Slow-to-fast fibre transitions and fibre atrophy are typical ultrastructure adaptations of especially slow muscles with short-term (i.e. 4--22 days) exposure to microgravity. However, the effects of spaceflight on respiratory muscles have not been established. The purpose this study was to quantitatively identify potential fibre transitions and muscle atrophy in the diaphragm---the most important respiratory muscle. Five adult rats orbited earth aboard NASA's 14 day Spacelab Life Sciences 11(SLS2) mission and were compared to a flight control group of five ground-based rats. Fibre type profile and fibre cross sectional areas (CSA) of the right crural region were immunohistochemically determined. No significant changes to myosin heavy chain (MHC) based fibre type distribution were detected, but there was a non-significant decrease in CSA of all pure fibres. These findings suggest that short duration spaceflight may not dramatically effect neuromuscular patterns of activity in rat diaphragm.
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