Effects of elimination of activity on contractile and histochemical properties of rat soleus muscle.

Morphological, contractile, and histochemical properties of rat soleus muscle were studied after 2 or 4 weeks of complete elimination of neuromuscular activity. Inactivity was induced by chronic perfusion of tetrodotoxin (TTX) to the sciatic nerve. Significant reductions in muscle mass and fiber size were found after 2 or 4 weeks of disuse. Correspondingly, the percentage of dark-staining alkaline myosin ATPase fibers was increased from about 20% to 40% after 4 weeks of treatment. The capacity of soleus to generate tension when stimulated through the nerve was significantly impaired at frequencies greater than 20 Hz. Nevertheless, when the curarized muscle was stimulated directly, tension developed at frequencies above 20 Hz relative to peak tension was similar to control values. Absolute tetanic tension was significantly reduced after 2 or 4 weeks of treatment. These reductions could be only partly explained by muscle atrophy, resulting in specific tensions or approximately 55% of control after 2 or 4 weeks of treatment. Measures of the time course of the isometric twitch were found not to be reliable indicators of the contractile speed in TTX-treated soleus. Significant increases in the rate of tetanic tension development, expressed relative to peak tension, and the velocity of unloaded shortening, were seen after 4 weeks of disuse. These results reveal the extent to which virtually complete neuromuscular inactivity leads to chronic deficits in neuromuscular transmission and changes in both the net amount and quality of contractile proteins of rat soleus muscle.