Deciphering pro-arrhythmogenic mechanisms of EPAC in human atrial cardiomyocytes

Exchange protein directly activated by cAMP (EPAC) 1 and EPAC2 are involved in electrophysiological modulation in ventricular cardiomyocytes. Their putative contribution in supra-ventricular arrhythmogenic processes has been suggested in animal models. However, nothing is known about the electrophysiological remodelling and the underlying signalling pathway regulated by EPACs in human atrial cardiomyocyte. Action potentials (AP) and K⁺ currents (I_K) were recorded with the patch-clamp technique in enzymatically freshly isolated human atrial cardiomyocytes. Acute EPAC activation with the EPAC agonist 8-(4-chlorophenylthio)-2'-O-methyl-cAMP acetoxymethyl ester (8-CPTAM; 10 µmol/l) lengthened APs by inhibition of the repolarizing K⁺ currents in myocytes obtained from sinus rhythm (SR) patients. The selective EPAC1 pharmacological blocker AM-001 (20 µmol/l) or the EPAC2 inhibitor ESI-05 (25 µmol/l) prevented the effect of 8-CPTAM on APs and I_K, indicating that both EPAC isoforms participate in this electrophysiological regulation. Mechanistically, the effects of EPAC1 and EPAC2 proteins on the inhibition of three major components of K⁺ currents, I_to, I_KDR, and I_KUR, were Ca²⁺-independent but involved Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) and the AMP-activated protein kinase (AMPK)-nitric oxide synthase (NOS)-protein kinase G (PKG) axis. Interestingly, immunoblot analysis showed that EPAC1 but not EPAC2 was overexpressed in the atria of atrial fibrillation (AF) patients. Finally, the application of AM-001 consecutively to the 8-CPTAM treatment significantly corrected the EPAC-dependent downregulation of I_K in AF cardiomyocytes. Our results uncover that EPAC activation influences I_K by CaMKII and the AMPK-NOS-PKG signalling pathways in human atrial cardiomyocytes. Moreover, our findings suggest that EPAC1 over-activation in AF cardiomyocytes promotes the electrophysiological remodelling underlying the initiation of AF. KEY POINTS: Activation of exchange proteins directly activated by cAMP (EPAC) lengthens action potentials (AP) in human atrial cardiac myocytes. This AP duration increase is mediated by an inhibition of the repolarizing K⁺ current. Using EPAC1 and EPAC2 pharmacological inhibitors (AM-001 and ESI-05, respectively), we show that both EPAC1 and EPAC2 isoforms are involved in these electrophysiological effects. Mechanistically, EPAC-induced K⁺ current inhibition signalling involves both CaMKII and AMPK-NOS-PKG pathways. EPAC1 but not EPAC2 is overexpressed in atrial samples of patients with atrial fibrillation (AF). A selective pharmacological inhibitor of EPAC1, AM-001 prevents the downregulation of K⁺ current in cardiomyocytes from AF patients. Our results suggest that over-activation of EPAC1 and its signalling represent a cellular mechanism for atrial arrythmia and might be a potential target for the treatment of AF.