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.

Glucagon-like peptide 1 (GLP-1), a gut incretin hormone that stimulates insulin secretion, also activates antiapoptotic signaling pathways such as phosphoinositide 3-kinase and mitogen-activated protein kinase in pancreatic and insulinoma cells. Since these kinases have been shown to protect against myocardial injury, we hypothesized that GLP-1 could directly protect the heart against such injury via these prosurvival signaling pathways. Both isolated perfused rat heart and whole animal models of ischemia/reperfusion were used, with infarct size measured as the end point of injury. In both studies, GLP-1 added before ischemia demonstrated a significant reduction in infarction compared with the valine pyrrolidide (an inhibitor of its breakdown) or saline groups. This protection was abolished in the in vitro hearts by the GLP-1 receptor antagonist exendin

Pharmacological activation of the prosurvival kinases Akt and ERK-1/2 at reperfusion, after a period of lethal ischemia, protects the heart against ischemia-reperfusion injury. We hypothesized that ischemic preconditioning (IPC) protects the heart by phosphorylating the prosurvival kinases Akt and ERK-1/2 at reperfusion. In isolated perfused Sprague-Dawley rat hearts subjected to 35 min of lethal ischemia, the phosphorylation states of Akt, ERK-1/2, and p70 S6 kinase (p70S6K) were determined after 15 min of reperfusion, and infarct size was measured after 120 min of reperfusion. IPC induced a biphasic response in Akt and ERK-1/2 phosphorylation during the preconditioning and reperfusion phases after the period of lethal ischemia. IPC induced a fourfold increase in Akt, ERK-1/2, and p70S6K phosphorylation at reperfusion and reduced the infarct risk-to-volume ratio (56.9 Ϯ 5.7 and 20.9 Ϯ 3.6% for control and IPC, respectively, P Ͻ 0.01). Inhibiting the IPCinduced phosphorylation of Akt, ERK-1/2, and p70S6K at reperfusion with the phosphatidylinositol 3-kinase (PI3K) inhibitor LY-294002 or the MEK-1/2 inhibitor PD-98059 abrogated IPC-induced protection (46.3 Ϯ 5.8, 49.2 Ϯ 4.0, and 20.9 Ϯ 3.6% for IPC ϩ LY-294002, IPC ϩ PD-98059, and IPC, respectively, P Ͻ 0.01), demonstrating that the phosphorylation of these kinases at reperfusion is required for IPC-induced protection.In conclusion, we demonstrate that the reperfusion phase following sustained ischemia plays an essential role in mediating IPC-induced protection. Specifically, we demonstrate that IPC protects the heart by phosphorylating the prosurvival kinases Akt and ERK-1/2 at reperfusion. mitogen-activated protein kinases; phosphatidylinositol 3-kinase-Akt; myocardial infarction; reperfusion injury TRANSIENT EPISODES of nonlethal ischemia and reperfusion confer profound protection on the myocardium in response to a prolonged lethal episode of ischemia-reperfusion, a phenomenon that has been termed ischemic preconditioning (IPC) (11). Studies have demonstrated that, after a lethal episode of ischemia, the pharmacological phosphorylation of certain prosurvival kinases, such as phosphatidylinositol 3-kinase (PI3K)-Akt and the mitogen-activated protein kinase p42/p44 extracellular signal-regulated kinases 1 and 2 (ERK-1/2), at reperfusion protects the heart against ischemia-reperfusion injury (7). In addition, it appears that there exists a complex interplay or "cross talk" between these two kinase cascades, at reperfusion, in the execution of their cardioprotective effects (5).Interestingly, we recently demonstrated that the PI3K-Akt and MEK-1/2-ERK-1/2 kinases, which we have termed the reperfusion injury salvage kinase pathway (7), are phosphorylated at reperfusion in response to an IPC stimulus (5). However, whether their phosphorylation at reperfusion is required for IPC-induced protection is unknown. Although previous studies have shown that the phosphorylation of the PI3K-Akt (10, 15) and MEK-1/2-ERK-1/2 (3) kinase cascades occurs in the setting of IPC, thes...