14 days of bed rest induces a decline in satellite cell content and robust XXXX of skeletal muscle fibers in middle-aged adults.

@entity9740 @entity26 @entity194
UNASSIGNED: Bed rest, a ground-based spaceflight analog, induces robust @entity194 of skeletal muscle, an effect that is exacerbated with increasing age. We examined the effect of 14 days of bed rest on skeletal muscle satellite cell content and fiber @entity194 in middle-aged adults, an understudied age demographic with few overt signs of muscle aging that is representative of astronauts who perform long-duration spaceflight. Muscle biopsies were obtained from the vastus lateralis of healthy middle-aged adults (n=7 [4M;3F]; age: 51  1 y) before (Pre-BR) and after (Post-BR) 14 days of bed rest. Immunohistochemical analyses were used to quantify @entity9740 ( @entity9740 ) isoform expression, cross-sectional area (CSA), satellite cell and myonuclear content, and capillary density. Peak @entity26 consumption, knee extensor strength, and body composition were also measured Pre- and Post-BR. Post-BR @entity9740 Type 2a fiber percentage was reduced and mean CSA decreased in all fiber types (-24  5%; P<0.05). Satellite cell content was also reduced Post-BR (-39  9%; P<0.05) and the change in satellite cell content was significantly correlated with the change in mean fiber CSA (R2=0.60; P<0.05). A decline in capillary density was observed Post-BR (-23  6%; P<0.05), and Post-BR capillary content was significantly associated with Post-BR peak aerobic capacity (R2=0.59; P<0.05). A subtle decline in myonuclear content occurred during bed rest (-5  1%; P<0.05). The rapid maladaptation of skeletal muscle to 14 days of mechanical unloading in middle-aged adults emphasizes the need for robust countermeasures to preserve muscle function in astronauts.
