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Are the Responses of Adductor Longus Muscle to Gravitational Unloading in Wistar Hannover Rats Region-specific?  T. Ohira¹, X.D. Wang², M. Terada¹, F. Kawano², Y. Higo¹, N. Nakai² , T. Ochiai³, J. Gyotoku⁴, N. Nishimoto¹ and Y. Ohira^{1, 2}.  Grad. Sch. of ¹Frontier Biosci. and ²Med., Osaka Univ., Osaka, ³Mitsubishi Heavy Industries, Kobe, and ⁴Tsurui Chem. Co., Hyogo, Japan.

   Pronounced atrophy and shift toward fast-twitch type were noted in adductor longus (AL) muscle of Wistar Hannover (WH) rats with unknown mechanism (J. Gravit. Physiol. 13: 79-80, 2006).  Thus, responses of electromyogram (EMG) and histochemical properties of caudal and rostral regions of AL to gravitational unloading by using parabolic flight using a jet airplane (Mitsubishi MU-300, Diamond Air Service, Nagoya, Japan) and/or hindlimb suspension were studied in 5-wk-old male WH rats.  Chronic hindlimb suspension with or without 16-day recovery was also performed for 16 days.  The expression of myosin heavy chain (MHC) and specific activities of succinate dehydrogenase and a-glycerophosphate dehydrogenase, as well as fiber cross-sectional area, were analyzed in serial cross-sections.  The numbers of myonuclei and satellite cells and length of sarcomeres in single muscle fibers, sampled from tendon-to-tendon, were also determined.  Significant shift of fibers toward fast-twitch type and atrophy were noted in the suspended groups.  The % of pure type I MHC fibers was decreased to approximately half of the pre-suspension level and type I+II fibers was increased.  Re-loading for 16 days was not enough for the complete recovery.  The responses of enzyme activities to unloading were not prominent.  These phenomena were closely related to decreased EMG activity and mechanical stress, caused by shortened sarcomere length, during unloading.  Unloading-related fiber atrophy was also associated with pronounced decrease of myonuclear numbers.  The patterns of responses were similar between two regions of muscle.

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