Dynamic exercise discloses different time-related responses in stress hormones

Objective: Responses to stressful events are generally regarded as reaction s of the organism to accommodate to or compensate for stress. This reaction is classically described as an activation of the sympathoadrenal system an d the hypothalamic-pituitary-adrenocortical (HPA) axis. Activation of the r elease of growth hormone and prolactin in blood also occurs during various types of stress. Assuming that the stress response is a neuroendocrine mech anism that occurs in anticipation of physical exercise, we investigated whe ther an incremental exercise protocol can be used as a model stressor to di sclose a distinct pattern of activation in these hormonal systems, which wo uld support the notion that these systems have different roles in preparing the organism for physical activity and recovery. Moreover, such a model ma y help improve our understanding of the endocrine expressions of psychologi cal stress. Methods: After an overnight fast, 8 healthy men (age, 19-26 yea rs) cycled at 40, 60, 80, and 100% of the power output at (V)over doto(2max ) in successive time blocks of 10 minutes each up to exhaustion. Venous blo od was sampled immediately before exercise, at the end of each block, and d uring the recovery phase 5 and 30 minutes after exercise. Plasma adrenalin and noradrenalin were measured by high-performance liquid chromatography; p lasma adrenocorticotropic hormone, beta -endorphin, cortisol, growth hormon e, and prolactin were measured by specific immunoassays. Heart rate and lev els of blood lactate and adrenalin were measured as markers of workload-rel ated responses. Results: Results showed that increases in heart rate, lacta te, adrenalin, noradrenalin, and growth hormone reflected the relative work load, in contrast to increases in adrenocorticotropic hormone, beta endorph in, and prolactin, which were observed only after exercise reached an inten sity of 80% (V)over doto(2max). 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 mu scular effort and protective against tissue damage. Conclusions: Measuremen t of the cumulative response to exercise shows that activation of stress ho rmones occurs at different time points, supporting the notion that these ho rmones 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 involv ement of different neurobiological and cognitive emotional mechanisms.