Block of transient outward-type cloned cardiac K⁺ channel currents by quinidine

The antiarrhythmic drug quinidine has been shown to block several types of K⁺ channel currents in cardiac preparations including the transient outward current (I(to)). To characterize the molecular mechanism of quinidine block, a cloned I(to)-type cardiac K⁺ channel (RHK1) was expressed in Xenopus oocytes, and drug effects were examined on whole-cell and single-channel currents. Extracellular application of quinidine reduced whole-cell RHK1 current amplitude in a concentration-dependent manner. The block was voltage dependent, with an IC₅₀ of 1.69 mM at 0 mV, and the value decreased to 875 μM at +60 mV. Quinidine significantly slowed the current inactivation time course during voltage-clamp pulses without changing the rate of activation or the steady-state inactivation. To test the channel-state dependence of quinidine block, the cells were 'rested' in the presence of quinidine (500 μM) for 2 to 3 minutes before applying depolarizing pulses to +60 mV. During the first pulse, the current inactivation rate was slower than control, but the peak current was only reduced by less than 5%. Subsequent pulses reduced the peak current amplitude to ≃50% of control. These results suggest that quinidine blocks the open channel and that the drug must first dissociate before the channel can close, thereby causing a crossover in current tracings. In measurements of single-channel current from cell-attached patches, open time was reduced by quinidine in a concentration-dependent manner. Single-channel current amplitude was not altered by quinidine. Application of quinidine to the intracellular side of inside-out patches had an effect similar to that obtained from cell-attached patches but at 10-fold lower concentrations. External quinidine may therefore have to pass into or through the cell membrane to reach its blocking site.