APPEARANCE OF COMPLEX BRANCHED MUSCLE-FIBERS IS ASSOCIATED WITH A SHIFT TO SLOW MUSCLE CHARACTERISTICS

To examine the effects of numerous complex branched fibers (CBF) on wh ole muscle contractile properties, we established a model of myopathic muscles containing a large number of CBF by repeated local injection of bupivacaine hydrochloride (Marcaine) into the plantaris (PLA) muscl e. Marcaine injections were administered once weekly for 10 weeks into the right PLA. muscles of Wistar male rats, The in situ contractile p roperties of Marcaine-injected PLA muscles (I-PLA) were examined under urethane anesthesia, and compared with the contralateral (control) PL A muscle (C-PLA), Numerical and morphological examination using the mo dified nitric acid fiber digestion method and scanning electron micros copy showed that Marcaine resulted in an 8-fold increase in the number of branched fibers in the I-PLA muscles and about 70% of these fibers were CBE The latter were composed of ten or more muscle fibers fused together along with many thin and thick, long and short daughter branc hes. the time to peak tension of twitch and tetanus, and 1/2 relaxatio n time were significantly longer in I-PLA muscles, representing a shif t to slow muscle characteristics. However, the total area of slow fibe rs/muscle cross-sectional area was similar in I-PLA and C-PLA muscles. Aggregates of same-type fibers (slow fibers) with small and large dia meters were observed, reflecting an expected cross-sectional property of CBE Our results suggest that the appearance of several CBF in a mus cle is associated with a shift towards slower contractile properties i n the affected muscle.