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Nature and Mode of Action of Hormones

Nature and Mode of Action of Hormones


One of the chemical messengers produced by endocrine glands, whose secretions are liberated directly into the bloodstream and transported to a distant part or parts of the body, where they exert a specific effect for the benefit of the body as a whole. The endocrine glands involved in the maintenance of normal body conditions are pituitary, thyroid, parathyroid, adrenal, pancreas, ovary and testis. However, these organs are not the only tissues concerned in the hormonal regulation of body processes. For example, the duodenal mucosa, which is not organized as an endocrine gland, elaborates a substance called secretin which stimulates the pancreas to produce its digestive juices. The placenta is also a very important hormone-producing tissue. See separate articles on the individual glands.


The hormones obtained from extracts of the endocrine glands may be classified into four groups according to their chemical constitution:

(1) phenol derivatives, such as epinephrine, norepinephrine, thyroxine, and triiodothyronine

(2) proteins, such as the anterior pituitary hormones, with the exception of adrenocorticotropic hormone (ACTH), human chorionic gonadotropin, pregnant-mare-serum gonadotropin, and thyroglobulin

(3) peptides, such as insulin, glucagon, ACTH, vasopressin, oxytocin, and secretin

(4) steroids, such as estrogens, androgens, progesterone, and corticoids. Hormones, with a few exceptions like pituitary growth hormone and insulin, may also be classified as either tropic hormones or target-organ hormones. The former work indirectly through the organs or glands which they stimulate, whereas the latter exert a direct effect on peripheral tissues.


Chemical nature of hormones

Chemically, most hormones belong to one of three major groups: proteins and peptides, steroids (fat-soluble molecules whose basic structure is a skeleton of four carbon rings), or derivatives of the amino acid tyrosine, characterized by a 6-carbon, or benzene, ring. There are some hormones, such as melatonin from the pineal gland and the locally acting prostaglandins, which cannot be included in any of these groups, but may share a number of their characteristics. The glands which produce protein and peptide hormones are the pituitary, certain cells of the thyroid, the parathyroids, and the pancreas. Steroids are produced by the cortex or outer layer of the adrenal gland and by the ovaries and testes. The tyrosine derivatives are the thyroid hormones, and the catecholamines (adrenaline and noradrenaline) which are produced in the medulla of the adrenal glands.


Knowledge of the chemical nature of a hormone is important as it enables one to predict how the hormone is produced, how rapidly it can be released in response to a stimulus, in what form it circulates in the blood, how it acts, the time course of its effect, and the route of administration therapeutically.


Hormone action

The chemical nature of the hormone also affects the mechanism of action. All hormones act on cells by way of their 'receptors'. Each hormone has its own receptor to which it binds, matching rather like a lock and key. This is why hormones circulating throughout the body in the blood may leave capillaries to enter the extracellular fluid of many tissues, but act only on those cells which possess the appropriate receptor. Proteins and peptides cannot enter the cell and so act via cell membrane receptors, producing their effects by 'second messengers', which are activated in the cell as soon as the hormone binds to the receptor. Thus peptide hormones can produce quite rapid responses. Steroid and thyroid hormones, by contrast, can enter the cell and bind to intracellular receptors, producing their effects by stimulating the production of new proteins. There is therefore a relatively long lag period before the response to these hormones is seen.


Hormones produce a variety of responses throughout the body and may be grouped according to their actions, although there is overlap between the groups.


First there are the metabolic hormones which control the digestion of food, its storage and use. Such hormones include those produced by the digestive tract, which control secretion of digestive juices and activity of the muscle in the wall of the tract; also the hormones which regulate blood glucose, namely insulin, (which lowers it), and glucagon, growth hormone, the thyroid hormones, and cortisol, which all raise it.


Second are the hormones which regulate the composition of the blood, and hence of all the body fluids. Excluding those that regulate the glucose content, these are: aldosterone and atrial natriuretic hormone (produced in the heart), which control the amount of sodium in the blood; vasopressin or antidiuretic hormone, which controls the amount of water; parathyroid hormone and vitamin D, which raise blood calcium; and calcitonin, which lowers blood calcium. It is perhaps surprising to learn that a vitamin can also be a hormone, but it is similar in many ways to the steroid hormones, and the active form is produced in one part of the body for action an another. The vitamin D taken in the diet or formed in the skin under the action of UV light is not the active form: this is produced after modification takes place first in the liver and then the kidney.


Next are the stress hormones, primarily adrenaline and noradrenaline, which are under the control of the autonomic nervous system: cortisol and a number of the pituitary hormones are also involved in the response to stress.


A further group are those responsible for growth, development, and reproduction. These include growth hormone itself, and the hormones controlling ovarian and testicular function (luteinizing hormone, LH, and follicular stimulating hormone, FSH) - all of which come from the pituitary - and the hypothalamic hormones, which in turn control these pituitary secretions. Included also are the steroid hormones, produced by the ovaries (oestrogens and progesterone) and testes (testosterone), and those hormones involved in birth and lactation, chiefly oxytocin and prolactin.


The final major group includes those hormones that control other endocrine systems, and therefore interact with the other groups. The pituitary hormones adrenocorticotrophic hormone (ACTH), thyroid stimulating hormone (TSH), and the gonadotrophic hormones LH and FSH control the release of some of the metabolic and stress hormones and of the reproductive hormones, whilst hypothalamic hormones in turn control pituitary function.


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