title: Fiber size, type, and myosin heavy chain content in rhesus hindlimb muscles after 2 weeks at 2 G.
abstract: BACKGROUND
  Fiber atrophy and an increase in the percentage of fast fibers have been observed in Rhesus leg muscles after spaceflight.
  
  
  OBJECTIVE
  Hypergravity will result in muscle fiber hypertrophy and an increase in the percentage of slow fibers.
  
  
  METHODS
  Open muscle biopsies were obtained from Rhesus soleus, medial gastrocnemius (MG), and tibialis anterior (TA) muscles before and after 14 d of centrifugation (2 G) and in time-matched controls. Cage activity levels were measured by telemetry.
  
  
  RESULTS
  Based on monoclonal antibody binding for myosin heavy chains (MHC), the fastest region of soleus contained a higher proportion of type I+II (27 vs. 13%) and had a tendency for a lower proportion of type I (38 vs. 61%, p = 0.10) fibers after than before centrifugation. There was a higher proportion of type I+II fibers in post- vs. pre-2 G (10 vs. 0.6%) MG biopsies. Fiber type distribution and MHC composition were unaffected in the TA. Overall, mean fiber sizes were unaffected by centrifugation. Average cage activity levels were 36% lower during than before 2 G.
  
  
  CONCLUSIONS
  Our hypothesis was rejected. The changes in the proportion of fibers expressing type I MHC are the reverse of that expected with chronic loading of extensors and, paradoxically, are similar to changes observed with chronic unloading, such as occurs during spaceflight, in this primate model. The data are consistent with the observed decrease in total daily activity levels.
authors.affiliation: Department of Physiological Science, University of California, Los Angeles 90095-1761, USA.
authors.forename: Morteza
authors.initials: M
authors.lastname: Tavakol
authors.forename: Roland R
authors.initials: RR
authors.lastname: Roy
authors.forename: Jung A
authors.initials: JA
authors.lastname: Kim
authors.forename: Hui
authors.initials: H
authors.lastname: Zhong
authors.forename: John A
authors.initials: JA
authors.lastname: Hodgson
authors.forename: Tana M
authors.initials: TM
authors.lastname: Hoban-Higgins
authors.forename: Charles A
authors.initials: CA
authors.lastname: Fuller
authors.forename: V Reggie
authors.initials: VR
authors.lastname: Edgerton
mesh_terms: D000222:Adaptation, Physiological; D000818:Animals; D002498:Centrifugation; D006614:Hindlimb; D018471:Hypergravity; D007150:Immunohistochemistry; D008253:Macaca mulatta; D008297:Male; D018485:Muscle Fibers, Skeletal / Q000473:pathology*; D018482:Muscle, Skeletal / Q000737:chemistry / Q000473:pathology*; D018995:Myosin Heavy Chains / Q000032:analysis*
chemical_list: D018995:Myosin Heavy Chains