Background and purpose: The principal cause of cardiomyocyte dysfunction and necrosis resulting from Ischemia/Reperfusion (I/R) injury is the production and release of Reactive Oxygen Species (ROS) from damaged mitochondria. Mitochondrial ROS-mediated oxidation of cellular proteins and lipids disrupts both cellular metabolism and organelle integrity, leading to depletion of ATP stores and an increase in intracellular calcium (Ca²⁺) levels. Thus, attenuation of I/R-induced ROS production has been a therapeutic strategy to salvage damaged cardiomyocytes and thereby limit cardiac functional impairment and infarct size [1]. Previous studies have tested the effectiveness of mitochondrial-targeted antioxidants. Mitoquinone (MitoQ) was effective in reducing I/R injury when given only prior to prolonged ischemia, while Szeto-Schiller (SS)-31 was effective when given either prior to ischemia or at the beginning of reperfusion. This study was undertaken to determine whether these agents are effective in limiting myocardial and hindlimb I/R injury when given during the first 5min of reperfusion only, and thereby support the premise that mitochondrial damage underlies reperfusion-induced cell death during the initial minutes of reperfusion.

Experimental approach: Male Sprague-Dawley rats (275-325 g) were randomized into myocardial or hindlimb I/R groups. Isolated, retrogradely perfused hearts were subjected to global I(30 min)/R(45 min). Hearts were treated with MitoQ (1-20 μM), SS-31 (10-100 μM), or plasma (control) added to the perfusate at the onset of reperfusion and was assessed for cardiac function and infarct size. In the hindlimb experiments, either hydrogen peroxide (H₂O₂) or Nitric Oxide (NO) sensors were placed randomly in both the right and left femoral veins of the same animal. One limb was subjected to I(30 min)/R(45 min) by reversibly clamping the femoral artery/vein, while the other limb served as a sham. The animal received an intravenous (i.v.) bolus of either MitoQ (1.8 mg kg^-1), SS-31 (2.5 mg kg^-1), or saline (control) at the beginning of reperfusion. The difference in blood H₂O₂ or NO between the femoral vein of ischemic and sham limbs in each animal was continuously measured to assess the effects of the drugs.

Key results: In the myocardial I/R model, MitoQ and SS-31 given upon reperfusion significantly improved cardiac function and reduced infarct size compared to untreated control hearts. In the hindlimb I/R model, elevations in blood H₂O₂ levels and reductions in blood NO, both indices of elevated ROS, were significantly attenuated by both MitoQ and SS-31 given upon reperfusion compared to saline treated control animals.

Conclusion and implications: The results indicate that mitochondrial-derived ROS is a major contributor to reperfusion injury and that MitoQ and SS-31 work expeditiously to attenuate ROS in that both agents improve cardiac function and limit infarct size when administered only at the onset of reperfusion. The data suggest that MitoQ or SS-31 would be an effective adjuvant to reduce ischemia reperfusion injury in acute myocardial infarction patients receiving percutaneous coronary intervention, thrombolytic therapy, or undergoing coronary by-pass surgery.