Plasma membranes of the gills of freshwater- and seawater-adapted tilapia were analyzed for Ca(2+)-ATPase and Na+/Ca2+ exchange activity. The relative importance of ATP-driven and Na(+)-gradient-driven Ca2+ transport in Ca2+ extrusion was evaluated on the basis of kinetic analyses in vitro. The Na+/Ca2+ exchangers in branchial membranes from freshwater or seawater fish displayed similar kinetics. The ATP-driven Ca2+ pump, however, showed a somewhat lower affinity for Ca2+ in membranes isolated from seawater gills than in membranes from freshwater gills; no difference in Vmax was found. The activity of the exchanger was estimated to be 50% of that of the ATP-driven pump at prevailing cytosolic Ca2+ concentrations (10(−7) mol l-1). Opercular ionocyte densities and branchial Na+/K(+)-ATPase content were not significantly different in fish residing in fresh water or sea water. We conclude that the gills of tilapia living for prolonged periods in fresh water or sea water do not differ in the make-up of their basolateral membrane with regard to Ca(2+)-ATPase, Na+/Ca2+ exchange and Na+/K(+)-ATPase activity. Apparently, the densities of these carriers suffice for calcium and sodium homeostasis under these vastly different ambient conditions.

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