Research on the sympathetic adrenal-medullary (SAM) system has its roots in the work by Walter B. Cannon in the beginning of this century (Cannon, 1914). On the basis of animal experiments, he described the fight-or-flight response or the emergency function of the adrenal medulla. The SAM system is activated when the individual is challenged in its control of the environment (Fig. 1, Henry, 1992). Via hypothalamus and the sympathetic nervous system, psychological stress stimulates the adrenal medulla to secrete the two catecholamines, epinephrine (adrenaline) and norepinephrine (noradrenalin), into the blood stream. This defence reaction prepares the body for battle.

The cardiovascular and neuroendocrine functions activated by the SAM system are aimed at mobilizing energy to the muscles and the heart and, at the same time, reducing blood flow to the internal organs and the gastro-intestinal system. In response to physical threat, this is an efficient means for survival by increasing the organism's capacity for fight or flight. Today, however, the SAM system is more often challenged by threats of a social or mental rather than physical nature. Short term activation of this system is necessary for adequate coping with environmental demands and for protection of the body, whereas possible health consequences of intense, repeated and/or sustained activation of this psychobiological program in response to psychosocial demands is a major objective for stress research and relevant for allostasis.

Numerous studies from laboratory experiments as well as from various natural settings illustrate the sensitivity of the SAM system to various psychosocial conditions, such as daily stress at work, at home, at school, at day-care centers, at hospitals, on commuter trains, on buses etc. (see reviews by Mason, 1968; Levi, 1972; Henry & Stephens, 1977; Ursin et al., 1978; Frankenhaeuser 1979; 1983; Usdin et al., 1980; Axelrode and Reisine, 1984; Lundberg, 1984).

The catecholamines and their concomitant effects on other physiological functions, such as blood pressure, heart rate and lipolysis, may serve as objective indicators of the stress that an individual is exposed to. However, these bodily effects are also assumed to link psychosocial stress to increased health risks. Longlasting elevated catecholamine levels are considered to contribute to the development of atherosclerosis and predispose to myocardial ischemia (Karasek et al., 1982; Krantz & Manuck, 1984; Rozanski et al., 1988). The elevated catecholamine levels also make the blood more prone to clotting, thus, reducing the risk of heavy bleeding in case of tissue damage but, at the same time, increasing the risk of arterial obstruction and myocardial infarcfion. The role of the catecholamines in hypertension is also of great interest (e.g., Nelesen & Dimsdale, 1994).

Less is known about the role of the catecholamines in other health problems. However, in the study of psychosocial aspects of musculoskeletal disorders (e.g., Moon & Sauter, 1996), it is generally assumed that psychological stress plays an important role by influencing various bodily functions including muscle tension and, thus, form a link to neck, shoulder and back pain problems (Lundberg & Melin, 2002). In keeping with this, jobs with a high prevalence of musculoskeletal disorders, such as repetitive assembly line work, are characterized by highly elevated sympathetic arousal (cf. Table 3) and slow unwinding after work (Johansson et al., 1978; Melin et al., 1997). In addition, in a laboratory experiment (Lundberg et al., 1994), positive correlations were found between blood pressure, norepinephrine and mentally induced EMG activity of the trapezius muscle.

Assessment

A small but relatively constant fraction of the circulating levels of epinephrine and norepinephrine in the blood is excreted into the urine (Frankenhaeuser, 1971; Levi, 1972). Consequently, assessment can be made in blood (usually plasma) as well as in urine. However, whereas epinephrine is mainly produced by the adrenal medulla, the major part of the circulating norepinephrine is produced by sympathetic nerve endings. Studies comparing urinary levels with corresponding hormone determinations in plasma are scarce, but available data indicates a significant positive relationship between changes in urinary and plasma catecholamines (Akerstedt et al., 1983; Steptoe, 1985).

The amount of urinary catecholamines excreted during a particular period of time can be determined from the concentration in the sample, multiplied by the total urine volume, or by relating the concentration to a reference substance such as creatinine excretion. Provided that reliable measurements are obtained and the subject is able to empty his/her bladder completely, the results from the different methods are almost identical. The catecholamines are usually determined by high performance liquid chromatography with electrochemical detection (Lundberg et al., 1988; Hjemdahl et al., 1990).

Methodological considerations

In order to reduce the influence of circadian rhythms and individual differences in baseline levels, it is recommended to express individual responses to stress in relation to his/her corresponding baseline level obtained during relaxation at the same time of the day on another day. Thus, percent change from baseline is usually a more relevant measure than absolute levels.

In stress research, like in many other research areas, most studies have been performed on men. However, in the early 1970's, investigators in Marianne Frankenhaeuser's group in Stockholm started to compare stress responses of males and females. In these early studies of sex differences in psychophysiological stress responses, it was consistently found that women were less reactive than men in terms of epinephrine secretion during experimental stress (e.g., Frankenhaeuser, Dunne & Lundberg, 1976; Johansson, 1972). Although women performed as well or usually even better than men on the various stress tests, they did not increase their epinephrine secretion very much. However, during more intense stress, such as a stressful examination (Frankenhaeuser, Rauste von Wright et al., 1978), female students were found to increase their epinephrine output significantly but, still, to a lesser extent than the male students did.

A possible explanation for these sex differences is that performance stress is less challenging to women than to men. Emotional stress has been found to have a more pronounced effect on catecholamine levels in women (Lundberg, de Châ teau, Winberg & Frankenhaeuser, 1981) and women in less traditional roles seem to respond to performance stress with the same epinephrine output as their male colleagues (Collins & Frankenhaeuser, 1978). More recent studies comparing men and women matched for education and occupational level show that women may respond by as much epinephrine output at work and during experimental stress as men do (e.g., Frankenhaeuser et al., 1989).

Although, the possible influence of biological factors, such as steroid sex hormones on catecholamine responses, cannot be excluded (Wasilewska, Kobus and Bargiel, 1980; Tersman, Collins and Eneroth, 1991), it seems as if psychological factors and gender role patterns are more important than biological factors for the sex differences in catecholamine responses.

In men, a significant positive correlation is usually found between perceived stress and physiological responses at work (e.g., Lundberg, Granqvist, Hansson, Magnusson & Wallin, 1989; Frankenhaeuser et al., 1962; Frankenhaeuser et al., 1989). However, in women, physiological stress levels at work seem to spill over into non-work situations (Rissler, 1977; Frankenhaeuser et al., 1989; Lundberg, 1996; Lundberg & Frankenhaeuser, 1999). This interaction between stress from paid employment and unpaid work at home is important to consider in the study of women's stress.

Epinephrine levels are significantly elevated by overstimulation as well as by understimulation compared to more optimal environmental conditions (Frankenhaeuser et al., 1971; Levi, 1972; Frankenhaeuser & Gardell, 1976). Work overload, a very high work pace, too much responsibility, and role conflicts as well as simple, monotonous and repetitive jobs or lack of meaningful activities (e.g., unemployment), may contribute to elevated epinephrine levels.

The acute response ("phasic" elevation according to Ursin et al., 1978) to a novel environmental situation diminishes as the individual habituates but, in contrast to cortisol levels which seem to return to baseline (Pollard, 1995), catecholamine levels remain chronically elevated also during normal work conditions ("tonic" elevation according to Ursin et al., 1978).

One example of an adequate or economic response to mental stress is presented in Fig. 2 (Forsman, 1983), which shows the epinephrine changes of healthy male students during successive periods of experimental stress and rest in the laboratory. The subjects were able to return to their baseline level each time the stress exposure ended.

Another example is shown in Fig. 3, illustrating the epinephrine output of 50 women giving birth to their first chld (Alehagen et al., 1999). Despite a 500 percent increase during labor and pushing, the epinephrine levels had returned to the pregnancy levels after a couple of days.

Considering the various cardiovascular and metabolic functions influenced by the catecholamines, this means that blue collar workers in general are exposed to a greater total physiological load than white collar workers. In addition, workers in repetitive jobs seem to have difficulties to unwind after work, i.e., their physiological arousal remains elevated at least 1-2 hours after work compared to the rapid unwinding of workers in more flexible jobs (Johansson et al., 1978; Lundberg et al., 1993; Melin et al., 1997). This means that workers in simple, monotonous and repetitive jobs not only have to pay a greater physiological toll at work but also have less chance for relaxation and recovery off the job (Melin & Lundberg, 1997). Physical stressors at work such as noise may further contribute to the total load on blue collar workers (Glass & Singer, 1972).

In view of traditional gender differences in responsibility for unpaid work at home (Hall, 1990; Kahn, 1991; Lundberg et al., 1994), the long term health risks for women in repetitive work seem to be of particular importance (Rcpetti et al., 1989; Rodin & Ickovics, 1990; Frankenhaeuser et al., 1991).

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