Transcellular Intestinal Calcium Transport in Freshwater and Seawater Fish and Its Dependence on Sodium/calcium Exchange

Transepithelial calcium uptake and transcellular calcium uptake mechanisms were compared in the proximal intestine of freshwaterand seawater-adapted tilapia, Oreochromis mossambicus. Stripped intestinal epithelium of seawater fish showed a higher paracellular permeability to calcium in vitro. Net transepithelial calcium uptake was 71% lower, reflecting a physiological response to the increased inward calcium gradient. Na+/K+-ATPase activity was significantly enhanced in enterocytes of seawateradapted fish, in line with the water transport function of the intestine in seawater fish. The Vmax and the Km values for Ca2+ of the ATP-dependent Ca2+ pump in seawater fish enterocytes were 28% and 27%, respectively, lower than in freshwater fish. The Km for Ca2+ of the Na+/Ca2+ exchanger was 22% lower, and a 57% decrease in the Vmax for Ca2+ of the exchanger was observed. Apparently, the density of exchanger molecules in the basolateral plasma membrane is reduced in seawater fish. From the correlation between the differences in net intestinal calcium uptake and Na+/Ca2+ exchange activity we conclude that Na+/Ca2+ exchange is the main basolateral effector of transcellular calcium uptake.