Bed rest reduces metabolic protein content and abolishes exercise-induced mRNA responses in human skeletal muscle

Background: The aim was to test the hypothesis that one week of bed rest will reduce mitochondrial number and expression and activity of oxidative proteins in human skeletal muscle, but that exercise-induced intracellular signaling as well as mRNA and microRNA (miR) responses are maintained after bed rest. Research Design and Methods: Twelve young, healthy, male subjects completed 7 days of bed rest with vastus lateralis muscle biopsies taken before and after bed rest. In addition, muscle biopsies were obtained from 6 of the subjects prior to, immediately after and 3h after 45 min one-legged knee extensor exercise performed before and after bed rest. Results: Maximal oxygen uptake decreased 5% and exercise endurance decreased non-significantly 25% by bed rest. Bed rest reduced skeletal muscle mitochondrial DNA/nuclear DNA content 15%, hexokinase II and sirtuin 1 protein content ~45%, 3-hydroxyacyl-CoA dehydrogenase and citrate synthase activity ~8% as well as miR-1 and miR-133a content ~10%. Cytochrome c and vascular endothelial growth factor (VEGF) protein content as well as capillarization did however not change significantly with bed rest. Acute exercise increased AMP-activated protein kinase phosphorylation, peroxisome proliferator activated receptor ¿ coactivator-1a and VEGF mRNA content in skeletal muscle before bed rest, but the responses were abolished after bed rest. Conclusion: The present findings indicate that only 7 days of physical inactivity reduce skeletal muscle metabolic capacity as well as abolish exercise-induced adaptive gene responses likely reflecting the interference with the ability of skeletal muscle to adapt to exercise.