Compared effects of hindlimb unloading versus terrestrial deafferentation on muscular properties of the rat soleus.

Hindlimb unloading is known to induce some neuromuscular changes especially in postural muscles such as the soleus. Our goal was to determine the role of proprioceptive inputs on these modifications by comparing soleus muscle properties of rats being either hindlimb unloaded or terrestrial deafferented. Under deep anesthesia, a first group of rats were submitted to a bilateral deafferentation (DEAF group, n = 6) performed by section of the dorsal roots L(3) to L(5) after laminectomy. A second group of rats was submitted to a hindlimb-unloading period (HU group, n = 6). After 14 days, the morphological and contractile properties as well as the content in myosin heavy-chain (MHC) isoforms were studied in the right soleus muscle in HU and DEAF groups. The results were compared to those obtained in control animals (CON group). After HU versus CON group, the soleus muscle was atrophied and presented a decrease in muscle forces in relation with a slow-to-fast transition characterized by a decrease in kinetic contractile parameters and by an overexpression in the fast MHC isoforms. The DEAF soleus muscle showed both a significant muscle atrophy and a loss of forces when compared with CON rats. The comparison between DEAF and HU rats indicated that some modifications occurring after HU are purely of motor origin (i.e., slow to fast transition), whereas muscle atrophy and decrease in muscle force are partly the result of an afferent silent. Our study underlined the importance of afferent input integrity in the maintenance of muscle characteristics.