title: Human muscle sympathetic neural and haemodynamic responses to tilt following spaceflight.
abstract: Orthostatic intolerance is common when astronauts return to Earth: after brief spaceflight, up to two-thirds are unable to remain standing for 10 min. Previous research suggests that susceptible individuals are unable to increase their systemic vascular resistance and plasma noradrenaline concentrations above pre-flight upright levels. In this study, we tested the hypothesis that adaptation to the microgravity of space impairs sympathetic neural responses to upright posture on Earth. We studied six astronauts approximately 72 and 23 days before and on landing day after the 16 day Neurolab space shuttle mission. We measured heart rate, arterial pressure and cardiac output, and calculated stroke volume and total peripheral resistance, during supine rest and 10 min of 60 deg upright tilt. Muscle sympathetic nerve activity was recorded in five subjects, as a direct measure of sympathetic nervous system responses. As in previous studies, mean (+/- S.E.M.) stroke volume was lower (46 +/- 5 vs. 76 +/- 3 ml, P = 0.017) and heart rate was higher (93 +/- 1 vs. 74 +/- 4 beats min(-1), P = 0.002) during tilt after spaceflight than before spaceflight. Total peripheral resistance during tilt post flight was higher in some, but not all astronauts (1674 +/- 256 vs. 1372 +/- 62 dynes s cm(-5), P = 0.32). No crew member exhibited orthostatic hypotension or presyncopal symptoms during the 10 min of postflight tilting. Muscle sympathetic nerve activity was higher post flight in all subjects, in supine (27 +/- 4 vs. 17 +/- 2 bursts min(-1), P = 0.04) and tilted (46 +/- 4 vs. 38 +/- 3 bursts min(-1), P = 0.01) positions. A strong (r(2) = 0.91-1.00) linear correlation between left ventricular stroke volume and muscle sympathetic nerve activity suggested that sympathetic responses were appropriate for the haemodynamic challenge of upright tilt and were unaffected by spaceflight. We conclude that after 16 days of spaceflight, muscle sympathetic nerve responses to upright tilt are normal.
authors.affiliation: Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas and the University of Texas Southwestern Medical Center at Dallas, Dallas, TX 75231, USA. benjaminlevine@texashealth.org
authors.forename: Benjamin D
authors.initials: BD
authors.lastname: Levine
authors.forename: James A
authors.initials: JA
authors.lastname: Pawelczyk
authors.forename: Andrew C
authors.initials: AC
authors.lastname: Ertl
authors.forename: James F
authors.initials: JF
authors.lastname: Cox
authors.forename: Julie H
authors.initials: JH
authors.lastname: Zuckerman
authors.forename: André
authors.initials: A
authors.lastname: Diedrich
authors.forename: Italo
authors.initials: I
authors.lastname: Biaggioni
authors.forename: Chester A
authors.initials: CA
authors.lastname: Ray
authors.forename: Michael L
authors.initials: ML
authors.lastname: Smith
authors.forename: Satoshi
authors.initials: S
authors.lastname: Iwase
authors.forename: Mitsuru
authors.initials: M
authors.lastname: Saito
authors.forename: Yoshiki
authors.initials: Y
authors.lastname: Sugiyama
authors.forename: Tadaaki
authors.initials: T
authors.lastname: Mano
authors.forename: Rong
authors.initials: R
authors.lastname: Zhang
authors.forename: Kenichi
authors.initials: K
authors.lastname: Iwasaki
authors.forename: Lynda D
authors.initials: LD
authors.lastname: Lane
authors.forename: Jay C
authors.initials: JC
authors.lastname: Buckey
authors.forename: William H
authors.initials: WH
authors.lastname: Cooke
authors.forename: Friedhelm J
authors.initials: FJ
authors.lastname: Baisch
authors.forename: Dwain L
authors.initials: DL
authors.lastname: Eckberg
authors.forename: C Gunnar
authors.initials: CG
authors.lastname: Blomqvist
mesh_terms: D000328:Adult; D018480:Astronauts; D018475:Head-Down Tilt / Q000502:physiology*; D006439:Hemodynamics / Q000502:physiology*; D006801:Humans; D008297:Male; D018482:Muscle, Skeletal / Q000294:innervation*; D013026:Space Flight; D016683:Supine Position / Q000502:physiology; D013564:Sympathetic Nervous System / Q000502:physiology*