Cortisol disposition and production rate in horses during rest and exercise.

This research examines how a strenuous 56-km endurance exercise affects cortisol levels in horses. It was observed that physical exercise led to a two to threefold spike in plasma cortisol, but these values quickly dropped to pre-exercise levels. The cortisol production rate was notably higher during exercise compared to rest.

Understanding Cortisol Disposition and Production During Exercise

The study focused on the following key areas:

• The cortisol in horses’ bodies due to a highly strenuous endurance exercise was measured. This 56-km exercise was observed to cause an immediate increase in plasma cortisol — a stress hormone produced by the adrenal glands.
• Six horses were involved in this study, and they were given a dose of [3H]cortisol before their exercise. The increase ranged between two- and threefold, yet the levels returned rapidly to pre-exercise status.

Cortisol Clearance and Steady-State Volume

Key findings on clearance and steady-state volume of cortisol are:

• The researchers noted that during exercise, the clearance (the process of removing cortisol from the bloodstream) and the steady-state volume (the even concentration of cortisol in the body) of total cortisol escalated.
• The exact figures jumped from 137 to 338 ml.kg-1.h-1 for clearance, and from 229 to 359 ml/kg for volume distribution.
• However, the terminal half-life of cortisol (the time taken for the concentration to reduce by half) lessened significantly from 1.55 to 0.97 hours.

Estimated Cortisol Production Rate

The study reported the following findings on cortisol production:

• At rest, the estimated cortisol production rate was at 4.41 micrograms.kg-1.h-1.
• During exercise, this figure increased significantly to 26.75 micrograms.kg-1.h-1. This demonstrates the weight of the impact exercise has on the adrenal secretion rate.

Implications to Assess Adrenal Gland Function

The researchers concluded that:

• Exercise induced a major increase (sixfold) in the adrenal secretion rate (rate of cortisol production) in horses.
• The critical observation, however, was that this increase wasn’t represented accurately by the more limited two- to threefold increase in plasma cortisol levels.
• Therefore, to evaluate the function and response of the adrenal gland during physical activity, plasma cortisol clearance must be measured.