Equine sweating responses to submaximal exercise during 21 days of heat acclimation
This study examined sweating responses in six exercise-trained horses during 21 consecutive days (4 h/day) of exposure to, and daily exercise in, hot humid conditions (32–34°C, 80–85% relative humidity). On days 0, 3, 7, 14, and 21, horses completed a standardized exercise test on a treadmill (6° incline) at a speed eliciting 50% of maximal O2 uptake until a pulmonary artery temperature of 41.5°C was attained. Sweat was collected at rest, every 5 min during exercise, and during 1 h of standing recovery for measurement of ion composition (Na+, K+, and Cl−) and sweating rate (SR). There was no change in the mean time to reach a pulmonary artery temperature of 41.5°C (range 19.09 ± 1.41 min on day 0 to 20.92 ± 1.98 min on day 3). Peak SR during exercise (ml ⋅ m−2 ⋅ min−1) increased on day 7 (57.5 ± 5.0) but was not different on day 21 (48.0 ± 4.7) compared with day 0 (52.0 ± 3.4). Heat acclimation resulted in a 17% decline in SR during recovery and decreases in body mass and sweat fluid losses during the standardized exercise test of 25 and 22%, respectively, by day 21. By day 21, there was also a 10% decrease in mean sweat Na+ concentration for a given SR during exercise and recovery; this contributed to an ∼26% decrease in calculated total sweat ion losses (3,112 ± 114 mmol on day 0 vs. 2,295 ± 107 mmol on day 21). By day 21, there was a decrease in sweating threshold (∼1°C) but no change in sweat sensitivity. It is concluded that horses responded to 21 days of acclimation to, and exercise in, hot humid conditions with a reduction in sweat ion losses attributed to decreases in sweat Na+concentration and SR during recovery.