Overwork effect on partially denervated rat soleus muscle

The purpose of this study was to assess the effect of overwork induced by synergistic tenotomy on the muscle weight (MW) and isometric tetanic tension (P₀) of the partially denervated rat soleus muscle. Forty-nine young adult male Sprague-Dawley rats were divided into six groups (n = 5 to 9): normal control (N), normal synergistically tenotomized (N-ST), L₄ denervated control (L₄), L₄ denervated synergistically tenotomized (L₄-ST), L₅ denervated control (L₅), and L₅ denervated synergistically tenotomized (L₅-ST). Bilateral L₄ or L₅ root transections produced partial denervation. To yield soleus muscle overwork the gastrocnemius and plantaris muscles were bilaterally tenotomized in all ST groups at the time of denervation. The soleus MW and P₀ were examined seven days following denervation and tenotomy. Compared to their respective controls, there was increased MW in the N-ST (43.5%, p<0.01) and the L₄-ST (38.6% p<0.01) groups. The L₅-ST group showed no significant change in MW compared to the L₅ group. Tetanic tension was increased only in the N-ST group. The L₄-ST and L₅-ST groups did not show any change in P₀ compared to their respective controls. Despite marked denervation, the L₅ group did not show any atrophy as compared to the N group, suggesting passive stretch of the denervated muscle fibers, hypertrophy of the innervated muscle fibers and peripheral sprouting prevented the loss of MW due to denervation. Overwork in this markedly denervated group could not produce any increase in the MW or P₀. It is suggested that peripheral sprouting and stretch-induced overwork of muscle fibers may be responsible for sparing of muscle atrophy and P₀.