Key result
Complete inactivity shrinks soleus fiber size ~75% and reduces type I MHC in rats.
Complete inactivity in adult rats leads to rapid soleus muscle atrophy and a slower shift in myosin heavy chain composition from type I to type IIx.
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Animal models demonstrate rapid soleus atrophy from inactivity; leaves open translation to human disuse or rehabilitation strategies.
Myosin heavy chain (MHC) and fiber size properties of the adult rat soleus were determined after 4-60 days of complete inactivity, i.e., lumbar spinal cord isolation. Soleus atrophy was rapid and progressive, i.e., 25% and 64% decrease in weight and 33% and 75% decrease in fiber size after 4 and 60 days of inactivity, respectively. Changes in MHC occurred at a slower rate than the atrophic response. After 15 days there was de novo expression of type IIx MHC (approximately 10%). By 60 days, type IIx MHC accounted for 33% of the total MHC content, and 7% of the fibers contained only type IIx MHC. The relative amount of type I MHC was reduced from 93% in control to 49% after 60 days of inactivity. Therefore, the effects of 60 days of inactivity suggest that during this time period at least 75% of fiber size and approximately 40% of type I MHC composition of the adult rat soleus can be attributed to activation-related events.
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Grossman et al. (1998) studied Inactivity. Complete inactivity (lumbar spinal cord isolation) vs. Control was evaluated on Myosin heavy chain (MHC) composition and fiber size properties of the soleus muscle. Complete inactivity via lumbar spinal cord isolation for 60 days in adult rats resulted in a 75% decrease in soleus fiber size and a reduction of type I MHC from 93% to 49%.
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