Differential effects of quinidine and flecainide on plateau duration of human atrial action potential.

Quinidine and flecainide, two class-I antiarrhythmics increase action potential duration (APD) at 90% repolarization and cellular refractory period in human atrial fibers without significant change in resting potential. On the other hand, quinidine decreases APD at 50%, whereas flecainide slightly increases, which suggests different effects on Ca2+ current. Using isolation cell procedure and whole cell recording, we found that 10 microM quinidine (34.77 +/- 6.5%, n = 5) and 0.5 microM flecainide (50.46 +/- 6.2%, n = 4) decrease calcium current in human atrium. It is concluded that, at therapeutical concentrations, quinidine and flecainide modify the action potential plateau phase in a different manner, which is not only related to the calcium current decrease.