Comparison of the adrenocortical response to both pharmacological and physiological stresses in sport horses.

Seven sport horses were investigated to compare physiological and pharmacological stresses on the plasma glucocorticosteroid levels. Venous blood was sampled and analysed for cortisol concentrations before and after each type of stress. The physiological stress was induced by the completion of a cross-country and the pharmacological one, by an intravenous ACTH administration (200 micrograms) in the same horses, one week after the course. Both exercise and ACTH injection induced a highly significant cortisol increase of 79 +/- 0.10% and 160 +/- 0.18%, respectively. The relative plasma cortisol increases due to exercise on the one hand, and due to ACTH administration on the other hand, were correlated with a coefficient of 0.82. It was concluded that the plasma cortisol increase induced by a standardised pharmacological stress could be used in order to predict the exercise-induced stress response.