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

The effects of training and over-training on plasma cortisol and P-endorphi n (PEP) concentrations at rest and after standardised exercise tests and th e cortisol responses to,adrenocorticotropin (ACTH) administration were inve stigated in standardbred horses. Twelve horses were divided randomly into c ontrol and over-trained (OT) groups after 17 weeks slow- and moderate-inten sity treadmill training, The standardised treadmill exercise test consisted of 2 min at velocities corresponding to 30, 50, 70 and 100% of maximum O-2 consumption. Overtraining, defined as a significant decrease in body weigh t and treadmill run-time-to-fatigue in an incremental velocity test, occurr ed in the OT group after 32 weeks of training exercise, Peak cortisol conce ntrations 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 conc entrations over a 120-min period after exercise decreased from 258+/-11.7 t o 192+/-16.6 nmol l(-1) (P<0.05).: Mean and total cortisol and PEP concentr ations in resting horses were not significantly different after over-traini ng. Peak cortisol concentrations after adrenocorticotropin (ACTH) administr ation were not significantly different in the over-trained group. Dysfuncti on of the hypothalamo-pituitary-adrenocortical axis occurs in over-trained horses, but this adaptation is not associated with a change in the adrenoco rtical responsiveness to ACTH.