Objective: Responses to stressful events are generally regarded as reactions of the organism to accommodate to or compensate for stress. This reaction is classically described as an activation of the sympathoadrenal system and the hypothalamic-pituitary-adrenocortical (HPA) axis. Activation of the release of growth hormone and prolactin in blood also occurs during various types of stress. Assuming that the stress response is a neuroendocrine mechanism that occurs in anticipation of physical exercise, we investigated whether an incremental exercise protocol can be used as a model stressor to disclose a distinct pattern of activation in these hormonal systems, which would support the notion that these systems have different roles in preparing the organism for physical activity and recovery. Moreover, such a model may help improve our understanding of the endocrine expressions of psychological stress.
Methods: After an overnight fast, 8 healthy men (age, 19-26 years) cycled at 40, 60, 80, and 100% of the power output at VO2max in successive time blocks of 10 minutes each up to exhaustion. Venous blood was sampled immediately before exercise, at the end of each block, and during the recovery phase 5 and 30 minutes after exercise. Plasma adrenalin and noradrenalin were measured by high-performance liquid chromatography; plasma adrenocorticotropic hormone, beta-endorphin, cortisol, growth hormone, and prolactin were measured by specific immunoassays. Heart rate and levels of blood lactate and adrenalin were measured as markers of workload-related responses.
Results: Results showed that increases in heart rate, lactate, adrenalin, noradrenalin, and growth hormone reflected the relative workload, in contrast to increases in adrenocorticotropic hormone, beta endorphin, and prolactin, which were observed only after exercise reached an intensity of 80% VO2max. Increases in cortisol were found just after exhaustion. The delayed response of cortisol may be initiated by a drop in blood glucose levels but may also be considered preparatory to vigorous muscular effort and protective against tissue damage.
Conclusions: Measurement of the cumulative response to exercise shows that activation of stress hormones occurs at different time points, supporting the notion that these hormones have different roles in preparing the organism for physical activity and recovery: ie, workload- and effort-related adaptation on one hand and protection against disturbed homeostasis on the other. The delayed response of the HPA axis during incremental exercise contrasts with the nondelayed HPA axis response observed during psychological stress and points to involvement of different neurobiological and cognitive emotional mechanisms.