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

The intestinal absorption of Ca2+ occurs by both a saturable, transcellular 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 the active extrusion from the cell into the circulatory system. Each step in the transcellular movement of Ca2+ has a vitamin D-dependent component. The paracellular path also appears to be affected by vitamin D status. This review emphasizes some aspects of the Ca2+ absorptive process that require resolution and/or further experimental support. The following are discussed: evidence for participation in the active transport of Ca2+ by all segments of the small intestine; a hypothetical model for the feedback control of entry of luminal Ca2+; the current views on vitamin D-dependent movement of Ca2+ through the cytosolic compartment of the enterocyte; the stimulated synthesis of the plasma membrane Ca2+ pump and its gene expression by vitamin D; and the vitamin D-dependency of the paracellular transfer of Ca2+ with a comment on the physiological significance of the rapid response of the Ca2+ absorptive system in vitamin D-replete animals to 1,25-dihydroxyvitamin D.