Accentuated vagal antagonism paradoxically increases ryanodine receptor calcium leak in long-term exercised Calsequestrin2 knockout mice

BACKGROUND Long-term aerobic exercise alters autonomic balance, which may not be favorable in heart rate (HR)-dependent arrhythmic diseases including catecholaminergic polymorphic ventricular tachycardia (CPVT) because of preexisting bradycardia and increased sensitivity to parasympathetic stimulation. OBJECTIVE The purpose of this study was to determine whether long-term exercise-induced autonomic adaptations modify CPVT susceptibility. METHODS We determined exercise-induced parasympathetic effects on HR, arrhythmia incidence, and intracellular sarcoplasmic reticulum (SR) Ca2+ leak in atrial (ACM) and ventricular (VCM) cardiomyocytes, in exercised (EX) calsequestrin knockout (CASQ2(-/-)) mice, a model of CPVT. RESULTS Although 8-week treadmill running improved exercise capacity in EX CPVT mice, the incidence and duration of ventricular tachycardia also increased. HR variability analyses revealed an increased high-frequency component of the power spectrum and root mean square of successive differences in R-R intervals indicating accentuated vagal antagonism during beta-adrenergic stimulation resulting in negligible HR acceleration. In EX CASQ2(-/-) VCM, peak amplitude of Ca2+ transient (CaT) increased, whereas SR Ca2+ content decreased. Aberrant Ca2+ sparks occurred at baseline, which was exacerbated with isoproterenol. Notably, although 10 mM of the cholinergic agonist carbachol prevented isoproterenolinduced Ca2+ waves in ACM, CaT amplitude, SR Ca2+ load, and isoproterenol-induced Ca2+ waves paradoxically increased in VCM. In parallel, ventricular ryanodine receptor (RyR2) protein expression increased, whereas protein kinase A- and calmodulindependent protein kinase II-mediated phosphorylation of RyR2 was not significantly altered, which could imply an increased number of leaky channels. CONCLUSION Our novel results suggest that long-term exercise in CASQ2(-/-) mice increases susceptibility to ventricular arrhythmias by accentuating vagal antagonism during b-adrenergic challenge, which prevents HR acceleration and exacerbates abnormal RyR2 Ca2+ leak in EX CASQ2(-/-) VCM.