The stimulation of 1,25-dihydroxycholecalciferol metabolism in vitamin D-deficient rats by 1,25-dihydroxycholecalciferol treatment.

Daily oral administration of 1,25-dihydroxycholecalciferol to vitamin D-deficient rats increases the rate of disappearance of [(3)H]1,25-dihydroxycholecalciferol and increases the rate of appearance of metabolites both less polar and more polar than 1,25-dihydroxycholecalciferol in the intestine, bone, liver, kidney, plasma, and muscle. Since 1,25-dihydroxycholecalciferol is believed to be the metabolically active form of vitamin D in the stimulation of intestinal calcium transport and bone calcium mobilization, these results provide an explanation for the fact that daily oral administration of 1,25-dihydroxycholecalciferol is relatively ineffective in the maintenance of serum calcium and in the calcification of bone in rats.