The metabolic processes within a given cell are frequently regulated by signals from outside the cell. A usual means of intercellular communication takes place through the workings of the endocrine system, in which the ductless glands produce hormones as intercellular messengers.
Hormones are transported from the sites of their synthesis to the
sites of action by the bloodstream (Figure 24.5). In terms of their chemical
structure, some typical hormones are steroids, such as estrogens, androgens,
and mineralocorticoids; polypeptides, such as insulin and endorphins; and amino
acid derivatives, such as epinephrine and norepinephrine (Table 24.3).
Hormones have several important functions in the body. They help main-tain homeostasis, the balance of biological activities in the body. The effect of insulin in keeping the blood glucose level within narrow limits is an example of this function. The operation of epinephrine and norepinephrine in the “fight-or-flight” response is an example of the way in which hormones medi-ate responses to external stimuli. Finally, hormones play roles in growth and development, as seen in the roles of growth hormone and the sex hormones. The methods and insights of biochemistry and physiology alike have helped illuminate the workings of the endocrine system.
The release of hormones exerts control on the cells of target organs;
other control mechanisms, however, determine the workings of the endocrine
gland that releases the hormone in question. Simple feedback mechanisms, in
which the action of the hormone leads to feedback inhibition of the release of
hor-mone, can be postulated (Figure 24.6). The workings of the endocrine system
are, in fact, much less simple, with the added complexity allowing for a
greater degree of control. To illustrate with a rather restricted example,
insulin is released in response to a rapid rise in the level of blood glucose.
In the absence of control mechanisms, an excess of insulin can produce hypoglycemia, the condition of low
blood glucose. In addition to negative feedback control on the release of
insulin, the action of the hormone glucagon tends to increase the level of
glucose in the bloodstream. The two hormones together regulate blood glucose.
A more sophisticated control system involves the action of the hypothalamus, the pituitary, and specific endocrine glands (Figure 24.7). The central nervous system sends a signal to the hypothalamus.
The hypothalamus secretes a hormone-releasing factor, which in turn stimulates release of a
trophic hor-mone by the anterior pituitary (Table 24.3). (The action of the
hypothalamus on the posterior pituitary is mediated by nerve impulses.) Trophic hormones act on specific endocrine glands, which release the
hormones to be transported to target organs. Note that feedback control is
exerted at every stage of the process. Even more fine-tuning is possible with
zymogen activation mechanisms, which exist for many well-known hormones.
The chemical natures of hormones play a predictably important role
in their roles in cell signaling. Steroid hormones, for example, can enter the
cell directly through the plasma membrane or can bind to plasma membrane
receptors. Nonsteroid hormones enter the cell exclusively as a result of
binding to plasma membrane receptors (Figure 24.8).
The releasing factors and trophic hormones listed in Table 24.3
tend to be polypeptides, but the chemical natures of the hormones released by
specific endocrine glands show greater variation. Thyroxine, for example,
produced by the thyroid, is an iodinated derivative of the amino acid tyrosine.
Abnormally low levels of thyroxine lead to hypothyroidism,
characterized by lethargy and obesity, whereas increased levels produce the
opposite effect (hyperthyroidism).
Low levels of iodine in the diet often lead to hypothyroid-ism and an enlarged
thyroid gland (goiter). This
condition has largely been eliminated by the addition of sodium iodide to
commercial table salt (“iodized” salt). (It is virtually impossible to find
table salt that is not iodized.)
Steroid hormones are produced by the adrenal cortex and the gonads (testes in males, ovaries in females). The adrenocortical hormones include glucocorticoids, which affect carbohydrate metabolism, modulate inflammatoryreactions, and are involved in reactions to stress. The mineralocorticoids control the level of excretion of water and salt by the kidneys. If the adrenal cortex does not function adequately, one result is Addison’s disease, characterized by hypogly-cemia, weakness, and increased susceptibility to stress. This disease is eventually fatal unless it is treated by administration of mineralocorticoids and glucocor-ticoids to make up for what is missing. The opposite condition, hyperfunction ofthe adrenal cortex, is frequently caused by a tumor of the adrenal cortex or of the pituitary. The characteristic clinical manifestation is Cushing’s syndrome, marked by hyperglycemia, water retention, and the easily recognized “moon face.”
The adrenal cortex produces some steroid sex hormones, the androgens and estrogens, but the main site of production is the gonads. Estrogens
are requiredfor female sexual maturation and function, but not for embryonic
sexual devel-opment of female mammals. Animals that are male genetically appear
to be females if they are deprived of androgens during embryonic development.
As a final example, we shall discuss growth hormone (GH), which is a
polypeptide. When overproduction of GH occurs, it is usually because of a
pituitary tumor. If this condition occurs while the skeleton is still growing,
the result is gigantism. If the
skeleton has stopped growing before the onset of GH overproduction, the result
is acromegaly, characterized by
enlarged hands, feet, and facial features. Underproduction of GH leads to dwarfism, but this condition can be
treated by the injection of human GH before the skeleton reaches maturity.
Animal GH is ineffective in treating dwarfism in humans. Supplies of human GH
were very lim-ited when it could be obtained only from cadavers, but it can now
be synthesized by recombinant DNA techniques. Human growth hormone (HGH) has
recently become avail-able to individuals who believe it will help alleviate
the effects of aging. It was known that the level of HGH decreases after middle
age is reached. Many have assumed that the availability of growth hormone, if
one could afford it, would be a virtual fountain of youth. Even though few
results are conclusive at this time, HGH is being prescribed, and the medical
community has adopted rules for its use. For example, doctors will consider
prescribing it only for patients over age
The same hormone is also used illegally by endurance athletes, and
there is currently no reliable test to stop this illegal use.