The Vestibulosympathetic Reflex is Attenuated after Prolonged Head‐Down Bed Rest

Post spaceflight orthostatic intolerance is associated with an inability to increase peripheral vascular resistance (PVR). Simulated microgravity (6° head‐down bed rest, HDBR) studies have demonstrated inadequate increases in muscle sympathetic nerve activity (MSNA) in subjects that experience orthostatic intolerance during head‐up tilt. Activation of the vestibulosympathetic reflex (VSR) using head‐down rotation increases MSNA and PVR in humans. We tested the hypothesis that the VSR would be attenuated after approximately 35 days of HDBR. MSNA (microneurography) and leg blood flow (Doppler ultrasound of the popliteal artery) were measured before and after 36±1 days of HDBR in 7 subjects. Peripheral vascular conductance (PVC) was calculated by limb blood flow/mean arterial pressure. Before HDBR, HDR significantly increased MSNA (Δ80±25% bursts/min, Δ83±12% total activity; p<0.01) and tended to decrease PVC (Δ13±5%; p=0.09). After HDBR, the increase in MSNA elicited by HDR was attenuated compared to before HDBR. MSNA increased burst frequency and total activity only by Δ23±13% and Δ34±22% during HDR after HDBR, respectively. This result demonstrates a significant attenuation of the VSR after prolonged HDBR. Despite this decrease in MSNA, PVC did not change in response to the VSR after HDBR. These data provide support for the concept that attenuation of the VSR could contribute to an inadequate increase in MSNA observed post‐HDBR in those subjects that experience orthostatic intolerance. NIH DC006459 , HL077670 & NSBRI CA00404