Sodium-Extruding and Calcium-Extruding Sodium/Calcium Exchangers Display Similar Calcium Affinities

Na+/Ca2+ exchange activities in purely inside-out and mixed inside-out and right-side-out fish enterocyte basolateral plasma membrane vesicle preparations display equal affinities for Ca2+, showing that only the intracellular Ca2+ transport site of the Na+/Ca2+ exchanger is detected in experiments on vesicle preparations with mixed orientation. Therefore, Ca2+ pump and Na+/Ca2+ exchange activity may be compared directly without correction for vesicle orientation. The Na+/Ca2+ exchange activity in fish enterocyte vesicles is compared to the activity found in dog erythrocyte vesicles. The calcium-extruding exchanger in fish basolateral plasma membranes shows values of Km and V(max) for calcium similar to those found for the sodium-extruding exchanger in dog erythrocyte membranes, indicating that differences in electrochemical gradients underlie the difference in cellular function of the two exchangers.