Effects of atropine on thermoregulatory responses to exercise in different environments

The thermoregulatory effects of atropine (2 mg im) were examined in six heat-acclimated subjects during exercise in three environments, which provided different evaporative capacities, but similar heat stress as indicated by the wet bulb globe temperature index (WBGT). Subjects walked in environments of Ta=42.3°C, Tdp=14.6°C, WBGT=29.1°C (HD); Ta=33.9°C, Tdp=23.5°C, WBGT=28.9°C (WM); Ta=30.4°C, Tdp=23.8°C, WBGT=27.4°C, (WW) after atropine and saline injections. In comparison to saline, atropine elevated rectal temperature (Tre) (p<0.05) in HD. Additionally, atropine elevated (p<0.01) mean skin temperature (T̄(sk)), and heart rate (HR) in all three environments relative to saline. Whole body sweating rate (Ṁ(sw)) was 45% lower (p<0.01) in each environment after atropine relative to saline. Exercise time was reduced from saline values (p<0.05) by 26.5 min in the HD after atropine. Within the atropine treatments, T(re) was higher (p<0.05) in HD (0.6°C) than WW, and HR was higher (p<0.05) in HD (23 b·min^-1) and WM (14 b·min^-1) than WW. T̄(sk) was higher (p<0.01) in WM than WW (1.2°C) and in HD than WM (1.5°C). Exercise time was 26.5 min longer (p<0.05) in WW than HD in the atropine experiments. The results indicated that Ṁ(sw) depression by atropine in all three environments had its greatest effect on thermoregulation in HD where evaporation was critical to heat dissipation; Ṁ(sw) reduction had less effect in WW and WM where sensible heat loss accounted for a greater portion of heat exchange.