Interaction between theophylline and ouabain in the rabbit heart: effect on (Na+ + K+)-ATPase

In electrically stimulated rabbit ventricular strips, theophylline (0.3-30 mM) antagonized the increased contractility produced by ouabain (0.8 microM). Initial velocity of specific [3H]ouabain binding to homogenates prepared from the muscle strips was used to determine the fraction of binding sites occupied by ouabain. Theophylline decreased the binding of ouabain to (Na+ + K+)-ATPase. It is concluded that the effect of theophylline on ouabain-induced positive inotropism may be mediated by decreased ouabain binding to (Na+ + K+)-ATPase.