Postconditioning: A Form of “Modified Reperfusion” Protects the Myocardium by Activating the Phosphatidylinositol 3-Kinase–Akt Pathway

Brief intermittent episodes of ischemia and reperfusion, at the onset of reperfusion after a prolonged period of ischemia, confer cardioprotection, a phenomenon termed “ischemic postconditioning” (Postcond). We hypothesized that this phenomenon may just represent a modified form of reperfusion that activates the reperfusion injury salvage kinase (RISK) pathway. Isolated perfused rat hearts were subjected to: (a) 35 minutes of ischemia and 120 minutes of reperfusion, and infarct size was determined by tetrazolium staining; or (b) 35 minutes of ischemia and 7 minutes of reperfusion, and the phosphorylation states of Akt, endothelial NO synthase (eNOS), and p70S6K were determined. Postcond reduced infarct size from 51.2±3.4% to 31.5±4.1% (P<0.01), an effect comparable with ischemic preconditioning (IPC; 27.5±2.3%; P<0.01). Of interest, the combined protective effects of IPC and Postcond were not additive (30.1±4.8% with IPC+Postcond; P=NS). Inhibiting phosphatidylinositol 3-kinase (PI3K) at reperfusion using LY or Wortmannin (Wort) during the first 15 minutes of reperfusion completely abolished Postcond-induced protection (31.5±4.1% with Postcond versus 51.7±4.5% with Postcond+LY, P<0.01; 56.2±10.1% with Postcond+ Wort; P<0.01), suggesting that Postcond protects the heart by activating PI3K–Akt. Western blot analysis demonstrated that Postcond induced a significant increase in phosphorylation of Akt, eNOS, and p70S6K in an LY- and Wort-sensitive manner. In conclusion, we show for the first time that ischemic Postcond protects the myocardium by activating the prosurvival kinases PI3K–Akt, eNOS, and p70S6K in accordance with the RISK pathway.