Purkinje cell calcium dysregulation is the cellular mechanism that underlies catecholaminergic polymorphic ventricular tachycardia

Background: Inherited arrhythmias can be caused by mutations in the cardiac ryanodine receptor (RyR2). The cellular source of these arrhythmias is unknown. Isolated RyR2^R4496C mouse ventricular myocytes display arrhythmogenic activity related to spontaneous Ca²⁺ release during diastole. On the other hand, recent whole-heart epicardial and endocardial optical mapping data demonstrate that ventricular arrhythmias in the RyR2 ^R4496C mouse model of catecholaminergic polymorphic ventricular tachycardia (CPVT) originate in the His-Purkinje system, suggesting that Purkinje cells, and not ventricular myocytes, may be the cellular source of arrhythmogenic activity. The relative effect of the RyR2^R4496C mutation on calcium homeostasis in ventricular myocytes versus Purkinje cells is unknown. Objective: This study sought to determine which cardiac cell type is more severely affected, in terms of calcium handling, by expression of the RyR2^R4496C mutant channel: the ventricular myocytes or the Purkinje cells. Methods and Results: To discriminate Purkinje cells from ventricular myocytes, we crossed the RyR2^R4496C mouse model of CPVT with the Cx40^EGFP/+ transgenic mouse. This genetic cross yields Purkinje cells that express eGFP, and therefore fluoresce green when excited by the appropriate wavelength; ventricular myocytes, which do not express connexin 40, are not green. Intracellular calcium was measured in each cell type using calcium-sensitive probes. Purkinje cells of the RyR2^R4496C mouse model of CPVT show an approximately 2× greater rate (P <.05) and approximately 2× to 3× greater amplitude (P <.000001) of spontaneous calcium release events than ventricular myocytes isolated from the same heart. Conclusion: These results demonstrate that focally activated arrhythmias originate in the specialized electrical conducting cells of the His-Purkinje system in the RyR2^R4496C mouse model of CPVT.