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

Summary

Ca²⁺ shifts in isolated, perfused ventricular muscle of guinea-pig hearts were investigated with the aid of⁴⁵Ca under the conditions of complete equilibration of preparations with isotopecontaining solutions. The content of⁴⁵Ca 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 with⁴⁵Ca containing solution. The difference of 0.93 mmol/kg w.w. is equivalent to fraction Ca₂ (15) and is labelled accordingly. Ouabain (1 μM) increased the⁴⁵Ca 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 oubain 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.