Potassium Channel Subunit Remodeling in Rabbits Exposed to Long-Term Bradycardia or Tachycardia

Abstract

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 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.