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

Abstract

Acute RyR2 activation by exchange protein directly activated by cAMP (Epac) reversibly perturbs myocyte Ca2+ homeostasis, slows myocardial action potential conduction, and exerts pro-arrhythmic effects. Loose patch-clamp studies, preserving in vivo extracellular and intracellular conditions, investigated Na+ current in intact cardiomyocytes in murine atrial and ventricular preparations following Epac activation. Depolarising steps to varying test voltages activated typical voltage-dependent Na+ currents. Plots of peak current against depolarisation from resting potential gave pretreatment maximum atrial and ventricular currents of -20.23±1.48(17) and -29.8±2.4(10) pA/μm2 (means± SEM (n)). Challenge by 8- CPT (1 μM) reduced these currents to -11.21±0.91(12) (p<0.004) and -19.3±1.6(11) pA/μm2 (p<0.04) respectively. Currents following further addition of the RyR2 inhibitor dantrolene (10 μM) (-19.91±2.84(13) and -26.6±1.7(17)), and dantrolene whether alone (-19.53±1.97(8) and -27.6±1.9(14)) or combined with 8-CPT (-19.93±2.59(12) and -29.9±2.5(11)), were indistinguishable from pretreatment values (all p>>0.05). Assessment of the inactivation that followed by applying subsequent steps to a fixed voltage 100 mV positive to resting potential gave concordant results. Half-maximal inactivation voltages and steepness factors, and time constants for Na+ current recovery from inactivation in double-pulse experiments, were similar through all the pharmacological conditions. Intracellular sharp microelectrode membrane potential recordings in intact Langendorff-perfused preparations demonstrated concordant variations in maximum rates of atrial and ventricular action potential upstroke, (dV/dt)max. We thus demonstrate an acute, reversible, Na+ channel inhibition offering a possible mechanism for previously reported pro-arrhythmic slowing of AP propagation following modifications of Ca2+ homeostasis, complementing earlier findings from chronic alterations in Ca2+ homeostasis in genetically modified RyR2-P2328S hearts.