The effects of acute exposure to high altitude on muscle protein synthesis rates in human volunteers were examined after active and passive ascent.
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 [2H5ring] phenylalanine.
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.
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.
1Department of Internal Medicine, University of Berne, Inselspital, Berne, SWITZERLAND; 2Cantonal Hospital, Winterthur, SWITZERLAND; 3Department of Animal Sciences, University of Illinois, Urbana, Illinois, IL; 4Department of Surgery, State University of New York at Stony Brook, Stony Brook, NY; and 5Department of Sports Medicine, University of Heidelberg, Heidelberg, GERMANY
Address for correspondence: Dr. R. Imoberdorf, Department of Internal Medicine, Cantonal Hospital Winterthur, CH-8401 Winterthur, Switzerland; E-mail: email@example.com.
Submitted for publication May 2005.
Accepted for publication November 2005.