Mitochondrial Mechanisms Of Intermittent High Altitude Hypoxia Against Ischemia/Reperfusion Injury

Objectives: Many studies have shown that intermittent high altitude hypoxictraining markedly enhances myocardial tolerance against ishemia/reperfusion.Protection is evidenced by better recovery of myocardial contractile function,reduction of ischemia-reperfusion induced arrhythmias, and decrease of infarct size.To date, very little is known about the role of mitochondria on IHA hypoxiccardioprotection. The aim of this study was to investigate the effects of IHA hypoxiaon the mitochondrial respiratory, mitochondrial permeability transition pore (MPTP),mitochondrial ATP-sensitive potassium channels (mitoKATP), and protein expressionof mitochondrial and their roles played in preventing ischemia-reperfusion injury.Methods: Adult male Sprague-Dawley rats were divided randomly into two groups:a normoxic group and an IHA hypoxic group. For IHA hypoxic study, rats wereexposed to hypoxia simulated 5000 m in a hypobaric chamber for 6 h/day, lasting 42days. Langendorff-perfused hearts from both groups were subjected to 30 min ofglobal ischemia followed by 120 min reperfusion and the functional changes wereinvestigated. The infarct size was measured by triphenyltetrazolium chloride (TTC)staining. Mitochondria were isolated by differential centrifugation from rat hearts.The change in absorption at 540 nm was recorded at different Ca²⁺ concentrations.Cytochrome c released from mitochondria was detected via Western blot. The changesof fluorescence and cell length were monitored using a video-edge detection systemafter loading with indo-1. Proteomic analysis was applied for exploring mitochondrialproteins, which correlate with IHA hypoxic training.Results: Ischemia-reperfusion significantly inhibited the recovery of cardiacfunction of the normoxic hearts, whereas IHA hypoxia significantly improved thefunctional recovery of Langendorff hearts on reperfusion and limited infarct size.Administration of atractyloside immediately upon reperfusion abolished the protectiveeffects of IHA hypoxia. In isolated myocytes, IHA hypoxia significantly improved therecovery of cell length, lowered ischemia-reperfusion-induced [Ca²⁺]c and [Ca²⁺]moverloading. Opening the MPTP with atractyloside immediately at reperfusionabolished these cardioprotective effects of IHA hypoxia, but had no appreciableinfluence on those of normoxic hearts. IHA hypoxia prolongs the time taken to induceMPTP opening and the time taken to induce rigor contracture when myocytessubjected to oxidative stress. At different Ca2+ concentrations, ?A540 from IHAhypoxic mitochondria was significantly lower than that of normoxic mitochondria.Compared with normoxia, IHA hypoxia significantly reduced cytochrome c release atthe same [Ca2+]. The release of cytochrome c has no different after treated withatractyloside in both normoxic and IHA hypoxic groups.MitoKATP specific antagonist 5-HD eliminated the beneficial effects of IHAhypoxia on the functional recovery after ischemia. Simulated ischemia-reperfusioninduced an significant increase of [Ca2+]c and [Ca2+]m in normoxic myocytes, but littleaffect [Ca2+]c and [Ca2+]m in IHA hypoxic training. These cardioprotective effectswere abolished by 5-HD.The differentially expressed mitochondria proteins were analyzed by proteomicresearch. IHA hypoxic training increases several enzymes which correlate with energymetabolism, including malate dehydrogenase, aldehyde dehydrogenase, ubiquinol-cytochrome c reductase iron-sulfur subunit, aconitase, electronatransfer flavoprotein αsubunit, and ATP synthase β chain etc. IHA hypoxia also increases the expression of amolecular chaperone—HSP60 and an antioxidant protein--peroxiredoxin 5. However,IHA hypoxia decreases the expression of aspartate aminotransferase.Conclusion: These results indicated that IHA hypoxic training alleviatedischemia-reperfusion-induced injury. The maintenance of mitochondrial integrity ofboth structure and function plays an important role during ischemia-reperfusion. Onone hand, IHA hypoxia enhances tolerance of mitochondria to Ca2+ overload, whichmay reduce the probability of MPTP opening. On the other hand, IHA hypoxiaaffiliates activation of mitoKATP channels and reduces Ca2+ overloading duringischemia-reperfusion. In addition, the alterations in mitochondrial proteome inmyocardium provide significant information for understanding the mechanism ofcardioprotection afforded by IHA hypoxia.