title: AVP response to LBNP is attenuated in a hyperbaric environment.
abstract: BACKGROUND
  Accumulated evidence indicates attenuated vasopressin (AVP) release associated with hyperbaria in humans. The present study was designed to test the hypothesis that plasma AVP response to central hypovolemia was attenuated in a hyperbaric environment.
  
  
  METHODS
  Response of AVP to central hypovolemic stress, in the form of lower-body negative pressure (LBNP), was measured in 10 men at 1 and 3 atmosphere absolute (ATA) air. The stress consisted of 4 min each of control and LBNP at -20 and -40 mmHg. Plasma AVP, plasma norepinephrine (NE), plasma osmolality, and hematocrit (Hct) were measured. Thoracic impedance (Z0), and leg volume were also determined.
  
  
  RESULTS
  The baseline AVP (0.9 +/- 0.3 pg x ml(-1)) at 1 ATA increased significantly in response to -40 mmHg LBNP (1.8 +/- 0.6 pg x ml(-1)), but the increase was attenuated at 3 ATA (1.2 +/- 0.8 pg x ml(-1)). The baseline NE at 3 ATA was lower by 22.4 pg x ml(-1) than that of 1 ATA (226.5 pg x ml(-1)). NE increased with increasing LBNP at both environmental pressures, but the increase was independent of the pressure. Changes in Z0, reflecting thoracic blood volume, were identical in both environments. Mean arterial pressure, plasma osmolality, and Hct remained constant throughout the experimental period. Leg volume increased stress dependently during LBNP (approximately 34 ml at -20 mmHg; approximately 74 ml at -40 mmHg) and the increment was similar in the two environments.
  
  
  CONCLUSIONS
  The present study revealed that hyperbaria attenuated the response of AVP and NE to hypovolemic stress. The attenuated AVP response to LBNP was associated with neither plasma osmolality nor intrathoracic blood shift.
authors.affiliation: Department of Sports Life Style Management, National Institute of Fitness and Sports, Kanoya, Japan.
authors.forename: Sueko
authors.initials: S
authors.lastname: Sagawa
authors.forename: Katsuya
authors.initials: K
authors.lastname: Yamauchi
authors.forename: Yuka
authors.initials: Y
authors.lastname: Tsutsui
authors.forename: Yutaka
authors.initials: Y
authors.lastname: Endo
authors.forename: Keizo
authors.initials: K
authors.lastname: Shiraki
mesh_terms: D000328:Adult; D001274:Atmospheric Pressure; D006400:Hematocrit; D006801:Humans; D020896:Hypovolemia / Q000503:physiopathology*; D008297:Male; D009638:Norepinephrine / Q000097:blood; D011311:Pressoreceptors / Q000502:physiology; D012039:Regional Blood Flow; D013909:Thorax / Q000098:blood supply; D014667:Vasopressins / Q000097:blood* / Q000378:metabolism*
chemical_list: D014667:Vasopressins; D009638:Norepinephrine