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

The research done in this article primarily studies the impact of over-training on the plasma cortisol and beta-endorphin levels in racehorses, and also examines their adrenal responsiveness to ACTH. Results showed that over-trained horses displayed signs of hypothalamo-pituitary-adrenocortical axis dysfunction, but their response to ACTH did not change.

Details of the Study

• The research involved a total of twelve standardbred horses that were divided into control and over-trained(OT) groups.
• Both groups underwent 17 weeks of slow- to moderate-intensity treadmill training post which the standardised exercise tests were administered.
• The exercise tests involved durations of 2 minutes at speeds matching 30%, 50%, 70%, and 100% of the maximum oxygen consumption.

Findings

• Over-training was identified as a significant reduction in body weight as well as treadmill run-time-to-fatigue in the velocity test. This condition manifested in the OT group after 32 weeks of the training routine.
• Peak cortisol concentrations after exercise dropped significantly in the OT group, from 320+/-15.6 nmol l(-1) at week 8 to 245+/-17.0 nmol l(-1) at week 32. The mean cortisol concentrations over 120 minutes after exercising fell from 258+/-11.7 to 192+/-16.6 nmol l(-1).
• In restive states, average and total cortisol and beta-endorphin concentrations did not vary significantly after over-training.
• Peak cortisol concentrations after ACTH administration displayed no significant variance in the over-trained group.

Conclusions

• The study concludes a dysfunction of the hypothalamo-pituitary-adrenocortical axis in over-trained horses. This was evident through the noted decrease in cortisol concentrations post exercise.
• The adrenocortical responsiveness to ACTH, however, did not change in over-trained horses signifying that the adaptation on overtraining is not associated with a change in this response.