Decreased thin filament density and length in human atrophic soleus muscle fibers after spaceflight.
Soleus muscle fibers were examined electron microscopically from pre- and postflight biopsies of four astronauts orbited for 17 days during the Life and Microgravity Sciences Spacelab Mission (June 1996). Myofilament density and spacing were normalized to a 2. 4-microm sarcomere length. Thick filament density ( approximately 1, 062 filaments/microm(2)) and spacing ( approximately 32.5 nm) were unchanged by spaceflight. Preflight thin filament density (2, 976/microm(2)) decreased significantly (P < 0.01) to 2,215/microm(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 increase 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%.
Riley
D A
DA
Department of Cell Biology, Neurobiology, and Anatomy, Medical College of Wisconsin, Milwaukee 53226, USA. dariley@mcw.edu
Bain
J L
JL
Thompson
J L
JL
Fitts
R H
RH
Widrick
J J
JJ
Trappe
S W
SW
Trappe
T A
TA
Costill
D L
DL
D008841:Actin Cytoskeleton / Q000378:metabolism*; D018480:Astronauts; D006801:Humans; D008297:Male; D008854:Microscopy, Electron; D018485:Muscle Fibers, Skeletal / Q000737:chemistry / Q000378:metabolism*; D018482:Muscle, Skeletal / Q000737:chemistry / Q000378:metabolism*; D009133:Muscular Atrophy / Q000503:physiopathology*; D009210:Myofibrils / Q000378:metabolism / Q000648:ultrastructure; D012518:Sarcomeres / Q000378:metabolism / Q000648:ultrastructure; D013026:Space Flight; D014893:Weightlessness