title: Cardiovascular deconditioning through head-down tilt bed rest increases blood pressure variability and plasma renin activity.
abstract: BACKGROUND
  The renin-angiotensin-aldosterone system may be of primary importance in the mechanism of bed rest cardiovascular deconditioning.
  
  
  OBJECTIVE
  This study was designed to test the hypothesis that bed rest cardiovascular deconditioning does not result simply from plasma volume loss, but is also at least partially attributable to a persistent disequilibrium of the neuroendocrine mediators of plasma volume homeostasis. We examined whether changes in the renin-angiotensin-aldosterone system occur in association with the cardiovascular deconditioning and hemodynamic instability induced by antiorthostatic 6 degrees head-down tilt bed rest.
  
  
  METHODS
  Normal male volunteers (n = 10) were tested before, during, and after 14 d of head-down tilt with a high (150 mEq.d-1) salt intake, using head-down tilt as a model of cardiovascular deconditioning and lower body negative pressure (LBNP) as a model of orthostatic stress.
  
  
  RESULTS
  Resting plasma renin activity was 2.22 +/- 0.85 ng.ml-1.h-1 (+/- SD) at baseline and increased to 4.14 +/- 1.21 ng.ml-1.h-1 at the end of head-down tilt (p < 0.05), but urine aldosterone, plasma aldosterone, and urine sodium did not change with head-down tilt. Although the plasma norepinephrine response to LBNP was accentuated, resting adrenergic tone did not change during head-down tilt. Cardiovascular deconditioning was associated with an increase in blood pressure variability during LBNP as assessed by both beat-to-beat standard deviation and spectral analysis.
  
  
  CONCLUSIONS
  These data support a proposed link between blood pressure variability and the renin-angiotensin system in cardiovascular deconditioning.
authors.affiliation: Department of Internal Medicine, Medical College of Virginia, Richmond, USA.
authors.forename: J F
authors.initials: JF
authors.lastname: Schmedtje
authors.forename: W L
authors.initials: WL
authors.lastname: Liu
authors.forename: A A
authors.initials: AA
authors.lastname: Taylor
mesh_terms: D000328:Adult; D017704:Baroreflex / Q000502:physiology; D001510:Bed Rest; D001794:Blood Pressure; D018493:Cardiovascular Deconditioning / Q000502:physiology*; D004837:Epinephrine / Q000097:blood; D018475:Head-Down Tilt / Q000502:physiology*; D006801:Humans; D008297:Male; D009638:Norepinephrine / Q000097:blood; D010953:Plasma Volume; D012083:Renin / Q000097:blood*; D012084:Renin-Angiotensin System / Q000502:physiology
chemical_list: D012083:Renin; D009638:Norepinephrine; D004837:Epinephrine