Voltage Dependence of Cardiac Delayed Rectifier Block by Methanesulfonamide Class III Antiarrhythmic Agents

Voltage-clamp experiments were performed on isolated guinea pig ventricular myocytes to examine the voltage dependence of delayed rectifier block by methanesulfonamide channel blockers. Voltage-dependent channel block, in which block decreases as membrane potential is made more positive, could contribute to the phenomenon of reverse use dependence, in which the magnitude of the drug-induced prolongation in action potential duration (APD) is inversely proportional to stimulation rate. To determine whether such a voltage dependence exists, concentration-response curves were generated for dofetilide, E-4031, sematilide, and D,L-sotalol at test potentials ranging from 0 to 60 mV. All agents blocked current in a manner consistent with selective blockade of the rapidly activating component of delayed rectifier current, IKr. The rank order of potency was E-4031 approximately dofetilide > sematilide > sotalol. Block of tail currents by this class of compounds was more potent after test potentials to +60 mV than after those < or = 0-10 mV. These data are inconsistent with voltage-dependent channel block being a contributing factor to reverse use-dependence and suggest that other mechanisms must be responsible for this phenomenon.