Plasma cortisol concentration in Thorougbred horses during and after standardized exercise tests on a treadmill and effect of conditioning on basal cortisol values

The behaviour of cortisol concentration in plasma of clinically normal horses performing two different exercise tests on a treadmill was investigated to study the possible value of measuring cortisol for performance diagnosis. In addition, the horses were exercised with different conditioning programmes and their effect on basal plasma cortisol concentration was measured. Finally, the repeatability of the cortisol values measured before and after exercise were examined. Horses were always exercised on a treadmill. The multiple step exercise test consisted of five gallop workouts lasting five minutes each. The velocity in the first step was 6.0 m/s. Each consecutive step velocity was increased by 0.5 m/s. The two-speed exercise test consisted of two runs. In the first run horses were galloped over 1,200 m at a constant velocity of 10, 10.5 or 11 m/s. Thereafter horses were walked for 30 minutes. The second run over 1,200 m was conducted at speeds of 13, 13.5 or 14 m/s. Each conditioning programme examined consisted of eleven repetitions of exercise of 5, 15 or 25 minutes duration at a velocity at which, mathematically, horses had a blood lactate concentration of 2.5 or 4 mmol/l. During the multiple step test the mean cortisol concentration in plasma showed a tendency to increase (p = 0.07). The mean peak of the cortisol concentration in plasma was measured 10 minutes after the test. Thereafter mean cortisol concentration continuously decreased and the day after the test values were similar to those before the test. After the first run of the two-speed test mean plasma cortisol concentration tended to increase (p = 0.08), while significant changes were registered after the second run (p < 0.01): It decreased up to the fifth minute after exercise (p < 0.05) and increased threreafter reaching a plateau between the 15th and 45th minute after exercise (p < 0.05). The morning after the test mean plasma cortisol concentration had returned to the values measured the morning before the test. None of the conditioning programmes had an effect on the mean plasma concentration of cortisol in the horses at rest. The mean coefficient of variation of plasma cortisol concentration in five horses before exercise was 34.1% and 36.4% after exercise. The large individual variability of plasma cortisol concentrations before and after exercise does not allow a good repeatability of results, and enforces the need for multiple blood sampling during exercise. The multiple step exercise test as well as the two-speed exercise test induced increases of the plasma cortisol concentration after exercise. The changes measured supply a basis for studies on treadmills on the value of monitoring the plasma cortisol concentration of sport horses for performance diagnosis.