The present study was designed to determine whether conversion of fiber types (type I to II) induced by hindlimb unloading (HU) is identical along the length of fibers in the soleus (SOL) muscle. After 1 and 2 wk HU, SOL was divided into three (proximal, middle and distal) regions and the fiber type composition was determined using myosin adenosinetriphosphatase (ATPase) . There was no significant change in fiber type distribution after 1 wk of HU. The percentage of type I fibers significantly decreased in the proximal and middle regions and that of type IIc fibers increased in all regions after 2 wk of HU. The percentage of type IIc fibers greatly increased in the proximal and middle regions compared with the distal region. Serial cross-sections every 300μm were processed for myosin ATPase from the middle to the distal region, and then analyzed to show the differences in staining characteristics along the length of single fibers. After 2 wk of HU, some of the fibers (3.1±1.2%) displayed changes in the staining characteristics of myosin ATPase (pH 10.3) ; 84% of these fibers identified type IIc fibers in the middle region were observed as type I fibers in the distal region. These results suggest that muscle fibers in unloaded SOL may not transform synchronously or uniformly along their length

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, 252-288g 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 ha 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.

Using several electrophoretic techniques, this study examined the effects of 3 weeks hindlimb suspension on the patterns of isomyosins, myosin heavy chain (HC) isoforms and myosin light chain (LC) isoforms in the soleus muscle of the rat. The suspended soleus showed a shift in the HC isoform distribution with a marked increase in fast HC isoforms and a commensurate decrease in HCI. In addition, the change in the fast HC isoforms consisted of the expression of HCIId and HC IIb absent in the normal soleus. In contrast to HC isoforms, suspension did not lead to appreciable changes in LC isoform distribution. Analyses of electrophoresis under nondenaturing conditions demonstrated that the normal soleus expressing HCI and HCIIa isoforms contained two isomyooins. Although, of the two isomyosins observed in the normal soleus, the faster migrating band most likely represented the HCIIa-based one (FMas), its mobility was not identical with that of the HCIIa-based isomyosin (FMaf) found in fast-twitch muscles, migrating in the order FMaf>FMas. FMas was designated as intermediate isomyosin (IM) . Some of the suspended soleus contained slow isomyosin (SM) and IM whereas the others comprised FM 3 and/or FM 2 as well as SM and IM. In spite of the expression of HCIIb and HCIId in the suspended soleus, FM 3 and FM 2 observed in these muscles exhibited distinct mobilities from either HCIId-based or HCIIb-based isomyosins comprised in fast-twitch muscles. These results suggest that some of newly expressed HCIId and/or HCIIb isoforms in the suspended soleus are associated with not only fast but also slow LC isoforms and function as a constitutive element of the myosin molecule.