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Responses of Back Muscle Fibers in Sprague-Dawley Rats to Spaceflight.  M. Terada¹, F. Kawano², H. Kume², T. Ohira¹, Y. Higo¹, Y. Nakajima³, X.D. Wang², N. Nakai², W.E. Hinds³ and Y. Ohira^{1, 2}.  Grad. Sch. ¹Frontier Biosci. & ²Med., and ³Sch. Sci, Osaka Univ., Toyonaka City, Osaka, Japan, and ³Ames Res. Ctr., Moffet Field, CA, U.S.A.

   Effects of 14 days of spaceflight (Spacelab Life Sciences-2) on the properties of back muscle (internal oblique) fibers were studied in male adult Sprague-Dawley rats.  They were randomly separated into 5 groups; preflight vivarium ground controls killed ~2 h after launch [launch day (L+0)], flight (F) groups killed either ~5 h after return (R+0) or 9 days after touchdown/return (R+9), and synchronous ground controls (SC) for R+0 and R+9.  The experimental rats in groups R+0 and R+9 were exposed to microgravity for 14 days on board of the space shuttle Columbia (Space Transport System-58).  Ground-based rats were maintained under conditions similar to those of the F rats during the mission and served as synchronous controls.  The muscle was sampled from rat killed by decapitation.  Single muscle fiber segments were isolated in low calcium relaxing solution.  Myosin heavy chain (MHC) expression, fiber cross-sectional area (CSA), and myonuclear number were analyzed.  The percent distribution of fibers expressing type I, I+II, and II MHC in group L+0 was 3.8, 3.3, and 92.9 %, respectively.  The mean number of myonuclei in the combined fibers was 84.2/mm fiber length in this group.  The fiber phenotype and myonuclear number did not change in response growth and spaceflight.  On the other hand, flight-related growth inhibition was noted in fiber CSA.  Even though the mean CSA in F-R+0 was 32% greater than in L+0, it was 18% less than in SC-R+0 (p<0.05).  The mean CSA in group F-R+9 was identical to that in the age-matched control.  It was suggested that the growth of back muscle fibers was inhibited during 14 days of spaceflight but this inhibition was reversible.   

(Supported by National Space Development Agency of Japan and the Grant-in-Aid for Scientific Research (S, 19100009) from Japan Society for the Promotion of Science)