THE EFFECT OF DENERVATION ON MYOSIN ISOFORM SYNTHESIS IN RABBIT SLOW-TYPE AND FAST-TYPE MUSCLES DURING TERMINAL DIFFERENTIATION - DENERVATION INDUCES DIFFERENTIATION INTO SLOW-TYPE MUSCLES

The soleus and gastrocnemius medialis of eight-day-old rabbits were de nervated and the effects were examined after fifty-two days by biochem ical, cytochemical and mechanical methods. The contralateral soleus ex hibited the properties of slow-type muscle, namely a predominance of s low-type myosin isoforms and slow-type oxidative fibers, slow twitch a nd low maximal velocity for shortening. The contralateral gastrocnemiu s exhibited the properties of fast-type muscle, namely a predominance of fast-type myosin isoforms and fast-type non-oxidative fibers, fast twitch and high maximal velocity of shortening. Denervation of muscles caused the differentiation of the two muscles towards slow-type muscl es. Both denervated soleus and gastrocnemius muscles exhibited a predo minance of slow-type myosins (either the normal type, made up of slow heavy and light chains, or the hybrid type, made up of slow heavy and regulatory light chains and fast essential light chains), a predominan ce of slow-type fibers, and slow mechanical properties. Thus, innervat ion in rabbit appears to be a determining factor for differentiation i nto fast-type muscle, but it is not necessary for differentiation into slow-type muscle. This conclusion contradicts the findings of previou s studies in rat and thus raises new questions concerning the role of nerves in controlling the expression of myosin isoforms and the differ entiation of muscle fibers.