Cardioprotection after acute exposure to simulated high altitude in rats. Role of nitric oxide

Aim: In previous studies, upregulation of NOS during acclimatization of rats to sustained hypobaric hypoxia was associated to cardioprotection, evaluated as an increased tolerance of myocardium to hypoxia/reoxygenation. The objective of the present work was to investigate the effect of acute hypobaric hypoxia and the role of endogenous NO concerning cardiac tolerance to hypoxia/reoxygenation under beta-adrenergic stimulation. Methods: Rats were submitted to 58.7 kPa in a hypopressure chamber for 48 h whereas their normoxic controls remained at 101.3 kPa. By adding NOS substrate L-arg, or blocker L-NNA, isometric mechanical activity of papillary muscles isolated from left ventricle was evaluated at maximal or minimal production of NO, respectively, under beta-adrenergic stimulation by isoproterenol, followed by 60/30 min of hypoxia/reoxygenation. Activities of NOS and cytochrome oxidase were evaluated by spectrophotometric methods and expression of HIF1-alpha and NOS isoforms by western blot. Eosin and hematoxiline staining were used for histological studies. Results: Cytosolic expression of HIF1-alpha, nNOS and eNOS, and NO production were higher in left ventricle of hypoxic rats. Mitochondria] cytochrome oxidase activity was decreased by hypobaric hypoxia and this effect was reversed by L-NNA. After H/R, recovery of developed tension in papillary muscles from normoxic rats was 51-60% (regardless NO modulation) while in hypobaric hypoxia was 70% +/- 3 (L-arg) and 54% +/- 1 (L-NNA). Other mechanical parameters showed similar results. Preserved histological architecture was observed only in L-arg papillary muscles of hypoxic rats. Conclusion: Exposure of rats to hypobaric hypoxia for only 2 days increased NO synthesis leading to cardio-protection.