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FUNCTIONAL ALTERATIONS OF THE RAT SOLEUS MUSCLE DURING EARLY RELOADING AFTER HINDLIMB SUSPENSION. K. Lee, H. Lee and I. Choi. Department of Life Science, College of Liberal Arts and Science, Yonsei University, Republic of Korea.

   To assess alteration in muscle functional properties during early reloading, we examined contractility and fatigability of the rat soleus muscle during 1 h, 5 h, 1 d, and 2 wk of reloading after 3 wk of hindlimb suspension. Compared to age-matched controls, reloading for 1 h – 1 d resulted in a further decrease in tetanic tension (P_o, ~17%) and an increase in shortening velocity (V, up to 27%) from those after 3 wk unloading. Both P_o and V returned to control levels after 2 weeks of reloading. During a series of 2-sec electric stimulation and 2-sec rest periods, muscle tension in unloading-reloading (UR) groups showed a relative decrease similar to the controls. However, the magnitude of depression in tension over the 2-sec exertion after 60 sec of stimulation was greater in the UR groups, indicating lower fatigue resistance than the controls. Muscle lactate concentration peaked during the 3-wk unloading, remained high at 1 h reloading, and returned to the control level after 5 h reloading. Electron microscopic analyses on the soleus sections show significant misalignment of myofibrils during the unloading and early reloading period. Thus, the muscle properties are affected significantly within the first hour after reloading, and the low fatigue resistance may indicate poor performance of limb muscles in the case of emergency in space or return to the earth.

(Supported by KOSEF R05-2003-000-10589-0 to I. Choi.)