Central core degeneration after tenotomy in soleus muscles of hyperthyroid rats.

Tenotomy of the rat soleus muscle is followed by a central degeneration of slow, fatigue-resistant muscle fibers. Previous experiments showed that fast, fatigable fibers of the gastrocnemius when transformed to slow, fatigue-resistant fibers by cross-reinnervation also develop lesions after tenotomy. The experiments described in this communication were carried out to discover whether the susceptibility of fibers to lesions was determined by their fiber type or the nature of their innervation. Rats were rendered hyperthyroid by the administration of sodium 3,3',5-triiodo-L-thyronine (T3) for 7 to 10 weeks. Tenotomy of the soleus muscles was then carried out and the experimental and contralateral muscles were removed and stained for myosin ATPase activity after a further 2 weeks. The hyperthyroid state of each animal was confirmed by the assay of succinate dehydrogenase activity of liver and the contralateral muscle. After acid preincubation, whole muscle fiber type counts of contralateral muscles showed a statistically significant change from a predominantly acid-stable population of fibers to acid-labile fibers. In addition, many fibers of intermediate staining properties were seen. When the experimental muscles were examined, all three varieties of fiber showed central degeneration. The nature of the fiber type change induced by T3 and the role that innervation might play in this is discussed. It was concluded that the susceptibility of fibers to the lesions that follow tenotomy is dependent on the nature of their innervation rather than their fiber type.