Hypothalamic-pituitary-adrenal axis response and glucose homeostasis during exercise to fatigue before and after training in old and young adult Standardbred mares

BACKGROUND: While reduced cortisol responses to exercise have been described in older horses, the effects of aging and training on the hypothalamic-pituitary-adrenal axis (HPA axis) and downstream metabolic signaling are currently unknown. AIMS / OBJECTIVES: This study tested the hypothesis that age and exercise training alter the response of the HPA axis, insulin, and glucose during an acute exercise challenge to fatigue. METHODS: Six old (22.0 ± 0.7 years) and six young adult (7.3 ± 0.6 years) unfit Standardbred mares were tested. Mares ran a graded exercise test before (GXT1) and after (GXT2) 8 weeks of training (∼60% of HRmax, 3 days/week). Data was analyzed using mixed-effects models with significance of fixed effects determined by ANOVA. RESULTS: There was a significant effect of training on plasma glucose, with higher concentrations during GXT1 vs. GXT2 (P = 0.01), but there was no effect of age. Young horses had lower plasma insulin concentrations than old horses during GXT2 (P = 0.005), but not during GXT1. Age or training did not significantly affect plasma cortisol concentrations, but plasma ACTH was higher during GXT2 vs. GXT1 (P = 0.03), with no effects of age. At VO₂max, the relationships between ACTH and cortisol were negative during GXT1 and positive during GXT2 for both age groups; however, these findings were not statistically significant. CONCLUSION: Training, but not aging, alters horses' ACTH and glucose responses to acute exercise. Aging affects horses' insulin response during acute exercise after training, with young horses having lower insulin responses post training than old horses.