ADAPTATION OF FIBERS IN FAST-TWITCH MUSCLES OF RATS TO SPACEFLIGHT AND HINDLIMB SUSPENSION

The adaptation of single fibers in medial gastrocnemius (MG), a fast-twitch extensor, and tibialis anterior (TA), a fast-twitch flexor, was studied after 14 days of spaceflight (COSMOS 2044) or hindlimb suspension. Cross-sectional area (CSA) and succinate dehydrogenase (SDH), alpha-glycerophosphate dehydrogenase (GPD), and myofibrillar adenosinetriphosphatase (ATPase) activities were determined in fibers identified in frozen serial cross sections. Fibers were categorized as light, dark, or intermediate on the basis of myosin ATPase staining and alkaline preincubation and immunohistochemically as reacting with slow, fast, or both slow and fast myosin heavy chain monoclonal antibodies. Because there was a close relationship between these two means of categorizing fibers, all fibers were categorized on the basis of the immunohistochemical reaction. The percentage of slow- and fast-twitch fibers of the MG and TA were unchanged in either group. Mean fiber size of all fibers, irrespective of type, was unaffected in either muscle after flight or suspension. The fibers that expressed both fast and slow myosin heavy chains were smaller than control in the MG of both experimental groups. Compared with control, the SDH and total SDH activities in the MG were significantly less in suspended rats, with the fast-twitch fibers showing the largest difference. The ATPase activity in the MG was higher in flight than in control or suspended rats. There were no significant effects of flight on fibers of the TA. In contrast, the TA in suspended rats had higher GPD activities than either control or flight rats. No change in the GPD-to-SDH ratio was observed after flight in either muscle, whereas in fast-twitch fibers this ratio was elevated in the suspended compared with control rats. In general, the adaptations were more marked in the MG than in the TA and in the suspended than in the flight rats. Also, changes in enzyme activities and cross-sectional area were not in parallel, suggesting independent regulation of the metabolic and size properties of fibers in fast-twitch unloaded muscles. Finally, in comparison to a previous 12.5-day flight (COSMOS 1887), the adaptations in the muscle fibers of the MG were more moderate.