Effects of heat acclimation on atropine-impaired thermoregulation

The effects of saline or atropine injection (2 mg, im) on eccrine sweating and performance time in seven healthy male subjects were evaluated during treadmill walking (1.34 m X s-1) in a hot-dry environment (Ta = 49 degrees C, Tdp = 20.5 degrees C) before and after heat acclimation (HA). Mean skin temperature (Tsk), rectal temperature (Tre), and heart rate (HR) were continuously measured. Sweat loss from the skin (Msw) was calculated by changes in body weight. HA resulted in decreased (p less than 0.05) Tre (0.4 degrees C) and HR (17 b X min-1), and increased (p less than 0.05) Msw (16 g X m-2 X h-1) during the saline experiments. Pre-acclimation, Msw was reduced (p less than 0.01) 65% (151 g X m-2 X h-1) with atropine, which resulted in higher (p less than 0.01) Tre (0.4 degrees C) and Tsk (2.8 degrees C). HR was increased 48% (53 b X min-1) by atropine pre-acclimation (p less than 0.01). Post-acclimation, atropine reduced (p less than 0.01) Msw 33% (100 g X m-2 X h-1) and increased (p less than 0.01) HR 63% (62 b X min-1) compared to saline exposures. The change in Tre X min-1 (delta Tre/delta t) was lower (p less than 0.05) in atropine-injected subjects following heat acclimation, and their worktime was improved by an average of 23.5 min (p = 0.08). These data demonstrate that heat acclimation improves the endurance time of atropine-treated subjects in a hot-dry environment. This improvement was, in part, due to the potentiation of sweat gland activity enabling greater evaporative cooling for the same dose of atropine.