increased the PR from 943 +/- 165 to 1,500 +/- 337 pg.min-1.kg-1 (P less than or equal to 0.001). With 1,25(OH)2D3 infusion, urinary excretion of metabolites of [3H]25(OH)D3 rose rapidly, and kidney homogenates from these rats demonstrated vigorous side-chain oxidation of [3H]25(OH)D3. With low-Ca diet, urinary tritium excretion increased more gradually, and no direct side-chain oxidation of [3H]25(OH)D3 occurred in vitro. The increased MCR of 25(OH)D3 with low-Ca diet could be accounted for by enhanced synthesis of 1,25(OH)2D3 and subsequent degradation in target tissues.
authors.affiliation: Clinical Nutrition Research Unit, University of Chicago, Illinois 60637.
authors.forename: M J
authors.initials: MJ
authors.lastname: Bolt
authors.forename: W E
authors.initials: WE
authors.lastname: Jensen
authors.forename: M D
authors.initials: MD
authors.lastname: Sitrin
mesh_terms: D000818:Animals; D001711:Biotransformation; D002112:Calcifediol / Q000097:blood*; D002117:Calcitriol / Q000008:administration & dosage / Q000097:blood / Q000494:pharmacology*; D002118:Calcium / Q000172:deficiency; D002136:Calcium, Dietary / Q000494:pharmacology*; D007262:Infusions, Intravenous; D007668:Kidney / Q000378:metabolism; D008297:Male; D008657:Metabolic Clearance Rate; D011865:Radioisotope Dilution Technique; D051381:Rats; D011919:Rats, Inbred Strains; D014316:Tritium
chemical_list: D002136:Calcium, Dietary; D014316:Tritium; D002117:Calcitriol; D002112:Calcifediol; D002118:Calcium