Abstract
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
