Inhibited SERCA Activity Retards Decline of Repeated Short Tetani Force in Atrophic Soleus

Jiao Bo; Ma Xiaowu; Li Hui; Yu Zhibin

Abstract：Objective To confirm that the relaxation duration can be used as a functional marker of sarcoplasmic reticulum Ca2+ -ATPase (SERCA) activity and to elucidate the role of SERCA activity on the rate of force decline during repeated short tetani in the atrophic soleus induced with electric-stimulated intermittent.Methods Tail-suspended rats were used to simulate weightlessness on the ground.The isolated muscle strips were perfused to measure the changes in contractile function.Results Four-week unloaded soleus showed a significant shortening in twitch and tetanic time-from-peak-force-to-75％-relaxation (TR75) and a faster decline of force in 27 ℃ perfusion condition compared with the control group.The shortened TR75 was related to a high SERCA activity in the unloaded soleus.On the contrary,cyclopiazonic acid,an inhibitor of SERCA,prolonged TR75 and reduced the rate of force decline to the control level in the unloaded soleus.Lower perfusion temperature (22 ℃ ) decreased SERCA activity and the rate of force decline,but higher temperature (35 ℃ )did the opposing effects in the control soleus.The rate of force decline was not affected by low concentration of caffeine in the unloaded soleus.However,the elevated extracellular Ca2+ concentration and A23187 reduced the rate of force decline in the unloaded soleus.Conclusion Reduced TR75 is related to high SERCA activity in atrophic soleus.TR75 may express the functional marker of SERCA activity.Slightly inhibited SERCA activity may maintain a higher Ca2 + concentration of intracellular muscular fibers to reduce the rate of force decline in the titanic contraction.