Differentiation of fast and slow muscles in the rat after neonatal denervation: a physiological study.

Whether an intact innervation is essential for postnatal muscle differentiation was examined in the rat by recording physiological contraction parameters. Muscles in one leg were denervated neonatally (within 24 h of birth) and, between 3-28 days after the operation, their contractions were compared with those of the contralateral control muscles. Experiments were performed on the extensor digitorum longus (edl, a fast muscle) and the soleus (a slow muscle) muscles and contractions were recorded in vitro, at 35 degrees C and with direct stimulation. When compared with the control muscles, 3-4-day-old neonatally denervated fast and slow muscles had longer twitch contractions, higher twitch/tetanus ratios and certain other specific differences in their contraction parameters. These denervation-induced changes in neonatal muscles were essentially similar to those produced 3-7 days after denervation in the differentiated (4-week-old) fast muscle. Despite differences in their absolute values, the contraction parameters of neonatally denervated and control edl muscles changed similarly during development, indicating that postnatal differentiation of fast muscle fibres is independent of a neuronal influence. In the case of the neonatally denervated soleus muscle, the developmental changes in contraction parameters, i.e. shortening of the twitch duration, increase of rate of rise and rate of relaxation in the tetanus and increase of the maximum shortening velocity, were more pronounced than in the control slow muscle; also, there were similarities with the pattern of fast muscle differentiation. Thus, muscle fibre differentiation in soleus becomes altered towards that of a fast muscle after neonatal denervation.