The effect of low-Na perfusion and D-600 on 45Ca exchange and contractions in the ventricular myocardium of the guinea-pig heart.

The effect of low-Na (50 mM) solution and D-600 (1 mg/l) on the beat-to-beat exchange of 45Ca in the isolated, perfused ventricles and on the cellular AP-s and contractions of the papillary muscles of guinea-pig hearts was investigated. D-600 completely inhibited excitation-dependent 45Ca exchange and almost completely inhibited contractions. Perfusion with low-Na solution resulted in a transient increase of 45Ca exchange by 30% and an increase of the contractile force by 80%. Low-Na solution transiently restored 45Ca exchange in the ventricles treated with D-600 by 50% but it did not restore contractions. It is proposed that Na--Ca exchange is not important in the tested preparations under the physiological conditions. When this exchange is enhanced by decrease in [Na+]o the resulting influx of Ca2+ does not activate contraction, but is probably used to fill the intracellular Ca stores.