Muscle fiber transformation in capsaicin-treated rats

Rats were injected with capsaicin or solvent on the 2nd day of life. Nine weeks later both animal groups were further subdivided: one was untreated and the other group underwent surgery consisting in either (a) removal of one sternohyoid superior muscle or (b) a sham operation. The sternohyoid superior muscles were removed at 3-day intervals and investigated for their myofibrillar ATPase reaction. Capsaicin application alone produced a primary (baseline) muscle fiber transformation from fast to slow which was reflected in a considerable increase in the type IA fiber population (almost lacking in normal controls). However, at all stages investigated the degree of transformation (i.e., the amount of IA fibers) displayed a great individual variability. In addition to this basic transformation, both surgical procedures elicited a concordant behavior of the muscles and an undulating time course of the number of type IA fibers. This secondary reversible muscle fiber transformation was superimposed on the primary one. There is evidence that the operative treatments caused a synchronization of the on-going transformation processes in the labile primary transformed fiber population rather than fiber changes per se.