title: Clonidine decreases vasoconstriction and shivering thresholds, without affecting the sweating threshold.
abstract: OBJECTIVE
  This study was conducted to test the hypothesis that clonidine produces a dose-dependent increase in the sweating threshold and dose-dependent decreases in vasoconstriction and shivering thresholds.
  
  
  METHODS
  Six healthy subjects (two female) were studied on four days after taking clonidine in oral doses of either 0 (control), 3, 6 or 9 micrograms.kg-1. The order followed a balanced design in a double-blind fashion. Oesophageal temperature and mean skin temperature (from 12 sites) were measured. Subjects were seated in 37 degrees C water which was gradually warmed until sweating occurred (sweat rate increased above 50 g.m-2.h-1). The water was then cooled gradually until thresholds for vasoconstriction (onset of sustained decrease in fingertip blood flow) and shivering (sustained elevation in metabolism) were determined. Thresholds were then referred to as the core temperature, adjusted to a designated mean skin temperature of 33 degrees C.
  
  
  RESULTS
  High dose clonidine similarly decreased the adjusted core temperature thresholds for vasoconstriction by 1.16 +/- 0.30 degrees C and for shivering by 1.63 +/- 0.23 degrees C (P < 0.01). The dose response effects were linear for both cold responses with vasoconstriction and shivering thresholds decreasing by 0.13 +/- 0.05 and 0.19 +/- 0.09 degree C.microgram-1 respectively (P < 0.0001). The sweating threshold was unaffected by clonidine, however the interthreshold range between sweating and vasoconstriction thresholds increased from control (0.19 +/- 0.48 degree C) to high dose clonidine (1.31 +/- 0.54 degrees C).
  
  
  CONCLUSIONS
  The decreases in core temperature thresholds for cold responses and increased interthreshold range are consistent with the effects of several anaesthetic agents and opioids and is indicative of central thermoregulatory inhibition.
authors.affiliation: Laboratory for Exercise and Environmental Medicine, Health, Leisure and Human Performance Research Institute, Winnipeg, Manitoba, Canada.
authors.forename: G
authors.initials: G
authors.lastname: Nicolaou
authors.forename: A A
authors.initials: AA
authors.lastname: Chen
authors.forename: C E
authors.initials: CE
authors.lastname: Johnston
authors.forename: G P
authors.initials: GP
authors.lastname: Kenny
authors.forename: G K
authors.initials: GK
authors.lastname: Bristow
authors.forename: G G
authors.initials: GG
authors.lastname: Giesbrecht
mesh_terms: D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000328:Adult; D003000:Clonidine / Q000097:blood / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004311:Double-Blind Method; D005260:Female; D006801:Humans; D008297:Male; D012768:Shivering / Q000187:drug effects*; D013546:Sweating / Q000187:drug effects*; D014661:Vasoconstriction / Q000187:drug effects*
chemical_list: D000316:Adrenergic alpha-Agonists; D003000:Clonidine