Transcellular transport of calcium and inorganic phosphate in the small intestinal epithelium.

Transport mechanisms involved in the small intestinal handling of inorganic phosphate and calcium have been studied by different in vitro methods during the last few years. In concordance with studies on intact epithelial preparations, studies with brush-border and basal-lateral membrane vesicles isolated from the small intestinal epithelial cell revealed that transcellular calcium and inorganic phosphate fluxes are coupled to transcellular sodium flux, i.e., secondary active via coupling to the primary active sodium flux. A sodium-coupled mechanism in the brush-border membrane leads to cellular accumulation of inorganic phosphate. A sodium-coupled mechanism leads to extrusion of calcium from the cell into the serosal interstitium. A primary active transport mediated by the Ca-ATPase and located in the basal-lateral membrane also exists for calcium. Regulation of transcellular phosphate and calcium flux proceeds via altered influx rates at the luminal cell pole.