Ca2+ Signaling Triggered by Shear-Autocrine P2X Receptor Pathway in Rat Atrial Myocytes

Abstract

Background/Aims: The atrium is exposed to high shear stress during heart failure and valvular diseases. We aimed to understand atrial shear-induced Ca²⁺ signaling and its underlying mechanisms. Methods: Pressurized micro-flow was applied to single rat atrial myocytes, and Ca²⁺ signal, membrane potential, and ATP release were assessed using confocal imaging, patch clamp technique, and luciferin-luciferase assay, respectively. Results: Shear stress (∼16 dyn/cm²) induced global Ca²⁺ waves (∼0.1 events/s) from the periphery to the center of cells in a transverse direction (“T-wave”; ∼145 μm/s). Pharmacological interventions and simultaneous recording of membrane potential and Ca²⁺ demonstrated that shear-induced T-waves resulted from action potential (AP)-triggered Ca²⁺ release from the sarcoplasmic reticulum. T-waves were not sensitive to inhibitors of known shear signaling mechanisms except connexin hemichannels and ATP release. Shear stress caused ATP release from these myocytes (∼1.1x10^-17 moles/unit membrane, µm²); ATP release was increased by enhancement of connexin hemichannels and suppressed by inhibition of the hemichannels, but not affected by inhibitors of other ATP release pathways. Blockade of P2X receptor, but not pannexin or the Na⁺-Ca²⁺ exchanger, eliminated shear-induced T-wave initiation. Conclusion: Our data suggest that shear stress triggers APs and concomitant Ca²⁺ signaling via activation of P2X receptors by connexin hemichannel-mediated ATP release in atrial myocytes.