Flecainide exerts paradoxical effects on sodium currents and atrial arrhythmia in murine R yR2‐P2328S hearts

Abstract

AimsCardiac ryanodine receptor mutations are associated with catecholaminergic polymorphic ventricular tachycardia (CPVT), and some, including RyR2-P2328S, also predispose to atrial fibrillation. Recent work associates reduced atrial Nav1.5 currents in homozygous RyR2-P2328S (RyR2S/S) mice with slowed conduction and increased arrhythmogenicity. Yet clinically, and in murine models, the Nav1.5 blocker flecainide reduces ventricular arrhythmogenicity in CPVT. We aimed to determine whether, and how, flecainide influences atrial arrhythmogenicity in RyR2S/S mice and their wild-type (WT) littermates.MethodsWe explored effects of 1 μm flecainide on WT and RyR2S/S atria. Arrhythmic incidence, action potential (AP) conduction velocity (CV), atrial effective refractory period (AERP) and AP wavelength (λ = CV × AERP) were measured using multi-electrode array recordings in Langendorff-perfused hearts; Na+ currents (INa) were recorded using loose patch clamping of superfused atria.ResultsRyR2S/S showed more frequent atrial arrhythmias, slower CV, reduced INa and unchanged AERP compared to WT. Flecainide was anti-arrhythmic in RyR2S/S but pro-arrhythmic in WT. It increased INa in RyR2S/S atria, whereas it reduced INa as expected in WT. It increased AERP while sparing CV in RyR2S/S, but reduced CV while sparing AERP in WT. Thus, RyR2S/S hearts have low λ relative to WT; flecainide then increases λ in RyR2S/S but decreases λ in WT.ConclusionsFlecainide (1 μm) rescues the RyR2-P2328S atrial arrhythmogenic phenotype by restoring compromised INa and λ, changes recently attributed to increased sarcoplasmic reticular Ca2+ release. This contrasts with the increased arrhythmic incidence and reduced INa and λ with flecainide in WT.

Epac‐induced ryanodine receptor type 2 activation inhibits sodium currents in atrial and ventricular murine cardiomyocytes

“…In parallel with the present findings, RyR2 ‐P2328S cardiomyocytes also demonstrated acutely reduced Na v 1.5 function . This was partially rescued by pharmacological interventions reducing RyR2‐mediated Ca 2+ release . These findings together suggest direct effects of altered cytosolic [Ca 2+ ] upon Nav1.5 function .…”

“…Ag/AgCl electrodes maintained electrical connections to the organ bath and pipette. Loose patch‐clamp experiments were performed as previously described . The pipette was lowered onto the membrane surface and gentle suction applied to allow seal formation around the patch of membrane.…”

“…However, murine RyR2‐P2328S CPVT cardiac models showed reduced atrial and ventricular conduction velocities in common with Na v 1.5‐haploinsufficient Scn5a+/− hearts modelling the Brugada Syndrome . Pharmacological inhibition of RyR2‐mediated Ca 2+ release with flecainide partly rescued these effects . Furthermore, selective, acute RyR2 activation through the Epac pathway produced parallel pro‐arrhythmic effects .…”

Multiple targets for flecainide action: implications for cardiac arrhythmogenesis

“…These changes could be attributed to (a) chronically down‐regulated expression of Na v 1.5 channels, demonstrated in RyR2 ‐P2328S ventricles (Ning et al , ), and (b) acute actions of increased [Ca 2+ ] i upon Na v 1.5 channel function. Loose‐patch clamp recordings demonstrated reduced peak I Na in whole isolated RyR2 ‐P2328S compared with WT atria to extents comparable with those reported in Nav1.5 channel‐haploinsufficent Scn5a +/− hearts (King et al , ; Salvage et al , ) (Figure A, left traces). These conduction abnormalities could not be attributed to either fibrotic change or altered connexin expression.…”

“…Flecainide (1 μM) modified arrhythmic tendency and conduction velocity in RyR2 ‐P2328S hearts, in directions that paradoxically contrasted with its corresponding effects upon either WT and Scn5a +/− hearts. It exerted pro‐arrhythmic atrial and ventricular effects in Scn5a +/− and some WT hearts, although it produced consistently anti‐arrhythmic effects in RyR2‐ P2328S atria (Salvage et al , ). Multi‐electrode recording array studies demonstrated marked conduction slowing in RyR2‐ P2328S compared with WT atria.…”