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

Overview

• This study investigated how aging and exercise training influence hormonal and metabolic responses, specifically the hypothalamic-pituitary-adrenal (HPA) axis, insulin, and glucose levels, during exercise to fatigue in young and old Standardbred mares.
• The research found that exercise training modified ACTH and glucose responses, while aging influenced insulin levels after training, highlighting differences in metabolic adaptation between young and old horses.

Introduction and Research Questions

• The HPA axis plays a central role in the physiological stress response by regulating cortisol and ACTH secretion during exercise.
• Previous studies showed older horses have lower cortisol responses during exercise, but the effect of aging combined with training on the HPA axis and metabolism was unclear.
• This study aimed to test if both age and exercise training change the HPA axis response and subsequent glucose and insulin dynamics during an acute exercise challenge to fatigue.

Methods

• Subjects: Twelve unfit Standardbred mares divided into two age groups:
  • Old mares: average age 22.0 years
  • Young mares: average age 7.3 years
• Exercise protocol: Each mare performed a graded exercise test to exhaustion (GXT) twice:
  • GXT1 before training
  • GXT2 after 8 weeks of training
• Training regimen: Approximately 60% of maximal heart rate, 3 days per week, for 8 weeks.
• Measures taken: Plasma concentrations of ACTH, cortisol, insulin, and glucose were monitored throughout exercise tests.
• Statistical analysis: Mixed-effects models with ANOVA to evaluate the effects of age, training, and their interaction on measured parameters.

Results

• Effect of Training on Glucose:
  • Plasma glucose was significantly higher during the initial exercise test (GXT1) compared to after training (GXT2), suggesting improved glucose utilization with training (P = 0.01).
  • No effect of age on glucose levels during exercise was observed.
• Effect of Age on Insulin:
  • After training (GXT2), young horses showed lower insulin concentrations than old horses during exercise (P = 0.005), indicating age-related differences in insulin response to exercise post-training.
  • No significant age difference in insulin levels was found before training (GXT1).
• Effect of Training and Age on Cortisol and ACTH:
  • Plasma cortisol levels did not significantly change with training or age, contrary to previous assumptions of reduced cortisol with aging.
  • Plasma ACTH concentrations were higher after training (GXT2) compared to before training (GXT1) regardless of age (P = 0.03), suggesting training enhances ACTH response during exercise.
  • The correlation between ACTH and cortisol at maximal oxygen consumption (VOmax) changed: negative correlation pre-training and positive correlation post-training, though these changes were not statistically significant.

Conclusions and Implications

• Training modifies horses’ HPA axis responses by increasing ACTH release during acute exercise, which may contribute to improved stress and metabolic regulation with conditioning.
• Aging alone does not significantly alter cortisol or ACTH responses but does affect insulin response during exercise after training, with older horses maintaining higher insulin levels, indicating possible differences in insulin sensitivity or glucose metabolism with age.
• The reduction in plasma glucose levels post-training suggests enhanced glucose uptake or utilization during exercise after conditioning, independent of age.
• These findings enhance the understanding of how metabolic and hormonal responses adapt to exercise and aging in horses, which can inform training and management practices to optimize health and performance in older and younger equine athletes.