Influence of training on sweating responses during submaximal exercise in horses

Abstract

Sweating responses were examined in five horses during a standardized exercise test (SET) in hot conditions (32–34°C, 45–55% relative humidity) during 8 wk of exercise training (5 days/wk) in moderate conditions (19–21°C, 45–55% relative humidity). SETs consisting of 7 km at 50% maximal O2 consumption, determined 1 wk before training day (TD) 0, were completed on a treadmill set at a 6° incline on TD0, 14, 28, 42, and 56. Mean maximal O2consumption, measured 2 days before each SET, increased 19% [TD0 to 42: 135 ± 5 (SE) to 161 ± 4 ml · kg−1 · min−1]. Peak sweating rate (SR) during exercise increased on TD14, 28, 42, and 56 compared with TD0, whereas SRs and sweat losses in recovery decreased by TD28. By TD56, end-exercise rectal and pulmonary artery temperature decreased by 0.9 ± 0.1 and 1.2 ± 0.1°C, respectively, and mean change in body mass during the SET decreased by 23% (TD0: 10.1 ± 0.9; TD56: 7.7 ± 0.3 kg). Sweat Na+concentration during exercise decreased, whereas sweat K+concentration increased, and values for Cl− concentration in sweat were unchanged. Moderate-intensity training in cool conditions resulted in a 1.6-fold increase in sweating sensitivity evident by 4 wk and a 0.7 ± 0.1°C decrease in sweating threshold after 8 wk during exercise in hot, dry conditions. Altered sweating responses contributed to improved heat dissipation during exercise and a lower end-exercise core temperature. Despite higher SRs for a given core temperature during exercise, decreases in recovery SRs result in an overall reduction in sweat fluid losses but no change in total sweat ion losses after training.