Influence of two weeks of non-weight bearing on rat soleus motoneurons and muscle fibers.

BACKGROUND

Chronic unloading of the hindlimbs of rats results in atrophy and a shift in the metabolic and mechanical properties of the soleus muscle a predominantly slow postural muscle, towards those observed in faster muscles.

HYPOTHESIS

We hypothesized that the spinal motoneurons innervating the soleus muscle would also atrophy and show adaptations in their metabolic properties following a period of unloading.

METHODS

For 2 weeks, 10-week-old male Wistar rats were hindlimb suspended and the mean soma size and succinate dehydrogenase activity of motoneurons innervating the soleus muscle were examined. Retrograde labeling with nuclear yellow injected in the soleus muscle was used to identify the soleus motor pool. The mean fiber size and fiber type distribution from a central region of each soleus muscle was determined.

RESULTS

The soma size distribution, mean soma cross-sectional area and mean succinate dehydrogenase activity of motoneurons in the soleus motor pool were similar in control and hindlimb suspended rats. In contrast, compared to control the mean fiber size of each fiber type and the percentages of Type I fibers decreased and of Type IIa fibers increased in the soleus muscle following hindlimb suspension.

CONCLUSIONS

The results indicate that significant alterations in the phenotype and size of muscle fibers can occur without concomitant adaptations in the innervating motoneurons usually associated with motor unit types (i.e., soma size and oxidative capacity).