title: Role of sympathetic responses on the hemodynamic consequences of rapid changes in posture in humans.
abstract: Tolerance to +G(z) gravitational stress is reduced when +G(z) stress is preceded by exposure to hypogravity (fractional, 0, or negative G(z)). For example, there is an exaggerated fall in eye-level arterial pressure (ELAP) early on during +G(z) stress (head-up tilt; HUT) when this stress is immediately preceded by -G(z) stress (head-down tilt; HDT), termed the "push-pull effect." The aim of the present study was to test the hypothesis that sympathetic responses contribute to the push-pull effect. Young, healthy subjects (n = 7 males and 3 females) were subjected to 30 s of 30 degrees HUT from a horizontal position and to 30 s of 30 degrees HUT when HUT was immediately preceded by 20 s of -15 degrees HDT. Four bouts of HDT-HUT were alternated between five bouts of HUT in a counterbalanced design, and 1 min was allowed for recovery between tilts. This protocol was repeated during clonidine administration (2.5 microg/kg bolus over 30 min and then continuously at 0.36 microg x kg(-1) x h(-1)). Clonidine blunted the vasomotor responses to tilting, and this led to exaggerated changes in arterial pressure. Clonidine exerted little specific influence on the push-pull effect. Thus sympathetic responses appear neither to contribute to, nor protect against, the push-pull effect for the rate and duration of tilting imposed in the present study.
authors.affiliation: Department of Integrative Physiology, The University of Iowa, Iowa City, IA 52245, USA.
authors.forename: Don D
authors.initials: DD
authors.lastname: Sheriff
authors.forename: Inger Helene
authors.initials: IH
authors.lastname: Nådland
authors.forename: Karin
authors.initials: K
authors.lastname: Toska
mesh_terms: D000222:Adaptation, Physiological; D000293:Adolescent; D000316:Adrenergic alpha-Agonists / Q000008:administration & dosage; D000328:Adult; D001783:Blood Flow Velocity; D001794:Blood Pressure; D002302:Cardiac Output; D002319:Cardiovascular System / Q000000981:diagnostic imaging / Q000187:drug effects / Q000294:innervation*; D003000:Clonidine / Q000008:administration & dosage; D004562:Electrocardiography; D005260:Female; D018470:Gravity, Altered; D018475:Head-Down Tilt; D006339:Heart Rate; D006439:Hemodynamics / Q000187:drug effects; D006801:Humans; D007024:Hypotension, Orthostatic / Q000000981:diagnostic imaging / Q000503:physiopathology*; D007262:Infusions, Intravenous; D017078:Laser-Doppler Flowmetry; D035002:Lower Extremity / Q000098:blood supply*; D008297:Male; D054971:Orthostatic Intolerance / Q000000981:diagnostic imaging / Q000503:physiopathology*; D011187:Posture; D012039:Regional Blood Flow; D013564:Sympathetic Nervous System / Q000187:drug effects / Q000503:physiopathology*; D018667:Tilt-Table Test; D013997:Time Factors; D018617:Ultrasonography, Doppler, Pulsed; D055815:Young Adult
chemical_list: D000316:Adrenergic alpha-Agonists; D003000:Clonidine