Arrhythmic effects of Epac-mediated ryanodine receptor activation in Langendorff-perfused murine hearts are associated with reduced conduction velocity.

Mengye Li,Sandeep S Hothi,Christopher L‐H Huang,Samantha C Salvage,Andrew A Grace,Kamalan Jeevaratnam

doi:10.1111/1440-1681.12751

Mengye Li, Sandeep S Hothi + Show 4 more

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https://doi.org/10.1111/1440-1681.12751

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Abstract

SummaryRecent papers have attributed arrhythmic substrate in murine RyR2‐P2328S hearts to reduced action potential (AP) conduction velocities (CV), reflecting acute functional inhibition and/or reduced expression of sodium channels. We explored for acute effects of direct exchange protein directly activated by cAMP (Epac)‐mediated ryanodine receptor‐2 (RyR2) activation on arrhythmic substrate and CV. Monophasic action potential (MAP) recordings demonstrated that initial steady (8 Hz) extrinsic pacing elicited ventricular tachycardia (VT) in 0 of 18 Langendorff‐perfused wild‐type mouse ventricles before pharmacological intervention. The Epac activator 8‐CPT (8‐(4‐chlorophenylthio)‐2′‐O‐methyladenosine‐3′,5′‐cyclic monophosphate) (VT in 1 of 7 hearts), and the RyR2 blocker dantrolene, either alone (0 of 11) or with 8‐CPT (0 of 9) did not then increase VT incidence (P>.05). Both progressively increased pacing rates and programmed extrasystolic (S2) stimuli similarly produced no VT in untreated hearts (n=20 and n=9 respectively). 8‐CPT challenge then increased VT incidences (5 of 7 and 4 of 8 hearts respectively; P<.05). However, dantrolene, whether alone (0 of 10 and 1 of 13) or combined with 8‐CPT (0 of 10 and 0 of 13) did not increase VT incidence relative to those observed in untreated hearts (P>.05). 8‐CPT but not dantrolene, whether alone or combined with 8‐CPT, correspondingly increased AP latencies (1.14±0.04 (n=7), 1.04±0.03 (n=10), 1.09±0.05 (n=8) relative to respective control values). In contrast, AP durations, conditions for 2:1 conduction block and ventricular effective refractory periods remained unchanged throughout. We thus demonstrate for the first time that acute RyR2 activation reversibly induces VT in specific association with reduced CV.

Highlights

Abnormal Ca2+ homeostasis increases risks of cardiac arrhythmia.[1,2] For example, increases in cardiomyocyte intracellular [Ca2+] arising from increased ryanodine receptor-2 (RyR2)-mediated sarcoplasmic reticular Ca2+ release activity[2] could increase sodium–calcium exchanger activity, thereby producing delayed afterdepolarizations and premature ventricular beats.[3,4,5] When superimposed upon an arrhythmic686 |w il eyonlinelibrary.com/journal/cep Clin Exp Pharmacol Physiol. 2017;44:686–692
The resulting values of latency, APD90, basic cycle length (BCL) at onset of 2:1 block, and ventricular effective refractory periods (VERPs) were. Together these findings suggest actions of exchange protein directly activated by cAMP (Epac)-mediated RyR2 activation by 8-CPT both increases the incidence of arrhythmia following extrasystolic, and incremental pacing and reduces conduction velocities (CV) without affecting action potential (AP) recovery properties
It had no effect on the AP recovery parameters of APD90, BCL at onset of 2:1 block, and VERP

Summary

| INTRODUCTION

Abnormal Ca2+ homeostasis increases risks of cardiac arrhythmia.[1,2] For example, increases in cardiomyocyte intracellular [Ca2+] arising from increased ryanodine receptor-2 (RyR2)-mediated sarcoplasmic reticular Ca2+ release activity[2] could increase sodium–calcium exchanger activity, thereby producing delayed afterdepolarizations and premature ventricular beats.[3,4,5] When superimposed upon an arrhythmic. This consisted of runs of repeated stimulus trains, each composed of eight stimuli paced at 8 Hz followed by an S2 stimulus. The resulting values of latency, APD90, BCL at onset of 2:1 block, and VERP were Together these findings suggest actions of Epac-mediated RyR2 activation by 8-CPT both increases the incidence of arrhythmia following extrasystolic, and incremental pacing and reduces CV without affecting AP recovery properties. The findings are consistent with acute pro-arrhythmic actions of RyR2 receptor modulation on both CV and arrhythmia, adding to previous studies implicating chronic alterations in Na+ channel protein expression reported with genetic modifications of RyR2

Findings

| DISCUSSION

| METHODS