Pannexin-I/P2X 7 Purinergic Receptor Channels Mediate the Release of Cardioprotectants Induced by Ischemic Pre- and Postconditioning

Ischemic pre- and postconditioning protect ex vivo rat hearts from ischemia/reperfusion injury by promoting the release of cardioprotective agents by an unknown mechanism. Because P2X( 7) purinergic receptors are known to combine with pannexin-1 to form channels that allow adenosine triphosphate (ATP) release from cells, we hypothesized that these channels have a role in the release of multiple cardioprotectants during ischemic preconditioning (IPC). Addition of either a pannexin-1 hemichannel blocker (5 micromol/L carbenoxolone [CBX] or 0.4 micromol/L mefloquine [MF]) or a selective antagonist of the rat P2X(7) purinergic receptor (2 micromol/L brilliant blue G [BBG]) blocked IPC. These antagonists also blocked ischemic postconditioning. Preconditioning by exogenous addition of either sphingosine-1-phosphate or adenosine was not blocked by either CBX or BBG, indicating that they only affected the release of endogenous mediators, not any subsequent steps. To determine if only ATP release was mediated by pannexin-1/P2X(7) channels, we added an extra cycle of IPC to release sufficient quantities of additional cardioprotectants to eliminate the dependence on adenosine derivatives. This did not override the inhibition of IPC by CBX or MF, suggesting that the channel mediates the release of multiple cardioprotectants. Inhibitors of other P2X receptors, P2Y receptors, or connexins did not affect IPC. We conclude that a pannexin-1/P2X(7) channel is responsible for the release of cardioprotectants induced by ischemic pre- and postconditioning.