IKrvs.IKsblockade and arrhythmogenicity in normoxic rabbit Purkinje fibers: does it really make a difference?

The electrophysiological (standard intracellular microelectrode technique) and pro-arrhythmic (occurrence of early after-depolarization) effects of five class III agents acting on delayed rectifier current (I(K)), rapid (I(Kr)), and/or slow (I(Ks)) components have been studied in rabbit Purkinje fibers taken near the septum and submitted in vitro to reduced stimulation rate (from 1 to 0.5 Hz) in the absence or presence of epinephrine (10 nm) during normoxic conditions. There were two I(Kr) blockers (d-sotalol and dofetilide), two I(Ks) blockers (chromanol 293B and HMR 1556), and a non-selective I(K) blocker (azimilide). d-sotalol, dofetilide, and azimilide lengthened APD(60) and APD(90) in a concentration-dependent manner. Both d-sotalol and dofetilide showed pro-arrhythmia at highest concentrations and in the presence of epinephrine and lower stimulation rate. Despite azimilide markedly lengthened APD(90), it was globally less pro-arrhythmic than dofetilide. Thus, in normoxic rabbit Purkinje fibers, I(Kr) blockade prolonged action potential duration (APD) and increased the incidence of early after-depolarizations, particularly so in the presence of adrenergic stimulation and bradycardia, I(Ks) blockade did neither, and non-selective I(K) blockade (by azimilide) behaved principally as I(Kr) blockade. It is concluded that in normoxic rabbit Purkinje fibers, I(Ks) blockade was neutral, whereas I(Kr) blockade was pro-arrhythmic, which may make a difference worth exploration in more complex models.