CONTROL OF THE NA-CA EXCHANGER IN ISOLATED HEART-CELLS .2. BEAT-DEPENDENT ACTIVATION IN NORMAL-CELLS BY INTRACELLULAR CALCIUM

Electrical stimulation of isolated adult rat heart cells in suspension at 4 Hz resulted in a fourfold increase in the rate of sodium influx and efflux across the sarcolemma, with no change in total cell sodium, as measured with Na-22. The magnitude of stimulation-dependent sodium fluxes under these conditions averaged 17 nmol/min/mg protein. The increased rate of efflux was inhibited by tetrodotoxin, verapamil, or dichlorobenzamil and required extracellular calcium. The inhibition by tetrodotoxin was overcome by Bay K 8644. The basal rate of Na-22 efflux in cells at rest was inhibited only slightly by dichlorobenzamil. The stimulation-induced efflux was not inhibited by ouabain, but in the presence of ouabain, stimulation increased the rate of accumulation of total sodium by 4 nmol/min/mg. This increase was inhibited by tetrodotoxin or verapamil. A calcium-dependent increase in rate of Na-22 influx and efflux could also be induced by KCl addition. This was inhibited by verapamil and dichlorobenzamil but not by tetrodotoxin and was reversed by EGTA, but only after a delay. We conclude the following. 1) The Na-Ca exchanger in cells at rest is no more than 10% activated. 2) The exchanger becomes activated directly or indirectly by calcium that enters the cell through calcium channels during excitation. 3) In this preparation the major part of excitation-induced sodium fluxes are mediated by the Na-Ca exchanger, with only a relatively small direct participation of sodium channels. These channels participate indirectly by promoting calcium channel activation. 4) If all the calcium-dependent sodium fluxes were Na-Ca exchange, then calcium flux through the exchanger per beat would be about sevenfold larger than that through the calcium channels. An undetermined part of the calcium-dependent sodium fluxes, however, could be a direct Na-Na exchange through the activated Na-Ca exchanger.