Equine sweating responses to submaximal exercise during 21 days of heat acclimation

This study examined sweating responses in six exercise-trained horses durin g 21 consecutive days (4 h/day) of exposure to, and daily exercise in, hot humid conditions (32-34 degrees C, 80-85% relative humidity). On days 0, 3, 7, 14, and 21, horses completed a standardized exercise test on a treadmil l (6 degrees incline) at a speed eliciting 50% of maximal O-2 uptake until a pulmonary artery temperature of 41.5 degrees C was attained. Sweat was co llected at rest, every 5 min during exercise, and during 1 h of standing re covery for measurement of ion composition (Na+, K-divided by, and Cl-) and sweating rate (SR). There was no change in the mean time to reach a pulmona ry artery temperature of 41.5 degrees 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 1 7% 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+ conce ntration for a given SR during exercise and recovery; this contributed to a n similar to 26% decrease in calculated total sweat ion losses (3,112 +/- 1 14 mmol on day 0 vs. 2,295 +/- 107 mmol on day 21). By day 21, there was a decrease in sweating threshold (similar to 1 degrees C) but no change in sw eat sensitivity. It is concluded that horses responded to 21 days of acclim ation to, and exercise in, hot humid conditions with a reduction in sweat i on losses attributed to decreases in sweat Nat concentration and SR during recovery.