Lack of Triggered Automaticity Despite Repolarization Abnormalities due to Bepridil and Lidoflazine

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Abstract
Bepridil and lidoflazine are calcium channel antagonists that also prolong action potential duration, produce QT interval prolongation on the surface ECG, and have been associated with the distinctive form of ventricular tachycardia, torsade de pointes. It has been demonstrated that quinidine, which also prolongs QT interval and induces torsade de pointes, produces early afterdepolarizations and triggered activity in canine Purkinje fibers driven at slow rates. The effects of bepridil (1-10 microM) and of lidoflazine (5-15 microM) on the transmembrane action potential from canine Purkinje fibers were therefore studied. At long cycle lengths, unlike with quinidine, triggered activity arising during phase 3 was rare; phase 3 secondary plateaus (early afterdepolarizations) were, however, common. Arrest of transmembrane potential between -50 and -60 mV during stimulation at long cycle lengths was also frequently observed (7/20 of bepridil and 3/15 of lidoflazine-treated fibers). In preparations with prominent phase 3 secondary plateaus, addition of epinephrine resulted in triggered activity; similarly, epinephrine caused automaticity from a depolarized membrane potential in 8/9 fibers quiescent at -50 to -60 mV. Thus, the calcium channel blocking drugs bepridil and lidoflazine produced striking repolarization changes at slow stimulation rates, but unlike quinidine, triggered activity was rare. The effect of epinephrine on these drug-treated fibers suggests that the slow inward calcium current plays a role in the generation of triggered activity in the presence of prolonged repolarization.