The alteration by propranolol of the inotropic and bathmotropic effects of dihydro-ouabain on guinea-pig papillary muscle

Potassium changes the relationship between receptor occupancy and the inotropic effect of cardiac glycosides in guinea-pig myocardium

K+ (2.4-15.6 mmol l-1) antagonized the positive inotropic effect of dihydro-ouabain. The concentration-effect curves became steeper with the shift to higher concentrations of the glycoside. At 1.2 mmol l-1 Ca2+, an increase in K+ from 2.4 to 12 mmol l-1 required tenfold higher concentrations of dihydro-ouabain to produce equal inotropic effects. This factor was reduced to four at 3.2 mmol l-1 Ca2+. The same change in K+ concentration, at 1.2 mmol l-1 Ca2+, diminished the inotropic effect of ouabain on rested-state contractions by a factor of six. The positive inotropic effect of Ca2+ was also antagonized by K+ (1.2-12 mmol l-1). Reduction of Na+ from 140 to 70 mmol l-1 abolished the antagonistic action of K+ (1.2-8.0 mmol l-1). Moreover the inotropic effect of Ca2+ was enhanced. Reduction of Na+, from 140 to 70 mmol l-1, antagonized the positive inotropic effect of dihydro-ouabain more at low (2.4 mmol l-1) than at high (8.0 mmol l-1) K+. Accordingly, the extent of the dihydro-ouabain-K+ antagonism was reduced. When the K+ concentration was increased from 2.4 to 12 mmol l-1, [3H]-ouabain binding was reduced by a factor of three. This is less than the reduction in the inotropic effectiveness of ouabain or dihydro-ouabain. Reduction of stimulation frequency from 1 to 0.1215 Hz did not significantly alter the antagonistic effect of K+. Diminution of Vmax of the action potential was observed only at K+ concentrations greater than 5.9 mmol l-1, whereas the resting membrane potential was continuously depolarized over the entire range of K+ concentrations. The results support the view that the reduction in receptor affinity cannot be the sole cause of the antagonism between the glycoside and K+. Impairment of passive Na+ influx during diastole, due to the K+-dependent depolarization of the resting membrane potential, contributed to about one half of the glycoside-K+ antagonism.

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.