Abstract
The effects of three Class I antiarrhythmic agents, quinidine (Class 1A), lidocaine (Class 1B) and encainide (Class 1C) on the Mg2(+)-dependent, Na+/K+ activated adenosine triphosphatase were compared with those of ouabain in microsomal preparations of the guinea pig heart. Lidocaine and quinidine inhibited the ATP-hydrolysis by the Na(+)-K(+)-ATPase in a concentration-dependent fashion similar to that of ouabain. The concentrations for half maximal inhibition of the enzyme activity were 4.1 +/- 0.7 mM for lidocaine and 0.46 +/- 0.12 mM for quinidine. Encainide exhibited stimulatory tendency at pharmacological concentrations, enhancing the enzyme activity by 2.5% at 1.0 microM and 25% at 1.0 mM. These results show that the Class I antiarrhythmic drugs may influence the Na(+)-K(+)-ATPase activity in different ways.