title: Evidence for increased beta-adrenoreceptor responsiveness induced by 14 days of simulated microgravity in humans.
abstract: We studied hemodynamic responses to alpha- and beta-receptor agonists in eight healthy men before and after 14 days of 6 degrees head-down tilt (HDT) to test the hypothesis that increased adrenoreceptor responsiveness is induced by prolonged exposure to simulated microgravity. Steady-state infusions of isoproterenol (Iso) at rates of 0.005, 0.01, and 0.02 microgram.kg-1.min-1 were used to assess beta 1- and beta 2-adrenoreceptor responsiveness. Infusions of phenylephrine (PE) at rates of 0.25, 0.50, and 1.00 microgram.kg-1.min-1 were used to assess responsiveness of alpha 1-vascular adrenoreceptors. Slopes calculated from linear regressions between Iso and PE doses and changes in beat-to-beat heart rate, blood pressure, and leg vascular resistance (occlusion plethysmography) for each subject were used as an index of alpha- and beta-adrenoreceptor responsiveness. HDT increased the slopes of heart rate (1,056 +/- 107 to 1,553 +/- 83 beats micrograms-1.kg-1.min-1; P = 0.014) and vasodilation (-469 +/- 111 to -1,446 +/- 309 peripheral resistance units.microgram-1.kg-1.min-1; P = 0.0224) to Iso infusion. There was no alteration in blood pressure or vascular resistance responses to PE infusion after HDT. Our results provide evidence that simulated microgravity causes selective increases in beta 1- and beta 2-adrenoreceptor responsiveness without affecting alpha 1-vascular adrenoreceptor responses.
authors.affiliation: Biomedical Operations and Research Office, National Aeronautics and Space Administration, Kennedy Space Center, Florida 32899, USA.
authors.forename: V A
authors.initials: VA
authors.lastname: Convertino
authors.forename: J L
authors.initials: JL
authors.lastname: Polet
authors.forename: K A
authors.initials: KA
authors.lastname: Engelke
authors.forename: G W
authors.initials: GW
authors.lastname: Hoffler
authors.forename: L D
authors.initials: LD
authors.lastname: Lane
authors.forename: C G
authors.initials: CG
authors.lastname: Blomqvist
mesh_terms: D000328:Adult; D017704:Baroreflex / Q000187:drug effects; D001794:Blood Pressure; D004305:Dose-Response Relationship, Drug; D018475:Head-Down Tilt / Q000502:physiology*; D006339:Heart Rate; D006439:Hemodynamics / Q000187:drug effects / Q000502:physiology*; D006706:Homeostasis; D006801:Humans; D007262:Infusions, Intravenous; D007545:Isoproterenol / Q000008:administration & dosage / Q000494:pharmacology*; D007866:Leg / Q000098:blood supply; D008297:Male; D009638:Norepinephrine / Q000097:blood; D010656:Phenylephrine / Q000008:administration & dosage / Q000494:pharmacology*; D018340:Receptors, Adrenergic, alpha-1 / Q000502:physiology*; D011943:Receptors, Adrenergic, beta / Q000502:physiology*; D018342:Receptors, Adrenergic, beta-1 / Q000502:physiology; D018343:Receptors, Adrenergic, beta-2 / Q000502:physiology; D012044:Regression Analysis; D018561:Space Simulation; D013997:Time Factors; D014655:Vascular Resistance; D014893:Weightlessness
chemical_list: D018340:Receptors, Adrenergic, alpha-1; D011943:Receptors, Adrenergic, beta; D018342:Receptors, Adrenergic, beta-1; D018343:Receptors, Adrenergic, beta-2; D010656:Phenylephrine; D007545:Isoproterenol; D009638:Norepinephrine