Shear stress activates P2X4 receptor currents via triggering Cx43 hemichannels in atrial myocytes: possible role on atrial adaptation to pressure overload

Increase of afterload induces cardiac hypertrophy leading to contractile failure and dilation. Atria to receive venus return should adapt to such mechanical changes including shear stress. However, the atrial cell responses to shear stress are poorly understood. Recent evidence suggests that atrial myocytes release ATP fast under shear stress via connexin43 (Cx43) hemichannels, which triggers action potentials and Ca2+ waves. We explored cellular mechanism for graded depolarization under shear stress and its implication on atrial pathogenesis during progressive increase in afterload. Using whole-cell patch clamp combined with micro fluid-jet system, we found in rat atrial myocytes that shear stress activates non-selective cation currents (IS,Cs) that were enhanced by zero external Ca2+, but eliminated by La3+, high concentrations of carbenoxolone, anti-Cx43 antibodies, TAT-Gap19 or Cx43 conditional knockout (cKO), suggesting a key role of Cx43 hemichannel. Intracellular calcein fluorescence imaging to assess gap junction hemichannel activity revealed that shear stress induces a calcein efflux from atrial myocytes which was increased by zero external Ca2+ and suppressed by Cx43 hemichannel inhibitions (La3+, Gap19 or Cx43 cKO). The IS,Cs was eliminated by ATP-metabolizing apyrase and suppressed by antagonists of P2 receptors, mainly P2X4 receptor (P2X4R), indicating a role of autocrine P2X4R activation. Furthermore, in a rat model of left atrial (LA) hypertrophy, induced by transverse aortic constriction (TAC) for 5-weeks, IS,Cs and its P2X4R current component, and P2X4R protein level were largely increased with unaltered hemichannel function. In severely dilated left atria following 20-week-TAC, Cx43 hemichannel activity and P2X4R-currents of IS,Cs were attenuated again with lowered protein levels for Cx43 and P2X4R. Cx43 hemichannel-P2X4R signaling could account for increase of atrial excitability under shear stress. Characteristic alterations in this response during progressive pressure overload suggest that it plays a role in atrial compensatory adaptation to hypertension.