Heat storage in horses during submaximal exercise before and after humid heat acclimation

Departments of 1 Clinical Studies and 2 Pathobiology, Ontario Veterinary College, and 3 Human Biology and Nutritional Sciences, University of Guelph, Guelph, Ontario N1G 2W1, Canada The effect of humid heat acclimation on thermoregulatory responses to humid and dry exercise-heat stress was studied in six exercise-trained Thoroughbred horses. Horses were heat acclimated by performing moderate-intensity exercise for 21 days in heat and humidity (HH) [34.2-35.7°C; 84-86% relative humidity (RH); wet bulb globe temperature (WBGT) index ~32°C]. Horses completed exercise tests at 50% of peak O 2 uptake until a pulmonary arterial temperature (T pa ) of 41.5°C was attained in cool dry (CD) (20-21.5°C; 45-50% RH; WBGT ~16°C), hot dry (HD 0) [32-34°C room temperature (RT); 45-55% RH; WBGT ~25°C], and HH conditions (HH 0), and during the second hour of HH on days 3, 7, 14 , and 21 , and in HD on the 18th day (HD 18) of heat acclimation. The ratios of required evaporative capacity to maximal evaporative capacity of the environment (E req /E max ) for CD, HD, and HH were ~1.2, 1.6, and 2.5, respectively. Preexercise T pa and rectal temperature were ~0.5°C lower ( P