Postrest inotropy in rabbit ventricle: Na+-Ca2+ exchange determines sarcoplasmic reticulum Ca2+ content.

To determine whether Na+-Ca2+ exchange is a physiologically significant Ca2+ efflux mechanism in rabbit ventricle, we investigated the effects exerted on postrest contractions by interventions that alter the transmembrane distribution of Na+ or Ca2+ so as to retard Ca2+ efflux via this system. Contractions elicited after rest periods of 0.25-10 min in duration were studied. The following interventions increased postrest contractions much more than those elicited by rhythmic stimulation: 1) Na+ pump inhibition by cardiac glycosides or by a reduction in extracellular K+, 2) reduction of extracellular Na+ (maintaining a constant [Ca2+]-to-[Na+]2 ratio), and 3) elevation of extracellular Ca2+. In contrast, isoproterenol, norepinephrine, and histamine produced comparable increases in both rhythmically stimulated and postrest contractions, suggesting that the postrest contractile potentiation was not just the result of a general increase in inotropic state. Ryanodine, which appears to antagonize sarcoplasmic reticulum (SR) Ca2+ release in cardiac muscle, markedly reduced the amplitude of the postrest contractions, but only modestly decreased rhythmically stimulated responses. Results suggest 1) that Ca2+ released from SR is involved in postrest response, 2) that Na+-Ca2+ exchange serves as a Ca2+ efflux pathway in normally polarized resting rabbit ventricle, and 3) that this activity in part determines the amount of Ca2+ available for release from SR.