title: Muscle sympathetic responses during orthostasis in heat-stressed individuals.
abstract: OBJECTIVE
  Whole-body heat stress compromises the control of blood pressure during an orthostatic challenge, although the extent to which this occurs can vary greatly between individuals. The mechanism(s) responsible for these varying responses remain unclear. This study tested the hypothesis that the individuals who are best able to tolerate an orthostatic challenge while heat stressed are the ones with the largest increase in sympathetic activity during orthostasis, indexed from recordings of muscle sympathetic nerve activity (MSNA).
  
  
  METHODS
  MSNA, arterial blood pressure, and heart rate were recorded from 11 healthy volunteers throughout passive whole-body heating and during 15 min of 60° head-up tilt (HUT) or until the onset of pre-syncopal symptoms.
  
  
  RESULTS
  Whole-body heating significantly increased core temperature (~0.9°C), supine heart rate and MSNA. Eight of 11 subjects developed pre-syncopal symptoms resulting in early termination of HUT. The HUT tolerance time was positively correlated (R = 0.82, P = 0.01) with the increase in MSNA by HUT.
  
  
  CONCLUSIONS
  These data suggest that the individuals with the largest increase in MSNA during upright tilt have the greatest capacity to withstand the orthostatic challenge while heat stressed.
authors.affiliation: Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital Dallas, USA.
authors.forename: Jian
authors.initials: J
authors.lastname: Cui
authors.forename: Manabu
authors.initials: M
authors.lastname: Shibasaki
authors.forename: David A
authors.initials: DA
authors.lastname: Low
authors.forename: David M
authors.initials: DM
authors.lastname: Keller
authors.forename: Scott L
authors.initials: SL
authors.lastname: Davis
authors.forename: Craig G
authors.initials: CG
authors.lastname: Crandall
mesh_terms: D000328:Adult; D004244:Dizziness / Q000150:complications*; D005260:Female; D018882:Heat Stress Disorders / Q000473:pathology*; D006801:Humans; D008297:Male; D018482:Muscle, Skeletal / Q000503:physiopathology*; D013564:Sympathetic Nervous System / Q000473:pathology*