In vivo exposure to chronic
hypoxia (CH) depresses myocardial performance and tolerance to
ischemia, but daily reoxyenation during CH (CHR) confers cardioprotection. To elucidate the underlying mechanism, we tested the role of phosphatidylinositol-3-kinase-protein
kinase B (Akt) and p42/p44
extracellular signal-regulated kinases (ERK1/2), which are known to be associated with protection against
ischemia/reperfusion (I/R). Male Sprague-Dawley rats were maintained for two weeks under CH (10% O(2)) or CHR (as CH but with one-hour daily exposure to room air). Then, hearts were either frozen for biochemical analyses or Langendorff-perfused to determine performance (intraventricular balloon) and tolerance to 30-min global
ischemia and 45-min reperfusion, assessed as recovery of performance after I/R and
infarct size (tetrazolium staining). Additional hearts were perfused in the presence of 15 micromol/L
LY-294002 (inhibitor of Akt), 10 micromol/L
UO-126 (inhibitor of ERK1/2) or 10 micromol/L
PD-98059 (less-specific inhibitor of ERK1/2) given 15 min before
ischemia and throughout the first 20 min of reperfusion. Whereas total Akt and ERK1/2 were unaffected by CH and CHR in vivo, in CHR hearts the phosphorylation of both
proteins was higher than in CH hearts. This was accompanied by better performance after I/R (heart rate x developed pressure), lower end-diastolic pressure and reduced
infarct size. Whereas the treatment with
LY-294002 decreased the phosphorylation of Akt only, the treatment with
UO-126 decreased ERK1/2, and that with
PD-98059 decreased both Akt and ERK1/2. In all cases, the cardioprotective effect led by CHR was lost. In conclusion, in vivo daily reoxygenation during CH enhances Akt and ERK1/2 signaling. This response was accompanied by a complex phenotype consisting in improved resistance to stress, better myocardial performance and lower
infarct size after I/R. Selective inhibition of Akt and ERK1/2 phosphorylation abolishes the beneficial effects of the reoxygenation. Therefore, Akt and ERK1/2 have an important role to mediate cardioprotection by reoxygenation during CH in vivo.