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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.

Objectives: Recruitment of the survival kinase cascades, PI3K-Akt and Raf-MEK1/2-Erk1/2, at the time of reperfusion, following a lethal ischemic insult, may mediate the protection associated with ischemic preconditioning (IPC). The exact interplay between these two kinase cascades in mediating this effect is not clear. We examine the 'cross-talk' between these kinase cascades in their contribution to IPC-induced protection., Methods and Results: In isolated perfused rat hearts subjected to 35 min of lethal ischemia +/- ischemic preconditioning, the phosphorylation states of Akt, Erk1/2, p70S6K were determined after 15 min of reperfusion, and infarct size was measured after 120 min of reperfusion. IPC induced a threefold increase in Akt, Erk1/2, and p70S6K phosphorylation, at reperfusion. We found that inhibiting the PI3K-Akt (using LY294008) at reperfusion induced the phosphorylation of Erk1/2-p70S6K, and conversely, that inhibiting the MEK1/2-Erk1/2 pathway (using PD 98059) at reperfusion, induced the phosphorylation of Akt, suggesting 'cross-talk' between the two kinase pathways. However, this effect was not accompanied by a reduction in infarct size (43.1 +/- 7.2% with LY 294008 and 57.7 +/- 7.0% with PD 98059 vs. 46.3 +/- 5.8% in control; P = NS), suggesting that both the kinase cascades may need to be activated to mediate IPC-induced protection. IPC reduced the infarct-risk volume ratio to 17.8 +/- 2.3% from 46.3 +/- 5.8% in control (P < 0.01). Inhibiting p70S6K, a kinase situated downstream of both PI3K and Erk1/2, using rapamycin, abolished IPC-induced protection (46.0 +/- 7.7% with IPC+RAPA vs. 17.8 +/- 2.3% with IPC; P < 0.01)., Conclusions: We report that, the survival kinase cascades PI3K-Akt and MEK1/2-Erk1/2, which are recruited at the time of reperfusion in response to ischemic preconditioning, exhibit 'cross-talk' such that inhibiting one cascade activates the other and vice versa. Furthermore, at the time of reperfusion, these kinase cascades mediate IPC-induced protection, by acting in concert via p70S6K., (Copyright 2004 European Society of Cardiology)

Mitochondrial permeability transition (mPT) is a crucial event in the progression to cell death in the setting of ischemia-reperfusion. We have used a model system in which mPT can be reliably and reproducibly induced to test the hypothesis that the profound protection associated with the phenomenon of myocardial preconditioning is mediated by suppression of the mPT. Adult rat myocytes were loaded with the fluorescent probe tetramethylrhodamine methyl ester, which generates oxidative stress on laser illumination, thus inducing the mPT (indicated by collapse of the mitochondrial membrane potential) and ATP depletion, seen as rigor contracture. The known inhibitors of the mPT, cyclosporin A (0.2 microM) and N-methyl-4-valine-cyclosporin A (0.4 microM), increased the time taken to induce the mPT by 1.8- and 2.9-fold, respectively, compared with control (P < 0.001) and rigor contracture by 1.5-fold compared with control (P < 0.001). Hypoxic preconditioning (HP) and pharmacological preconditioning, using diazoxide (30 microM) or nicorandil (100 microM), also increased the time taken to induce the mPT by 2.0-, 2.1-, and 1.5-fold, respectively (P < 0.001), and rigor contracture by 1.9-, 1.7-, and 1.5-fold, respectively, compared with control (P < 0.001). Effects of HP, diazoxide, and nicorandil were abolished in the presence of mitochondrial ATP-sensitive K(+) (K(ATP)) channel blockers glibenclamide (10 microM) and 5-hydroxydecanoate (100 microM) but were maintained in the presence of the sarcolemmal K(ATP) channel blocker HMR-1098 (10 microM). In conclusion, preconditioning protects the myocardium by reducing the probability of the mPT, which normally occurs during ischemia-reperfusion in response to oxidative stress.

Reperfusion is a pre-requisite to salvaging viable myocardium, following an acute myocardial infarction. Reperfusion of ischaemic myocardium, however, is not without risk, as the act of reperfusion itself can paradoxically result in myocyte death: a phenomenon termed lethal reperfusion-induced injury. Therapeutic strategies that target and attenuate reperfusion-induced cell death may provide novel pharmacological agents, which can be used as an adjunct to current reperfusion therapy, to limit myocardial infarction. Recent evidence has implicated apoptotic cell death during the phase of reperfusion as an important contributor to lethal reperfusion-induced injury. Targeting anti-apoptotic mechanisms of cellular protection at the time of reperfusion may therefore offer a potential approach to attenuating reperfusion-induced cell death. In this regard, ischaemia-reperfusion has been shown to activate the anti-apoptotic pro-survival kinase signalling cascades, phosphatidylinositol-3-OH kinase (PI3K)-Akt and p42/p44 extra-cellular signal-regulated kinases (Erk 1/2), both of which have been implicated in cellular survival. Activating these pro-survival kinase cascades at the time of reperfusion has been demonstrated to confer protection against reperfusion-induced injury. We and others have shown that insulin, insulin-like growth factor-1 (IGF-1), transforming growth factor-beta1 (TGF-beta1), cardiotrophin-1 (CT-1), urocortin, atorvastatin and bradykinin protect the heart, by activating the PI3K-Akt and/or Erk 1/2 kinase cascades, when given at the commencement of reperfusion, following a lethal ischaemic insult. Pharmacological manipulation and up-regulation of these pro-survival kinase cascades, which we refer to as the Reperfusion Injury Salvage Kinase (RISK) pathway, as an adjunct to reperfusion may therefore protect the myocardium from lethal reperfusion-induced cell death and provide a novel strategy to salvaging viable myocardium and limiting infarct size.

Objective: The opening of the mitochondrial permeability transition pore (mPTP) in the first few minutes of post-ischaemic reperfusion is a critical determinant of reperfusion-induced cell death. We hypothesised that the novel immunosuppressant, sanglifehrin-A (SFA), given at the time of reperfusion, protects the myocardium from ischaemia-reperfusion injury, by suppressing mPTP opening., Methods: Isolated perfused rat hearts were subjected to 35 min ischaemia/120 min reperfusion, and were treated with (1) SFA (1.0 microM) or (2) DMSO vehicle for the first 15 min of reperfusion or (3) SFA (1.0 microM) after the first 15 min of reperfusion. We examined the effect of SFA on mPTP opening directly, using a myocyte model of oxidative stress. Laser illumination of adult rat myocytes loaded with the fluorophore, TMRM, generates oxidative stress, which induces mPTP opening (represented by mitochondrial membrane depolarisation) followed by rigour contracture., Results: In the isolated perfused heart model, SFA, given during the first 15 min of post-ischaemic reperfusion, reduced the infarct-risk volume ratio from 43.9+/-2.5% in the control group to 23.8+/-4.2% with SFA (p=0.001). However, when SFA was given after the first 15 min of reperfusion, there was no change in infarct size (43.8+/-5.7% with SFA vs. 43.9+/-2.5% in control; p=NS), suggesting that SFA has to be present during the first 15 min of reperfusion to induce protection. In the isolated adult myocyte model, SFA was shown to inhibit mPTP opening in the setting of oxidative stress, represented by an increase in the ROS threshold required to induce: mitochondrial membrane depolarisation (from 269+/-21 to 777+/-100 s; p<0.001) and rigour contracture (from 613+/-14 to 1329+/-129 s; p<0.001)., Conclusions: Inhibiting mPTP opening during the first few minutes of reperfusion, using sanglifehrin-A, limits infarct size and protects myocytes from oxidative stress.

Objective: We propose that ischemic preconditioning (IPC) and mitochondrial K(ATP) channel activation protect the myocardium by inhibiting mitochondrial permeability transition pore (MPTP) opening at reperfusion., Methods: Isolated rat hearts were subjected to 35 min ischemia/120 min reperfusion and assigned to the following groups: (1) control; (2) IPC of 2x5 min each of preceding global ischemia; (3,4,5) 0.2 micromol/l cyclosporin A (CsA, which inhibits MPTP opening), 5 micromol/l FK506 (which inhibits the phosphatase calcineurin without inhibiting MPTP opening), or 20 micromol/l atractyloside (Atr, a MPTP opener) given at reperfusion; (6,7) pre-treatment with 30 micromol/l diazoxide (Diaz, a mitochondrial K(ATP) channel opener) or 200 nmol/l 2 chloro-N(6)-cyclopentyl-adenosine (CCPA, an adenosine A1 receptor agonist); (8) IPC+Atr; (9) Diaz+Atr; (10) CCPA+Atr. The effect of mitochondrial K(ATP) channel activation on calcium-induced MPTP opening in isolated calcein-loaded mitochondria was also assessed., Results: IPC, CsA when given at reperfusion, and pre-treatment with diazoxide or CCPA all limited infarct size (19.9+/-2.6% in IPC; 24.6+/-1.9% in CsA, 18.0+/-1.7% in Diaz, 20.4+/-3.3% in CCPA vs. 44.7+/-2.0% in control, P<0.0001). Opening the MPTP with atractyloside at reperfusion abolished this cardio-protective effect (47.7+/-1.8% in IPC+Atr, 42.3+/-3.2% in Diaz+Atr, 51.2+/-1.6% in CCPA+Atr). Atractyloside and FK506, given at reperfusion, did not influence infarct size (45.7+/-2.1% in Atr and 43.1+/-3.6% in FK506 vs. 44.7+/-2.0% in control, P=NS). Diazoxide (30 micromol/l) was shown to reduce calcium-induced MPTP opening by 52.5+/-8.0% in calcein-loaded mitochondria. 5-Hydroxydecanoic acid (100 micromol/l) was able to abolish the cardio-protective effects of both diazoxide and IPC., Conclusion: One interpretation of these data is that IPC and mitochondrial K(ATP) channel activation may protect the myocardium by inhibiting MPTP opening at reperfusion.