Impaired reflex splanchnic venous constriction may contribute to post‐flight orthostatic intolerance in female astronauts

Abstract

Astronauts, particularly females, experience post-flight orthostatic intolerance. There is evidence that blood pooling in the splanchnic vasculature, rather than the legs, may be important in females. We proposed that simulated microgravity (hindlimb unloading HLU) would impair reflex mesenteric venoconstriction and mobilization of splanchnic blood. Using intravital microscopy, we investigated the effect of hemorrhage-induced sympathetic activation on mesenteric microvascular tone. Virgin female rats were subjected to HLU at 30o head-down tilt. After 14 days, the animals were anesthetized, positioned on the stage of an inverted microscope, and the mesenteric vascular arcade was viewed using a CCD. After stabilization, they were subjected to a controlled hemorrhage (0.75mL blood/100g body wt.). Real-time alterations in vessel diameter before, during and after hemorrhage were measured by computerized image analysis. As anticipated, hemorrhage induced mesenteric venous constriction in the control animals (no HLU). By contrast, the HLU animals exhibited a significant increase in mesenteric venous diameter. There were no significant changes in mesenteric arterial tone. We propose that impaired reflex mesenteric venous constriction and inadequate mobilization of splanchnic blood may contribute to post-spaceflight orthostatic hypotension, particularly in females. Canadian Space Agency