Vestibular Control of Intermediate- and Long-Term Cardiovascular Responses to Experimental Orthostasis

Sustained orthostasis elicits the elevation of arterial bloodpressure (BP) via sympathetic activation in conscious Wistar ratsfor at least 2 hours. We tested the hypothesis whether vestibularapparatus plays a role in BP and heart rate (HR) control inresponse to prolonged gravitational stress. BP and HR responsesto 45º head-up for either 2 or 24 hours were monitored bytelemetry. Vestibular lesions (VL) were performed by a modifiedmicrosurgical-chemical technique. Horizontal BP and HR were notinfluenced by VL preceding 2-hour tilt. VL abolished the sustained2-hour BP response to head-up tilt (8.3±0.9 mm Hg relative tohorizontal values) while suppressed HR transiently only. VLeliminated diurnal BP fluctuations and decreased HR in horizontalposition for 24 hours. Head-up tilt for 24 hours increased BP andHR progressively in intact animals, raising their daily averagevalue by 5.6±0.7 mm Hg and 22.2±6 BPM, respectively. VLresulted in an initial BP rise followed by progressive BP reductionin response to long-term head-up tilt (4±2.2 mm Hg) withouteliminating the tachycardia (34.4±5.4 BPM). Thus, blockade oflabyrinthine inputs attenuates the BP responses elicited by bothintermediate and long-term gravitational stress of orthostatictype. However, other sensory inputs derived from non-vestibularcues (e.g. proprioceptive, visual, visceral, cutaneous etc.) seemto be effective enough to maintain BP normal.