Atrial myocytes are continuously exposed to shear stress during cardiac cycles. Previous reports have shown that shear stress induces two different types of global Ca2+ signaling in atrial myocytes-longitudinal Ca2+ waves (L-waves) and action potential-involved transverse waves (T-waves), and suggested an underlying role of the autocrine activation of P2 receptors. We explored the correlations between ATP release and Ca2+ wave generation in atrial myocytes and investigated why the cells develop two Ca2+-wave types during the same shear force. We examined whether ATP release correlates with different shear-stress (~16 dyn/cm2)-mediated Ca2+ signaling by simultaneous measurement of local Ca2+ and ATP release in individual atrial myocytes using two-dimensional confocal imaging and sniffer patch techniques, respectively. Functional P2X7-receptor-expressing HEK293 cells were established as sniffer cells, which generated currents in real time in response to ATP released from a closely positioned atrial myocyte. Both shear-stress-induced L- and T-waves were preceded by sniffer currents with no difference in the current magnitude. Left atrial (LA) myocytes had two- to three-fold larger sniffer currents than right atrial (RA) cells, as was confirmed by ATP chemiluminescence assay. Shear-stress-induced ATP release was eliminated by connexin (Cx) 43 hemichannel inhibition using La3+, Gap19, or knock-down of Cx43 expression. The level of phosphorylated Cx43 at Ser386 (p-Cx43Ser368), but not total Cx43, was higher in LA versus RA myocytes. Most LA cells (~70%) developed L-waves, whereas most RA myocytes (~80%) presented T-waves. Shear-stress-induced T-waves were completely removed by inhibition of P2X4R, which were most abundant in rat atrial cells. Expression of P2X4R was higher in RA than LA myocytes, whereas expression of P2Y1R, the mediator of L-waves, was higher in LA than RA myocytes. ATP release mainly triggers L-waves in LA myocytes and T-waves in RA myocytes under the same shear force, partly because of the differential expression of P2Y1R and P2X4R between LA and RA myocytes. Higher ATP release in LA myocytes under shear stress may not contribute to determination of the wave pattern.
Keywords: ATP release; Ca(2+) wave; Left and right atrial myocyte; P2X4 receptor; Shear stress; connexin43.
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