A comparison of rat myosin from fast and slow skeletal muscle and the effect of disuse.

Certain enzymatic and structural features of myosin, purified from rat skeletal muscles representative of the fast twitch glycolytic (type IIb), the fast twitch oxidative (type IIa), and the slow twitch oxidative (type I) fiber, were determined and the results were compared with the measured contractile properties. Good correlation was found between the shortening velocities and Caz+-activated ATPase activity for each fiber type. Fast twitch white (type IIb) and mixed fast twitch red (type IIa/IIb) muscles could not be distinguished physiologically and showed three identical isomyosins (FM1, FM2, and FM3) by nondenaturing electrophoresis. The relative abundance of fast twitch light chains (LC) in rat white muscle (type IIb) is comparable with other mammalian fast twitch muscles, except for the reduced amount of LCsf in the rat. The low level of LCsf in the rat is corroborated by correlation between light chain distribution and the ratio of fast myosin isomyosins. Fast twitch red type IIa and fast twitch white type IIb muscles have similar myosin ATPase activities and maximal shortening velocity, but differ in terms of isomyosin profile and in the percentage of light chains and light chain stoichiometry. Short term hind limb immobilization caused prolongation of contraction time and one-half relaxation time in the fast twitch muscles and a reduction of these contractile properties in slow twitch soleus. Furthermore, the increased maximum shortening velocity in the immobilized soleus could be correlated with increased Ca2+-ATPase, but no change was observed in the enzymatic activity of the fast twitch muscles. No alteration in light chain distribution with disuse was observed in any of the fiber types, The myosin from slow twitch soleus could be distinguished from fast twitch myosins on the basis of the pattern of peptides generated by proteolysis of the heavy chains. Six weeks of hind limb immobilization resulted in both an increased ATPase activity and an altered heavy chain primary structure in the slow twitch soleus muscle.