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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.
Barcenas-Ruiz et al. (Fri,) studied this question.
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