Effect of Recovery Mode Following Hind-Limb Suspension on Soleus Muscle Compositum in the Rat

Abstract

The purpose of this study was to compare the effects of two different recovery modes from hind-limb suspension-induced hypodynamia on whole body and muscle (soleus) growth as well as soleus composition and size changes of different fiber types within this same muscle. Following 28 days of tail-suspension, rats were returned to their cages and sedentarily recovered (HS), or were exercised by running on a treadmill 5 days/wk, at progressively increasing workloads (HR) for one month. Sedentary and running control groups of animals (CS, CR) were also evaluated for comparative purposes. The exercise program, which was identical for CR and HR groups, had no effect on body wt., soleus wt., soleus muscle composition or fiber size in CR rats. Atrophied soleus muscle and reduced soleus wt./body wt. ratio (both 60% of control) had returned to control values by day 7 of recovery in both suspended groups despite the fact that whole body wt. gain was significantly reduced (p < 0.05) in HR as compared to HS rats. Atrophied soleus Type I fiber mean cross-sectional area in both HR and HS groups demonstrated similar and significant (p < 0.01) increases during recovery. Increases in Type IIa and IIc fiber area during this same period were significant only in the HR group. While the percentage area of muscle composed of Type I fibers increased in both hypodynamic groups during recovery, the reduction in area percentage of muscle made up of Type IIa fibers was again only significant in the HR group. With respect to Type IIc fiber percentage, a biphasic response was seen in the soleus of both HS and HR groups during the recovery period. Type IIc fiber area percentage significantly increased by day 7 (HR) and day 14 (HR, HS) of recovery in both suspended groups when compared to CR rats but had returned to control values (CS) by day 28. Additionally large pale-centered targetoid fibers within the same muscle, previously associated with denervation, were observed in HR and HS rats during the recovery period. The results indicate that exercise recovery following suspension-induced hind-limb atrophy produces additional changes in soleus fiber size and composition not seen in a paired sedentary recovery group or control rats exposed to the same exercise regimen.

Key words

Hind-limb Suspension - muscle atrophy - fiber type - morphometry - targetoid fiber