Training- and competition-related changes on circulating adrenocorticotropic hormone and cortisol response in Thoroughbred Racehorses

Background: Exercise is a physiological challenge inducing intensity-dependent hypothalamic-pituitary-adrenal (HPA) axis responses. The hypothesis was that training and racing sessions induce different HPA axis activation. Aims/objective: To assess whether different exercise sessions affect equine adrenal responsiveness according to age and sex. Methods: Ten clinically healthy Thoroughbred racehorses (6 females and 4 males) were recruited. Four were 2 years old and six were 3 years old. Horses were subjected to two exercise conditions: training and competitive racing sessions. Blood samples were collected at rest and at 5- and 30-min post-exercise to assess circulating adrenocorticotropin (ACTH) and cortisol changes. To determine the effects of exercise, age, and sex on hormonal responses over baseline values, one-way analysis of variance (ANOVA) was applied. Results: Compared to baseline concentrations, ACTH and cortisol concentrations increased at 5 and 30 min (P < .001) after both training and racing sessions. Compared with training, racing horses had greater ACTH increases at baseline (Δ% 64) and 5 min (Δ% 110), and 30 min (Δ% 283) post-exercise. Cortisol concentrations increased more after racing than training at 5 min (Δ% 89) and 30 min (Δ% 128). Conclusion: Results showed that training and competitive racing differentially affected the HPA axis response, with a potential preparatory or anticipatory reaction to exercise before racing.