Pharmacological Enhancement of Small Conductance Ca ²⁺ -Activated K ⁺ Channels Suppresses Cardiac Arrhythmias in a Mouse Model of Catecholaminergic Polymorphic Ventricular Tachycardia

BACKGROUND: Sarcolemmal small conductance Ca 2+ -activated K + channels have the unique capacity to translate intracellular Ca 2+ signal into repolarization, while mitochondrial SK channels can link Ca 2+ cycling to mitochondrial function. We hypothesize that pharmacological enhancement of SK channels can be protective against malignant cardiac arrhythmias associated with disturbances in Ca 2+ handling machinery. METHODS: A mouse CASQ2 KO (calsequestrin type 2 knockout) model of catecholaminergic polymorphic ventricular tachycardia (CPVT) was used for in vivo ECG recordings and for cell electrophysiology, Ca 2+ , and reactive oxygen species imaging in isolated ventricular myocytes (VMs). RESULTS: Bidirectional and polymorphic ventricular tachycardias in CASQ2 KO mice induced by stress challenge (epinephrine+caffeine cocktail) were attenuated by injection of NS309, a specific SK channel enhancer. Voltage-clamp experiments in isolated VMs treated with β-adrenergic agonist isoproterenol showed a reduction of sarcolemmal SK channel current (I SK ) density in CPVT VMs. Application of NS309 to CPVT VMs increased I SK . Current-clamp experiments demonstrated a significant reduction of arrhythmogenic delayed afterdepolarizations and spontaneous Ca 2+ waves in isoproterenol-challenged CPVT VMs pretreated with NS309. Importantly, subsequent application of membrane-impermeable SK channel inhibitor apamin did not reverse the protective effects of NS309, suggesting important roles of mitochondrial SK channels in intracellular Ca 2+ handling rescue. SK channel enhancement reversed the increased rate of reactive oxygen species production by mitochondria in CPVT VMs. It also reversed increased cardiac RyR2 (ryanodine receptor 2) oxidation measured in samples from CPVT hearts of the animals after the stress challenge. Electron microscopy studies showed a significant widening of mitochondria cristae in the ventricular tissue from CPVT mice, which led to a decrease in quaternary supercomplexes of electron transport chain, resulting in impairment of ATP production in VMs under β-adrenergic stimulation. Application of NS309 facilitated cristae flattening in CPVT ventricular tissue and restored supercomplexes and ATP production in VMs from diseased animals. CONCLUSIONS: Sarcolemmal SK channel enhancement reduces arrhythmic potential by restoring repolarization force in CPVT VMs. Activation of mitochondrial SK channels attenuates mitochondria structural changes in CPVT, restoring more efficient electron transport chain assembly into supercomplexes and reducing mito-reactive oxygen species production. This decreases RyR2 oxidation and thus channel activity, reducing spontaneous Ca 2+ waves underlying arrhythmogenic delayed afterdepolarizations.