Biological activity of 24,25-dihydroxycholecalciferol in chicks and rats

The ability of 24R, 25- and 24S, 25-dihydroxycholecalciferol to stimulate intestinal calcium transport and bone calcium mobilization in chicks was measured. Enhancement of intestinal calcium transport by 325 or 130 nmoles of either compound was maximal by 24 hours. The effects of these compounds on bone calcium mobilization were also maximal by 24 to 36 hours. When lower doses were tested, 2 nmoles of the 24R, 25-dihydroxycholecalciferol significantly stimulated intestinal calcium transport, whereas 130 nmoles of the S isomer were required for a significant response. Neither steroid had a significant effect on bone calcium mobilization when doses of less than 130 nmoles were given. When chicks received orally 32.5, 325 or 1,625 pmoles of 24R, 25-dihydroxycholecalciferol daily from hatching for 4 weeks, several parameters showed a dose-related response. These included growth, serum calcium, bone ash, renal 25-hydroxycholecalciferol-1-hydroxylase and intestinal vitamin D-dependent calcium binding protein. Rats given 32.5 to 1,625 pmoles of 24R, 25-dihydroxycholecalciferol for 3 or 6 weeks were equivalent to vitamin D-treated controls in terms of growth and serum calcium levels. It is concluded that within the lower dose ranges (2 to 30 pmoles) the R isomer of 24,25-dihydroxycholecalciferol is more active in stimulating intestinal calcium transport than the S isomer but that neither compound increases bone calcium mobilization at these dose levels. Also, the rat is more responsive in terms of growth and serum calcium, to small dialy doses of 24R, 25-dihydroxycholecalciferol than is the chick.