CORTISOL DISPOSITION AND PRODUCTION-RATE IN HORSES DURING REST AND EXERCISE

The influence of a 56-km endurance exercise on cortisol kinetics and p roduction rate was evaluated in six horses administered [H-3]cortisol. Exercise resulted in an immediate two- to threefold increase in plasm a cortisol, with values returning very rapidly to preexercise levels. During exercise, clearance and steady-state volume of distribution of total cortisol were greatly increased (338 +/- 95 vs. 137 +/- 34 ml . kg(-1) . h(-1) for clearance and 359 +/- 82 vs. 229 +/- 18 ml/kg for v olume of distribution), whereas the terminal half-life decreased signi ficantly (0.97 +/- 0.16 vs. 1.55 +/- 0.33 h). The estimated cortisol p roduction rate was 4.41 +/- 1.06 mu g . kg(-1) . h(-1) at rest and 26. 75 +/- 5.11 mu g . kg(-1) . h(-1) during exercise. We conclude that ex ercise triggers a large increase (x6) in the adrenal secretion rate, w hich is not accurately reflected by the more limited increase (x2-3) i n plasma cortisol concentration, the actual measurement of plasma cort isol clearance being a prerequisite to assessment of adrenal gland fun ction during exercise.