Title: Microgravity Induced Changes in Aortic Stiffness and its Role in Orthostatic Intolerance

Microgravity (μG) induced orthostatic intolerance (OI) in astronauts is characterized by a marked decrease in cardiac output (CO) in response to an orthostatic stress. Since CO is highly dependent on venous return, alterations in the resistance to venous return (RVR) may be important in contributing to OI. The RVR is directly dependent on arterial compliance (Ca), where aortic compliance (Cao) contributes up to 60% of Ca. We tested the hypothesis that μG induced changes in Ca may represent a protective mechanism against OI. A retrospective analysis on hemodynamic data collected from astronauts after 5-18 day spaceflight missions revealed that orthostatically tolerant (OT) astronauts showed a significant decrease in Ca after spaceflight while OI astronauts showed a slight increase in Ca. A ground based animal model simulating μG, hindlimb unweighted (HLU) rats, was used to explore this phenomenon. Two independent assessments of Cao, in vivo pulse wave velocity (PWV) of the thoracic aorta and in vitro pressure-diameter squared relationship (PDSR) measurements of the excised thoracic aorta were determined. PWV showed a significant increase in aortic stiffness compared to control despite unchanged blood pressures. This increase in aortic stiffness was confirmed by the PDSR analysis. Thus both actual μG in humans and simulated μG in rats induces changes in Cao. The difference in Ca in OT and OI astronaut suggests that the μG induced decrease in Ca is a protective adaptation to spaceflight that reduces the RVR and allows for the maintenance of adequate CO in response to an orthostatic stress.