FORCE-VELOCITY AND POWER CHARACTERISTICS OF RAT SOLEUS MUSCLE-FIBERS AFTER HINDLIMB SUSPENSION

The effects of 1, 2, and 3 wk of hindlimb suspension (HS) on force-vel ocity and power characteristics of single rat soleus fibers were deter mined. After 1, 2, or 3 wk of HS, small fiber bundles were isolated, p laced in skinning solution, and stored at -20 degrees C until studied. Single fibers were isolated and placed between a motor arm and force transducer, functional properties were studied, and fiber protein cont ent was subsequently analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Additional fibers were isolated from soleus of control and after 1 and 3 wk of HS, and fiber type distribut ion and myosin light chain stoichiometry were determined from SDS-PAGE analysis. After 1 wk of HS, percent type I fibers declined from 82 to 74%, whereas hybrid fibers increased from 10 to 18%. Percent fast typ e II fibers increased from 8% in control and 1 wk of HS to 26% by 3 wk of HS. Most fibers showed an increased unloaded maximal shortening ve locity (V-o), but myosin heavy chain remained entirely slow type I. Th e mechanism for increased V-o is unknown. There was a progressive decr ease in fiber diameter (14, 30, and 38%) and peak force (38, 56, and 6 3%) after 1, 2, and 3 wk of HS, respectively. One week of HS resulted in a shift of the force-velocity curve, and between 2 and 3 wk of HS t he curve shifted further such that V-o was higher than control at all relative loads <45% peak isometric force. Peak absolute power output o f soleus fibers progressively decreased through 2 wk of HS but showed no further change at 3 wk. The results suggest that between 2 and 3 wk the I-IS-induced alterations in the force-velocity relationship act t o maintain the power output of single soleus fibers despite a continue d reduction in fiber force.