Kinetics, stoichiometry and role of the Na-Ca exchange mechanism in isolated cardiac myocytes

Compelling evidence has existed for more than a decade for a sodium/calcium (Na-Ca) exchange mechanism in the surface membrane of mammalian heart muscle cells which exchanges about three sodium ions for each calcium ion. Although it is known that cardiac muscle contraction is regulated by a transient increase in intracellular calcium (Ca2+]i) triggered by the action potential, the contribution of the Na-Ca exchanger to the Ca2+]i transient and to calcium extrusion during rest is unclear. To clarify these questions, changes in Ca2+]i were measured with indo-1 in single cardiac myocytes which were voltage clamped and dialysed with a physiological level of sodium. We find that Ca entry through the Na-Ca exchanger is too slow to affect markedly the rate of rise of the normal Ca2+]i transient. On repolarization, Ca extrusion by the exchanger causes Ca2+]i to decline with a time constant of 0.5 s at -80 mV. The rate of decline can be slowed e-fold with a 77-mV depolarization. Calcium extrusion by the exchanger can account for about 15% of the rate of decline of the Ca2+]i transient (the remainder being calcium resequestration by the sarcoplasmic reticulum (SR]. The ability of the cell to extrude calcium was greatly reduced on inhibiting the exchanger by removing external sodium, which itself led to an increase in resting Ca2+]i. This finding is in contrast to the suggestion that calcium extrusion at rest is mediated mainly by a sarcolemmal Ca-ATPase.(ABSTRACT TRUNCATED AT 250 WORDS)