Abstract 2277: Verapamil Prevents Fatal Arrhythmia by Blocking Cardiac Ryanodine Receptor in a Mouse Model of Catecholaminergic Polymorphic Ventricular Tachycardia Carrying Calsequestrin-2 Mutation
Catecholaminergic polymorphic ventricular tachycardia (CPVT), a familial arrhythmia syndrome caused by ryanodine receptor (RyR2) or calsequestrin-2 (CASQ2) gene mutations, is characterized by stress induced sudden death. Previously we reported efficacy of verapamil in modifying the abnormal intracellular calcium and in reducing arrhythmias in a Casq2-null mouse, a model of CPVT. The present study aimed to determine mechanisms by which verapamil prevents CPVT.
[Methods & Results] Verapamil was administered orally for 2 weeks in adult Casq2-null mice. From treated and untreated mutant ventricles, sarcoplasmic reticulum (SR) proteins were extracted. SR Ca2+ATPase, phospholamban, FKBP12.6, CASQ2, calreticulin, phosphorylated and dephosphorylated RyR2 assessed by Western-blot were unaltered by verapamil treatment. Intracellular Ca2+ contents were analyzed in adult wildtype (wt) and mutant cardiomyocytes in response to rapid administration of caffeine (20mM), in regular buffer or in verapamil-containing buffer (1microM). Total SR Ca2+ content in wt cardiomycytes treated with verapamil or untreated were not statistically different. Untreated Casq2-null cardiomyocytes had significantly lower levels of caffeine-induced SR Ca2+ release than wt (p<0.001) due to Ca2+ leak through the RyR2. In contrast, Casq2-null cardiomyocytes with verapamil had significantly increased SR Ca2+ content compared to untreated mutant cells (p<0.001). Verapamil-treated mutant cells had SR Ca2+ content comparable to wt cells.
[Conclusion] Verapamil effects on Ca2+ handling and arrhythmia in Casq2-null mice are independent of SR protein expression or phosphorylation. Verapamil normalized total SR Ca2+ content in mutant cardiomyocytes, likely by reducing RyR2 leak of Ca2+. SR Ca2+ content restoration by verapamil in Casq2-null mice indicates that CASQ2 function mainly as a regulatory protein of the SR Ca2+ release channel rather than Ca2+ storage protein.