8.    Cardiac vulnerability during sexual maturation. Role of nitric oxide.

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 (H/R). This effect started to be shown only after several weeks of chronic hypoxia. However, in a recent work we
showed that 48 h of exposure to hypobaric hypoxia conferred cardioprotection mediated by the nitric oxide system in 3 month-old rats. Because chronic hypoxia had been initiated in seven-week-old rats, in the present work we evaluated the effect of acute exposure of animals to 4400 m simulated altitude on the functional
capacity of NOS in the response of cardiac contractility to H/R. Sexually immature male rats were submitted to 58.7 kPa for 48 h (HH) in a hypopressure chamber and their controls remained at 101.3 kPa (C). Mechanical left ventricle pressure (LVP) and energetic performance (Ht) of hearts perfused by Langendorff technique and exposed to H/R were evaluated. Results showed that LVP was similar in HH and C hearts, but Ht tended to be higher in HH than in C (12.2 ± 1.1 vs. 10.0 ± 0.4 mW/g). After H/R, LVP increased similarly in both groups (60%), whereas Ht was 8.6 ± 0.5 vs. 10.9 ± 0.7 mW/g (p<0.05) for C and HH, respectively. Expression of NOS
isoforms was evaluated by western blot in left ventricle cytosol, showing decreases of 31% and 18% in the HH group in eNOS and nNOS expression, respectively. To evaluate the functional capacity of NOS, papillary muscle.isometric mechanical activity (developed tension and maximal rates of contraction and relaxation) from
left ventricles was determined in the presence of NOS substrate (L-arg) or inhibitor (L-NNA) before and during H/R, under β-adrenergic stimulation. After H/R, the contractile recovery was 98% and 53% in C, and 41% and 40% in HH in the presence of L-arg or L-NNA, respectively. Oxygen consumption measured by highresolution
respirometry and ATP production determined by chemiluminiscence in isolated mitochondria from left ventricles were not modified by 48 h of hypobaric hypoxia. In conclusion, sexually immature rats, whether they were normoxic or exposed to 48 h of hypobaric hypoxia, showed low tolerance to H/R, although it could be improved in normoxic rats, which had higher levels of NOS expression than HH ones, by the addition of L-arg. We hypothesize that, at this stage, cardioprotection conferred by gestational hypoxia would have vanished, whereas
the molecular basis for endogenous cardioprotective mechanisms may be starting to develop. Age should be keep in mind when considering a possible clinical approach for cardiovascular hypoxic risk using short hypobaric hypoxia, as well as with other preconditioning models.