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 (Ito). To characterize the molecular mechanism of quinidine block, a cloned Ito-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 microM 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 microM) 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 approximately 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.
Yatani
A
A
Department of Pharmacology and Cell Biophysics, University of Cincinnati, College of Medicine 45267-0575.
Wakamori
M
M
Mikala
G
G
Bahinski
A
A
D000818:Animals; D003001:Cloning, Molecular; D004594:Electrophysiology; D007267:Injections; D008955:Models, Cardiovascular; D009206:Myocardium / Q000378:metabolism*; D009865:Oocytes; D015221:Potassium Channels / Q000187:drug effects* / Q000502:physiology*; D011802:Quinidine / Q000494:pharmacology*; D013997:Time Factors; D014981:Xenopus
D015221:Potassium Channels; D011802:Quinidine