Class I antiarrhythmic drug effects on ouabain binding to guinea pig cardiac Na⁺-K⁺ATPase

The notion that the inhibition of the Mg 2+ -dependent ATP-hydrolytic function of the myocardial Na + -K + ATPase by class I antiarrhythmic agents occurs as a result of their binding to the same receptor sites as the digitalis glycosides was tested by performing competitive binding assays of [ 3 H]ouabain (OUA) with eight drugs: disopyramide, encainide, lidocaine, lorcainide, phenytoin, procainamide, quinidine, and tocainide in guinea pig heart microsomal preparations. In the first set of experiments, 10-200 µM concentrations of the drugs were preincubated with the enzyme and displacement assays performed with 250 nM OUA. The drugs showed receptor occupancy of 19-32% at 50 µM, 25-44% at 100 µM, and 37-56% at 200 µM. Then, 10-500 nM concentrations of OUA were preincubated with the enzyme, and competitive assays were performed using 200 µM concentrations of the drugs. OUA occupied 39-51% of the receptor sites at 100 nM, 44-67% at 250 nM, and 62-82% at 500 nM, displacing the drugs in a concentration-dependent fashion. The results show that antiarrhythmic drugs interact with the same or similar receptor sites as ouabain on the Na + -K + ATPase, pointing to a possible contribution of these interactions to the mechanism for their inhibitory actions on the enzyme, and perhaps their arrhythmogenic effects.