Effects of cool and hot humid environmental conditions on neuroendocrine responses of horses to treadmill exercise.
Authors: Williams R J, Marlin D J, Smith N, Harris R C, Haresign W, Davies Morel M C
Journal: Veterinary journal (London, England : 1997)
Summary
# Editorial Summary: Neuroendocrine Responses to Exercise in Heat and Humidity Understanding how environmental stress interacts with exercise physiology is crucial for managing horses competing in hot, humid conditions or preparing them for such challenges. Williams and colleagues examined five horses performing a competition-standard exercise test (simulating a three-day event speed and endurance phase) in both cool conditions (20°C/40% relative humidity) and hot humid conditions (30°C/80% RH), with testing repeated before and after a 15-day heat acclimation protocol. Hot humid conditions substantially elevated catecholamine responses—adrenaline and noradrenaline concentrations increased significantly during and after exercise, with adrenaline remaining elevated even five minutes into recovery—whilst also elevating beta-endorphin and cortisol levels compared to cool conditions. Following the 15-day acclimation period, the neuroendocrine response pattern shifted meaningfully: adrenaline concentrations at two minutes recovery and cortisol during the initial phases of exercise increased further, suggesting these hormones facilitate thermal adaptation, though noradrenaline and beta-endorphin responses showed minimal change post-acclimation. For practitioners preparing competition horses for heat exposure, these findings indicate that plasma adrenaline and noradrenaline may be useful markers of the adaptive process during heat acclimation, whilst plasma cortisol appears less specific as a heat stress indicator; this work underscores the physiological reality that heat stress compounds exercise demand and warrants systematic acclimation protocols in vulnerable individuals.
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Practical Takeaways
- •Horses performing endurance exercise in hot, humid conditions show markedly elevated stress hormone responses; conditioning in these conditions may help modulate this response over time.
- •Beta-endorphin changes may offer a more sensitive marker of heat adaptation than cortisol alone when assessing whether horses are tolerating thermal stress effectively.
- •Heat acclimation over 15 days alters the neuroendocrine response pattern to exercise; this should be considered when preparing competition horses for warm-climate events.
Key Findings
- •Plasma adrenaline and noradrenaline concentrations were significantly elevated during hot humid exercise compared to cool dry conditions, with adrenaline remaining elevated 5 minutes into recovery (P<0.001).
- •A 15-day heat acclimation period significantly increased plasma adrenaline at 2 min recovery (P<0.001) and cortisol at end of Phase B (P<0.01) compared to pre-acclimation.
- •Plasma beta-endorphin concentrations increased during hot humid pre-acclimation exercise and showed a trend toward lower levels post-acclimation, suggesting potential use as a thermal tolerance indicator.
- •Cortisol alone appears limited as a specific indicator of heat stress and thermal tolerance in exercising horses before or after acclimation.