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 microM) 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 x 10(-5) M (n = 4). The steady-state inactivation curve (hx) 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 approximately 200 ms) were applied repetitively at intervals of 300 ms approximately 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.