Effects of short-term and prolonged bed rest on the vestibulosympathetic reflex

Dyckman DJ, Sauder CL, Ray CA. Effects of short-term and prolonged bed rest on the vestibulosympathetic reflex. Am J Physiol Heart Circ Physiol 302: H368-H374, 2012. First published October 21, 2011; doi: 10.1152/ajpheart.00193.2011.-The mechanism(s) for post-bed rest (BR) orthostatic intolerance is equivocal. The vestibulosympathetic reflex contributes to postural blood pressure regulation. It was hypothesized that muscle sympathetic nerve responses to otolith stimulation would be attenuated by prolonged head-down BR. Arterial blood pressure, heart rate, muscle sympathetic nerve activity (MSNA), and peripheral vascular conductance were measured during head-down rotation (HDR; otolith organ stimulation) in the prone posture before and after short-duration (24 h; n = 22) and prolonged (36 +/- 1 day; n = 8) BR. Head-up tilt at 80 was performed to assess orthostatic tolerance. After short-duration BR, MSNA responses to HDR were preserved (Delta 5 +/- 1 bursts/min, Delta 53 +/- 13% burst frequency, Delta 65 +/- 13% total activity; P similar to 0.001). After prolonged BR, MSNA responses to HDR were attenuated similar to 50%. MSNA increased by Delta 8 +/- 2 vs. Delta 3 +/- 2 bursts/min and Delta 83 +/- 12 vs. Delta 34 +/- 22% total activity during HDR before and after prolonged BR, respectively. Moreover, these results were observed in three subjects tested again after 75 +/- 1 days of BR. This reduction in MSNA responses to otolith organ stimulation at 5 wk occurred with reductions in head-up tilt duration. These results indicate that prolonged BR (similar to 5 wk) unlike short-term BR (24 h) attenuates the vestibulosympathetic reflex and possibly contributes to orthostatic intolerance following BR in humans. These results suggest a novel mechanism in the development of orthostatic intolerance in humans.