DENERVATION-INDUCED REGION-SPECIFIC CHANGES IN FIBER TYPES IN THE SOLEUS AND PLANTARIS MUSCLES OF RATS

Muscle fibre composition was compared among the proximal (25%), middle (50%) and distal (75%) regions of muscle to investigate whether dener vation induces region-specific changes of fibre types in the soleus an d plantaris muscles of rats. Decreases in mass were observed in both m uscles after denervation. In the soleus muscle, denervation increased the percentage of type I fibres with a concomitant increase in the pro portion of type IIC and IIA fibres. The extent of such transformations was greater in the proximal region than the middle and distal regions . In normal plantaris muscle, the middle region showed a higher propor tion of type IIA fibres with a lower percentage of type IIB fibres rec iprocally than other regions. These regional differences in fibre type s were not detected in the 4-week denervated plantaris muscle. These f indings suggest that denervation-induced transformations from type I t o type II fibres begin in the proximal region in the soleus muscle of rats. In addition, regional differences in fibre types along the muscl e length could be regulated by neuromuscular activity through normal i nnervation in the plantaris muscle.