Hypothermia induced by chlorpromazine or L-tryptophan: effects on treadmill performance in the heat.

To study the effects of preinduced hypothermia on the physiological and thermoregulatory responses to exercise in the heat rats were intravenously administered either 100 micrograms of chlorpromazine (CPZ) or 200 mg/kg of L-tryptophan (L-Trp) under restraint in a cold (4 degrees C) environment. When rectal temperatures (Tre) reached 32-33 degrees C the rats were removed to a hot environment (35 degrees C) where they ran on a level treadmill (9.14 m/min) to hyperthermic exhaustion (Tre, 42.5-43 degrees C). Both CPZ and L-trp hypothermia was effective in increasing significantly (P less than 0.001) the time to hyperthermic exhaustion. However, the maximal Tre and skin temperatures (Tsk) attained were unaffected by either treatment. When the rats exercised on the treadmill, increments (degrees C/min) in Tre and Tsk were significantly (P less than 0.02, minimal) greater for the initially hypothermic animals compared to normothermic controls. Cooling rates were unaffected by either treatment. We concluded from these studies that, although preinduced hypothermia is extremely effective in prolonging the time to hyperthermic exhaustion, no additional beneficial thermoregulatory responses accrued as a result of this treatment.