Sarcoplasmic reticulum and Na+/Ca2+ exchanger function during early and late relaxation in ventricular myocytes

The relative importance of the Na+/Ca2+ exchanger in the initial and terminal phases of relaxation and the decline in the [Ca2+](i) transient was investigated in adult rabbit ventricular myocytes loaded with the Ca2+ indicator flue 3. For electrically stimulated contractions, the peak intracellular Ca2+ concentration ([Ca2+](i)) was 700 +/- 87 nM and end-diastolic [Ca2+](i) was 239 +/- 30 nM (0.25 Hz, 37 degrees C, 1.08 mM extracellular Ca2+ concentration; n = 14). Abrupt inhibition of Na+/Ca2+ exchange was produced by removal of extracellular Na+ (KCl substitution) and Ca2+ [2 mM Ca2+-free ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid] by means of a rapid switcher device (SW). Abrupt exposure to high K+ induced an action potential, although sufficient Ca2+ remained adjacent to the sarcolemma to induce a contraction (SW beat) and [Ca2+](i) transient that were identical in amplitude to those induced by electrical stimulation (ES beat). The initial relaxation and decline in the [Ca2+](i) transient was not significantly prolonged by abrupt elimination of the Na+/Ca2+ exchanger, but the rate and extent of the terminal phase of the decline in the [Ca2+](i) transient were significantly reduced. The first derivative of [Ca2+](i) with respect to time versus [Ca2+](i) during the decline of the [Ca2+](i) transient attributable to sarcoplasmic reticulum (SR) function was estimated from the average SW transients, and that attributable to Na+/Ca2+ exchange was estimated from the difference between SW and ES transients. By this analysis, the Na+/Ca2+ exchanger produces 13% of the first half of the decline in [Ca2+](i) and 45% of the second half of the decline. We conclude that abrupt inhibition of forward Na+/Ca2+ exchange does not significantly affect the amplitude or the initial rate of decline of the [Ca2+](i) transient and relaxation. However, its contribution to the reduction of [Ca2+](i) becomes apparent late during the [Ca2+](i) transient, when cytosolic [Ca2+](i) has been reduced.