Enzyme activity and myoglobin concentration in rat myocardium and skeletal muscles after passive intermittent simulated altitude exposure.

We studied the effect of intermittent hypobaric hypoxia exposure on lactate dehydrogenase and citrate synthase activities, together with myoglobin content, of rat myocardium, tibialis anterior, and diaphragm muscles. The intermittent hypoxia exposure programme consisted of daily 4-h sessions in a hypobaric chamber (5000 m) over a period of 22 days. Samples were taken at the end of the programme, and 20 and 40 days later, and compared with those of control animals. In myocardium, lactate dehydrogenase activity was significantly depressed in animals 20 days post-exposure (314.6 +/- 15.3 IU . g(-1)) compared with control animals (400 +/- 14.3 IU . g(-1)), while citrate synthase activity and myoglobin concentration showed a significant stepwise increase from control animals (88.2 +/- 3.6 IU . g(-1) and 4.38 +/- 0.13 microm . mg(-1)) to animals 20 days (104.7 +/- 3.7 IU . g(-1) and 5.01 +/- 0.17 microm . mg(-1)) and 40 days post-exposure (108.8 +/- 6.5 IU . g(-1) and 5.11 +/- 0.22 microm . mg(-1)). In contrast, no differences were found in diaphragm and tibialis anterior muscles. Our results show that intermittent hypobaric hypoxia exposure increased the oxidative character of myocardium even 20 days after the hypoxic stimulus has ceased, and that this effect lasts for more than 40 days for citrate synthase activity and myoglobin concentration. These findings support our previous results on skeletal and cardiac muscle capillarization after passive intermittent simulated altitude exposure, thus providing morphofunctional and biochemical evidence for increased cardiac aerobic efficiency.