Responses of ankle plantar-flexors to various inhibitions of muscular activities in rats]

Effects of hindlimb suspension, tenotomy, denervation, and/or the combination of these models on plantar-flexors were studied in adult rats. Suspension-induced atrophy was not promoted by addition of tenotomy. But the magnitude of the atrophy was advanced if denervation or both denervation and tenotomy were combined with 5-day hindlimb suspension. Similar effects were noted in the cross-sectional area of single muscle fibers, especially of slow-twitch fibers. A shift of muscle fiber type from slow- to fast-twitch type was also induced mainly in soleus. The atrophy and fiber transformation were closely associated with a passive shortening of muscle due to the plantar-flexion of ankle and/or tenotomy and a disappeared electrical activity caused by denervation. The fiber atrophy, but not the shift of fiber type, was further advanced by the combination of tenotomy and denervation. It is suggested that muscle atrophy is caused by the decreased fiber size and protein content. The water content was also reduced proportionally.