Sodium-calcium exchange in heart: membrane currents and changes in [Ca2+]i

Recordings have been made of changes in intracellular calcium ion concentration (Ca2+]i) that can be attributed to the operation of an electrogenic, voltage-dependent sodium-calcium (Na-Ca) exchanger in mammalian heart cells. Guinea pig ventricular myocytes under voltage clamp were perfused internally with fura-2, a fluorescent Ca2+-indicator, and changes in Ca2+]i and membrane current that resulted from Na-Ca exchange were identified through the use of various organic channel blockers and impermeant ions. Depolarization of cells elicited slow increases in Ca2+]i, with the maximum increase depending on internal Na+], external Ca2+], and membrane voltage. Repolarization was associated with net Ca2+ efflux and a decline in the inward current that developed instantaneously upon repolarization. The relation between Ca2+]i and current was linear, and the slope was made steeper by hyperpolarization.