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 · s^-1) in a hot, dry environment (Ta=49°C, Tdp=20.5°C) before and after heat acclimation (HA). Mean skin temperature (T̄sk), rectal temperature (Tre), and heart rate (HR) were continuously measured. Sweat loss from the skin (Ṁsw) was calculated by changes in body weight. HA resulted in decreased (p<0.05) Tre (0.4°C) and HR (17 b · min^-1), and increased (p<0.05) Ṁsw (16 g · m^-2 · h^-1) during the saline experiments. Pre-acclimation, Ṁsw was reduced (p<0.01) 65% (151 g · m^-2 · h^-1) with atropine, which resulted in higher (p<0.01) Tre (0.4°C) and T̄sk (2.8°C). HR was increased 48% (53 b · min^-1) by atropine pre-acclimation (p<0.01). Post-acclimation, atropine reduced (p<0.01) Ṁsw 33% (100 g · m^-2 · h^-1) and increased (p<0.01) HR 63% (62 b · min^-1) compared to saline exposures. The change in Tre · min^-1 (Δ Tre/Δ t) was lower (p<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.