Electromechanical characterization of the effects of racemic sotalol and its optical isomers on isolated canine ventricular trabecular muscles and Purkinje strands

In both isolated canine ventricular trabecular muscle and Purkinje strand preparations, dl-sotalol and its two optical isomers d- and l-sotalol produced a concentration-dependent increase in action potential duration while other transmembrane electrical characteristics were not significantly affected. The magnitude of the increase in action potential durations was greater in Purkinje strand preparations. In Purkinje strand preparations, the effect was rate dependent (i.e., the increase in duration was proportionately greater when stimulation frequency was slowed). From the concentration of each compound calculated to produce a 50% maximal increase in Purkinje fiber action potential durations, d-sotalol appeared to be one to three times more potent than either l-sotalol or the racemate. Each compound appeared to increase force development in ventricular trabecular muscle preparations stimulated at a frequency of 2 Hz. Increased force development was only observed in Purkinje strand preparations stimulated at slower rates (0.5-0.33 Hz). These results are unlike those produced by other beta-adrenergic blockers and suggest that the antiarrhythmic effects of sotalol are related primarily to its effect of action potential duration. The estimated potency ratios established for the effect of dl-sotalol and its optical isomers on both trabeculae and Purkinje fiber action potential durations (d greater than dl-l) may indicate that these effects are unrelated to the beta-adrenergic blocking properties of these compounds. The differential effect of sotalol on isolated trabeculae and Purkinje strand preparations may help to explain the clinically reported phenomenon of sotalol-induced torsade de pointes.