| ABSTRACT Staying in microgravity, man and animals will have adverse changes in a variety of physiological systems. Among them, cardiovascular deconditioning and skeletal muscle atrophy, which would seriously impair the effectiveness of extravehicular activities and the safety during the returning to the Earth, are worth being paid special attention to. Many countermeasures have been adopted to prevent astronauts from these changes in the past several decades. Among them, mostly have been exercise-based countermeasures. The counteracting effects of them are very limited. In the 21st century, mankind will realize the long-term manned spaceflight to Mars. With the operation of the international space station (ISS), the question of how to counteract these adverse effects of long spaceflight is in a great need. Many investigators agree with that the gravity-based multi-system countermeasure would be a solution. There are two types of artificialgravity (AG), i.e., the continuous AG (CAG) provided by full-time spinning of the entire space system/habitat and the intermittent AG (IAG) by incorporating a short-arm centrifuge into the spacecraft. However, whether it is practical, the following questions must be solved firstly. l)Is IAG helpful in alleviating or preventing the adverse effects on heart and vessels, skeletal muscles, bones, and etc induced by weightlessness? 2) "How much, how long and how often" of IAG is in need to provide a full prevention of adverse effects of weightlessness? 3) What is the most effective and optimized prescription of IAG in counteracting the adverse effects of weightlessness? And whether some combined countermeasures are needed?Based on these questions, the first part of the present work was to investigate and compare the counteracting effects of daily +45 head-up tilt (HUT) and standing (STD) of various duration, and daily 1 h centrifugation (CEN) by -1.5G\ or -2.6Gx with each other in preventing muscle atrophy induced by simulated weightlessness. The second part was to compare STDi of 1, 2, or 4 h/d in ameliorating the myocardial contractility depression induced by simulated weightlessness.The main results of the present work are as follows:1. General Features of the IAG simulation models: During the simulation period, the body weight (BW) of SUS rats increased steadily, and showed no significant differences as compared with that of the CON. But in groups of different countermeasures, the BWs were all affected, especially in SUS+HUT (4 h/d) and CEN (2.6G) groups. After four weeks of simulated weightlessness, the wet weights of testes among different groups were significantly less than CON. There were nosignificant differences in the wet weights of adrenals among different groups.2. Effectiveness of simulated IAG in counteracting of the muscle atrophy induced by simulated microgravity2.1 Changes without intermittent gravitation: Wet weight and relative weight (wet weight/body weight) of muscles except TA decreased significantly after 4 weeks of hindlimb suspension. Muscles as SOL and MG, which play an antigravity, postural role in 1 G environment atrophied more than those type II fiber predominant muscles as PLA and EDL. The CSA of type I fibers in different muscles of SUS group decreased significantly than those of CON. The CSA of type II fibers except in EDL was obviously less than that of CON.2.2 Changes in muscle weight with intermittent gravitation: Head-up tilt, standing and centrifugation of different durations per day could ameliorate the atrophy of SOL greatly. The longer the exposure time per day was, the more it could prevent. In terms of changes in relative weight of muscles, head-up tilt, standing and centrifugation all showed counteracting effect in the prevention of the atrophy of muscles, except TA. The results have suggested that daily STD or HUT for four hours can prevent the atrophy of muscles thoroughly.2.3 Changes in muscle fibers with intermittent gravitation: Daily HUT or STD for 2 or 4 hours prevented the CSA changes of ty...
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