Falling back to Earth: Altered splanchnic vasoreactivity may contribute to microgravity-induced orthostatic intolerance

摘要

The adaptations of the human cardiovascular system to microgravity, which are well tolerated while in space, can lead to dizziness, blurred vision, and fainting upon reentry. Specifically, orthostatic hypotension, a fall in blood pressure upon standing, poses a major problem for astronauts returning to Earth, and presents a hazard should an emergency ever arise during landing. Female astronauts are affected more frequently than males, suggesting underlying gender-related differences in cardiovascular regulation. One such difference has already been demonstrated: in response to an orthostatic challenge, males experience blood pooling primarily in the legs, while females show much more blood pooling in the abdominal region. Normally, the splanchnic blood vessels should reflexively constrict, initiating mobilization of the blood to supply the brain. In order to determine whether attenuation of this reflex is responsible for post-flight orthostatic intolerance (particularly in females), we examined in vitro vascular reactivity in male and female rats subjected to simulated microgravity (hind-limb unloading; HLU). The constrictor response of third-order mesenteric blood vessels to phenylephrine was measured by wire myograph. The sensitivity of female arteries to phenylephrine was attenuated following HLU. Maximal tensile response to phenylephrine was unchanged in these vessels, while maximal tensile response to high KCI increased. In male arteries, HLU did not significantly change any of these parameters. These results indicate that, after HLU, the mesenteric arteries of females (but not males) can be expected to show reduced sensitivity to noradrenergic sympathetic outflow. This may partially underlie the gender disparity in post-flight orthostatic hypotension.