title: Catecholamine response to maximal exercise following 16 days of simulated microgravity.
abstract: BACKGROUND
  We tested the hypothesis that the elevated peak heart rate (HR) response to maximal exercise following microgravity exposure is associated with increased plasma levels of catecholamines.
  
  
  METHODS
  To do this, plasma norepinephrine (NE) and epinephrine (E) were measured in venous blood samples obtained from 7 subjects before and immediately after graded supine cycle exercise to volitional fatigue performed prior to and at the conclusion of 16 d of 6 degrees head-down tilt (HDT).
  
  
  RESULTS
  Resting HR was increased (p = 0.014) following HDT (56 +/- 3 to 63 +/- 2 bpm), but pre-exercise NE concentration (217 +/- 18 pg.ml-1) was not different (p = 0.224) from that measured before HDT (183 +/- 23 pg.ml-1). Peak exercise HR was greater (p = 0.001) after simulated microgravity exposure compared to pre-HDT (181 +/- 3 bpm vs. 172 +/- 4 bpm). NE measured at volitional fatigue was 64% higher (p = 0.004) after HDT compared to that measured prior to HDT (1337 +/- 181 vs. 2191 +/- 189 pg.ml-1 for pre- and post-HDT, respectively). Plasma E concentration at exhaustion was unchanged by microgravity exposure.
  
  
  CONCLUSIONS
  Our observation of greater levels of plasma NE and peak exercise HR suggests that sympathetic influences on cardiac chronotropic control may be altered by exposure to prolonged microgravity.
authors.affiliation: Department of Physiology, University of Florida, Gainesville, USA.
authors.forename: K A
authors.initials: KA
authors.lastname: Engelke
authors.forename: V A
authors.initials: VA
authors.lastname: Convertino
mesh_terms: D000328:Adult; D004837:Epinephrine / Q000097:blood*; D015444:Exercise / Q000502:physiology*; D005080:Exercise Test; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D008297:Male; D009638:Norepinephrine / Q000097:blood*; D014893:Weightlessness
chemical_list: D009638:Norepinephrine; D004837:Epinephrine