EPAC proteins act as downstream effectors of cAMP pathway. EPAC activation lengthens Action Potential Duration (APD) and dysruptes Ca2+ homeostasis in ventricular myocytes. In atria, APD modulations contribute to pathogenesis of Atrial Fibrillation (AF). Recent studies revealed EPAC proteins as a putative AF promotor in mice by APD lengthening. However, the EPAC-induced variation of human atrial cardiomyocyte electrophysiology is not yet described.

To evaluate the influence of EPAC proteins in human atrial cardiomyocytes electrophysiology.

Right auricular appendages were collected from patients in Sinus Rhythm (SR) and AF undergoing open-heart surgery. Action Potentials and K+ currents (IK) were recorded in human isolated cardiomyocytes or in cultured HL-1 atrial myocytes with the patch-clamp technique. EPAC proteins were acutely activated or inhibited by pharmacological tools. EPAC proteins levels and phosphorylation states of NO-synthases were measured by Western-Blot.

In human atrial cardiomyocytes, EPAC activation with 8-CPTAM (10μM) lengthened APD by downregulation of both transient peak and sustained (IKsus) components of repolarizing K+ currents. Pre-treatment of myocytes with selective EPAC1 or EPAC2 pharmacological blockers (AM-001, 20μM and ESI-05, 25μM, respectively) revealed that both EPAC isoforms participate in this effect. Mechanistically, pharmacological inhibition of NO-synthases (NOS) and PKG significantly reduced the EPAC-induced decrease of IKsus. The inhibition of AMPK, a positive regulator of NOS activity, significantly reduced the EPAC-dependent IKsus alteration, whereas the blockade of Akt, a NOS activator, failed to influence EPAC effect. In HL-1 myocytes, a similar downregulation of IKsus occured after EPAC activation with 8-CPTAM. In addition, 8-CPTAM treatment significantly increased the phosphorylation level of the endothelial isoform of NOS in its activating site Ser1177, whereas AMPK inhibition prevented this increase. Of note, 8-CPTAM treatment did not modify Reactive Oxygen Species levels in HL-1 cells. Interestingly, we found that the EPAC1 isoform, but not EPAC2, was overexpressed in right auricular appendages from AF patients compared to SR patients. Consistent with this finding, the EPAC1 blocker AM-001 significantly rectified the IK alteration in cardiomyocytes from AF patients treated with 8-CPTAM, but not in cells from SR patients.

EPAC stimulation lengthens APD by decreasing IK through an AMPK-eNOS-PKG pathway in human atrial cardiomyocytes. Finally, alteration of EPAC1 signaling could promote AF onset.