Effects Of Intermittent Head-up Tilt On Compliance And Tectology Of Veins In Rabbits After Simulated Weightlessness

Human beings survive and evolve under 1G gravity. All the physiological functions have adapted well to this condition. When astronauts have been exposed to microgravity for a long time during spaceflight, obvious changes of physiological system can be observed. Adaptive changes in cardiovascular function occurr under microgravity during spaceflight. Adverse effects might occur to human health and safety when astronauts return to the normal gravity condition on the earth. Postflight orthostatic intolerance, characterized by tachycardia, hypotension and occasional faint in orthostatic position, is one of the most often observed problems and affects almost every astronaut. At present, the orthostatic intolerance can not be explained with any single factor. The reasons may include many factors, for example, the changes of venous compliance of lower limbs, the changes of cardiac function, the decrease of blood volume or body fluid and the changes of endocrine system. Changes of cardiovascular function and construction may be one of the leading mechanisms. Quite a few researchs have been done on the cardiac and arterial function, but the role of the vein, which can affect blood volume and strokevolume, still remains unclear.The orthostatic intolerance is the main symptom of cardiovascular dysfunction after exposing to weightlessness. It may threaten the safety of cosmonauts during their returning to the surface of the earth, reduce their ability of leaving the space capsule urgently and the ability of readaptating the environment of 1G after spaceflight. Therefore, it is crucial to find the effective ways to prevent cosmonauts from orthostatic intolerance. Some countermeasures have been used in spaceflight, for example, exercise, supplying enough nutrition, LBNP (lower body negative pressure) and so on. But there is not an efficient countermeasure which can entirely prevent the happening of orthostatic intolerance. The artificial weight, as a method of weight simulating, is gaining more and more attention. The most commonly used methods include intermittent head-up tilt, standing-up and centrifugal machine produced artificial gravity, et al.In our study, the head-down tilt (-20°) rabbits (offered by animal experimental center of the Fourth Military Medical University) were used for simulated weightlessness. The head-up tilt (+45°) model was used as countermeasure. To investigate the changes of compliance and structure of different veins under weightlessness simulation and countermeasure conditions and the mechanism, we studied the curves of compression-displacement relationship, the histomorphological changes and the expression of ET-1 in femoral, jugular and mesenteric veins in different groups.The main findings are as follows:1. Effects of intermittent head-up tilt on compliance of vein in rabbitsafter simulated weightlessness. We investigated the effects of 2 h/d 45° head-up tilt on compliance of femoral, jugular and mesenteric veins after 21 d -20° head-down tilt. The results showed that, at the same compression, the circumferential displacement of femoral vein and longitudinal displacement of mesenteric vein in the simulated weightlessness group increased more obviously than that in control group (P <0.05 or P<0.01), but no significant changes were observed in the longitudinal displacement of femoral vein between the simulated weightlessness group and control group. The circumferential and longitudinal displacement of jugular vein in simulated weightlessness group decreased more significantly than that in control group(P <0.05 or P <0.01). At the same stress, the circumferential displacement of femoral vein and longitudinal displacement of mesenteric vein in countermeasure group decreased more significantly than that in simulated weightlessness group(P <0.05 or P <0.01), while there were no obvious differences in the longitudinal displacement of femoral vein between countermeasure group and simulated weightlessness group. The circumferential and longitudinal displacement of jugular vein in countermeasure group increased more significantly than that in simulated weightlessness group(P <0.05 or P <0.01). There were no significant differences between countermeasure group and control group in femoral, jugular and mesenteric veins. It suggested that 21 d simulated weightlessness might lead to a significant decrease of jugular venous compliance while an increase of femoral and mesenteric venous compliance in rabbits. Training of 45° head-up tilt 2 h per day could partly counteract the effects of simulated weightless on the compliance of the femoral, jugular and mesenteric vein.2. Effects of intermittent head-up tilt on the histomorphological changes of femoral vein and jugular vein in rabbits under simulated weightlessness. We investigated the effects of 2 h/d 45° head-up tilt on the microstructure of femoral vein and jugular vein (studied by light microscope and transmission electron microscope) after 21d simulated weightlessness. The result shows that, some of the endothelial cells were brushed off and the smooth muscle cells were necrotic in femoral vein in simulated weightlessness group. The vasal wall became thinner. While the endothelial cells of jugular vein were hyperplastic and the internal elastic lamina became thickened. No significant changes were observed in femoral vein and jugular vein in the countermeasure group. Using transmission electron microscope, it is observed that the mitochondrias became swollen and chromatin gathered around the karyotheca in the smooth muscle cells of femoral vein in simulated weightlessness group. The quantity of endoplasmic reticulum in smooth muscle cells in jugular vein increased and the cells turned into excretory type. The ultrastructure of femoral vein and jugular vein in countermeasure group had no obvious differences with that in control group. It suggested that 21 d simulated weightlessness might induce a hyperplastic change in femoral vein and atrophic change in jugular vein of rabbit. Two-hour 45° head-up tilt every day might partly counteract the changes induced by simulated weightlessness.3. Effects of intermittent head-up tilt on the endothelin-1 expression of femoral vein and jugular vein in rabbits under simulated weightlessness. We investigated the effects of 2 h/d 45° head-up tilt on the ET expression of different vein after simulated weightlessness. The results...