Author(s): MUKAI, K., EBISUDA, Y., TAKAHASHI, Y., SUGIYAMA, F., KITAOKA, Y., Institution: EQUINE RESEARCH INSTITUTE, JAPAN RACING ASSOCIATION, Country: JAPAN, Abstract-ID: 602

INTRODUCTION:
Heat acclimation improves exercise performance in hot environments and reduces the risk of heat-related illness. Heat stress is associated with beneficial adaptations in skeletal muscle, including mitochondrial biogenesis. However, the effects of heat acclimation on the skeletal muscle metabolome remain unclear, and a comprehensive understanding of the underlying mechanisms is needed. Therefore, this study investigated the effects of heat acclimation on the skeletal muscle metabolome in horses.
METHODS:
Five trained Thoroughbred horses (two geldings and three mares, 6–8 years old) completed three weeks of heat acclimation (three sessions per week). The training protocol consisted of high-intensity interval training (3 × 60 s at 100% VO₂max with 120 s of recovery at 30% VO₂max) performed under hot conditions (wet bulb globe temperature [WBGT]: 30°C). Pulmonary artery temperature during exercise was measured, and venous blood samples were collected immediately after exercise for plasma lactate analysis. Biopsy samples from the middle gluteal muscle were collected pre- and immediately post-exercise on the first day (Day 1) and the final day (Day 9) of heat acclimation for metabolomic analysis. The effects of heat acclimation, exercise, and their interaction were analyzed using mixed-effects models (P < 0.05).
RESULTS:
After heat acclimation, peak pulmonary artery temperature and plasma lactate concentration were reduced from 41.7 to 41.1°C and from 18.7 to 12.1 mmol/L, respectively. After exercise, 14 and 19 of 116 metabolites were significantly increased on Day 1 and Day 9, respectively, whereas 4 and 7 metabolites were significantly decreased on Day 1 and Day 9, respectively. Pre-exercise NADP⁺ concentration was lower on Day 1 than on Day 9, whereas post-exercise XDP concentration was lower on Day 9 than on Day 1. Post-exercise citric acid concentration was higher on Day 9 than on Day 1. The lactate/pyruvate ratio was increased after exercise only on Day 1, whereas UDP-glucose, glutathione (GSSG), and total glutamate-related amino acids were decreased after exercise only on Day 9.
CONCLUSION:
These findings suggest that heat acclimation induces a metabolic reprogramming in skeletal muscle characterized by enhanced mitochondrial oxidation and improved redox regulation, resulting in more efficient and tightly regulated metabolic responses to exercise. This study provides new insights into the metabolic mechanisms underlying heat acclimation in equine skeletal muscle.