The influence of (+)-propranolol on the inotropic effect of dihydro-ouabain in relation to stimulation frequency.

1. The influence of (+)-propranolol, 10(-6) to 4 x 10(-5) mol/l, upon the positive inotropic effect of dihydro-ouabain was studied on guinea-pig papillary muscle. At 1 Hz, negative inotropically effective concentrations of (+)-propranolol decreased the inotropic effectiveness of the glycoside. 2. For equivalent diminution of the basal force of contraction, either by reduction of stimulation frequency or by increasing concentrations of (+)-propranolol, a similar decrease in the inotropic effectiveness of dihydro-ouabain was observed. In contract, reduction of [Ca2+]o had almost no effect on the effectiveness of the glycoside. 3. The inhibitory effect of (+)-propranolol on the inotropic effectiveness of dihydro-ouabain declined with reduction of stimulation frequency. At any stimulation frequency tested, the inhibitory effect of (+)-propranolol was proportional to the effect of a reduction of that particular frequency to the rested-state condition in the absence of (+)-propranolol. 4. The results are consistent with the hypothesis that the inhibition of the excitation-coupled sodium influx by (+)-propranolol is the cause of its inhibiting effect on the effectiveness of dihydro-ouabain.