Prostacyclin attenuates oxidative damage of myocytes by opening mitochondrial ATP-sensitive K⁺ channels via the EP₃ receptor

Prostacyclin (PGI₂) and the PGE family alleviate myocardial ischemia-reperfusion injury and limit oxidative damage. The cardioprotective effects of PGI₂ have been traditionally ascribed to activation of IP receptors. Recent advances in prostanoid research have revealed that PGI ₂ can bind not only to IP, but also to EP, receptors, suggesting cross talk between PGI₂ and PGEs. The mechanism(s) whereby PGI ₂ protects myocytes from oxidative damage and the specific receptors involved remain unknown. Thus fresh isolated adult rat myocytes were exposed to 200 μM H₂O₂ with or without carbaprostacyclin (cPGI₂), IP-selective agonists, and ONO-AE-248 (an EP ₃-selective agonist). Cell viability was assessed by trypan blue exclusion after 30 min of H₂O₂ superfusion. cPGI ₂ and ONO-AE-248 significantly improved cell survival during H ₂O₂ superfusion; IP-selective agonists did not. The protective effect of cPGI₂ and ONO-AE-248 was completely abrogated by pretreatment with 5-hydroxydecanoate or glibenclamide. In the second series of experiments, the mitochondrial ATP-sensitive K⁺ (K_ATP) channel opener diazoxide (Dx) reversibly oxidized flavoproteins in control myocytes. Exposure to prostanoid analogs alone had no effect on flavoprotein fluorescence. A second application of Dx in the presence of cPGI₂ or ONO-AE-248 significantly increased flavoprotein fluorescence compared with Dx alone, but IP-selective agonists did not. This study demonstrates that PGI ₂ analogs protect cardiac myocytes from oxidative stress mainly via activation of EP₃. The data also indicate that activation of EP ₃ receptors primes the opening of mitochondrial K_ATP channels and that this mechanism is essential for EP₃-dependent protection.