Subsidiary class III effects of beta blockers? A comparison of atenolol, metoprolol, nadolol, oxprenolol and sotalol.

The electrophysiological effects of various concentrations of atenolol, metoprolol, nadolol, D-oxprenolol, L-oxprenolol, and D, L-oxprenolol and D-sotalol, L-sotalol, and D, L-sotalol were compared in rabbit right ventricular papillary muscle studied in vitro using intracellular microelectrodes. An assessment of the relative beta blocking effects of D-sotalol and D, L-sotalol against the inotropic action of isoprenaline was made in the same preparation. Of the drugs tested, only oxprenolol and sotalol showed prolongation of action potential duration and effective refractory period (class III action), with oxprenolol showing, in addition, depression of maximal upstroke velocity and the presence of post-repolarisation refractoriness (class I action). When contrasted at clinically relevant concentrations, only sotalol retained a class III effect, without any actions on variables related to fast inward current. The effects of oxprenolol and sotalol were not found to be stereospecific. The mechanical experiments indicate that, in this preparation, D-sotalol has approximately one-fourteenth of the beta blocking potency of the racemic compound. It therefore merits further consideration as a useful alternative class III antiarrhythmic agent, which would be free from the side effects of beta receptor blocking treatment.