Frequent stimulation of the guinea-pig myocardium raises the inotropic efficacy of tissue-bound ouabain.

3H-Ouabain binding to frequently (1 Hz) stimulated papillary muscles from reserpine-pretreated guinea pigs was evaluated at ouabain concentrations of 18.5 and 200 nmol/l. Myocardial activity increased the amount of 3H-ouabain bound to the tissue in comparison with quiescent preparations. Since the shape of the time course of ouabain binding changed with frequent stimulation, a greater number of ouabain-accessible binding sites of the Na pump as induced by the rise in intracellular Na with frequent stimulation cannot be the sole mechanism of the frequency dependence. In view of their stimulatory properties on the Na pump the effects of intracellular Na and extracellular K could be equivalent. By contrast, both interventions were differently effective. The K antagonism on 3H-ouabain binding was independent from stimulation frequency. Furthermore, the shape of the time course of binding was not altered by [K]o. As evidenced by the dependence of half-times to steady-state effect on muscle diameter, the apparent rate of diffusion of ouabain was accelerated with the frequency of contractions. This acceleration could have interfered with the time course of binding at frequent stimulation. After correlating the time courses of positive inotropic effect and ouabain binding (concentration of ouabain in the medium 200 nmol/l), frequent stimulation was found to raise the inotropic efficacy of tissue-bound ouabain. The relation of excitation-dependent Na influx to the saturable, ouabain-inhibited, Na pump explained the frequency dependence of the intropic efficacy of ouabain; that is, the observed change of efficacy was consistent with Na-pump saturation in dependence on intracellular Na.