ENZYMIC PROPERTIES OF MYOSIN IN FAST AND SLOW TWITCH MUSCLES OF CAT FOLLOWING CROSS-INNERVATION

1. Following cross‐innervation of the cat flexor digitorum longus muscle ( F.D.L., fast‐twitch) and soleus muscle (slow‐twitch) the isometric twitch speed of the F.D.L. muscle decreases while the contraction time of soleus increases. The tetanus—twitch ratio of soleus is also increased. 2. Myosin or myofibrils from normal or self‐reinnervated fast‐twitch muscles of the cat split adenosine triphosphate (ATP) more rapidly than myosin or myofibrils from slow twitch muscles. 3. Following cross‐innervation the myosin and myofibrillar ATPase activity of flexor digitorum longus muscle falls, to become nearly equal to that of normal or self‐reinnervated soleus muscle. The ATPase activity of cross‐innervated soleus muscle increases only slightly. © 1969 The Physiological Society