In-vivo and ex-vivo studies on region-specific remodeling of large elastic arteries due to simulated weightlessness and its prevention by gravity-based countermeasure / 生理学报

The present study was designed to test the hypothesis that a medium-term simulated microgravity can induce region-specific remodeling in large elastic arteries with their innermost smooth muscle (SM) layers being most profoundly affected. The second purpose was to examine whether these changes can be prevented by a simulated intermittent artificial gravity (IAG). The third purpose was to elucidate whether vascular local renin-angiotensin system (L-RAS) plays an important role in the regional vascular remodeling and its prevention by the gravity-based countermeasure. This study consisted of two interconnected series of in-vivo and ex-vivo experiments. In the in-vivo experiments, the tail-suspended, hindlimb unloaded rat model was used to simulate microgravity-induced cardiovascular deconditioning for 28 days (SUS group); and during the simulation period, another group was subjected to daily 1-hour dorso-ventral (-G(x)) gravitation provided by restoring to normal standing posture (S + D group). The activity of vascular L-RAS was evaluated by examining the gene and protein expression of angiotensinogen (Ao) and angiotensin II receptor type 1 (AT1R) in the arterial wall tissue. The results showed that SUS induced an increase in the media thickness of the common carotid artery due to hypertrophy of the four SM layers and a decrease in the total cross-sectional area of the nine SM layers of the abdominal aorta without significant change in its media thickness. And for both arteries, the most prominent changes were in the innermost SM layers. Immunohistochemistry and in situ hybridization revealed that SUS induced an up- and down-regulation of Ao and AT1R expression in the vessel wall of common carotid artery and abdominal aorta, respectively, which was further confirmed by Western blot analysis and real time PCR analysis. Daily 1-hour restoring to normal standing posture over 28 days fully prevented these remodeling and L-RAS changes in the large elastic arteries that might occur due to SUS alone. In the ex-vivo experiments, to elucidate the important role of transmural pressure in vascular regional remodeling and differential regulation of L-RAS activity, we established an organ culture system in which rat common carotid artery, held at in-vivo length, can be perfused and pressurized at varied flow and pressure for 7 days. In arteries perfused at a flow rate of 7.9 mL/min and pressurized at 150 mmHg, but not at 0 or 80 mmHg, for 3 days led to an augmentation of c-fibronectin (c-FN) expression, which was also more markedly expressed in the innermost SM layers, and an increase in Ang II production detected in the perfusion fluid. However, the enhanced c-FN expression and increased Ang II production that might occur due to a sustained high perfusion pressure alone were fully prevented by daily restoration to 0 or 80 mmHg for a short duration. These findings from in-vivo and ex-vivo experiments have provided evidence supporting our hypothesis that redistribution of transmural pressures might be the primary factor that initiates region-specific remodeling of arteries during microgravity and the mechanism of IAG is associated with an intermittent restoration of the transmural pressures to their normal distribution. And they also provide support to the hypothesis that L-RAS plays an important role in vascular adaptation to microgravity and its prevention by the IAG countermeasure.

Mechanical properties and composition of mesenteric small arteries of simulated microgravity rats with and without daily -G(x) gravitation / 生理学报

The aim of the present study was to evaluate the active and passive mechanical properties and wall collagen and elastin contents of mesenteric small arteries (MSAs) isolated from rats of 28-day simulated microgravity (SUS), countermeasure [S + D: SUS plus 1 h/d -G(x) to simulate intermittent artificial gravity (IAG)] and control (CON) groups. Three mechanical parameters were calculated: the overall stiffness (β), circumferential stress (σ(θ))-strain (ε(θ)) relationship and pressure-dependent incremental elastic modulus (E(inc,p)). Vessel wall collagen and elastin percentage were quantified by electron microscopy. The results demonstrate that the active mechanical behavior of MSAs differs noticeably among the three groups: the active stress-strain curve of SUS vessels is very close to the passive curve, whereas the active σ(θ)-ε(θ) curves of CON and S + D vessels are shifted leftward and display a parabolic shape, indicating that for MSAs isolated from S + D, but not those from SUS rats, the pressure-induced myogenic constriction can effectively stiffen the vessel wall as the CON vessels. The passive mechanical behavior of MSAs does not show significant differences among the three groups. However, the percentage of collagen is decreased in the wall of SUS and S + D compared with CON vessels in the following order: SUS < S + D < CON. Thus, the relationship between passive mechanical behavior and compositional changes may be complex and yet depends on factors other than the quantity of collagen and elastin. These findings have provided biomechanical data for the understanding of the mechanism of postflight orthostatic intolerance and its gravity-based countermeasure.

Short-term simulated weightlessness enhances response of L-type calcium channel to angiotensin II in cerebral vascular smooth muscle cells in rats / 生理学报

Some studies suggest that the calcium channels and rennin-angiotensin system (RAS) play pivotal roles in the region-specific vascular adaptation due to simulated weightlessness. This study was designed to clarify if angiotensin II (Ang II) was involved in the adaptational change of the L-type calcium channel (Ca(L)) in the cerebral arterial vascular smooth muscle cells (VSMCs) under simulated weightlessness. Tail suspension (SUS) for 3 d was used to simulate immediate early cardiovascular changes to weightlessness. Then VSMCs in cerebral basilar artery were enzymatically isolated using papain, and Ca(L) current (barium instead of calcium as current carrier) in VSMCs was measured by whole-cell patch-clamp techniques. The results showed that 3-day simulated weightlessness significantly increased current density of Ca(L). However, I-V relationships of normalized peak current densities and steady-state activation curves of Ca(L) were not affected by simulated weightlessness. Although Ang II significantly increased current densities of Ca(L) in both SUS and control rats, the increase of Ca(L) current density in SUS rats was much more than that in control rats. These results suggest that 3-day simulated weightlessness induces the adaptational change of Ca(L) in cerebral VSMCs including increased response to Ang II, indicating that Ang II may play an important role in the adaptational change of cerebral arteries under microgravity.

Differential effect of simulated microgravity on myogenic tone of middle cerebral and mesenteric small arteries in rats / 生理学报

The aim of the present study was to investigate the effect of a short-term (3-day) simulated microgravity with and without daily dorsoventral gravitation (-G(x)) for 1 h on myogenic tone and vasoconstrictor responsiveness of the middle cerebral artery and mesenteric third-order small artery in rats. The tail-suspension (SUS) model was used to simulate cardiovascular deconditioning due to microgravity. Daily restoring to normal standing (STD) posture for 1 h was adopted to provide -G(x) as the countermeasure. Segments of middle cerebral artery and mesenteric third-order small artery were isolated and cannulated. Vascular diameters in response to increased intraluminal pressure (from 20 mmHg to 120 mmHg, by 20 mmHg steps) of isolated arteries under no-flow conditions were recorded by a Pressure Myograph System in both physiologic salt solution (PSS) (active diameter, Da) and calcium-free PSS (passive diameter, Dp). The myogenic tone was calculated by (Dp-Da)/Dpx100%. Vasoconstrictor responsiveness of the isolated middle cerebral artery to serotonin and that of small mesenteric artery to phenylephrine were assessed in the PSS under an intraluminal pressure of 40 mmHg. The results showed that SUS induced an enhancement of the myogenic tone and vasoconstrictor responsiveness in the isolated middle cerebral artery but a depression of those in the small mesenteric artery. Daily STD for 1 h prevented the depression of myogenic tone and vasoconstrictor responsiveness in the small mesenteric artery, but did not prevent the functional enhancement in the middle cerebral artery. These data suggest that a short-term simulated microgravity may result in different alterations in the function of the cerebral artery and the resistance vessel in the hind-body. Moreover, only the decrease of function in these resistance vessels, not in the cerebral arteries, can be prevented by such a countermeasure of daily STD for 1 h.

Comparison of biomechanical behavior of cerebral and mesenteric small arteries of simulated microgravity rats / 生理学报

The aim of the present study was to further elucidate the mechanisms of vascular adaptation to microgravity and its gravity-based countermeasure by a biomechanical approach. Active (the dissected vessel segment was superfused with PPS) and passive (while it was superfused with Ca(2+)-free PPS) biomechanical properties of mesenteric third-order small arteries and middle cerebral arteries isolated from 3-day simulated microgravity (SUS), countermeasure (STD, daily 1 h of -G(x) gravitation), and control (CON) groups of rats were studied. The following mechanical parameters were calculated: the overall stiffness parameter of passive vessels (beta), circumferential stress (sigma(theta))-strain (epsilon(theta)) relationship, and pressure-dependent incremental elastic modulus (E(inc,p)) of both active and passive vessels, and vascular smooth muscle (VSM) activity-dependent incremental modulus (E(inc,a)). Results from the analysis of active biomechanical properties revealed the contribution of vascular smooth muscle (VSM) tone during the early adaptation to microgravity: (1) For mesenteric small arteries, active circumferential sigma(theta) -epsilon(theta) curve of SUS group was comparable with that of the passive vessels, indicating that the function of VSM to restore the normal stress distribution is compromised; however, this mal-adaptation was fully prevented by the countermeasure of daily 1 h of -G(x) gravitation; (2) For the middle cerebral arteries, active circumferential sigma(theta) -epsilon(theta) relation of SUS group was shifted to the left side of the passive curve and epsilon(theta) was kept at a nearly constant level with the corresponding sigma(theta) being at its normal range; furthermore, the enhanced myogenic tone responsiveness was not prevented by daily short-duration -G(x). Analysis of the passive biomechanical properties has suggested remodeling changes in matrix components of different types of vessels, which might be significant if the exposure duration was further prolonged. In brief, studies of vascular biomechanics are of particular importance in elucidating the mechanisms underlying vascular adaptation to microgravity and its gravity-based countermeasure.