Different electrophysiological effects of the levo- and dextro-rotatory isomers of mexiletine in isolated rabbit cardiac muscle

Racemic mexiletine is a widely used antiarrhythmic agent that blocks sodium channels. The effects of R-(−) and S-(+) mexiletine stereoisomers on maximum rate of depolarization ([Formula: see text]), conduction time, and repolarization have not yet been investigated in isolated cardiac preparations. We studied the effect of the R-(−) and S-(+) mexiletine on rabbit cardiac action potential parameters by using the conventional microelectrode technique. Both enantiomers at 20 μmol/L of therapeutically and experimentally relevant concentration, significantly depressed the [Formula: see text] at fast heart rates (BCLs 300–700 ms). R-(−) mexiletine has more potent inhibitory effect than S-(+) mexiletine. Both R-(−) and S-(+) mexiletine significantly inhibited the [Formula: see text] of early extrasystoles measured at 70 ms diastolic interval induced by S1–S2 stimuli. R-(−) mexiletine has more pronounced inhibitory effect than S-(+) mexiletine. Both R-(−) and S-(+) mexiletine increased significantly the ERP/APD 90 ratio. The time constant (τ) of recovery of [Formula: see text] was found to be τ = 376.0 ± 77.8 ms for R-(−) mexiletine and τ = 227.1 ± 23.4 ms for S-(+) mexiletine, which indicates a slower offset kinetics for R-(−) mexiletine from sodium channels than that of the S-(+) enantiomer. These data suggest that R-(−) mexiletine might be a more potent antiarrhythmic agent than S-(+) mexiletine.