The long-term objective of the Principal Investigator is to evaluate vestibular reflex mechanisms that regulate the cardiovascular system in humans. During the past funding period, we demonstrated that activation of the otolith organs mediated prompt and marked increases in sympathetic nerve activity and limb vascular resistance. The main hypothesis for this project is that the vestibulosympathetic reflex participates importantly in orthostasis by activating the sympathetic nervous system in humans. It is hypothesized that attenuations in the vestibulosympathetic reflex contributes to orthostatic intolerance in humans. To test these hypotheses, we propose studies with the following specific aims: 1. To determine muscle sympathetic nerve activity (MSNA) responses during dynamic pitch rotation. Transition from the supine to upright posture and ambulation dynamically stimulate the otolith organs Therefore, it is important to determine whether dynamic activation of the otolith organs elicits increases in sympathetic vasoconstrictor outflow. We hypothesize greater increases in MSNA will be elicited by dynamic rather than static engagement of the otolith organs. 2. To determine MSNA responses to otolith organ stimulation in the prone, sitting, and upright positions. Because postural blood pressure control is most important in the upright posture, we will test the vestibulosympathetic reflex during head rotations in the sitting and standing positions. We hypothesize that increases in MSNA by the vestibulosympathetic reflex via head-down rotation is independent of body posture. 3. To determine visceral vascular responses to static head-down rotation. Because visceral vascular vasoconstriction contributes importantly to postural blood pressure regulation, we hypothesize that vestibular activation will mediate increases in visceral vascular resistance. 4. To examine sympathetic and hemodynamic responses to otolith organ stimulation in subjects with a predisposition to orthostatic intolerance. We hypothesize that MSNA responses will be attenuated and blood pressure reduced during activation of the vestibulosympathetic in subjects with orthostatic intolerance (i.e., vestibular deficient and idiopathic orthostatic hypotension subjects, and endurance trained athletes). The design of these studies will increase our understanding of how the vestibular system contributes to orthostatic blood pressure regulation in humans. [unreadable] [unreadable]