[Decrease in tetanic tension in 4-week tail-suspended rat soleus and analysis of its underlying mechanisms].

Objective. To observe the dynamic changes in tetanic tension in 4-week tail-suspended rat soleus (SOL) and extensor digitorum longus muscle (EDL) and to elucidate its underlying mechanisms. Method. After 4 weeks of tail suspension SOL and EDL of the rats were isolated and perfused. Their isometric twitch and tetanic tensions were recorded. Result. Atrophy occurred in the SOL, but not in the EDL. The maximum twitch tension (Pt) decreased significantly and was more dependent on stimulation voltage in SOL of tail-suspended rat. Time to maximum twitch tension (TPT) and time from maximum twitch tension to half relaxation (TR50) were reduced in the SOL. Pt also decreased in the EDL, but TPT and TR50 did not change in the EDL. The maximum tetanic tension reduced in SOL and EDL of tail-suspended rats. The decline rate of tetanic tension in SOL was faster than the control. The profile of tetanic contraction curve in atrophic SOL approached that in EDL. Conclusion. The above results suggested that the underlying mechanisms of reduced maximum twitch and tetanic tension in 4-week tail-suspended rat SOL may involve the reduction in number of cross-bridge per cross-section area or force produced per cross-bridge. The rapid decline rate in atrophic SOL may be related to the shift in myosin heavy chain and troponin I isoforms or to decrease in excitability.