Ouabain evoked a higher increase in contractility for a given degree of Na+/K+ pump inhibition compared to dihydroouabain and low-K solutions, indicating an additional mechanism for its inotropic effect.
The positive inotropic effect of ouabain involves mechanisms beyond just the inhibition of the Na+/K+ pump, unlike dihydroouabain.
Isolated left atria from guinea pigs were stimulated at 3.3 Hz and bathed at 30 degrees C in Tyrode's solution containing 6 mM KCl. After equilibration, this solution was replaced by a low-K solution or by Tyrode's solution containing ouabain or dihydroouabain. These treatments evoked an increase in the contractility of the atria. The time to peak increase was about 30 min, and the inotropic effect was sustained for at least 40 min. After 30 min, 42K was added to the bathing solution in order to estimate the activity of the Na+/K+ pump. A linear relationship was observed between the degree of inhibition of the Na+/K+ pump and the increase in systolic tension. The regression line was the same for low-K solutions and dihydroouabain but not for ouabain. For a given degree of inhibition of the pump, ouabain evoked a higher increase in contractility. These findings indicate that inhibition of the Na+/K+ pump can be the only mechanism responsible for the positive inotropic effect of dihydroouabain but cannot be the sole mechanism for that of ouabain.
Godfraind et al. (Thu,) reported a other. Ouabain or dihydroouabain vs. Low-K solution or standard Tyrode's solution was evaluated on Increase in systolic tension and inhibition of the Na+/K+ pump. Ouabain evoked a higher increase in contractility for a given degree of Na+/K+ pump inhibition compared to dihydroouabain and low-K solutions, indicating an additional mechanism for its inotropic effect.
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