Plasma cortisol and β-endorphin concentrations in trained and over-trained standardbred racehorses

The effects of training and over-training on plasma cortisol and β-endorphin (βEP) concentrations at rest and after standardised exercise tests and the cortisol responses to adrenocorticotropin (ACTH) administration were investigated in standardbred horses. Twelve horses were divided randomly into control and over-trained (OT) groups after 17 weeks slow- and moderate-intensity treadmill training. The standardised treadmill exercise test consisted of 2 min at velocities corresponding to 30, 50, 70 and 100% of maximum O2 consumption. Over-training, defined as a significant decrease in body weight and treadmill run-time-to-fatigue in an incremental velocity test, occurred in the OT group after 32 weeks of training exercise. Peak cortisol concentrations after exercise decreased significantly in the OT group from 320±15.6 at week 8 to 245±17.0 nmol l–1 at week 32, and mean cortisol concentrations over a 120-min period after exercise decreased from 258±11.7 to 192±16.6 nmol l–1 (P<0.05). Mean and total cortisol and βEP concentrations in resting horses were not significantly different after over-training. Peak cortisol concentrations after adrenocorticotropin (ACTH) administration were not significantly different in the over-trained group. Dysfunction of the hypothalamo-pituitary-adrenocortical axis occurs in over-trained horses, but this adaptation is not associated with a change in the adrenocortical responsiveness to ACTH.