Alterations of phosphatidylethanolamine N-methylation in rat heart by quinidine.

: To elucidate the molecular mechanism underlying the adverse depression of myocardial contractility observed during antiarrhythmic therapy of quinidine, we investigated its action on the phosphatidylethanolamine N-methyltransferase (EC 2.1.1.17) activities of cardiac subcellular membranes. Rat heart sarcolemma, mitochondria, and microsomes (sarcoplasmic reticular fragments) were isolated, and the three catalytic sites for N-methylation activities were examined with 0.055 (site I), 10 (site II), and 150 (site III) microM concentrations of S-adenosyl-L-[methyl-3H]methionine as a methyl donor. Total methyl group incorporation into sarcolemmal phosphatidylethanolamine was depressed by 10(-6)-10(-3) M quinidine at sites II and III. The activity of site I was stimulated at low (10(-9) M) concentrations and inhibited at high concentrations of the drug. A similar behaviour was observed with procainamide, although the inhibitory effect was less pronounced and was not additive with quinidine. Quinidine-induced inhibition was associated with a depression of Vmax, while the apparent affinity for S-adenosyl-L-methionine was unaltered. Analysis of individual methylated phospholipids confirmed inhibition by quinidine at sites II and III in sarcolemma. Microsomal phosphatidylethanolamine N-methylation was affected by 10(-6) M quinidine only at site II, whereas no changes were noted in mitochondria. Quinidine also inhibited both the positive inotropic response and concomitant increase in tissue N-methylated phospholipids observed upon L-methionine perfusion of rat heart. These results suggest that quinidine alters the intramembranal level of N-methylated phospholipids, and this may serve as a biochemical mechanism contributing to its negative inotropic effect.