Dynamic changes in isometric twitch of hindlimb skeletal muscles from rats tail-suspended for different period

Objective To observe dynamic changes in isometric twitch of soleus and extensor digitorum longus(EDL) from rats tail-suspended for 1, 2, and 4 weeks. Methods The isometric twitch function of isolated soleus and EDL were examined under non-circulative perfusion condition. Results The soleus/body weight(BW) was 29.11±0.02 % in 1 week tail-suspended (SUS) rats, significantly lower than that of the control (CON) (37.73±0.02 %, P0.05). The soleus/BW in 2 and 4 weeks SUS rats was further reduced by 16.37±0.01 and 15.63±0.01 % respectively (P0.01). The isometric developed tension (DT) began to show trend of decrease in soleus after one week of unloading (CON: 1.76±0.18 g/mm2 vs SUS: 1.46±0.26 g/mm2, P0.05) and significantly decreased to 1.25±0.09 g/mm2 (P0.05) and 0.90±0.06 g/mm2 (P0.01) after two and four weeks of unloading respectively. The isometric DT had no change in EDL after 1 week of unloading, but significantly decreased after two and four weeks of unloading (P0.01). The time to 50 % peak tension (TP 50) during twitch contraction of soleus showed no significant change in all time periods of unloading, but the time to peak tension (TPT) was progressively shortened from 45.00±2.23 ms in CON to 30.00±2.58, 28.00±7.34 and 26.67±2.11 ms after one, two and four weeks of SUS respectively.The time from peak tension to 75 % relaxation (TR 75) of soleus was also shortened from 78.33±9.46 ms in CON to 56.67±4.21, 48.00±6.63 and 46.67±6.15 ms after one, two and four weeks of SUS respectively (P0.05～0.01). The TP 50, TPT and TR 75of EDL were not changed in SUS as compared with CON. Conclusions The results suggested that the change of contractile- relatedregulative proteins in soleus may occur earlier in the course of unloading, and the change of contractile proteins may apppear later.