Enzymatic responses and adaptations to swimming training and hypobaric hypoxia in postnatal rats.

Twenty four male Sprague-Dawley rats, 35 days old, were randomly assigned to one of four groups: 2 resting control groups and 2 swimming groups. The sea level-control and the sea level-swimming groups were housed 5 weeks at 1,011 hPa (760 mmHg) while the hypoxic control and swimming groups were housed for 1 week at 678 hPa, followed by 4 weeks at 611 hPa. The swimming rats were subjected to a swimming program of 30 min, 6 days/week for 5 weeks. Both hypoxia groups developed significantly higher Hb and Hct levels than the sea level groups. The glycogen content in the extensor digitorum longus (EDL) and the deep portion of the vastus lateralis (DVL) muscles of the sea level-swimming group were significantly greater as compared to the hypoxia swimming group. The succinate dehydrogenase (SDH) activity in the sea level-control group was significantly lower in the EDL muscle than in the 3 other groups, and in the DVL muscle lower than that of the sea level-swimming group. Histochemically, hypoxia and swimming training induced significant increases in the fast-twitch-oxidative-glycolytic (FOG) fibers (6-11%) in soleus muscle, and decreases in the slow-twitch-oxidative (SO) fibers. The EDL muscles had significantly higher percentages of FOG fibers in the hypoxia and swimming groups than in the sea level-control group. On the basis of the present study it seems probable that hypoxia is a triggering factor for the conversions of muscle fiber types and the increase in oxidative capacity.