EFFECT OF HINDLIMB UNWEIGHTING ON SINGLE SOLEUS FIBER MAXIMAL SHORTENING VELOCITY AND ATPASE ACTIVITY

This study characterizes the time course of change in single soleus muscle fiber size and function elicited by hindlimb unweighting (HU) and analyzes the extent to which varying durations of HU altered maximal velocity of shortening (V(o)), myofibrillar adenosinetriphosphatase (ATPase), and relative content of slow and fast myosin in individual soleus fibers. After 1, 2, or 3 wk of HU, soleus muscle bundles were prepared and stored in skinning solution at -20-degrees-C. Single fibers were isolated and mounted between a motor arm and a transducer, an fiber force, V(o), and ATPase activity were measured. Fiber myosin content was determined by one-dimensional sodium dodecyl sulfate- (SDS) polyacrylamide gel electrophoresis. After 1, 2, and 3 wk of HU, soleus fibers exhibited a progressive reduction in fiber diameter (16, 22, and 42%, respectively) and peak force (42, 48, and 72%, respectively). Peak specific tension was significantly reduced after 1 wk of HU (18%) and showed no further change in 2-3 wk of HU. During 1 and 3 wk of HU, fiber V(o) and ATPase showed a significant increase. By 3 wk, V(o) had increased from 1.32 +/- 0.04 to 2.94 +/- 0.17 fiber lengths/s and fiber ATPase from 291 +/- 16 to 1,064 +/- 128 muM . min-1 . mm-3. The percent fibers expressing fast myosin heavy chain increased from 4% to 29% by 3 wk of HU, and V(o) and ATPase activity within a fiber were highly correlated. However, a large population of fibers after 1, 2, and 3 wk of HU showed increases in V(o) and ATPase but displayed the same myosin protein profile on SDS gels as control fibers. The mechanism eliciting increased fiber V(o) and ATPase activity was not obvious but may have been due to increases in fast myosin that went undetected on SDS gels and/or other factors unrelated to the myosin filament.