Constitutive Intracellular Na+ Excess in Purkinje Cells Promotes Arrhythmogenesis at Lower Levels of Stress than Ventricular Myocytes from Mice with Catecholaminergic Polymorphic Ventricular Tachycardia
Background—In catecholaminergic polymorphic ventricular tachycardia (CPVT) cardiac Purkinje cells (PCs) appear more susceptible to Ca2+ dysfunction than ventricular myocytes (VMs). The underlying mechanisms remain unknown. Using a CPVT mouse (RyR2R4496C+/Cx40eGFP), we tested whether PC intracellular Ca2+ ([Ca2+]i) dysregulation results from a constitutive [Na+]i surplus relative to VMs.
Methods and Results—Simultaneous optical mapping of voltage and [Ca2+]i in CPVT hearts showed that spontaneous Ca2+ release (SCR) preceded pacing-induced triggered activity at subendocardial PCs. On simultaneous current-clamp and Ca2+ imaging early (EADs) and delayed (DADs) afterdepolarizations trailed SCR and were more frequent in CPVT PCs than CPVT VMs. As a result of increased activity of mutant RyR2 channels, sarcoplasmic reticulum (SR) Ca2+ load, measured by caffeine-induced Ca2+ transients, was lower in CPVT VM and PCs than respective controls (Ctrls) and SR fractional release was greater in both CPVT PCs and VMs than respective Ctrls. [Na+]i was higher in both Ctrl and CPVT PCs than VMs, whereas the density of the Na+-Ca2+ exchanger (NCX) current was not different between PCs and VMs. Computer simulations using a PC model predicted that the elevated [Na+]i of PCs promoted DADs, which were always preceded by SCR events from hyperactive RyR2 channels. Increasing [Na+]i monotonically increased DAD frequency. Confocal imaging experiments showed that post-pacing Ca2+ spark frequency was highest in intact CPVT PCs, but such differences were reversed upon saponin-induced membrane permeabilization, indicating that differences in [Na+]i played a central role.
Conclusions—In CPVT mice, the constitutive [Na+]i excess of PCs promotes triggered activity and arrhythmogenesis at lower levels of stress than VMs.
- Received February 6, 2016.
- Revision received April 27, 2016.
- Accepted May 3, 2016.
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