| For the near half century of manned space exploration, the effects of weightlessness on human body has been an important subject for gravitational physiology research. Weightlessness or simulated weightlessness has been shown to cause atrophy, reduced functional capacity, and increased fatigue in rat hind limb skeletal muscles. The atrophy of hind limb muscle can reduce the "second pump" effect, it may be a reason for postflight orthostatic intolerance. For prolonged space flight, the atrophy and decreased function in antigravity skeletal muscles is a major problem that needs to be solved. To understand the features of decreased contractile function and increased fatigability in unloaded hindlimb skeletal muscles will help to develop effective counter-measures during space flight, contribute to the prevention and treatment of skeletal muscle atrophy in long-term bed-rest patients on the ground, and provide useful information for sports medicine.Contractility of isolated muscle was examined and basic molecular biology techniques were used for measuring the time course of changes incontractile function and fatigability of soleus (SOL) and extensor digitorum longus (EDL) muscles of tail-suspended rats. The results are summarized as follows:1) Progressive atrophy in unloaded soleus muscle: The SOL/BW was 29.11 ±0.02 % in one week tail-suspended (SUS) rats, significantly lower than the control (CON) (37.73±0.02%, P0.05). The SOL/BW in two and four weeks SUS rats was further reduced (16.37±0.01 and 15.63±0.01%, respectively, P<0.01).2) Decreased maximal isometric twitch tension and shortened twitch duration in unloaded soleus muscle: The isometric developed tension (DT) began to decrease in soleus after one week of unloading (CON: 1.76±0.18 g/mm2 vs. SUS: 1.46±0.26 g/mm2, P>0.05) and significantly decreased to 1.25±0.09 g/mm2 (P<0.05) and 0.90±0.06 g/mm2 (P<0.01) after two and four weeks of unloading, respectively. The isometric DT had no change in EDL after one or two weeks of unloading, but significantly decrease after four weeks of unloading (CON 1.20±0.09 g/mm2 vs. SUS 0.77±0.15 g/mm2, P<0.01).The time to 50% peak tension (TP50) during twitch contraction showed no significant change in soleus after one, two and four weeks 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 (TR75) also shortened from 78.33±9.46 to 56.67±4.21, 48.00±6.63 and 46.67±6.15 ms in soleus after one, two and four weeks of SUS, respectively (P<0.05-0.01). The TP50, TPT and TR75 of EDL were not changed in SUS as compared with CON.3) Reduced maximal contractile tension and increased fatigability in unloaded soleus muscle during high-frequency tetanic contraction: In high-frequency tetanic contraction, the maximal tension (Po) of SOL reduced by 32.8%, 60.1% and 77.6 % after one week (P<0.05), two and four weeks of (P<0.01) SUS, respectively. The Po of EDL had no change after one and two weeks of SUS, but significantly reduced by 55.1 % after four weeks of SUS.The tension at the thirty-third second of tetanic contraction (P33) dropped by 40.9% as compared to Po in soleus after one week of unloading in contrast to 18.0-26.0% drop in the control (P<0.05). The P33/Po further decreased by 51.1% and 73.1% in SOL after two and four weeks of SUS, respectively. The fatigability in EDL did not change through out the experiment (P>0.05).4) Increase in optimal simulating frequency for intermittent tetanic contraction of unloaded SOL and EDL muscles: The optimal simulating frequency of SOL increased from 60 Hz in control to 100 Hz after four weeks of unloading. The optimal simulating frequency in EDL increased from 100 Hz to 140 Hz after four weeks of SUS.5) Decreased soleus Po of intermittent tetanic contraction after four weeks of unloading: The Po of intermittent tetanic contraction was... |
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