VITAMIN-D AND INTESTINAL CALCIUM-TRANSPORT - FACTS, SPECULATIONS AND HYPOTHESES

The intestinal absorption of Ca2+ occurs by both a saturable, transcel lular process and a nonsaturable, paracellular path. The transcellular path is a multistep process, comprised of the transfer of luminal Ca2 + into the enterocyte, the translocation of Ca2+ from point of entry ( the microvillus border or membrane) to the basolateral membrane, and t he active extrusion from the cell into the circulatory system. Each st ep in the transcellular movement of Ca2+ has a vitamin D-dependent com ponent. The paracellular path also appears to be affected by vitamin D status. This review emphasizes some aspects of the Ca2+ absorptive pr ocess that require resolution and/or further experimental support. The following are discussed: evidence for participation in the active tra nsport of Ca2+ by all segments of the small intestine; a hypothetical model for the feedback control of entry of luminal Ca2+; the current v iews on vitamin D-dependent movement of Ca2+ through the cytosolic com partment of the enterocyte; the stimulated synthesis of the plasma mem brane Ca2+ pump and its gene expression by vitamin D; and the vitamin D-dependency of the paracellular transfer of Ca2+ with a comment on th e physiological significance of the rapid response of the Ca2+ absorpt ive system in vitamin D-replete animals to 1,25-dihydroxyvitamin D.