COMPARISON OF SOLEUS MUSCLES FROM RATS EXPOSED TO MICROGRAVITY FOR 10VERSUS 14 DAYS

The effects of two different duration spaceflights on the extent of at rophy, fiber type composition, and myosin heavy chain (MHC) content of rat soleus muscles were compared. Adult male Fisher rats (n=12) were aboard flight STS-57 and exposed to 10 days of microgravity and adult ovariectomized female Spraque-Dawley rats (n=12) were aboard flight ST S-62 for 14 days. Soleus muscles were bilaterally removed from the fli ght and control animals and frozen for subsequent analyses. Muscle wet weights, fiber types (I, IC, IIC, and LIA), cross-sectional area, and MHC content were determined. Although a significant difference was fo und between the soleus wet weights of the two ground-based control gro ups, they were similar with regard to MHC content (ca 90% MHCI and ca 10% MHCIIa) and fiber type composition. Unloading of the muscles cause d slow-to-fast transformations which included a decrease in the percen tage of type I fibers and MHCI, an increase in fibers classified as ty pe IC, and the expression of two fast myosin heavy chains not found in the control rat soleus muscles (MHCIId and MHCIIb). Although the amou nt of atrophy (ca 26%) and the extent of slow-to-fast transformation ( decrease in the percentage of MHCI from 90% to 82.5%) in the soleus mu scles were similar between the two spaceflights, the percentages of th e fast MHCs differed. After 14 days of spaceflight, the percentage of MHCIIa was significantly lower and the percentages of MHCIId and MHCII b were significantly higher than the corresponding MHC content of the soleus muscles from the 10-day animals. Indeed, MHCIId became the pred ominant fast MHC after 14 days in space. These data suggest fast-to-fa ster transformations continued during the longer spaceflight.