Dynamics of postdenervation atrophy of young and old skeletal muscles: differential responses of fiber types and muscle types.

We investigated the dynamics of muscle fiber atrophy in denervated fast and slow muscles of young and old rats. Hind limbs of 4-month-old and 24-month-old male rats were denervated, and soleus and tibialis anterior muscles were examined morphometrically 1 and 2 months after denervation. In all denervated muscles, type II muscle fibers underwent rapid atrophy, although muscle-specific differences in rate were observed. In both young and old denervated soleus muscles, the type I fibers underwent a pattern of atrophy closely paralleling that of the type II fibers, but in the tibialis anterior muscle, the mean cross-sectional area of the type I fibers actually increased during the first 2 months postdenervation. This study has shown that, among different muscles and between young and old rats, there is considerable variation in the response of the muscle fibers to denervation and that one cannot generalize from one muscle or one age to another.