Plasma adrenocorticotropin, cortisol and catecholamines response to various exercises.

The responses of plasma adrenocorticotropin (ACTH), cortisol, noradrenaline and adrenaline in 5 Thoroughbred horses to an incremental exercise and 2 relative workload exercises, at 105 and 80% maximal oxygen consumption (VO2max), on a treadmill were examined. These hormone concentrations increased (P < 0.05) with each exercise and the maximal plasma concentrations of ACTH, cortisol were observed between 5 and 30 min after the end of the exercise, while maximal catecholamine concentrations occurred just at exhaustion time. The plasma ACTH, noradrenaline and adrenaline responses during exercise were more sensitive to the intensity of exercise than that of cortisol and showed a significant correlation with blood lactate concentrations (r = 0.605, P < 0.001 for ACTH; r = 0.718, P < 0.001 for noradrenaline; r = 0.738, P < 0.001 for adrenaline). The plasma cortisol response appeared to be connected with the duration of exercise (r = 0.71, P < 0.05). The recovery of these hormones was related to the exercise styles. These results suggest that the autonomic nervous system and the pituitary-adrenal axis of the horse are efficiently stimulated by various treadmill exercises, and these hormones may be used in the evaluation of exercise-induced stress.