Effects of chronic electrostimulation on rat soleus skinned fibers during hindlimb suspension.

In order to counteract the changes of the contractile properties of the rat soleus occurring during 10 days of hypokinesia-hypodynamia, due to hindlimb suspension (HS), two different patterns of electrostimulation were applied to the tibial nerve. The contractile properties of single chemically skinned muscle fibers were investigated using the tension-pCa relationship characteristics, the similar or different calcium and strontium affinities, and by measuring the P/tmax kinetic parameters. Our results showed that a pattern similar to firing rates of motoneurons innervating slow twitch muscles inhibited the slow to fast fiber changes observed during HS, whereas a pattern similar to firing rates of motoneurons from fast twitch muscles seemed to favor these changes. Since neither pattern maintained the isometric contractile force developed by the soleus fibers, we concluded that the decrease in mechanical strains imposed on the muscle during unloading was the main factor for the development of atrophy, while the kinetic changes might be predominantly modulated by the nervous command.