INFLUENCE OF 2 WEEKS OF NON-WEIGHT BEARING ON RAT SOLEUS MOTONEURONS AND MUSCLE-FIBERS

Background: Chronic unloading of the hindlimbs of rats results in atro phy and a shift in the metabolic and mechanical properties of the sole us muscle, a predominantly slow postural muscle, towards those observe d in faster muscles. Hypothesis: We hypothesized that the spinal moton eurons innervating the soleus muscle would also atrophy and show adapt ations in their metabolic properties following a period of unloading. Methods: For 2 weeks, 10-week-old mate Wistar rats were hindlimb suspe nded and the mean soma size and succinate dehydrogenase activity of mo toneurons innervating the soleus muscle were examined. Retrograde labe ling with nuclear yellow injected in the soleus muscle was used to ide ntify the soleus motor pool. The mean fiber size and fiber type distri bution from a central region of each soleus muscle was determined. Res ults: The soma size distribution, mean soma cross-sectional area and m ean succinate dehydrogenase activity of motoneurons in the soleus moto r pool were similar in control and hindlimb suspended rats. In contras t, compared to control the mean fiber size of each fiber type and the percentages of Type I fibers decreased and of Type IIa fibers increase d in the soleus muscle following hindlimb suspension. Conclusions: The results indicate that significant alterations in the phenotype and si ze of muscle fibers can occur without concomitant adaptations in the i nnervating motoneurons usually associated with motor unit types (i.e., soma size and oxidative capacity).