The effects of an endurance ride on neutrophil functions in endurance-trained horses were evaluated and related to metabolic changes and changes in cortisol concentrations. Blood samples were taken from 7 horses (aged 9–15 years) one day before, and then 30–60 min, 1 day and 8 days after the ride. The race resulted in elevated serum cortisol levels (<465 nmol/l) and an increased neutrophil:lymphocyte ratio. Immediately post race, the neutrophil ability to engulf yeast was increased. One day after the race, a decrease in leukotriene B4 production (approximately 40%) and in the respiratory burst (approximately 75%) was observed.
Blood glucose concentration remained unchanged, as did serum lactate, which was low. After the race, the muscle glycogen stores were about 33% of the values noted after 8 days of recovery. Histochemical staining showed that 22 ± 2% of the muscle fibres were totally depleted of glycogen. CK activity increased significantly from 40 ± 10 mmol/min pre-race to 228 ± 38 mmol/min post race. It was concluded that both hormonal and metabolic changes occurred during an endurance ride, which not only triggered neutrophil activation, but also induced alterations in their functional capacities.