EFFECTS OF MUSCLE LENGTH ON THE RESPONSE TO UNLOADING

Muscle mass, distribution of fiber types, fiber cross-sectional area ( CSA) and selected enzyme activities were determined in rats hindlimb-s uspended free of immobilization (Susp-Free), suspended with the ankle dorsiflexed (Susp-DF soleus stretched) or plantarflexed (Susp-PF, sole us shortened) for 10 days and compared to cage-control (Con) rats. Red uction of muscle weight associated with suspension was prevented in Su sp-DF rats. The mean CSAs of slow fibers were Con=Susp-DF>Susp-PF>Susp -Free and of fast and intermediate fibers tended to be Susp-DF>Con>Sus p-PF=Susp-Free. Mean activities of succinate dehydrogenase (SDH), alph a-glycerophosphate dehydrogenase (GPD) and myofibrillar adenosine trip hosphatase (mATPase) in slow and fast fibers were similar in Con and S usp-Free rats. Mean SDH activity in slow fibers was higher in Susp-DF and Susp-PF than in Con and Susp-Free. No significant differences in S DH activities of fast fibers were observed among groups. GPD activity was higher in slow fibers of Susp-DF and Susp-PF compared to Con. The mATPase activity was higher in slow fibers of Susp-DF compared to Con and Susp-Free rats and lower in fast fibers of Susp-DF compared to Con rats. Thus, when compared to control, the patterns of adaptation were more similar in the Susp-DF and Susp-PF than in the Susp-Free. Althou gh these results are consistent with previous studies demonstrating th at the load placed on a muscle can affect protein metabolism, the dire ction and magnitude of the adaptive responses observed in the present study were closely associated with the chronically imposed changes in muscle length, i.e. fixed at either a shortened or a lengthened positi on.