Plasma cortisol concentration in thoroughbred 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 hors es performing two different exercise tests an a treadmill was investigated to study the possible value of measuring cortisol for performance diagnosis . In addition, the horses were exercised with different conditioning progra mmes and their effect on basal plasma cortisol concentration was measured. Finally, the repeatability of the cortisol values measured before and after exercise were examined. Hones were always exercised on a treadmill. The mu ltiple step exercise test consisted of five gallop workouts lasting five mi nutes' each. The velocity in the first step was 6.0 mis. Each consecutive s tep velocity was increased by 0.5 m/s. The two-speed exercise test consiste d of two runs. In the first Fun hones were galloped over 1,200 m at a const ant velocity of 10, 10.5 or 1 1 m/s. Thereafter hones were walked for 30 mi nutes. The second run over 1,200 m was conducted at speeds of 13, 13.5 or 1 4 m/s. Each conditioning programme examined consisted of eleven repetitions of exercise of 5, 15 or 25 minutes' duration at a velocity at which, mathe matically, hones had a blood lactate concentration of 2.5 or 4 mmol/l. During the multiple step test the mean cortisol concentration in plasma sho wed a tendency to increase (p = 0.07). The mean peak of the cortisol concen tration in plasma was measured 10 minutes after the test. Thereafter mean c ortisol concentration continuously decreased and the day after the test val ues were similar to those before the test. After the first run of the two-s peed 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 increase d threreafter reaching a plateau between the 15th and 45th minute after exe rcise (p < 0.05). The morning after the test mean plasma cortisol concentra tion had returned to the values measured the morning before the test. None of the conditioning programmes had on effect on the mean plasma concentrati on of cortisol in the horses of rest. The mean coefficient of variation of plasma cortisol concentration in five hones before exercise was 34.1% and 3 6.4% after exercise. The large individual variability of plasma cortisol concentrations before a nd after exercise does not allow a good repeatability of results, and enfor ces 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 supp ly a basis for studies on treadmills on the value of monitoring the plasma cortisol concentration of sport horses for performance diagnosis.