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Program in Physical Therapy, University of Minnesota, Minneapolis, Minnesota 55455
This investigation compared how hindlimb
unweighting (HU) affected the contractile function of single soleus
muscle fibers from 12- and 30-mo-old Fischer 344 Brown Norway F1 Hybrid
rats. After 1 wk of HU, functional properties of single
permeabilized fibers were studied, and, subsequently, the fiber type
was established by myosin heavy chain (MHC) analysis. After HU, the
relative mass of soleus declined by 12 and 19% and the relative mass
of the gastrocnemius declined by 15 and 13% in 12- and 30-mo-old
animals, respectively. In 12-mo-old animals, the peak active force
(5.0 ± 0.2 ×10
4 vs. 3.8 ± 0.2 ×104 N) and the
peak specific tension (92 ± 4 vs. 78 ± 3 kN/m2) were significantly
reduced in the MHC type I fibers by 24 and 15%, respectively. In
30-mo-old animals, the peak active force declined by 40% (4.7 ± 0.2 ×104
vs. 2.8 ± 0. 3 ×104 N) and the peak
specific tension declined by 30% (79 ± 5 vs. 55 ± 4 kN/m2). The maximal unloaded
shortening velocity of the MHC type I fibers increased in 12-mo-old
animals (from 1.65 ± 0.12 to 2.59 ± 0.26 fiber lengths/s) and
in 30-mo-old animals (from 0.90 ± 0.09 to 1.50 ± 0.10 fiber
lengths/s) after HU. Collectively, these data suggest that
the effects of HU on single soleus skeletal muscle fiber function occur
in both age groups; however, the single MHC type I fibers from the
older animals show greater changes than do single MHC type I fibers
from younger animals.
contractile properties; inactivity; aging; fiber types; Fischer 344 Brown Norway F1 Hybrid
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