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

The effect of humid heat acclimation on thermoregulatory responses to humid and dry exercise-heat stress was studied in six exercise-trained Thoroughb red horses. Horses were heat acclimated by performing moderate-intensity ex ercise for 21 days in heat and humidity (HH) [34.2-35.7 degreesC; 84-86% re lative humidity (RH); wet bulb globe temperature (WBGT) index similar to 32 degreesC]. Horses completed exercise tests at 50% of peak O-2 uptake until a pulmonary arterial temperature (T-pa) of 41.5 degreesC was attained in c ool dry (CD) (20-21.5 degreesC; 45-50% RH; WBGT similar to 16 degreesC), ho t dry (HD 0) [32-34 degreesC room temperature (RT); 45-55% RH; WBGT similar to 25 degreesC], and HH conditions (HH 0), and during the second hour of K K on days 3, 7, 14, and 21, and in HD on the 18th day (HD 18) of heat accli mation. The ratios of required evaporative capacity to maximal evaporative capacity of the environment (E-req/E-max) for CD, HD, and HH were similar t o1.2, 1.6, and 2.5, respectively. Preexercise T-pa and rectal temperature w ere similar to0.5 degreesC lower (P < 0.05) on days 7, 14, and 21 compared with day 0. With exercise in HH, there was no effect of heat acclimation on the rate of rise in T-pa (and therefore exercise duration) nor the rate of heat storage. In contrast, exercise duration was longer, rate of rise in T -pa was significantly slower, and rate of heat storage was decreased on KD 18 compared with HD 0. It was concluded that, during uncompensable heat str ess in horses, heat acclimation provided modest heat strain advantages when E-req/E-max was <similar to>1.6, but at higher E-req/E-max no advantages w ere observed.