The Alterations Of Vascular Oxidative Stress In Simulated Microgravity Rats And The Countermeasure Effect Of Intermittent Artificial Gravity

Exposure to microgravity or simulated microgravity may result in variousstructural and functional changes in astronauts’cardiovascular system. Postflightstructural and functional imbalance of cardiovascular system may decrease theastronauts’ working ability or even influence aerospace safety, among whichpostflight orthostatic intolerance and decreasing aerobic capacity are the mostobvious. As shown in various studies, multiple mechanisms are involved inpost-flight orthostatic intolerance, including increased leg compliance,attenuated carotid baroreceptor-heart rate （HR） responsiveness, and etc., amongwhich hypovolemia induced by microgravity is the most important factor.Previous studies have revealed that hydrostatic pressure gradient disappearedunder the condition of microgravity, which induced the changes of transmuralpressure of the arterial system. Compared with1G gravity, pressure of cerebraland fore body blood vessels exhibited the state of “hypertension”, while that ofafter body showed the state of “hypotension”, inducing the differential structuraland functional adaptive changes of the vessels. Recent studies have shown thatthe structure of the fore body arterial vessels exhibited hypertrophic changeswith increasing vasoconstrictive response, while the after body arterial vessels showed atrophic changes with decreasing vasoconstrictive response. Multiplemechanisms are involved in structual remodeling and functional adjustment ofarterial system. Previous studies have demonstrated that the functional changesof ion channels in vascular smooth muscle may induce the functional adaptivechange of arterial blood vessels in hindlimb unweighted （SUS） rats. In addition,the change of NO synthase expression may also lead to structual remodeling andfunctional adjustment of arterial blood vessels. Moreover, recent studies haveshown that local renin-angiotensin system （L-RAS） expressed differentiallyamong different arterial blood vessels in SUS rats. The main composition ofRAS were overexpressed in the fore body of SUS rats but low expressed in theafter body. When investigating the expression of RAS components in thecerebral artery in SUS rats, we found that simulated microgravity could increasethe mRNA and protein expression of angiotensin （Ang）, angiotensin II type1receptor （AT1R） and angiotensin converting enzyme （ACE）. In addition, we alsofound that significant correlations existed between the change of RAS andvasodilatation. Oxidative stress generally existed in organisms, the imbalance ofwhich would aggravate the injury and remodeling of blood vessels with theproduction of ROS induced by the changes of Ang II and blood flow, especiallyin diseases such as hypertension and atherosclerosis. Besides, current studieshave indicated that O2-· had significant impact on vascular function and theexpression of NOS. Our previous study has demonstrated that oxidative stressinduced injury existed in cerebral and common carotid arteries in SUS rats andmay be involved in the structual remodeling and functional adjustment of bloodvessels through the regulation of bioactivity of NO. In space travel, astronautsgenerally adopt exercise as countermeasures against weightlessness. But thiskind of countermeasure could not prevent the adverse effect brought by longterm spaceflight. More countermeasures are now widely investigated, such asinterrupted artificial gravity. Previous work in our lab have shown that1hourstanding test every day could ameliorate the increased vasoconstrictive response, hypertrophic changes in fore body and decreased vasoconstrictive response,atrophic changes in after body in rats induced by SUS. Does oxidative stressplay an important roles in structual remodeling and functional adjustment ofarterial system in SUS rats? How does oxidative stress influence the structuraland functional remodeling of arterial blood vessels? Could1hour standing testevery day ameliorate the impact on vascular from weightlessness and influencethe level of oxidative stress?To investigate the impact of oxidative stress on structual remodeling andfunctional adjustment of arterial blood vessels under the condition ofweightlessness, we have implemented a series of experiments as follows:（1） Function of middle cerebral artery, common carotid artery and abdominalartery in21d SUS rats was measured by using isolated arterial tissue bath.Apocynin and SOD were adopted to see if they could ameliorate thevasodilatation of the three arteries. And1-hour standing test wasimplemented to determine if interrupted artificial gravity couldcountermeasure against the adverse effect induced by simulatedmicrogravity.（2） Oxidation-sensitive dye DHE was used to detect the level of O₂^-· in middlecerebral artery, common carotid artery and abdominal artery from21-daySUS rats, STD rats and CON rats. Malondialdehyde （MDA） content wasdetected by a detecting reagent kit to determine the extent of oxidative stressinjury in arterial blood vessels.（3） Western blot was used to detect the expression of NOX4and p22^phoxinmiddle cerebral artery, common carotid artery and abdominal artery21-daySUS rats, STD rats and CON rats. Mechanisms involved in the change ofoxidative stress level in SUS rats were further discussed and the impact of1hour standing test was observed.Main findings of this study are as follows:（1） Vasodilatation of arterial blood vessels in rats changed under the state of simulated microgravity.The invitro isometric tension recording of arteries have shown thatacetylcholine-induced vasodilation was decreased in middle cerebral arteries,common carotid arteries and abdominal arteries of21d SUS rats. Incubationwith apocynin and SOD could ameliorate the impaired concentration-dependentvasodilatation response to acetylcholine in the three arteries significantly.1hourstanding test every day could ameliorate vasodilatation of arterial blood vessels.These results have indicated that oxidative stress may be involved in the changeof vasodilatation of arterial blood vessels in rats under the state of simulatedmicrogravity.（2） The level of superoxide anions was increased and oxidative stress wasenhanced in arterial blood vessels in rats under the state of simulatedmicrogravity.The results of DHE staining have demonstrated that compared with CONgroup, the level of O2·-increased significantly in middle cerebral arteries,common carotid arteries and abdominal arteries in21d SUS group, while1hourstanding test every day could increase the oxidative stress in all3vessels, morethan that in SUS group. As the results of MDA showed, compared with CONgroup, the content of MDA increased significantly in middle cerebral arteries ofboth SUS and STD group. While in common carotid and abdominal arteries,compared with CON group, the content of MDA significantly increased in SUSgroup and that of STD group decreased nearly back to normal nevel. As theresults of western blot showed, compared with CON group, the expression ofp22^phoxsignificantly increased in middle cerebral, common carotid andabdominal arteries increased significantly in SUS group, while in STD group theexpression of p22^phoxwas significantly higher than that in SUS group. Theseresults have indicated that NADPH oxidase may be involved in the regulation ofoxidative stress injury of arterial blood vessels under the state of simulatedmicrogravity. These results suggest that the21d simulated microgravity can induceimpairment in endothelium-dependent relaxation of middle cerebral, commoncarotid arteries and abdominal aorta, which was associated with enhancedoxidative stress, higher expression of of NADPH oxidases. Daily STD for1hour could ameliorate the endothelium-dependent relaxation in middle cerebral,common carotid arteries and abdominal aorta of SUS rats, but the level ofoxidative stress in STD rats was still higher than that in CON group and theexpression of NADPH oxidase changed relatively.