[63]

LACK OF VESTIBULAR OTOLITH PARTICIPATION IN HUMAN ORTHOSTATIC BLOOD PRESSURE CONTROL.   D.E. Watenpaugh, A. Cothron, S.L. Wasmund, W.L. Wasmund, R. Carter III, N.K. Muenter, and M.L. Smith.  Department of Integrative Physiology, University of North Texas Health Science Center, Fort Worth. 

Evidence exists that the vestibular system influences the cardiovascular system.  We hypothesized that supine-to-upright vestibular otolith organ stimulation contributes significantly to human cardiovascular responses to orthostasis.  Twelve blindfolded subjects underwent the following three 60° upright tilting procedures for three min each, in random order:  1) with the head kept in line with the body, such that the head and body moved from horizontal to 60° above horizontal (0 to 0.87 Gz otolith organ stimulation);  2) with the neck flexed passively from 0° to 30° relative to the body during 60° tilt, such that the head moved from horizontal to 90° above horizontal (0 to 1 Gz otolith organ stimulation);  and  3) with the neck flexed 30° during supine baseline conditions, and the neck then passively extended to -30° during 60° body tilting, such that the head remained at 30° above horizontal throughout body tilting (constant 0.5 Gz otolith organ stimulation).  This tilt condition provided the cardiovascular challenge of whole body tilting without any concurrent increase in otolith Gz stimulation.  All 3 types of tilt increased thoracic impedance, muscle sympathetic nerve activity (160-180%, N = 8 of 12), arterial pressure (~13%), and heart rate (~19%) relative to supine conditions (all P < 0.04).  None of these responses differed significantly between the 3 tilt conditions.  These data suggest the otolith organ does not play an important role in human.