The present study was conducted in order to describe human hypothalamo-pituitary adrenal (HPA) axis adaptation in a model of repeated physical stress (endurance training) that causes a moderate increase in cortisol levels.SUBJECTS
We performed the same stimulation tests (adrenal stimulation with ACTH or pituitary stimulation with combined CRH/LVP) in a population of 8 endurance-trained athletes in two distinct situations: resting (baseline cortisol values) and 2 h after the end of strenuous exercise (increased cortisol values) to evaluate the HPA axis sensitivity to endogenous sustained increases in cortisol concentrations.MEASUREMENTS
During these tests, saliva and plasma cortisol (Fs and Fp, respectively) were assessed and compared.RESULT
Cortisol values in both plasma and saliva at the end of 2 h of exercise were significantly higher than in rested controls: Fs 11.5 ± 1.3 vs 6.5 ± 0.8 nmol.l−1 and Fp 428 ± 36 vs 279 ± 27 nmol.l−1 (post exercise vs post rest sessions, respectively, P < 0.001 for both). After either hormone test (CRH/LVP or ACTH), cortisol levels in plasma and saliva increased similarly when rest was compared to post exercise.RESULT
Saliva variations (Δ%) under exogenous hormone stimulation were dramatically greater than plasma variations. For example, under ACTH stimulation, the relative increments in cortisol were on control day: ΔFs 980 ± 139 vs ΔFp 218 ± 43% (saliva vs plasma, respectively, P < 0.05) and on exercise day: ΔFs 605 ± 89 vs ΔFp 102 ± 14% (saliva vs plasma, respectively, P < 0.05).CONCLUSIONS
In endurance-trained athletes, displaying a moderate but sustained endogenous cortisol increase: (1) ACTH responses following pituitary stimulation are not blunted, (2) cortisol responses following maximal adrenal stimulation are not blunted. Our results favour the hypothesis of a decreased pituitary sensitivity to cortisol negative feedback whereas the hypothesis of a major decreased adrenal sensitivity to ACTH was discarded. The greater ability of saliva assays to detect a cortisol increase strongly supports its use in the study of HPA physiology, whether under basal or dynamic conditions.