Influence of reduced neuromuscular activity on the development of neonatal rat hindlimb muscle

Normal ankle flexion and extension was inhibited in neonatal rats by applying an adhesive cast on the day of birth such that the tibialis anterior (TA) muscle of one hindlimb was immobilized in a shortened position. Casts were changed daily for 3 weeks, at which time animals were sacrificed. Immobilized TA muscles were significantly lighter in weight, and whole muscle and teased fiber fascicular lengths were significantly shorter. No effects were seen on fiber diameter profiles, total muscle fiber number or muscle cross-sectional area. Immobilized TA muscles had significantly lower phosphofructokinase activities, and lower slow-twitch fibers than contralateral muscles. The mitochondrial enzyme succinic dehydrogenase was unaffected by the reduced activity. It is apparent that, in a developing fast-twitch muscle such as the TA, myofibrillar ATPase histochemical patterns and glycolytic enzyme activity (phosphofructokinase) may be regulated by factors dependent upon neuromuscular activity. Mitochondrial enzymes, such as succinic dehydrogenase, seem to be regulated during development by factors which are not as sensitive to activity.