The effect of quinidine on the fast-activating, fast-inactivating potassium current (IK(f] in acutely dissociated melanotrophs of the adult rat pituitary was examined. Macroscopic currents were measured by use of the whole-cell configuration of the patch clamp technique. 2. Bath application of quinidine caused a dose-dependent reduction of the peak amplitude of IK(f). The Kd for blockade of IK(f) at 0 mV was estimated to be 41 +/- 5.6 microM. 3. Quinidine elicited a dose-dependent increase of the rate of the decay of IK(f) and this effect was enhanced by membrane depolarization. The possibility that this phenomenon reflects an open channel blocking reaction is discussed. 4. Quinidine also caused a 5 mV hyperpolarizing shift of the steady-state inactivation curve and increased the half-time for recovery from inactivation. Quinidine did not affect the onset of inactivation measured at -30 mV. 5. Internal quinidine did not appear substantially to affect either the peak amplitude or kinetics of IK(f). 6. A study of some structural analogues showed that hydroquinidine and quinacrine had effects similar to those of quinidine. The effect of quinacrine on the amplitude and kinetics of IK(f) was also pH-dependent. Cinchonine, which bears a close structural resemblance to quinidine, was much less effective as a blocker of IK(f).
