The inotropic effect of ouabain and its antagonism by dihydroouabain in rat isolated atria and ventricles in relation to specific binding sites.

The inotropic effect of ouabain has been studied in rat ventricles and atria. The concentration-effect curve of ouabain may be fitted by a model assuming the existence of two saturable components. The component with the higher sensitivity to ouabain accounted for 30% of the maximal increase in systolic tension in ventricles and for only 5% in atria. Increase in diastolic tension was only apparent at ouabain concentrations required to observe the low sensitivity component. [3H]-ouabain binding has been examined in microsomes prepared from atria and ventricles. High and low affinity binding sites have been observed. The ratio of high and low affinity ouabain binding sites was 4 fold lower in microsomes from rat atria than from rat ventricles. This could account for the difference in the response of these two tissues to the inotropic action of ouabain. In ventricular strips the high sensitivity component was much less apparent in the presence of dihydroouabain than with ouabain. When ventricular strips were preincubated in the presence of dihydroouabain 3 microM, the increase in systolic tension evoked by ouabain 1 microM was significantly reduced. Cumulative concentration-effect curve studies showed dihydroouabain antagonism to the high sensitivity component.