Correlation between inhibition of (Na+, K+)-membrane-ATPase and positive inotropic activity of cardenolides in isolated papillary muscles of guinea pig

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Abstract
Summary Concentrations of 17 cardenolides, cardenolide glucuronides and sulfates producing halfmaximal inhibition of (Na+, K+)-membrane-ATPase from different organs and animal species were determined in vitro. In addition the concentrations that increased the contractility of guinea pig isolated papillary muscles to a particular level were investigated. Comparisons between ATPase-inhibiting and positive inotropic cardiac activities showed extensive parallelism: the correlation coefficients after log/log transformation were between 0.92 and 0.97. The same close correlations are found if dissociation constants of cardenolide receptor complexes and concentrations causing 86Rb-uptake inhibition in human erythrocytes are examined. The concentrations necessary for inhibition of (Na+, K+)-membrane-ATPase of the guinea pig heart and the concentrations required to achieve a defined positive inotropic effect in guinea pig papillary muscle showed a log/log correlation coefficient of 0.97 (P<0.001). In both tests the potencies covered more than three orders of magnitude. The results support Repke's hypothesis on the digitalis receptor.