Introduction
The endocrine system consists of a number of glands that secrete liquid into the bloodstream (rather than through a duct into one of the body cavities). The chemical products elaborated by these glands are called hormones, and they have profound effects upon their target cells and organs. The overall function of the endocrine system is that of homeostatic control, and the level of each hormone is regulated via a complex monitoring and feedback mechanism. As an example, the parathyroid glands regulate the level of blood calcium, thereby controlling the overall level of excitability of the nervous system. Under the control of the parathyroids, calcium can be rapidly moved in or out of the bones to maintain an animal's blood levels within appropriate limits.
Some glands such as the thyroid, parathyroid, and the islet cells of the pancreas, secrete primarily a single hormone which has a relatively specific function. As examples, the thyroid hormone regulates oxidative metabolism, and the islet-cell hormone assists in glucose metabolism. The adrenal gland is more complex; it is a "defensive" gland, and is activated in stressful circumstances in which the organism must decide whether to fight or flee. In both cases, the adrenal promotes the general body functions that facilitate such reactions. The adrenal has two distinct anatomical portions (the cortex and medulla) and rich connections to the nervous system. The cortex secretes the glucocorticoids (corticoids), hormones that are involved in coping with stressful situations arising out of circumstances that are not immediately life-threatening. In contrast, the adrenal medulla secretes the catecholamine class of hormones-the most active are epinephrine and norepinephrine-which promote practically instantaneous preparations for fight and flight.
The pituitary, a small tissue mass located at the base of the brain below the hypothalamus, is the most important and complex endocrine gland. It secretes at least eight hormones that orchestrate the response of the other glands and that produce effects on general body functions such as growth and water balance. Pituitary activity varies dynamically depend stream hormone levels.
The interaction between EMFs and this interrelated group of endocrine glands-which themselves are only partially understood-is very complex. It could involve particular glands such as the calcium-parathyroid-bone axis. On the other hand, the brain itself might be sensitive to alterations in the electromagnetic environment. Such a sensitivity could result in activation of a number of hormonal systems by virtue of the direct connection between the brain and the pituitary. If an EMF constituted a threat to the integrity of the organism, the pituitary-adrenal stress response system would be called into action. Indeed, the bulk of the endocrine system studies have involved the pituitary-adrenal system. These studies illustrate the difficulty in establishing the precise causal chain of events in the functioning of a hormone system, as would be required to determine the level at which the EMF acts in the first instance.
Friedman and Carey measured the corticoid production in monkeys exposed to a 200-gauss DC magnetic field for 4 hour/day (3). Daily urine collections were combined into 72-hour period specimens to provide sufficient volume for biochemical determination of corticoids (presumed to reflect the levels in the blood). The pre-experimental level and the levels found during the four subsequent specimen periods are shown in figure 6.I. As judged by the increase in corticoids, there was a stress response which lasted for about the first 6 days and then subsided despite the continued exposure.
Corticoid synthesis by the adrenal cortex is controlled by the pituitary. When it is appropriate, a hypothalamic releasing factor stimulates the pituitary to produce adrenocorticotropin (ACTH) which in tum stimulates and controls adrenal corticoid production. Thus, Friedman's results were consistent with an effect at any level in the hormonal system. In a series of experiments, we exposed rats to 15,000 v/m at 60 Hz to determine whether the field produced an effect on the serum corticoids concomitantly with an effect on the pituitary (4). We found that following 30 days' continuous exposure, the corticoid levels were generally lower and the final pituitary weight was higher in the exposed animals (Table 6.1). These results indicated that both pituitary and adrenal function were altered following exposure, but, because of the feedback nature of the hormonal system, it was not possible to determine which tissue was affected initially. Recently Novitskiy (23) measured the endocrine system response at each level following brief exposure of rats to 10-1000 µW/ cm2 at 2.4 GHz. He found increased levels of serum corticoids, pituitary ACTH, and ACTH releasing-factor in the hypothalamus.
Fig. 6.1. Urine corticoid levels in monkeys during exposure to a DC magnetic field.
Table 6.1. BIOLOGICAL EFFECTS OF EXPOSURE T0 A 60-Hz ELECTRIC FIELD
Because the pituitary's activities are synchronized with the nervous system via intimate chemical and neuronal pathways in the hypothalamus, any EMF impact involving pituitary function would be expected to reach beyond ACTH and the classic stress-response system. There is some direct evidence that other pituitary secretions are affected by EMF's. For example, antldiuretic hormone (ADH) Is a pitultary secretlon that partlclpates in the regulation of the body's water balance. An increase in ADH fosters the reabsorption of water by the kidney's distal renal tubules thereby leading to a reduction in diuresis (flow of urine). Several studies have reported that EMFs increase serum ADH levels (5, 6) and reduce diuresis (6, 7). In many cases, however, the evidence of EMF impacts involving the pituitary is indirect and consists of effects on growth, metabolism, the cardiovascular and hematopoietic systems, and other body functions and systems that are under the influence and control of the endocrine system. In the remainder of this chapter we describe the EMF studies that involve the endocrine glands-principally the adrenal and thyroid. In succeeding chapters we present evidence of the effects of EMFs on other body functions and systems.