Arrhythmogenic actions of the Ca²⁺ channel agonist FPL‐64716 in Langendorff‐perfused murine hearts

Abstract

The experiments explored the extent to which alterations in L-type Ca2+ channel-mediated Ca2+ entry triggers Ca2+-mediated arrhythmogenesis in Langendorff-perfused murine hearts through use of the specific L-type Ca2+ channel modulator FPL-64716 (FPL). Introduction of FPL (1 μm) resulted in a gradual development (>10 min) of diastolic electrical events and alternans in spontaneously beating hearts from which monophasic action potentials were recorded. In regularly paced hearts, they additionally led to non-sustained and sustained ventricular tachycardia (nsVT and sVT). Programmed electrical stimulation (PES) resulted in nsVT and sVT after 5–10 and >10 min perfusion, respectively. Pretreatments with nifedipine, diltiazem and cyclopiazonic acid abolished arrhythmogenic tendency induced by subsequent introduction of FPL, consistent with its dependence upon both extracellular Ca2+ entry and the degree of filling of the sarcoplasmic reticular Ca2+ store. Values for action potential duration at 90% repolarization when any of these agents were applied to FPL-treated hearts became indistinguishable from those shown by untreated control hearts, in contrast to earlier reports of their altering in long QT syndrome type 3 and hypokalaemic murine models for re-entrant arrhythmogenesis. These arrhythmic effects instead correlated with alterations in Ca2+ homeostasis at the single-cell level found in investigations of the effects of both FPL and the same agents in regularly stimulated fluo−3 loaded myocytes. These findings are compatible with a prolonged extracellular Ca2+ entry that potentially results in an intracellular Ca2+ overload and produces the cardiac arrhythmogenecity following addition of FPL.

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

“…18,19 Such findings suggest that acutely altered Ca 2+ homeostasis might result in arrhythmic substrate arising from delayed conduction, as previously suggested for the pro-arrhythmic Brugada syndrome [1][2][3][4][5] in contrast to the recovery abnormalities associated with the LQT syndromes. 6,45 The pro-arrhythmic effects of acute abnormalities in cardiomyocyte Ca 2+ homeostasis provoked by adrenergic stimulation, caffeine-mediated RyR2 stimulation 10 or modified extracellular Ca 2+ entry, 11,12 have hitherto been primarily associated with arrhythmic triggering by delayed afterdepolarisation effects arising from the consequently altered Na + -Ca 2+ exchanger activity.…”

“…They recapitulate mutations associated with the chronic pro‐arrhythmic condition of human catecholaminergic polymorphic ventricular tachycardia (CPVT) . Triggering events have also been reported following acute adrenergic activation produced by modifications of RyR2‐mediated SR Ca 2+ release and surface Ca 2+ channel properties in wild‐type hearts expressing normal RyR2 and calsequestrin‐2 . More recent studies selectively activated RyR2‐Ca 2+ release channels using the phosphokinase A (PKA)‐independent exchange protein directly activated by cAMP (Epac) pathway .…”

“…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 .…”

“…8,9 Triggering events have also been reported following acute adrenergic activation produced by modifications of RyR2-mediated SR Ca 2+ release 10 and surface Ca 2+ channel properties in wild-type hearts expressing normal RyR2 and calsequestrin-2. 11,12 More recent studies selectively activated RyR2-Ca 2+ release channels using the phosphokinase A (PKA)-independent exchange protein directly activated by cAMP (Epac) pathway. [13][14][15] This increased Ca 2+ spark frequencies in adult rat cardiac myocytes 16 and amplitudes of Ca 2+ -dependent Ca 2+ release after isoproterenol treatment 17 in murine ventricular cardiomyocytes.…”

“…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. 8,12,19 However, murine RyR2-P2328S CPVT cardiac models showed reduced atrial 20…”