Effect of Vitamin D Metabolites on Intestinal Calcium Absorption and Calcium-Binding Protein in Young and Adult Rats*

Abstract

Intestinal calcium absorption and the capacity of the intestine to adapt to a low calcium diet both decrease with age in rats. The purpose of this study was to determine if these decreases are due to a decreased response of the adult rat intestine to the vitamin D metabolites 25-hydroxyvitamin D₃ (25-OH-D₃) and 1,25-dihydroxyvitamin D₃ [1,25-(OH)₂D₃]. Young (1.5 months old, 100 g) and adult (12 months old, 350 g) F344 male rats were fed an 0.8% strontium diet for 6 days to induce a l,25-(OH)₂D_a deficiency. After oral dosing with 1,25- (OHhDn, active calcium transport was measured by the everted gut sac technique, and the vitamin D-dependent calcium-binding protein (CaBP) content of the intestine was quantified by radial immunodiffusion. In both young and adult rats, transport was maximal 6 h after dosing, and CaBP content was maximal 16-24 h after dosing. In dose-response studies, young and adult rats responded to 1,25-(OH)₂D:) (0.3-3000 pg/g BW) with significantly increased active calcium transport and intestinal CaBP content when measured 16 h after dosing. The response of the adult rat was equal to that of the young rats in terms of CaBP content. However, active transport was less in the adult than in the young rats at each dose of 1,25-(OH)₂D₃ studied. In contrast, adult rats fed a low calcium diet for 10 days and dosed orally with 25-OHD.) showed no increases in intestinal calcium transport or CaBP content 24 or 48 h after dosing. It is concluded that the adult intestine does respond to exogenous oral l,25-(OH)₂D₃ but not to oral 25-OH-D₃. (Endocrinology106: 469, 1980)