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Am J Physiol Cell Physiol (October 6, 2004). doi:10.1152/ajpcell.00386.2004
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Submitted on August 9, 2004
Accepted on October 4, 2004

Skeletal muscle fiber atrophy: altered thin filament density changes slow fiber force and shortening velocity

Danny A Riley1*, James L Bain1, Janell G Romatowski2, and Robert H Fitts2

1 Cell Biology, Neurobiology & Anatomy, Medical College of Wisconsin, Milwaukee, WI, USA
2 Biological Sciences, Marquette University, Milwaukee, WI, USA

* To whom correspondence should be addressed. E-mail: dariley{at}mcw.edu.

Single skinned fibers from rat soleus and adductor longus (AL) muscles of weightbearing Control and after 14-day hindlimb suspension unloading (HSU) were studied physiologically and ultrastructurally to investigate how slow fibers increased shortening velocity (Vo) without fast myosin. We hypothesized that unloading and shortening of soleus during HSU reduced densities of thin filaments generating wider myofilament separations which increased Vo and decreased specific tension (kN/m2). During HSU, plantarflexion shortened soleus working length 23%. AL length was unchanged. Both muscles atrophied as shown by reductions in fiber cross sectional area. For AL, the 60% atrophy accounted fully for the 58% decrease in absolute tension (mN). In soleus, the 67% decline in absolute tension resulted from 58% atrophy plus a 17% reduction in specific tension. Soleus fibers exhibited a 25% reduction in thin filaments, whereas there was no change in AL thin filament density. Loss of thin filaments is consistent with a reduced cross bridge formation explaining the fall in specific tension. Vo increased 27% in soleus but was unchanged in AL. The Vo of Control and HSU fibers was inversely correlated (R= -0.83) with thin filament density and directly correlated (R=0.78) with thick to thin filament spacing distance in a non-linear fashion. These data indicate that reduction in thin filament density contributes to an increased velocity of shortening in slow fibers. Osmotically compacting myofilaments with 5% dextran returned density, spacing, specific tension and slowed Vo to near Control levels and provided evidence for myofilament spacing modulating tension and Vo.




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