Muscle disuse atrophy is not accompanied by changes in skeletal muscle satellite cell content

Muscle disuse leads to a considerable loss of skeletal muscle mass and strength. However, the cellular mechanisms underlying disuse-induced muscle fibre atrophy remain to be elucidated. Therefore, we assessed the impact of muscle disuse on quadriceps muscle cross-sectional area (CSA), muscle fibre size and satellite cell content, and associated myocellular signalling pathways. Methods: Twelve healthy, young (24+/-1 y) men were subjected to 2 wks of one-legged knee immobilisation via a full leg cast. Before and immediately after the immobilisation period and after six weeks of natural rehabilitation, muscle strength (one-repetition maximum), muscle cross-sectional area (single slice CT-scan), and muscle fibre type characteristics (muscle biopsies) were assessed. Protein and/or mRNA expression of key genes (i.e. MyoD, myogenin and myostatin) in the satellite cell regulatory pathways were determined using Western blotting and rtPCR analyses, respectively. Results: Quadriceps CSA declined following immobilisation by 8+/-2% (P<0.05). In line, both type I and type II muscle fibre size decreased by 7+/-3% and 13+/-4%, respectively (P<0.05). No changes were observed in satellite cell content following immobilisation in either type I or type II muscle fibres. Muscle myogenin mRNA expression had doubled (P<0.05) while myostatin protein expression had decreased by 30+/-9% (P<0.05) following immobilisation. Muscle mass and strength returned to baseline values within six weeks of recovery without any specific rehabilitative program. Conclusion: Two weeks of muscle disuse leads to considerable loss of skeletal muscle mass and strength. The loss of muscle mass is attributed to both type I and type II muscle fibre atrophy, and is not accompanied by a decline in satellite cell content.