Norepinephrine upregulates volume-regulated chloride current by PKA-independent cAMP/Epac pathway in human atrial myocytes.

BACKGROUND AND PURPOSE Adrenergic regulation of cell volume-regulated chloride current (I-Cl.vol) is species-dependent. The present study investigates the mechanism underlying adrenergic regulation of I-Cl.vol in human atrial myocytes. EXPERIMENTAL APPROACH Conventional whole-cell patch voltage-clamp techniques were used to record membrane current in human atrial myocytes. I-Cl.vol was evoked by hyposmotic bath solution (0.6 times isosmotic, 0.6T). KEY RESULTS I-Cl.vol was augmented by noradrenaline (1 mu M) during cell swelling in 0.6T but not under isosmotic (1 T) conditions. Up-regulation of I-Cl.vol in 0.6T was blocked by the beta-adrenoceptor antagonist propranolol (2 mu M), but not by the alpha(1)-adrenoceptor antagonist prazosin (2 mu M). This beta-adrenergic response involved cAMP but was independent of PKA; the protein kinase inhibitor H-89 (2 mu M) or PKI (10 mu M in pipette solution) failed to prevent I-Cl.vol up-regulation by noradrenaline. Moreover, the PI3K/PKB inhibitor LY294002 (50 mu M) and the PKG inhibitor KT5823 (10 mu M) did not affect noradrenaline-induced increases in I-Cl.vol. Interestingly, the exchange protein directly activated by cAMP (Epac) agonist 8-pCPT-2'-O-Me-cAMP (50 mu M) also up-regulated I-Cl.vol, and the noradrenaline-induced increase of I-Cl.vol in 0.6T was reversed or prevented by the Epac inhibitor ESI-09 (10 mu M). CONCLUSION AND IMPLICATIONS These data show that I-Cl.vol evoked by cell swelling of human atrial myocytes is up-regulated by noradrenaline via a PKA-independent cAMP/Epac pathway in human atrial myocytes. cAMP/Epac-induced I-Cl.vol is expected to shorten action potential duration during human atrial myocytes swelling and may be involved in abnormal cardiac electrical activity during cardiac pathologies that evoke beta-adrenoceptor signalling.