Background— Sustained heart rate abnormalities produce electrical remodeling and susceptibility to arrhythmia. Uncontrolled tachycardia produces heart failure and ventricular tachyarrhythmia susceptibility, whereas bradycardia promotes spontaneous torsade de pointes (TdP). This study compared arrhythmic phenotypes and molecular electrophysiological remodeling produced by tachycardia versus bradycardia in rabbits.

Methods and Results— We evaluated mRNA and protein expression of subunits underlying rapid (I_Kr) and slow (I_Ks) delayed-rectifier and transient-outward K⁺ currents in ventricular tissues from sinus rhythm control rabbits and rabbits with AV block submitted to 3-week ventricular pacing either at 60 to 90 bpm (bradypaced) or at 350 to 370 bpm (tachypaced). QT intervals at matched ventricular pacing rates were longer in bradypaced than tachypaced rabbits (eg, by ≈50% at 60 bpm; P<0.01). KvLQT1 and minK mRNA and protein levels were downregulated in both bradypaced and tachypaced rabbits, whereas ERG was significantly downregulated in bradypaced rabbits only. Kv4.3 and Kv1.4 were downregulated by tachypacing only. Patch-clamp experiments showed that I_Ks was reduced in both but I_Kr was decreased in bradypaced rabbits only. Continuous monitoring revealed spontaneous TdP in 75% of bradypaced but only isolated ventricular ectopy in tachypaced rabbits. Administration of dofetilide (0.02 mg/kg) to mimic I_Kr downregulation produced ultimately lethal TdP in all tachypaced rabbits.

Conclusions— Sustained tachycardia and bradycardia downregulate I_Ks subunits, but bradycardia also suppresses ERG/I_Kr, causing prominent repolarization delays and spontaneous TdP. Susceptibility of tachycardia/heart failure rabbits to malignant tachyarrhythmias is induced by exposure to I_Kr blockers. These results point to a crucial role for delayed-rectifier subunit remodeling in TdP susceptibility associated with rate-related cardiac remodeling.