Isoproterenol Antagonizes Prolongation of Refractory Period by the Class III Antiarrhythmic Agent E-4031 in Guinea Pig Myocytes Mechanism of Action

The mechanism by which isoproterenol (ISO) prevents the prolongation of action potential duration (APD) and refractory period (RP) by the class III antiarrhythmic agent E-4031 was studied. E-4031 (1 μM) increased RP by 501% with no effect on contractile force in papillary muscles isolated from guinea pig heart. ISO (1 μM) increased force of contraction more than fivefold and decreased RP by 25%. The prolongation of RP by E-4031 was prevented by pretreatment of muscles with ISO. The prolongation of APD in isolated guinea pig ventricular myocytes by 5 1 μM E-4031 also was antagonized by prior exposure of the cells to 1 μM ISO. Instantaneous currents and delayed rectifier K currents, IK, were measured in isolated myocytes using the suction microelectrode voltage-clamp technique. Currents were measured in response to 225 -msec depolarizing pulses from a holding potential of -40 mV. Previous studies have demonstrated that IK in these cells results from activation of two distinct outward K currents, IKs and IKr (specifically blocked by E-4031). ISO doubled the magnitude of IKS without significant effect on IKr. The instantaneous current, putatively identified as a Cl current, also was doubled by ISO but was unaffected by E-4031. The augmented conductance of IKS and instantaneous current by ISO results in a decrease in RP. The small effect of E-4031 on APD and RP in the presence of ISO results from the smaller contribution of IKr relative to the augmented repolarizing currents.