The skeletal muscle atrophy that occurs in response to microgravity is manifested as microscopic changes in muscle morphology and as changes in muscle function. The objective of this study was to define the morphologic and biochemical changes.
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Muscle samples were taken from 8 astronauts (3 from a 5-day mission and 5 from an 11-day mission) 21 days before flight and on landing day. The samples were taken by biopsy of a thigh muscle (the vastus lateralis). After the site was shaved, cleansed and anesthetized, a cut was made with a scalpel and muscle tissue was drawn by suction into a biopsy needle. The samples were quickly frozen in liquid nitrogen and stored at -70<°>C for later microscopic examination and histochemical studies. Muscle fibers were classified by staining tissue sections for myofibrillar adenosine triphosphatase (ATPase), and the activities of ATPase, succinate dehydrogenase (SDH), and alpha-glycerophosphate dehydrogenase (GPD) were determined by histochemical methods.
The mean cross-sectional area of muscle fibers was less after flight than before (15% less for slow-twitch fibers, 22% less for fast-twitch fibers). The loss varied from astronaut to astronaut, probably at least partly due to differences in exercise or in the use of lower body negative pressure. The number of capillaries per fiber also decreased, keeping the number of capillaries per unit of cross-sectional area the same. The number of fast-twitch fibers increased relative to the number of slow-twitch fibers. These changes were especially remarkable because they occurred after flights of only 5 days. Myofibrillar ATPase activity of type II but not type I fibers had increased after flight. SDH activity was not affected by spaceflight. GPD activity in type I fibers was 80% higher after flight than before flight. The results suggested that during spaceflight, rapid adaptive changes occurred in the size, vascularity, and metabolic properties of skeletal muscle.
Day MK, Allen DL, Mohajerani L, Greenisen MC, Roy RR, Edgerton VR. Adaptations of human skeletal muscle fibers to spaceflight. Journal of Gravitational Physiology.
Edgerton VR, Zhou M-Y, Ohira Y, Klitgaard H, Jiang B. Bell G, Harris B, Saltin B, Gollnick PD, Roy RR, Day MK, Greenisen M. Human fiber size and enzymatic properties after 5 and 11 days of spaceflight. Journal of Applied Physiology.
Moore AD Jr, Charles JB, Lee SMC, Siconolfi SF, Greenisen MC. Does bed rest produce changes in orthostatic function comparable to those induced by space flight? Acta Astronautica.
Zhou MY, Klitgaard H, Saltin B, Roy RR. Edgerton VR. Gollnick PD. Myosin heavy chain isoforms of human muscle after short-term spaceflight. Journal of Applied Physiology.
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Electrophoresis, polyacrylamide gel
Glucosephosphate dehydrogenase (GLDH)
Muscle fibers, fast-twitch
Muscle fibers, slow-twitch
Myosin heavy chains (MHC)
Succinate dehydrogenase (SDH)