Response to denervation of rabbit soleus and gastrocnemius muscles. Time-course study of postnatal changes in myosin isoforms, fiber types, and contractile properties.

In contrast to general belief, the response of rabbit muscles to denervation is maturation to slow-like type muscles [7]. We report now an investigation by biochemical, morphological, and mechanical studies of the time course effects of muscle denervation on the slow-type soleus and fast-type gastrocnemius to help elucidate the mechanism of maturation of rabbit denervated muscles to slow-like muscles. In both muscles, denervation induced selective progressive atrophy of most fast fibers and hypertrophy of many slow fibers which displayed wide Z-lines; this was accompanied by the appearance of hybrid LC1F- and LC1E-associated slow myosins. The percentage of slow myosins increased with age similarly in the contralateral and denervated soleus. On the other hand, the percentage of slow myosins remained low in the contralateral gastrocnemius, whereas it increased to 95% in the denervated gastrocnemius; in the denervated gastrocnemius, the percentage of slow myosins reached 50% at about 35 days postnatal. At this age, the maximal shortening velocity of the denervated gastrocnemius and its twitch contraction time were already those of a slow-type muscle. This suggests that in addition to myosin, other proteins contributed to the mechanical properties of the denervated gastrocnemius. Transformation of rabbit denervated muscles to slow-like type muscles, which are associated with a lower energy requirement and higher muscle endurance than fast-type muscles, may constitute an adequate model for human neuromuscular pathology.