The effects of verapamil and paired-pulse stimulation on mammalian ventricle.

Postextrasystolic potentiation induced by paired-pulse stimulation was studied in mammalian ventricle muscle in the presence of verapamil (0.2-4.0 muM). Verapamil exerts a negative inotropic effect on the potentiated contraction, but the force of the potentiated contraction in the presence of verapamil is greater than that of the normal, unpotentiated contraction in the absence of verapamil. The positive inotropic effect of paired stimulation appears to result both from an additional calcium influx appearing as a calcium current during the plateau of the premature action potential and from some additional mechanism, tentatively identified as a Na: Ca exchange. Our results suggest that paired-pulse stimulation could be used clinically to counteract the negative inotropic effects of verapamil; moreover, the antiarrhythmic action of verapamil might counteract any arrhythmias caused by paired pacing.