Skeletal Muscle mRNA Response to Hypobaric and Normobaric Hypoxia after Normoxic Endurance Exercise

Background: The physiological effects of hypoxia may be influenced by how hypoxia is achieved. The purpose of this study was to determine the effects of recovery in hypobaric hypoxia (HH), normobaric hypoxia (NH), and normobaric normoxia (NN) after endurance exercise on gene expression related to mitochondrial biogenesis, myogenesis, and proteolysis. Methods: Fifteen recreationally trained subjects each cycled for 1 hour before recovering for 4 hours in NN (laboratory atmospheric conditions, 975 m), HH (depressurized to simulate 4420 m), and NH (fraction of O₂ reduced to simulate 4420 m). Muscle biopsy samples were obtained before exercise and after 4 hours of recovery. Results: Blood oxygenation (SpO₂) was lower in HH (76.02 ± 0.58%) than NH (79.45 ± 0.56, p < 0.001), which were both lower than in NN (96.3 ± 0.17, p < 0.001). Heart rate was higher in HH (82 ± 2 bpm) than NH (77 ± 1 bpm, p < 0.001), which were both higher than in NN (67 ± 1 bpm, p < 0.001). Mitochondrial transcription factor A (TFAM) mRNA was lower after NN than HH (p = 0.034) or NH (p = 0.005), but was not different between HH and NH (p = 0.460). Myostatin (MSTN) mRNA decreased from pre-to postexercise (p < 0.001) in all conditions and was lower in HH compared with NH (p = 0.035) and NN (p = 0.017). No other differences were noted in genes related to mitochondrial biogenesis, myogenesis, or proteolysis (p > 0.05). Conclusion: TFAM mRNA is lower with hypoxia exposure, but effected by the type of hypoxia. MSTN gene expression is lower after exposure to HH than NH or NN. These data support previous work and caution the translation of NH data obtained in a NH environment to a HH environment.