title: Resetting of the arterial baroreflex increases orthostatic sympathetic activation and prevents postural hypotension in rabbits.
abstract: Since humans are under ceaseless orthostatic stress, the mechanism to maintain arterial pressure (AP) under orthostatic stress against gravitational fluid shift is of great importance. We hypothesized that (1) orthostatic stress resets the arterial baroreflex control of sympathetic nerve activity (SNA) to a higher SNA, and (2) resetting of the arterial baroreflex contributes to preventing postural hypotension. Renal SNA and AP were recorded in eight anaesthetized, vagotomized and aortic-denervated rabbits. Isolated intracarotid sinus pressure (CSP) was increased stepwise from 40 to 160 mmHg with increments of 20 mmHg (60 s for each CSP level) while the animal was placed supine and at 60 deg upright tilt. Upright tilt shifted the CSP-SNA relationship (the baroreflex neural arc) to a higher SNA, shifted the SNA-AP relationship (the baroreflex peripheral arc) to a lower AP, and consequently moved the operating point to marked high SNA while maintaining AP. A simulation study suggests that resetting in the neural arc would double the orthostatic activation of SNA and increase the operating AP in upright tilt by 10 mmHg, compared with the absence of resetting. In addition, upright tilt did not change the CSP-AP relationship (the baroreflex total arc). A simulation study suggests that although a downward shift of the peripheral arc could shift the total arc downward, resetting in the neural arc would compensate this fall and prevent the total arc from shifting downward to a lower AP. In conclusion, upright tilt increases SNA by resetting the baroreflex neural arc. This resetting may compensate for the reduced pressor responses to SNA in the peripheral cardiovascular system and contribute to preventing postural hypotension.
authors.affiliation: Department of Cardiovascular Dynamics, National Cardiovascular Centre Research Institute, Osaka 565-8565, Japan. kamiya@ri.ncvc.go.jp
authors.forename: Atsunori
authors.initials: A
authors.lastname: Kamiya
authors.forename: Toru
authors.initials: T
authors.lastname: Kawada
authors.forename: Kenta
authors.initials: K
authors.lastname: Yamamoto
authors.forename: Daisaku
authors.initials: D
authors.lastname: Michikami
authors.forename: Hideto
authors.initials: H
authors.lastname: Ariumi
authors.forename: Kazunori
authors.initials: K
authors.lastname: Uemura
authors.forename: Can
authors.initials: C
authors.lastname: Zheng
authors.forename: Syuji
authors.initials: S
authors.lastname: Shimizu
authors.forename: Takeshi
authors.initials: T
authors.lastname: Aiba
authors.forename: Tadayoshi
authors.initials: T
authors.lastname: Miyamoto
authors.forename: Masaru
authors.initials: M
authors.lastname: Sugimachi
authors.forename: Kenji
authors.initials: K
authors.lastname: Sunagawa
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology; D000818:Animals; D017704:Baroreflex; D001794:Blood Pressure; D004244:Dizziness / Q000150:complications / Q000503:physiopathology*; D007022:Hypotension / Q000503:physiopathology*; D007024:Hypotension, Orthostatic / Q000209:etiology / Q000503:physiopathology*; D011187:Posture; D011817:Rabbits; D013564:Sympathetic Nervous System