Postconditioning attenuates myocardial ischemia-reperfusion injury by inhibiting events in the early minutes of reperfusion

Objective: We previously showed that brief intermittent ischemia applied during the onset of reperfusion (i.e., postconditioning) is cardioprotective in a canine model of ischemia-reperfusion. This study tested the hypothesis that the early minutes of reperfusion (R) during which postconditioning (Post-con) is applied are critical to its cardioprotection. Methods: In anesthetized open-chest rats, the left coronary artery (LCA) was occluded for 30 min and reperfused for 3 h. All rats were randomly divided into six groups: Control (n = 8): no intervention at R; Ischemic preconditioning (IPC) (n = 8): the LCA was occluded for 5 min followed by 10 min of R before the index occlusion; Post-con 1 (n = 8): after LCA occlusion, three cycles of 10 s R followed by 10 s LCA re-occlusion were applied during the first minute of R; Post-con 2 (n = 8): Six cycles of 10 s R and 10 s re-occlusion were applied during the first 2 min of R; Delayed Post-con (n = 8): the ligature was loosened for full reflow for the first minute of R, after which the three-cycle Post-con algorithm was applied; Sham (n = 6): the surgical procedure was identical to other groups, but the LCA ligature was not ligated. Results: Infarct size (TTC staining) was 23% smaller in Postcon 1 (40 +/- 2%*) than in Control (52 +/- 3%), confirmed by plasma creatine kinase activity (18 +/- 2* vs. 46 +/- 6 IU/g protein). There was no further reduction in infarct size with 6 cycles of Post-con (40 +/- 2.9%, p>0.05 vs. Post-con 1). Meanwhile, infarct size reduction was significantly greater in the IPC group (17 +/- 3%) than in Post-con 1 (p<0.01). The plasma lipid peroxidation product malondialdehyde (MDA, muM/ml) was less after R in IPC and Post-con 1 (0.8 +/- 0.07* and 0.8 +/- 0.06*) vs. Control (1.21 +/- 0.08), consistent with a visual decrease in superoxide anion generation (dihydroethidium staining) in the AAR myocardium after 3 h of reperfusion. Neutrophil accumulation (myeloperoxidase activity, MPO, U/100 g tissue) in the AAR was less in IPC (1.4 +/- 0.3*) and Post-con 1 (2.5 +/- 0.3*) vs. Control (5.5 +/- 0.6). The reductions in infarct size, creatine kinase, MDA and DHE staining were lost with delayed Post-con, while MPO activity remained lower than in Control (3.2 +/- 0.4*). Conclusions: (1) Post-con at onset of R reduces myocardial injury; (2) cardioprotection may be mediated, in part, by inhibiting oxidant generation and oxidant mediated injury; (3) the first minute of R in the rat model is critical to cardioprotection by Post-con; and (4) cardioprotection by Post-con may be independent of neutrophil accumulation in AAR. *p<0.05 Postcon vs. Control. (C) 2004 European Society of Cardiology. Published by Elsevier B.V. All rights reserved.