QUINIDINE BLOCKS CARDIAC SODIUM CURRENT AFTER REMOVAL OF THE FAST INACTIVATION PROCESS WITH CHLORAMINE-T

To determine if the fast sodium current inactivation process is necessary for sodium current (INa) blockade by quinidine, we studied the effects of quinidine on INa in guinea-pig ventricular myocytes treated with chloramine-T, which removes the fast inactivation process of INa. Following exposure to chloramine-T (2 mm), INa amplitude was reduced at all voltages and INa decay was irreversibly prevented. Quinidine (10 μm) produced resting block of INa of 36 ± 2% (n = 5) at the peak potential of -30 mV in chloramine-T treated myocytes. Quinidine decreased INa in a dose-dependent manner. The half-blocking concentration (KD) was 1.9 ± 0.2 × 10-5m (n = 4). The steady-state inactivation curve (h∞) was shifted in the negative potential direction (-5.2 ± 0.4 mV, n = 4). Even after removal of the fast inactivation process of INa, use-dependent block was observed in the presence of quinidine when various depolarizing pulse durations (5 ms ∼ 200 ms) were applied repetitively at intervals of 300 ms ∼ 2 s. Longer depolarizing pulses and higher frequency pulse trains produced greater use-dependent block. Use-dependent block was also enhanced at more positive holding potentials. These results suggest that quinidine produces both resting block and use-dependent block of sodium channels in the absence of the fast INa inactivation process. © 1991.