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

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.

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

“…Reductions of Na + current of the kind described above have been associated with corresponding reductions in maximum action potential upstroke rate, (d V /d t ) max in turn resulting in the pro‐arrhythmic slowing of AP conduction reported in previous explorations of effects of Epac modulation . Figures and illustrate typical traces of atrial and ventricular action potential waveforms in response to regular 6 Hz pacing (A) and their first derivatives whose peaks reflect (d V /d t ) max (B).…”

“…It correspondingly, (ii) increased incidences of both triggered activity and ventricular tachycardia (VT) . However, it also (iii) associated such pro‐arrhythmic effects with decreased conduction velocities . Furthermore, (iv) these electrophysiological characteristics were rescued by the RyR2‐Ca 2+ release channel blocker dantrolene, despite (v) all these changes being associated with unaltered recovery characteristics as reflected in the cardiac action potential (AP) durations and ventricular effective refractory periods …”

“…Furthermore, selective, acute RyR2 activation through the Epac pathway produced parallel pro‐arrhythmic effects . It correspondingly produced decreases in AP conduction velocities that were partially reversed by the RyR2 antagonist dantrolene, with an absence of alterations in AP recovery characteristics …”

“…Fewer studies have explored arrhythmic substrate under conditions of altered Ca 2+ homeostasis. Neither chronic modifications in Ca 2+ homeostasis in RyR2‐P2328S models nor acute manipulations of Ca 2+ homeostasis in WT hearts altered AP recovery characteristics as reflected in AP durations (APD), refractory periods (ERP), or the relationships between these . 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 .…”

“…Finally, the results obtained with the approach adopted here could be directly compared to corresponding explorations in intact Langendorff perfused murine hearts. In addition to establishing the expected alterations in cellular Ca 2+ homeostasis, they used identical Epac activators and RyR2 antagonists in identical pharmacological protocols exploring the effects of Epac activation and their reversibility applying these to examine changes in AP conduction velocity and arrhythmic substrate …”