Morphological changes during fiber type transitions in low-frequency-stimulated rat fast-twitch muscle.

This study investigates morphological adaptations of rat extensor digitorum longus muscle to chronic low-frequency stimulation (10 Hz, 10 h/d, up to 61 +/- 7d). During the early stimulation period (2-4 d), increased basophilia and accumulation of RNA were seen predominantly in type-IIB fibers. Putative satellite cell activation, as indicated by 3H-thymidine incorporation, was also evident during this phase. By 12 d, fiber composition remained unaltered, but there was a decrease in the cross-sectional area of the type-IIB fibers. Following 28 d of low-frequency stimulation, the percentage of type-IIB fibers decreased from 43 +/- 3% to 0%, while type-IID fibers increased from 30 +/- 3% to 60 +/- 6%. The fraction of type-IIA fibers tended to increase (controls 19 +/- 3%; stimulated 29 +/- 4%), whereas that of the type-I fibers was unaltered (4 +/- 1%). At this time, the cross-sectional area of type-IID fibers was unaltered, but that of type-IIA and type-I fibers increased. Further stimulation resulted in a return of type-IID fibers to control levels (23 +/- 5%), and a marked increase in type-IIA fibers (45 +/- 8%). The percentage of type-I fibers increased from 4 +/- 1% to 8 +/- 1%. Throughout each stage of chronic stimulation, there was no histological evidence of fiber degeneration and regeneration. These results indicate that, in contrast to the rabbit, chronic low-frequency stimulation-induced fiber conversion in the rat extensor digitorum longus muscle is entirely due to fiber transformation.