Decreased thin filament density and length in human atrophic soleus musclefibers after spaceflight

Soleus muscle fibers were examined electron microscopically from pre- and p ostflight biopsies of four astronauts orbited for 17 days during the Life a nd Microgravity Sciences Spacelab Mission (June 1996). Myofilament density and spacing were normalized to a 2.4-mu m sarcomere length. Thick filament density (similar to 1,062 filaments/mu m(2)) and spacing (similar to 32.5 n m) were unchanged by spaceflight. Preflight thin filament density (2,976/mu m(2)) decreased significantly (P < 0.01) to 2,215/mu m(2) in the overlap A band region as a result of a 17% filament loss and a 9% increase in short filaments. Normal fibers had 13% short thin filaments. The 26% decrease in thin filaments is consistent with preliminary findings of a 14% increase in the myosin-to-actin ratio. Lower thin filament density was calculated to i ncrease thick-to-thin filament spacing in vivo from 17 to 23 nm. Decreased density is postulated to promote earlier cross-bridge detachment and faster contraction velocity. Atrophic fibers may be more susceptible to sarcomere reloading damage, because force per thin filament is estimated to increase by 23%.