The transformation of myosin in cross-innervated rat muscles.

1. The characteristics of isometric twitch and tetanic contractions have been determined for normal (N-EDL, N-SOL), self-innervated (S-EDL, S-SOL) and cross-innervated (X-EDL, X-SOL) extensor digitorum longus (EDL) and soleus (SOL) muscles of the rat at 35 degrees C. The muscles were then used for biochemical analyses of properties of myosin and actomyosin.2. The ATPase activities of myosin and actomyosin of X-EDL decreased to the level of those of N-SOL or S-SOL, and the ATPase activities of X-SOL approached those of N-EDL or S-EDL. Of the various ATPase activities, the actin- and Mg(2+)-activated ATPase activity of myosin and the Mg(2+)-activated ATPase activity of actomyosin showed the highest degree of correlation with the intrinsic speed of shortening of the muscles.3. Myosin of normal, self-innervated, and cross-innervated muscles combined with F-actin superprecipitated at rates which were proportional to the speed of muscle contraction.4. The pH profile curve and the ATP-induced dinitrophenylation reaction revealed that the structure of myosin of X-EDL was altered to that of N-SOL or S-SOL, and the structure of myosin of X-SOL was modified to that of N-EDL or S-EDL.5. No differences were found in the yield of myosin of normal, self-innervated, and cross-innervated extensor digitorum longus and soleus muscles.