Muscle fiber atrophy after cast immobilization in the rat.

The purpose of this study was to evaluate the effect of 6 weeks of bilateral cast immobilization on the adult Wistar rat plantarflexors. Knee and ankle joints were fixed in the neutral position, under sodium pentobarbital anesthesia. Controls (n = 7) and cast-immobilized (n = 7) rats were sacrificed at 6 weeks after the onset of the experimental period. The muscles were evaluated for muscle weight, protein content, and fiber histochemistry. There was equal weight loss in the gastrocnemius, soleus, and plantaris muscles, and greater loss of myofibrillar protein than of sarcoplasmic protein concentrations. Fiber atrophy of types I and II was equal in the soleus, and that of type II was greater than the type I atrophy in both regions of the plantaris. All soleus and 2 plantaris muscles demonstrated fiber damage. The results of this experiment suggest that the degree of fiber atrophy is specific to the fiber type and to its location in different muscles or muscle regions.