Abstract
1. 1. The effects of three class 1 antiarrhythmic drugs procainamide (class 1A), tocainide (class 1B) and lorcainide (class 1C) on microsomal Na +–K +-ATPase activity were compared with those of ouabain in guinea pig heart preparations. 2. 2. All three antiarrhythmic drugs exhibited concentration-dependent inhibitory actions on the enzyme activity in a fashion similar to that of ouabain. 3. 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 μM. 4. 4. The IC 50 values were 1.8 ± 0.5 μM for ouabain, 14.6 ± 3.4 μM for lorcainide, 2.8 ± 0.7 mM for tocainide and 6.7 ± 1.1 mM for procainamide. 5. 5. The results demonstrate that these antiarrhythmic agents inhibit the ouabain-sensitive myocardial Na +–K +-ATPase activity in vitro with comparatively varying potencies. 6. 6. These interactions may be pertinent to the proarrhythmic or arrhythmogenic effects of the class 1 type of antiarrhythmic drugs.
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