| Microgravity experienced by astronauts during space flight could induce a series ofadaptive changes in the cardiovascular system. When they return to the1G gravityenvironment on the ground or encounter with other orthostatic stress, a series ofcardiovascular dysfunction responses are aroused, among which the postflight orthostaticintolerance and reduced upright aerobic exercise capacity are the most important. Atpresent, scientists have not fully understood the mechanism of postflight cardiovasculardysfunction (PFCD), thus, it’s still one of the most important issues in the field ofgravitational physiology to explore how PFCD is generated. When exposed to microgravity, the body fluid in the astronauts shifts towards the head, resulting in relativeincreased fluid volume in the upper body whereas decreased fluid volume in the lowerbody. Subsequently, the normal hydrostatic pressure gradient formed in the1G gravityenvironment on the ground disappears and the hemodynamic condition changes.Eventually, a series of structural and functional remodeling of the arterial system isinduced. Results from numerous bed rest tests and postflight human studies have revealedthat, inadequate vasoconstrictive responses of the arterial system may be one of the majorexplanations for the occurrence of PFCD. Therefore, the present study mainly focused onthe mechanism for the vasoconstrictive changes of the arterial system induced byreal/simulated microgravity, in order to supply more evidence for the clarification of themechanism of PFCD and the proposal of new effective countermeasures.Previous vascular biological studies have demonstrated the important role for Rhokinase (Rho-associated coiled-coil forming kinase, ROCK) in the mediation of vascularsmooth muscle cells (VSMCs) contraction. ROCK serves as the first effector of RhoA andaccepts the regulation of the other members of the Rho family. Once activated by RhoA,ROCK could phosphorylate the myosin light chain phosphatase (MLCP) and inhibit itsdephosphorylative activity towards the myosin light chain (MLC), thus reinforce theVSMCs contraction. In addition, as reported by some other studies, ROCK could alsophosphorylate MLC directly and induce contraction. At present, extensive studies haveconfirmed the involvement of ROCK in the mediation of the abnormalities of arterialvasoconstriction in many cardiovascular diseases, such as hypertension and atherosclerosis.However, whether ROCK is involved in the abnormal arterial vasoconstriction induced byreal/simulated microgravity has not been intensively studied and needs furtherinvestigation.Studies from domestic and overseas labs, including our own lab, have confirmed theregional specific functional remodeling of the arterial system induced by simulatedmicrogravity in rats. Yet, related investigations have been mainly focused on the resistancearteries in the brain and hind body, studies on the large elastic arteries and medium-sizedmuscular arteries are relativly less involved and some of the results are still controversial. Therefore, in the present study,4week hindlimb unweighting rats were adopted as theanimal model to simulate the cardiovascular effects induced by microgravity, and the largeelastic arteries (common carotid artery, thoracic and abdominal aorta) and medium-sizedmuscular arteries (femoral artery) were taken as the objects of study. Isometric forcemeasurement was performed to observe the changes of both receptor-mediated andnon-receptor-mediated arterial vasoconstriction induced by simulated microgravity and toexamine the effects of Y-27632, a specific ROCK inhibitor, on the contraction of thevessels. Meanwhile, Western blot, immunohistochemistry (IHC) and ROCK activity assaywere applied to detect the expression of ROCK II and the activity of ROCK, in order todiscuss the relationships between the alterations of ROCK expression or activity and thechanges of arterial vasoconstriction induced by simulated microgravity in rats.The main findings of the present study are as follows:(1) Regional specific changes of the arterial vasoreactivity induced by simulatedmicrogravityAs shown in the results of isometric force measurement, in the fore body vessels,compared with the CON rats, PE-induced and KCl-induced vasoconstriction were bothsignificantly increased in the thoracic aorta, and showed an increasing tendency withoutstatistical significance in the common carotid artery in HU rats. In the hind body vessels,compared with CON rats, both the PE-induced and KCl-induced contraction weresignificantly decreased in the abdominal aorta and femoral artery in HU rats.(2) Diminution of the differences in vasoreactivity between simulated microgravityand normal rats by Y-27632After the inhibition of ROCK by Y-27632, the PE-induced and KCl-induceddifferences in vasoreactivity between simulated microgravity and normal rats were bothsignificantly diminished. In the fore body vessels, Y-27632almost completely eliminatedthe differences in vasoreactivity between groups induced by both PE and KCl in thecommon carotid artery and thoracic aorta. In the hind body vessels, Y-27632significantlydiminished such differences in vasoreactivity in the abdominal aorta and femoral artery.Moreover, compared with pre-incubation with Y-27632, the concentration of the agonists needed for the differences in vasoreactivity between groups were higher after theapplication of Y-27632, which showed a “hysteretic pattern”.(3) Alterations of ROCK II expression and ROCK activity induced by simulatedmicrogravity in the arterial system of ratsThe results of Western blot and IHC showed that, compared with CON rats, theexpression of ROCK II was significantly decreased in the common carotid and femoralartery but significantly increased in the abdominal aorta, yet remained unchanged in thethoracic aorta in HU rats. As shown in the results of ROCK activity assay, compared withthe CON rats, HU significantly decreased the phosphorylation of Thr696of the myosinlight chain phosphatase target-1(MYPT-1) in the common carotid and femoral artery andabdominal aorta, but significantly increased such phosphorylation level in the thoracicaorta. As shown in the results of MLC Ser19phosphorylation detection, compared withCON rats, the phosphorylation level of Ser19remained unchanged in the common carotidartery, and increased significantly in the thoracic aorta, yet decreased significantly in theabdominal aorta and femoral artery.To sum up with the above results, simulated microgravity has induced inc reasedvasoconstriction in the thoracic aorta and an increasing trend in the common carotid artery,yet decreased vasoconstriction in the hind body vessels (abdominal aorta and femoralartery) in rats. Moreover, blockade of ROCK could diminish or even eliminate thevasoconstrictive discrepancies between simulated microgravity and normal rats, indicatingthe involvement of ROCK in the mediation of the abnormal vasoconstriction induced bysimulated microgravity in rats. Furthermore, simulated microgravity has induced differentalterations of the expression and activity of ROCK in different vessels, suggesting thatROCK may regulate the inadequate vasoconstrictive responses through the way ofadjusting its own expression or activity. |
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