title: Effects of cool and hot humid environmental conditions on neuroendocrine responses of horses to treadmill exercise.
abstract: To determine the effects of exercise, high heat and humidity and acclimation on plasma adrenaline, noradrenaline, beta-endorphin and cortisol concentrations, five horses performed a competition exercise test (CET; designed to simulate the speed and endurance test of a three-day event) in cool dry (CD) (20 degrees C/40% RH) and hot humid (30 degrees C/80% RH) conditions before (pre-acclimation) and after (post-acclimation) a 15 day period of humid heat acclimation. Plasma adrenaline and noradrenaline concentrations pre-acclimation were significantly increased compared with exercise in the CD trial at the end of Phases C (P<0.05) and D (P<0.05 and P<0.01, respectively) and at 2 min recovery (P<0.01), with adrenaline concentrations still elevated after 5 min of recovery (P<0.001). Plasma beta-endorphin concentrations were increased at the end of Phases C (P<0.05) and X (P<0.01) and at 5 and 30 min recovery (P<0.05) in the pre-acclimation session. Plasma cortisol concentrations were elevated after the initial warm up period pre-acclimation (P<0.01) and at the end of Phase C (P<0.05), compared with the CD trial. A 15 day period of acclimation significantly increased plasma adrenaline concentrations at 2 min recovery (P<0.001) and plasma cortisol concentration at the end of Phase B (P<0.01) compared with pre-acclimation. Acclimation did not significantly influence noradrenaline or beta-endorphin responses to exercise, although there was a trend for plasma beta-endorphin to be lower at the end of Phases C and X and after 30 min recovery compared with pre-acclimation. Plasma adrenaline, noradrenaline, beta-endorphin and cortisol concentrations were increased by exercise in cool dry conditions and were further increased by the same exercise in hot humid conditions. Exercise responses post-acclimation suggest that adrenaline and noradrenaline may play a role in the adaptation of horses to thermal stress and that changes in plasma beta-endorphin concentrations could be used as a sensitive indicator of thermal tolerance before and after acclimation. The use of plasma cortisol as a specific indicator of heat stress and thermal tolerance before or after acclimation in exercising horses appears limited.
mesh_terms: D000064:Acclimatization / Q000502:physiology; D000818:Animals; D004298:Dopamine / Q000097:blood; D004837:Epinephrine / Q000097:blood; D005080:Exercise Test / Q000662:veterinary*; D005260:Female; D006736:Horses / Q000097:blood / Q000502:physiology*; D006358:Hot Temperature / Q000009:adverse effects; D006813:Humidity / Q000009:adverse effects; D006854:Hydrocortisone / Q000097:blood; D008297:Male; D009490:Neurosecretory Systems / Q000502:physiology*; D009638:Norepinephrine / Q000097:blood; D010805:Physical Conditioning, Animal / Q000502:physiology*; D013312:Stress, Physiological / Q000097:blood / Q000503:physiopathology / Q000662:veterinary; D001615:beta-Endorphin / Q000097:blood
chemical_list: D001615:beta-Endorphin; D004298:Dopamine; D006854:Hydrocortisone; D009638:Norepinephrine; D004837:Epinephrine