CLONIDINE DECREASES VASOCONSTRICTION AND SHIVERING THRESHOLDS, WITHOUT AFFECTING THE SWEATING THRESHOLD

Purpose: This study was conducted to lest the hypothesis that clonidin e produces a dose-dependent increase in the sweating threshold and dos e-dependent decreases in vasoconstriction and shivering thresholds. Me thods: Six healthy subjects (two female) were studied on four days aft er taking clonidine in oral doses of either 0 (control), 3, 6 or 9 mu g.kg(-1). The order followed a balanced design in a double-blind fashi on, Oesophageal temperature and mean skin temperature (from 12 sites) were measured. Subjects were seated in 37 degrees C water which was gr adually warmed until sweating occurred (sweat rate increased above 50 g.m(-2)h(-1)). The water was then cooled gradually until thresholds fo r vasoconstriction (onset of sustained decrease in fingertip blood flo w! 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 threshold s 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 li near for both cold responses with vasoconstriction and shivering thres holds decreasing by 0.13 +/- 0.05 and 0.19 +/- 0.09 degrees C.mu g(-1) respectively (P < 0.0001). The sweating threshold was unaffected by c lonidine, however the interthreshold range between sweating and vasoco nstriction thresholds increased from control (0.19 +/- 0.48 degrees C) to high dose clonidine (1.31 +/- 0.54 degrees C). Conclusion: The dec reases in core temperature thresholds For cold responses and increased interthreshold range are consistent with the effects of several anaes thetic agents and opioids and is indicative of central thermoregulator y inhibition.