The aim of this study was to determine, in the rat, the effects of chronic exposure (7-9 weeks) to normobaric
hypoxia (FIO2=0.13, equivalent to 3700 m altitude) on cardiac and skeletal muscle properties, on maximal
oxygen uptake (VO2max), and endurance time to exhaustion (ETE). In addition, we evaluated the impact of
endurance training (90 min of treadmill running per day, 5 days per week, for 9 weeks) on these parameters. The results were compared to normoxic rats fed ad libitum (NAL) and to normoxic pair-weight (NPW) animals in order to take into account the influence of
hypoxia on growth rate. It was found that, in sedentary rats,
hypoxia results in
stunted growth, adrenal
atrophy, a significant reduction of cross-sectional area of fast-twitch (type II) fibres, a reduced capillary-to-fibre ratio (C/F), and a reduced oxidative capacity (decreases in
citrate synthase and 3-hydroxy-
Acyl CoA dehydrogenase activities) of the plantaris muscle. These effects are mainly related to the anorexic effects of prolonged exposure to
hypoxia. Nevertheless, hypoxic (H) rats displayed higher VO2max and ETE values when compared either to NAL or to NPW animals.
Endurance training resulted, in all groups (H, NAL, NPW), in a significant change of the fibre type distribution of the plantaris which displayed an increased number of type IIA fibres and a decreased proportion of type IIB fibres. In addition, the C/F ratio and cross-sectional area of fast-twitch fibres were normalized by superimposition of training on
hypoxia. Both VO2max and ETE were significantly higher in trained H rats than in NAL, but these improvements were mainly related to the reduced
body weight induced by
hypoxia. These data suggest that the greater aerobic capacity and tolerance for prolonged exercise induced by chronic exposure to
hypoxia can be mainly accounted for by the anorexic effects of
hypoxia, although other factors (e.g. increase in
oxygen carrying capacity induced by
hypoxia acclimatization) may play a significant role in some circumstances (e.g. in sedentary rats).