Intermittent Hypoxia Reduces Myocardial Apoptosis Induced By Ischemia/reperfusion And Its Potential Mechanisms

Intermittent hypoxia (IH) has been shown to providemyocardial protection against ishemia/reperfusion (I/R) -induced injury. Cardiacmyocyte loss through apoptosis has been reported in I/R injury. Our aim was toinvestigate whether IH could attenuate I/R -induced apoptosis in cardiac myocytesand its potential mechanisms. Methods: Adult male Sprague-Dawley rats wereexposed to hypoxia simulated 5000 m in a hypobaric chamber for 6 h/day, lasting 42days. Normoxia group rats were kept under normoxic conditions. Isolatedperfused hearts from both groups were subjected to 30 min of global ischemiafollowed by 60 min reperfusion. Incidence of apoptosis in cardiac myocytes wasdetermined by terminal deoxynucleotidyl transferase mediated dUTP nick endlabeling (TUNEL) and DNA agarose gel electrophoresis. Expressions of apoptosisrelated proteins, including Bax, Bcl-2 and cytochrome-C (Cyt.C), in cytosolic andmembrane fraction were detected by Western Blotting. To simulate I/R, isolatedventricular myoctes from normxia and intermittent hpoxia rats were exposed tometabolic inhibition (MI) 20 min followed by washout with normal solution.Necrotic cells or apoptotic cells were identified by Hoechst/PI staining. Time-lapseanalysis of mitochondrial membrane potential (?Ψm) was evaluated by confocalmicroscopic analysis after loading with Rodamine 123. Results: After I/R,enhanced recovery of cardiac function was observed IH hearts compared withnormoxis group. I/R-induced apoptosis, as evidenced by TUNEL-positive nucleiand DNA fragmentation, was significantly reduced in IH group compared withnormoxia group. Expression of Bax in both cytosolic and membrane fractions weredecreased in IH hearts compared with normoxia group after I/R. Although I/R did 6not induce changes in the level of Bcl-2 expression in cytosolic fraction between IHand normoxia groups, the expression of Bcl-2 in membrane fraction was upregulatedin intermittent hypoxia group compared with normoxia group. The increase in therelease of mitochondrial Cyt.C into cytosol induced by I/R was significantly inhibitedby IH. The number of necrotic cells was significantly increased in normoxiccardiomyocytes after simulated I/R, but was alleviated by IH. Addition of mitoKATPspecific opener, Diazoxide significantly reduced I/R-induced necrosis in normoxiccardiomyocytes, but had no effect on IH cardiomyocytes. The necrosis induced bysimulated I/R in IH cardiomyocytes was raised after treatment with 5-HD, specificmitoKATP inhibitor, however, 5-HD had no effect on normoxia cardiomyocytes. Agreater depolarization of ?Ψm during simulated ischemia was observed in normoxiacardiomyocytes than that observed in IH cardiomyocytes. Conclusion: Theseresults indicated that the cardioprotection of intermittent hypoxia againstischemia/reperfusion injury appears to be in part due to reduce myocardial apoptosis.Intermittent hypoxia attenuated ischemia/reperfusion-induced apoptosis via increasingthe ratio of Bcl-2/Bax, inhibiting mitochondrial Cyt.C release, maintaining the?Ψm.It suggested IH is beneficial to preserving mitochondrial function and structure intact.