We examined the time course of soleus muscle fiber type composition. Soma area and succinate dehydrogenase (SDH) activity of soleus motoneurons during three weeks of hindlimb suspension (HS) in rats. Adult female Wistar rats (n = 34, 252 ∼ 288 g BW) were divided into four groups : control (n = 8), hindlimb suspended for one week (HS 1 wk, n = 8), two weeks (HS 2 wk, n = 9), and three weeks (HS 3 wk, n = 9). Soleus muscle fiber composition was calculated from transverse sections stained for myosin ATPase (preincubation pH 10.3, 4.3). The fiber type composition did not change in the HS 1 wk, but in the HS 2 wk and HS 3 wk, the proportion of type I fibers decreased and that of type IIc and IIa fibers increased. Using a fluorescent neuronal tracer nuclear yellow, motoneurons innervating the soleus muscle were identified, and the soma area and SDH activity were measured. The soma area did not change for up to two weeks of HS, but decreased in the HS 3 wk. Compared with control, SDH activity of soleus motoneurons decreased in the HS 1 wk. However, in the HS 2 wk, the activity increased to the level of control. In the HS 3 wk, the activity tended to increase further. Generally, muscle fibers and their motoneurons have unitary characteristics. However, these results suggest that change in soleus muscle fiber composition are not accompanied by changes in soma area and SDH activity in soleus motoneurons during three weeks of HS. © 1995, The Japanese Society of Physical Fitness and Sports Medicine. All rights reserved.
