COMPARATIVE TOCAINIDE AND IN GUINEA EFFECTS OF PROCAINAMIDE, LORCAINIDE ON Na+-K+-ATPASE PIG HEART PREPARATIONS

-1. The effects of three class I antiarrhythmic drugs procainamide (class IA), tocainide (class IB) and lorcainide (class 1C) on microsomal Na+-K+-ATPase activity were compared with those of ouabain in guinea pig heart preparations. 2. All three antiarrhythmic drugs exhibited concentration-dependent inhibitory actions on the enzyme activity in a fashion similar to that of ouabain. 3. The rank order of their potencies showed the following tendency: lorcainide >> tocainide > procainamide. However, while the actions of lorcainide were comparable to those of cardiotonic steroids, those of procainamide became significant only at concentrations above 80 #M. 4. The IC5o values were 1.8 + 0,5 #M for ouabain, 14.6 + 3.4#M for lorcainide, 2.8 + 0.7 mM for tocainide and 6.7 + 1.1 mM for procainamide. 5. The results demonstrate that these antiarrhythmic agents inhibit the ouabain-sensitive myocardial Na÷-K+-ATPase activity in vitro with comparatively varying potencies. 6. These interactions may be pertinent to the proarrhythmic or arrhythmogenic effects of the class 1 type of antiarrhythmic drugs.