Ca2+ transport capacity of sarcolemmal Na+-Ca2+ exchange. Extrapolation of vesicle data to in vivo conditions

Na+-Ca2+ exchange activity is high in cardiac sarcolemmal vesicles suggesting an important physiologic role. Vesicular Na+-Ca2+ exchange, however, is usually measured under conditions which are far from physiologic. Using sarcolemmal vesicles, we have estimated the possible significance of both Ca2+ influx and efflux mediated by Na+-Ca2+ exchange under approximate in vivo ionic conditions. In this situation, Na+-Ca2+ exchange activity is far from maximal with intracellular Mg2+ causing significant inhibition. The capacity of the Na+-Ca2+ exchange system to extrude intracellular Ca2+ (at [Ca2+] = 6.0 microM) is about 1.2 mumol Ca2+/kg wet weight/s and approximately equals the capacity of the sarcolemmal ATP-dependent Ca2+ pump. The capacity of the sarcoplasmic reticular Ca2+ pump to remove cytoplasmic Ca2+ is much larger. Significant Ca2+ influx through the exchanger is unlikely to occur in normal mammalian myocardium and would require reduced extracellular Na+ or elevated intracellular Na+.