The effects of 10 days of spaceflight on the shuttle Endeavour on predominantly fast-twitch muscles in the rat

The primary purpose of this investigation was to determine the effects of m icrogravity on muscle fibers of the predominantly fast-twitch muscles in th e rat. Cross sectional area and myosin heavy chain (MHC) composition were a ssessed in order to establish the acute effects of microgravity associated with spaceflight. The extensor digitorum longus (EDL) and gastrocnemius mus cles were removed from 12 male Fisher 344 rats which had undergone 10 days of spaceflight aboard the space shuttle Endeavour and from 12 age- and weig ht-matched control animals. Both groups of animals received similar amounts of food and water and were synchronized for photoperiods, environmental te mperature, and humidity. Significant (P<0.05) reductions in muscle fiber si ze were observed in the gastrocnemius (fiber types I, IIA, IIDB, and IIB) a nd EDL (fiber type LIE) muscles after spaceflight. Significant MHC isoform transformations also resulted during this brief period of microgravity expo sure with a significant decrease in MHC IId isoform in the EDL muscle. A si gnificant decrease was also observed in the MHC lid isoform in the superfic ial (white) component of the gastrocnemius muscle after spaceflight, althou gh no alterations in MHC profile were demonstrated in the deep (red) compon ent of this muscle. These findings highlight the rapid plasticity of skelet al muscle during short-term spaceflight, If such pronounced adaptations to spaceflight also occur in humans, then astronauts are likely to suffer seve re decrements in skeletal muscle performance with long-term space flight an d upon return to earth after both short- and long-term missions. Thus, coun termeasures aimed at slowing or even preventing muscle fiber atrophy are wa rranted.