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The inotropic actions of isoproterenol in cat papillary muscles or trabeculae bathed in a salt solution containing 4 mM KCl were compared to those in similar muscles bathed in a salt solution containing 22 mM KC1. Although isoproterenol evoked the same increase in force of contraction in both groups of muscles, the time course of contraction differed markedly. In muscles bathed in 4 mM KC1, isoproterenol caused a concentration-dependent decrease in time-to-peak force, but in muscles bathed in 22 mM KCl, isoproterenol caused a concentration-dependent increase in time-to-peak force. These data suggest that ventricular muscle activated by slow response action potentials may utilize a different mechanism of excitation-contraction coupling than do muscles activated by Na-dependent action potentials.

Inotropic actions of isoproterenol in cat ventricular muscle. Effects of extracellular potassium.