A Comparison of the Cellular Electrophysiology of Mexiletine and Sotalol, Singly and Combined, in Canine Purkinje Fibers

To determine if combinations of mexiletine and sotalol retain Class I and III electrophysiologic actions, using standard microelectrode techniques, we examined the electrophysiologic effects on canine Purkinje fibers of solutions of mexiletine (3.1-100 microM), sotalol (3.1-400 microM), and combinations of the two drugs, at stimulation frequencies of 1.5 and 2.0 Hz. The combinations consisted of 12.5 microM of one drug combined with 3.1, 12.5, and 50 microM of the other. Mexiletine caused a concentration dependent depression of Vmax, the degree of depression always being greater at the more rapid frequency. At concentrations above 50 microM, sotalol depressed Vmax slightly. Increasing the stimulation frequency did not result in further Vmax depression. The addition of sotalol did not alter the Vmax depression produced by mexiletine but prior exposure to sotalol attenuated the effect of subsequent mexiletine. Both drugs reduced action potential amplitude and in combination their effects on this parameter were additive. Sotalol prolonged and mexiletine shortened action potential duration. Low concentrations of mexiletine reversed the prolongation caused by sotalol, whereas the addition of sotalol did not alter the effect of mexiletine. Mexiletine shortened the effective refractory period at low concentrations and prolonged it at high concentrations. Sotalol prolonged the effective refractory period at all concentrations. Exposure to a low concentration of sotalol did not alter the effects on the effective refractory period of subsequent exposure to mexiletine but a low concentration of mexiletine reduced the prolongation from subsequent sotalol. Thus, the combination of mexiletine and sotalol may add a Class II action to the Class I effects of mexiletine but the mexiletine prevents the Class III effects of sotalol.