Studies on Calciferol Metabolism

Abstract

It has been previously established that vitamin D3 (cholecalciferol) must undergo an obligatory two step metabolism to first 25-hydroxycholecalciferol and then 1,25-dihydroxycholecalciferol prior to the initiation in the intestine of increased calcium transport. These steps are mediated successively by the liver and the kidney. The present report compares the time course of appearance of vitamin D metabolites in various subcellular fractions of the intestinal mucosa with the onset of increased intestinal calcium transport after physiological doses of radioactive 25-hydroxycholecalciferol or 1,25-dihydroxycholecalciferol. With both steroids, the intestinal nucleus and its chromatin fraction, but not that of the liver or kidney, preferentially accumulated 65 to 75% of the radioactivity present in the tissue. This radioactivity was found by chromatographic analysis to be exclusively 1,25-dihydroxycholecalciferol. Maximal nuclear accumulation (4.5 to 6 pmoles per chick intestinal chromatin) occurred after 10 to 12 hours or only 4 hours with doses of 0.32 nmole of 25-hydroxycholecalciferol or 1,25-dihydroxycholecalciferol, respectively. Under these conditions increased intestinal calcium transport is maximal 28 or 9 hours after 25-hydroxycholecalciferol or 1,25-dihydroxycholecalciferol, respectively. Additionally it was found that the binding capacity of the nuclear chromatin for the 1,25-dihydroxycholecalciferol became saturated after a dose of 1.3 nmoles of 25-hydroxycholecalciferol or a dose of only 0.025 nmole of 1,25-dihydroxycholecalciferol. This correlates closely with the doses of these steroids required to elicit a maximum intestinal calcium transport response. These results collectively support the concept that the nuclear localization of 1,25-dihydroxycholecalciferol in the intestinal mucosa is an integral step in the development of the physiological response to vitamin D by this tissue.