Effect of support stimulation on unloaded soleus in rat.

We studied the efficacy of plantar support in the prevention of atrophy in a disused soleus muscle during hindlimb suspension (HS). The 14-day investigation involved three groups of hindlimb-suspended male Wistar rats and a group of control rats (C). In all HS groups, the left hindlimbs (L) of the animals were left free. As for the right hindlimbs (R), they were either provided support by an adjustable platform (Sup), or immobilized at the ankle joint in a neutral position (Im), or both supported by the platform and immobilized (Sup+Im). Mass, cross-sectional area (CSA) and slow twitch (ST) fiber percentage (ST%) in the R soleus muscle were similar in the Sup and control groups. In the Sup+Im group, these parameters were significantly lower than in the Sup R and C groups. However, the CSA of ST fibers in the Sup+Im R soleus was significantly higher than in those hindlimbs that were left hanging free. Succinate dehydrogenase activity in ST fibers, and alpha-glycerophosphate dehydrogenase activity in fast-twitch fibers had decreased in the Sup R as compared with the controls. The maximal rate of ADP-stimulated mitochondrial respiration was increased in the free-hanging Sup L hindlimb in comparison with the control. In conclusion, during HS: (1) hindlimb support prevents slow-to-fast fiber transformation and losses in muscle mass and fiber CSA, but brings about a decrease in metabolic enzyme activity, and (2) hindlimb plantar support attenuates but does not fully prevent ST fiber atrophy in the immobilized soleus.