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Greater sensitivity of the vestibulosympathetic reflex in the upright posture in humans

¹Penn State Heart and Vascular Institute, ²Department of Cellular and Molecular Physiology, and ³Department of Pathology, General Clinical Research Center, Pennsylvania State University College of Medicine, The Milton S. Hershey Medical Center, Hershey, Pennsylvania

Submitted 29 February 2008 ; accepted in final form 29 April 2008

Otolith organs have been shown to activate the sympathetic nervous system in the prone position by head-down rotation (HDR) in humans. To date, otolithic stimulation by HDR has not been comprehensively studied in the upright posture. The purpose of the present study was to determine whether otolithic stimulation increases muscle sympathetic nerve activity (MSNA) in the upright posture. It was hypothesized that stimulation of the otolith organs would increase MSNA in the upright posture, despite increased baseline sympathetic activation due to unloading of the baroreceptors. MSNA, arterial blood pressure, heart rate, and degree of head rotation were measured during HDR in 18 volunteers (23 ± 1 yr) in different postures. Study 1 (n = 11) examined HDR in the prone and sitting positions and study 2 (n = 7) examined HDR in the prone and 60° head-up tilt positions. Baseline MSNA was 8 ± 4, 15 ± 4, and 33 ± 2 bursts/min for prone, sitting, and head-up tilt, respectively. HDR significantly increased MSNA in the prone (4 ± 1 and 105 ± 37% for burst frequency and total activity, respectively), sitting (5 ± 1 and 43 ± 12%), and head-up tilt (7 ± 1 and 110 ± 41%; P < 0.05). Sensitivity of the vestibulosympathetic reflex (%MSNA/HDR; degree of head rotation) was significantly greater in the sitting and head-up tilt than prone position (prone = 74 ± 22; sitting = 109 ± 30; head-up tilt = 276 ± 103; P < 0.05). These data indicate that stimulation of the otolith organs can mediate increases in MSNA in the upright posture and suggest a greater sensitivity of the vestibulosympathetic reflex in the upright posture in humans.

vestibular; otolith organs; orthostasis; muscle sympathetic nerve activity