Vitamin D-dependent active calcium transport: the role of CaBP.

Transepithelial calcium transport in the intestine involves an active and a passive route. The active route is totally vitamin D-dependent, transcellular, and is largely expressed in the proximal intestine. Of the three steps involved in transcellular transport--entry into the mucosal cell, intracellular movement, and extrusion at the basolateral pole of the cell--neither entry nor extrusion appears rate-limiting in the absence of vitamin D, even though both are enhanced as a result of the action of the vitamin D. However, intracellular calcium movement inside the mucosal cell can match the experimental Vm of transcellular transport only in the presence of the vitamin D-dependent calcium-binding protein (CaBP, Mr = 8.8kDa). CaBP is thought to act as the equivalent of a calcium ferry by amplifying the intracellular movement of calcium. Thus, the major action of vitamin D on cellular calcium transport is via its hormonal product, CaBP, which amplifies intracellular calcium movement by raising total and free calcium levels in the transporting cell.