Effects of quinidine on action potentials and ionic currents in isolated canine ventricular myocytes

We examined the effects of quinidine (5-20 μM) on transmembrane action potentials and ionic currents of isolated canine ventricular myocytes. Collagenase treatment of canine ventricular tissue produced a yield of 40-60% healthy cells. Myocytes had normal resting and action potentials as measured using conventional microelectrodes. Quinidine decreased V̇(max), amplitude, overshoot, and the duration of action potentials stimulated by passage of brief current pulses through the recording pipette. Recovery was complete after washout except that action potential duration was prolonged compared with control. A discontinuous single microelectrode voltage ('switch') clamp was used to measure ionic currents. Quinidine irreversibly reduced steady-state outward current as measured with three different voltage clamp protocols. Quinidine reversibly decreased peak calcium current as well as the slowly inactivating and/or steady-state inward currents in the plateau voltage range, presumably both 'late' sodium (tetrodotoxin-sensitive) and calcium (tetrodotoxin-insensitive) currents. The effect on calcium current showed both tonic and use-dependent block. Thus, quinidine had a multitude of actions on both inward and outward currents, which combine to produce the net effect of quinidine on action potential configuration.