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Dynamic Exercise Discloses Different Time-Related Responses in Stress Hormones

From the Department of Medical Physiology and Sports Medicine (W.R.dV., N.T.M.B, M.H.dR.), University Medical Center Utrecht; and the Department of Endocrinology (H.P.F.K.), University Hospital Utrecht, Utrecht, The Netherlands.

Address reprint requests to: Wouter R. de Vries, MD, PhD, Department of Medical Physiology and Sports Medicine, University Medical Center Utrecht, Universiteitsweg 100, De Uithof, 3584 CG Utrecht, The Netherlands. Email: W.R.devries{at}med.uu.nl

	ABSTRACT

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION ACKNOWLEDGMENTS REFERENCES

 
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 O_2max 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, ß-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, ß endorphin, and prolactin, which were observed only after exercise reached an intensity of 80% O_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 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.

Key Words: blood lactate • sympathoadrenal system • hypothalamus-pituitary-adrenocortical axis • homeostasis • growth hormone • exercise.

Abbreviations: ACTH = adrenocorticotropic hormone; ßE = ß-endorphin; CV = coefficient of variation; GH = growth hormone; HPA = hypothalamic-pituitary-adrenocortical; HR = heart rate; PRL = prolactin; SAS = sympathoadrenal system; O_2max = peak oxygen uptake.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION ACKNOWLEDGMENTS REFERENCES

 
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 neuroendocrine SAS and the HPA axis (1–3), resulting in increases in plasma levels of noradrenalin and adrenalin (SAS) and ACTH, ßE, and cortisol (HPA axis). It has also been shown that GH and PRL are stress related because their release increases in response to hypoglycemia, exercise, and surgery (4, 5).

Assuming that the stress response is a neuroendocrine mechanism that occurs in anticipation of physical action, this study was designed to investigate whether an incremental exercise protocol can be used as a model stressor to separate the pattern of activation of these hormonal systems. We used exercise as the stressor because of its reliability, reproducibility, and safety in research. Exercise causes stress of different degrees depending on the type and duration of exercise. It is mainly physical in character, but competitive events are also associated with psychological stress (6). Moreover, such a stress model may be helpful in understanding the endocrine expressions of psychological stress. Activation of the SAS (measured as changes in plasma levels of adrenalin and noradrenalin) can be categorized as "neural" activation of noradrenalin, which is more effort related, and "hormonal" activation of adrenalin, which is more workload related and contributes to the ergotropic tuning of the organism. As long as the organism remains between the physiological limits of compensation, there will be no activation of the HPA axis (measured as changes in plasma levels of ACTH, ßE, and cortisol). In cases of disturbed homeostasis, the HPA axis will be activated. Such a delayed response of the HPA axis is less likely to be observed during psychologically engendered forms of stress. Although common output pathways and peripheral mechanisms are involved in exercise and psychological stress, there is an important difference. Unlike exercise stress, psychological stress is phylogenetically associated with the classic flight-fight response, in which powerful negative emotional components seem to induce more direct activation of the HPA axis (7). We investigated whether there is a difference in timing between the responses of the SAS and those of the HPA axis as a function of relative workload. Such information could contribute to a better understanding of the different roles of these systems 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. We used changes in HR, blood lactate level, and adrenalin level as markers of workload-related responses. Up to now exercise-induced secretion of GH has been reported to have both similar and different time patterns compared with PRL (8–11). Therefore, we also investigated the responses of GH and PRL to incremental exercise to gain insight into their physiological roles during exercise. Moreover, the impact of purely psychological stress on these hormones is uncertain because GH and PRL responses are studied less frequently under such conditions (4).

	SUBJECTS AND METHODS

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Subjects
Eight healthy male students participated in this study, which was approved by the ethics committee of the University Hospital Utrecht. Aims and methods were explained to the subjects, who then gave informed consent. Pertinent physical characteristics of the subjects were as follows (values are mean SD): age, 21.9 2.4 years; height, 179 5.8 cm; body weight, 71.4 6.4 kg; body mass index, 22.3 1.2 kg/m²; percentage of body fat (estimated from the sum of 4 skin-fold measurements, Ref. 12), 14.4 2.9%.

Experimental Protocol
During the week before the experiment, a maximal exercise test on a cycle ergometer (Excalibur, Lode, The Netherlands) was performed to determine O_2max (Oxycon Beta, Mijnhardt, The Netherlands) and accessory maximal power output. Exercise was initially performed with a load of 50 W for 2 minutes at a pedal frequency of 70 to 90 rpm, and the load was then increased by 50 W every 2 minutes up to a level of 4 W per kilogram of body weight, followed by an increase of 25 W every 2 minutes until the subjects were exhausted.

On the day of the experiment, subjects reported to the laboratory between 8 and 9 AM after an overnight fast. To induce stress-related responses, an incremental exercise test on a cycle ergometer was applied as a stimulus ( Figure 1). Subjects cycled at 40, 60, 80, and 100% of the power output at O_2max in successive time blocks of 10 minutes. During the last block they continued to exercise to exhaustion. The exercise session was followed by a passive recovery period of 30 minutes, during which subjects rested in a sitting position. Such a protocol induces mainly physical stress. Because the subjects were familiar with the test procedures, psychological stress was limited to the period during which subjects exercised to exhaustion.

View larger version (14K): [in this window] [in a new window]   Fig. 1. Protocol of the incremental exercise test. Arrows indicate times of blood collection (before exercise, at the end of each block, and after 5 and 30 minutes of recovery). As an example, the duration of the last block (power output at 100% O2max) is set at 2 minutes.

Heart Rate
HR was measured continuously with a Polar Sporttester (PE 3000, Support) during exercise and the first 5 minutes of recovery.

Statistical Analysis
Statistical analyses were performed with SPSS software, version 4.01 (SPSS Inc., Chicago, IL). Differences in exercise-induced responses of blood lactate and hormones were analyzed with nonparametric statistics because assumptions of parametric statistics were not met for all data. Friedman two-way analysis of variance was used, with a level of significance of 5% (two tailed). Hormone response data are given as mean SEM, whereas other data are given as mean SD.

	RESULTS

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Maximal Exercise Test
O_2max was 4.2 0.6 liter/min, which corresponds to 59.2 5.9 ml/min per kg body weight. The accessory power output at O_2max was 359 42 W, which corresponds to 5.0 0.5 W/kg. For each subject the power output at O_2max was used to calculate power output at a level of 40, 60, and 80%.

Incremental Exercise Test
Mean HR increased from 68 14 beats/min before exercise to 204 11 beats/min at exhaustion (p < .01). After 5 minutes of recovery, HR declined to 142 17 beats/min (p < .01). Mean lactate levels increased from 0.6 0.4 mmol/liter before exercise to 8.5 0.3 at exhaustion (p < .01). After 5 minutes of recovery, lactate decreased to 7.3 0.2 (p < .01). Increases in mean hematocrit from 0.44 0.01 (before exercise) to 0.47 0.01 (after exercise, p < .01) demonstrated a hemoconcentration of about 7%. The mean duration of the test was 31 minutes, 16 seconds, with a range of 30 minutes, 36 seconds, to 33 minutes.

Responses of Stress Hormones
The cumulative responses to exercise of the hormones of the SAS and HPA axis are given in Table 1. Adrenalin and noradrenalin show workload- and effort-related increases, respectively, in contrast to the responses of ACTH, ßE, and cortisol, which increased only during or shortly after heavy exercise (>80% power output). Figure 2 shows the response of GH (top) and the workload-related activation pattern of adrenalin (bottom). GH levels did not change significantly from baseline (before exercise) up to 60% power output (p = .18). At 80 and 100%, GH levels increased significantly to a peak value of 87.1 26.5 (p = .06) at the moment of exhaustion. Peak levels returned to 39.4 8.1 (p < .01) during the recovery period.

View this table: [in this window] [in a new window]   Table 1. Plasma Hormone Responses of the SAS and HPA Axis: Data Are Given As Mean SEM As a Function of the Relative Work Load (in % of Power Output at 100% o2max) and Recovery (R, 5 and 30 Min After Exhaustion).

View larger version (12K): [in this window] [in a new window]   Fig. 2. Plasma concentrations of GH (top) and adrenalin (bottom) as a function of exercise and recovery. *Significantly different from immediately preceding level.

View larger version (13K): [in this window] [in a new window]   Fig. 3. Plasma concentrations of PRL (top) and ACTH (bottom) as a function of exercise and recovery. *Significantly different from immediately preceding level.

	DISCUSSION

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION ACKNOWLEDGMENTS REFERENCES

The type of exercise (ie, dynamic vs. static exercise, exercise in a supine vs. an upright position, the mass of muscle engaged in the work performance, and the intensity and duration of exercise) strongly influences plasma hormone levels. We used an incremental exercise protocol, thus eliciting a cumulative response for each level of exercise intensity. We were not interested in exercise responses to specific workloads from separate trials (15) but in the activation pattern of stress hormones, which includes a duration effect. It is well known that the secretion of stress hormones increases with increasing workload and that the threshold intensity depends on the duration of exercise. We applied a protocol with a duration of at least 30 minutes and time blocks of 10 minutes, taking into account the plasma half-lives of all measured hormones, which are less than 30 minutes except for cortisol, which has a half-life of 60 to 90 minutes (16). Other factors that influence plasma hormone levels during exercise are age, gender, fitness level, and anticipatory reactions of the subjects. In this study we used dynamic exercise (cycle ergometry) with the subject in the upright position because of its reliability, reproducibility, and safety. A maximal power output of 5.0 0.5 W/kg and a O_2max of 59.2 5.8 ml/min per kg provide evidence that the subjects were physically active. Because they were familiar with the test procedures, anticipatory responses were less likely. The maximal lactate level (8.5 0.3 mmol/liter) and the maximal HR (204 11 beats/min) indicate that the subjects exercised until they were exhausted. A contribution of psychological stress to the responses measured during the last phase of the exercise protocol cannot be ruled out, although this is less likely for GH and PRL responses (4). Because we studied changes in plasma concentration as a function of the relative instead of the absolute work intensity, responses were normalized for differences in body size and fitness level.

Workload- and effort-related increases in plasma adrenalin and noradrenalin may be primarily elicited by feed-forward stimulation from motor centers in the brain and by stimulation of afferents from the active muscles (17). An increase in sympathoadrenal activity during exercise is of importance for ergotropic tuning of the organism: ie, cardiovascular adaptation, thermoregulation, water and electrolyte balance, mobilization of glycogen and triglyceride from depots, stimulation of respiration, and increased contractility in skeletal muscles (16, 18, 19).

The HPA axis has been studied extensively because of the presumed importance of glucocorticoids in allowing the body to cope with stress. In this study, subjects were motivated to exercise to exhaustion. The physiological actions of cortisol include gluconeogenesis and protection not against the source of stress itself but against inflammatory reactions and activation of the immune system, which are inherent to a stressful event but could endanger the organism. The observed cortisol response during recovery may be initiated by a drop in blood glucose levels during exercise or recovery (20, 21). Nongenomic mechanisms (22) allow peripheral effects of cortisol within a time scale of seconds to minutes, preparing the body for vigorous muscular effort and protection against tissue damage. Munck et al. (23) proposed that the stress-related increase of cortisol prevents the inflammatory and immune reactions from overshooting, which would lead to disturbed homeostasis. However, such an effect should be sufficiently delayed in relation to the time that is needed to allow the appropriate defense mechanisms to be activated. The observation that activation of the HPA axis during and after incremental exercise lags behind that of the response of the SAS is in line with this reasoning. As noted in the Introduction, this delayed response of the HPA axis is less likely to be observed during psychologically engendered forms of stress (7, 24). The HPA activation associated with exercise is primarily due to demands for fuel by the muscles, signaled by dropping blood glucose levels, perhaps accompanied by mental effort. The implicated feedback mechanisms are integrated in the hypothalamus and the pituitary gland. On the other hand, during psychological stress higher brain centers are more involved, and these centers operate regardless of physiological demands. The uncertain outcomes in threatening situations are associated with negative affect, producing immediate activation of the HPA axis, which involves efferent pathways from the orbitofrontal cortex and the subcortical motivational centers, such as the amygdala and the septohippocampal complex (7).

We observed clearly different activation patterns of plasma GH and PRL during exercise (Figures 2 and 3, top). It is well known that the interaction of duration and intensity of exercise influences the responses of both hormones (8, 16, 25–28). Our data confirm observations that increases of plasma PRL are less marked than those of GH during intense, brief exercise (11, 29) and that PRL may peak after exercise ends (30). The difference in timing is remarkable given that both hormones have a plasma half-life of about 20 minutes. It is well known that the central neuroendocrine and metabolic regulations of GH and PRL differ markedly. For example, GH secretion is stimulated by GH-releasing hormone and inhibited by somatostatin, whereas PRL is predominantly tonically inhibited by central dopaminergic neurons. Moreover, GH stimulates adequate substrate delivery to the muscles during exercise and prepares the organism for recovery (in line with its workload-related time course), whereas no metabolic action of PRL is known in humans other than a possible modulating role in the action of the hypothalamic-pituitary-gonadal axis (31). The time course of activation of PRL could otherwise be indicative of a role of PRL in the modulation of effects of the HPA axis, which should be further investigated. Overall there is sparse evidence that mental stress per se acts as a simple secretagogue for GH and PRL (4).

In summary, the data show that an incremental exercise protocol can be used as a model stressor to separate activation of the different components of the classic stress response. These findings support the notion that stress hormones have a different role in preparing the organism for physical activity and recovery. The delayed response of the HPA axis during incremental exercise is in contrast to the nondelayed response observed in psychological forms of stress, which is typically accompanied by uncertainty and negative affect.

	ACKNOWLEDGMENTS

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION ACKNOWLEDGMENTS REFERENCES

 
We gratefully acknowledge the assistance of Inge Maitimu (Laboratory of Endocrinology, University Hospital Utrecht) in the biochemical analysis and thank Jack E. van Honk, PhD, (Department of Psychonomics, Utrecht University) for his help with preparation of the manuscript.

Received for publication July 14, 1999.

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