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Skeletal muscle atrophies with disuse while with increased use and increased load skeletal muscle exhibits hypertrophy, with an increase in the size of existing muscle fibers. One signaling pathway involved in regulating skeletal muscle atrophy and hypertrophy is the AKT/mTOR pathway. The mTOR pathway activity increases in response to muscle activity during hypertrophy and decreases in activity during atrophy. Blocking this pathway genetically or with the mTOR inhibitor rapamycin blocks hypertrophy and genetic activation of the pathway induces hypertrophy. One agent that promotes muscle hypertrophy is the growth factor IGF-1. IGF-1 activates AKT, GSK-3 beta, and mTOR to promote hypertrophy. In contrast, the calcineurin pathway is not involved in hypertrophy and is down-regulated by agents such as IGF-1 that promote hypertrophy. Calcineurin may modulate other aspects of muscle function such as the development of slow muscle fibers through transcriptional regulation. These pathways lead to regulation of protein translation, with increased translation apparently acting as a key regulatory point in skeletal muscle hypertrophy. Agents such as IGF-1 that stimulate skeletal muscle hypertrophy may provide treatments for muscle atrophy and wasting. (This definition may be outdated - see the DesignNote.)
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