Skeletal muscle protein synthesis after active or passive ascent to high altitude

Introduction: The effects of acute exposure to high altitude on muscle protein synthesis rates in human volunteers were examined after active and passive ascent. Methods: Measurements were made initially at low altitude (550 m) and again after ascent to high altitude (4559 m). To be able to separate the contribution of physical exercise, one group was flown by helicopter (air group, N = 8), whereas the other group climbed to high altitude (foot group, N = 9). Fractional rates of muscle protein synthesis rates (FSR) were determined from the incorporation of isotope into protein after injection of [²H ₅ring] phenylalanine. Results: In the air group, there was no change in FSR at high altitude, whereas in the foot group, there was a 35% increase in FSR (P < 0.05 for interaction) measured 19-23 h after the end of climbing. At high altitude, the degree of hypoxia and alkalosis were not different between the groups. The plasma concentration of insulin-like growth factor-1, free thyroxin, free triiodothyronine, and thyroid-stimulating hormone were not different between the groups. Urinary 24-h cortisol excretion increased significantly in both groups after ascent, but the increase in the foot group was significantly higher compared with the air group. Conclusion: Physical exercise appeared to be responsible for the observed increase in muscle FSR. The significantly higher increase of 24-h cortisol excretion in the foot group suggests that the increase in FSR occurred despite higher levels of glucocorticoids, which generally affect muscle protein turnover by inhibiting protein synthesis.