title: Increased beta-adrenergic responsiveness induced by 14 days exposure to simulated microgravity.
abstract: Increased sensitivity of end-organ responses to neuroendocrine stimuli as a result of prolonged exposure to the relative inactivity of microgravity has recently been hypothesized.  This notion is based on the inverse relationship between circulating norepinephrine and beta-adrenoreceptor sensitivity.  Beta-adrenoreceptor activity is reduced in individuals who have elevated plasma norepinephrine as as a result of regular exposure to upright posture and physical exercise.  In contrast, adrenoreceptor hypersensitivity has been reported in patients with dysautonomias in which circulating catecholamines are absent or reduced.  Taken together, these studies and the observation that circulating plasma norepinephrine has been reduced during spaceflight and in groundbased simulations of microgravity prompt the suggestion that adrenoreceptor hypersensitivity may be a consequence of the adaptation to spaceflight.  We conducted an experiment designed to measure cardiovascular responses to adrenoreceptor agonists in human subjects before and after prolonged exposure to 6 degrees head-down tilt (HDT) to test the hypothesis that adaptation to microgravity increases adrenoreceptor responsiveness, and that this adaptation is associated with reduced levels of circulating norepinephrine.
authors.affiliation: Physiology Research Branch, Clinical Sciences Division, Brooks Air Force Base, TX 78235, 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: D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000318:Adrenergic beta-Agonists / Q000494:pharmacology*; D000328:Adult; D004305:Dose-Response Relationship, Drug; D018475:Head-Down Tilt / Q000009:adverse effects*; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D007545:Isoproterenol / Q000494:pharmacology*; D007866:Leg / Q000098:blood supply; D008297:Male; D009638:Norepinephrine / Q000097:blood*; D010656:Phenylephrine / Q000494:pharmacology*; D018340:Receptors, Adrenergic, alpha-1 / Q000187:drug effects; D018342:Receptors, Adrenergic, beta-1 / Q000187:drug effects; D018343:Receptors, Adrenergic, beta-2 / Q000187:drug effects*; D014655:Vascular Resistance / Q000502:physiology; D018474:Weightlessness Simulation
chemical_list: D000316:Adrenergic alpha-Agonists; D000318:Adrenergic beta-Agonists; D018340:Receptors, Adrenergic, alpha-1; D018342:Receptors, Adrenergic, beta-1; D018343:Receptors, Adrenergic, beta-2; D010656:Phenylephrine; D007545:Isoproterenol; D009638:Norepinephrine