Effects of ouabain and low-Na+ perfusion on rest-decay and post-rest recovery of cellular Ca content in ventricular muscle of guinea-pig heart.

Ca2+ shifts in isolated, perfused ventricular muscle of guinea-pig hearts were investigated with the aid of 45Ca under the conditions of complete equilibration of preparations with isotope-containing containing solutions. The content of 45Ca in stimulated preparations (rate 60/min) was 1.30 +/- 0.12 mmol/kg of wet weight (w.w.). 6 min rest resulted in the drop of this content to 0.37 +/- 0.05 mmol/kg w.w. despite continued perfusion with 45Ca containing solution. The difference of 0.93 mmol/kg w.w. is equivalent to fraction Ca2 (15) and is labelled accordingly. Ouabain (1 microM) increased the 45Ca content to 1.53 +/- 0.15 mmol/kg w.w. in the stimulated and to 1.12 +/- 0.23 mmol/kg w.w. in the rested muscle. The respective values after low (50 mM) sodium perfusion were 1.70 +/- 0.11 and 1.07 +/- 0.13 mmol/kg w.w. The differences between the stimulated and rested preparations (Ca2 fraction) were 0.41 and 0.63 mmol/kg, respectively. In the control experiments the force of the first post-rest beat dropped to 20 +/- 5% of the force of steady-state beats. During ouabain and low-sodium perfusion, the force of the first contraction increased markedly and its peak was larger than that of the few subsequent beats. It is concluded that Na-Ca exchange is the important factor in the rate-dependent control of Ca2 fraction content and of contractile force.