High-altitude exposure decreases TfR protein content in skeletal muscle. The changes in TfR expression in muscle biopsies obtained from vastus lateralis under normoxic conditions at SL and during exposure to HA (4559 m) for 7 to 9 days are shown. (A) Equal amounts of proteins from extracts of muscle biopsies were loaded onto SDS polyacrylamide gels and immunoblotted with antibodies against TfR. The blots were reprobed with an antibody against α-tubulin (α-tub) as a loading control. Antigens were visualized by chemiluminescence. Extracts of participant nos. 2, 4, and 6 were processed separately. (B) Densitometric quantification of immunoblot analysis of TfR protein content at SL and during HA. Individual changes for all study participants but no. 7, whose sample could not be analyzed because of technical reasons, are shown. (C) TfR mRNA levels. The TfR mRNA content is normalized to total cDNA content and expressed relative to the value at SL, which was arbitrarily defined as 100. The values are means ± SE for 9 participants. The calculation of statistical differences from normoxia was performed using the Wilcoxon test. **P < .01.