Distinguishing unloading- versus reloading-induced changes in rat soleus muscle.

Previously, solei from rats orbited 12.5 days aboard Cosmos 1887 biosatellite were biopsied 48-56 hours postflight. These atrophic muscles showed severe pathology. Designing a ground-based model of that space flight, we tested the hypothesis that 48 hours of postflight muscle reloading induced pathological changes. Rats were subjected to 12.5 days of hindlimb suspension unloading and biopsied immediately after suspension or after returning to normal weightbearing 12, 24, or 48 hours. Soleus morphological changes were quantitated on histochemically and immunohistochemically stained cross-sections. Solei from 0-hour reloaded rats showed significantly decreased wet weights, diminished myofiber cross-sectional area, angular myofibers, myofibril disruption, and more myofibers expressing fast myosin. Compared with suspension alone (0-hour reloading), reloading 12-48 hours induced slightly increased soleus wet weights, myofiber swelling, significant interstitial tissue edema, macrophage activation, and monocyte infiltration. These results suggest the degree and type of muscle degenerative changes observed postflight depend on the duration of gravity readaptation before biopsy and not solely on exposure to microgravity.