Effects of membrane potential on sodium-dependent calcium uptake by sarcolemma-enriched preparations from canine ventricle.

The effect of membrane potential on sodium-dependent calcium uptake by vesicles in an isolated cardiac sarcolemma preparation was examined. Initial time course studies showed that the reaction deviated from initial velocity conditions within minutes. This appeared to be due, in part, to loss of the sodium gradient. Assays carried out to 10 sec revealed a linear component of uptake (2 to 10 sec) and a faster component (complete by 2 sec). The latter was eliminated by loading the preparation with ethyleneglycol-bis-(beta-aminoethyl ether)N,N'-tetraacetic acid (EGTA). This maneuver did not affect the slow component, and subsequent studies used preparations containing EGTA. Potassium Nernst potentials (EK), established by potassium gradients in the presence of valinomycin, were varied from -100 to +30 mV by changing [K+]o from 1.18 to 153.7 mM ([K+]i = 50 mM). The initial velocity of sodium-dependent calcium uptake was stimulated twofold by changing EK from -100 to 0 mV and another twofold by raising EK from 0 to +30 mV. For the total range of EK and [K+]o, 32 to 36% of the increase appeared to reflect stimulation by extravesicular potassium. The remainder appeared to be due to membrane potential. The profile of sodium-dependent calcium uptake versus EK suggested that calcium influx through electrogenic sodium/calcium exchange may be much more affected by the positive region of the cardiac action potential than by the negative region.