Hormone

A hormone is a chemical messenger that directs biological activity. All multicellular organisms produce hormones (including plants - see article phytohormone).

The best-known animal hormones are those made by endocrine glands of vertebrate animals, but hormones are made by nearly every organ system and tissue type in an animal body. Many hormones are secreted (released) directly into the bloodstream; some hormones, (sometimes called 'ectohormones'), aren't secreted into the blood stream, but travel by diffusion to their target cells, which may be nearby cells (paracrine action) in the same tissue, or cells of a distant organ of the body. Hormones act as signals to the target cells; their actions are determined not only by the amounts in which they are secreted, but also by their pattern of secretion, and exactly how they act depends on the signal transduction mechanisms of the target tissue.

Hormone actions vary widely, but can include stimulation or inhibition of growth, induction or suppression of apoptosis (programmed cell death), activation or inhibition of the immune system, regulating metabolism and preparation for a new activity (e.g., fighting, fleeing, mating) or phase of life (e.g., puberty, caring for offspring, menopause). In many cases, one hormone may regulate the production and release of other hormones. Many hormones can be described as acting to regulate metabolic activity of an organ or tissue. Hormones also control the reproductive cycle of virtually all multicellular organisms.

History

The concept of internal secretion was developed in the 19th century; Claude Bernard described it in 1855, but did not specifically address the possibility of secretions of one organ acting as messengers to others. Still, various endocrine conditions were recognised and even treated adequately (e.g., hypothyroidism with extract of thyroid glands). A major breakthrough was the identification of secretin, the hormone secreted by the duodenum that stimulates pancreatic secretions, by Ernest Starling and William Bayliss in 1902. Previously, the process had been considered (e.g. by Ivan Pavlov) to be regulated by the nervous system. Starling and Bayliss showed that injecting duodenal extract into dogs rapidly increased pancreatic secretions, raising the possibility of a chemical messenger. Starling is also credited with introducing the term hormone, having used it in a 1905 lecture. Later reports indicate it was suggested to him by the Cambridge physiologist William B. Hardy .

Physiology of hormones

Most cells are capable of producing one or more, and sometimes many, molecules that signal other cells to alter their growth, function, or metabolism. The classical endocrine glands and their hormone products are specialized to serve regulation on the overall organism level, but can often be used in other ways or only on the tissue level. The rate of production of a hormone is often regulated by a homeostatic control system, usually by negative feedback. Homeostatic regulation of hormones depends, apart from production, on the metabolism and excretion of hormones.

Hormone secretion can be stimulated and inhibited by:

• Other hormones (stimulating- or releasing-hormones)
• Plasma concentrations of ions or nutrients, as well as binding globulins
• Neurons and mental activity
• Environmental changes, e.g., of light or temperature

One special group of hormones is trophic hormones that stimulate the hormone production of other endocrine glands. For example, thyroid-releasing hormone is released by neurons in the hypothalamus into blood vessels at the base of the brain which travel to the anterior pituitary gland; there it stimulates the secretion of thyroid-stimulating hormone (TSH) into the systemic circulation. TSH then acts on another endocrine gland - the thyroid - to increase the secretion of thyroid hormones.

A recently-identified class of hormones is that of the "Hunger Hormones" - ghrelin and PYY 3-36 which are secreted from the stomach and gastrointestinal tract, and many neuropeptides such as orexin which are released in the brain - and 'satiety hormones' - e.g., leptin, secreted from fat cells (adipocytes}, and obestatin, a fragment of the precursor for ghrelin.

Types of hormones

Vertebrate hormones fall into three chemical classes:

1. Amine-derived hormones are derivatives of the amino acids tyrosine and tryptophan. Examples are the catecholamines (dopamine, epinephrine and norepinephrine) and thyroxine.

1. Peptide hormones consist of chains of amino acids. Examples are TRH and vasopressin. Peptides composed of scores or hundreds of amino acids are usually referred to as proteins, and examples include insulin, secreted by the pancreas and growth hormone, secreted from the anterior pituitary. More complex protein hormones have carbohydrate side chains and are called glycoprotein hormones. Luteinizing Hormone, Follicle-Stimulating Hormone and Thyroid-Stimulating Hormone are all glycoprotein hormones secreted from the anterior pituitary. Peptide hormones are all secreted by calcium-dependent exocytosis, and all act via specific, high affinity G-protein coupled receptors that are present on the cell membrane of the target cell.

1. Lipid and phospholipid-derived hormones derive from lipids such as linoleic acid and phospholipids such as arachidonic acid. The main classes are the steroid hormones that derive from cholesterol and the eicosanoids. Examples of steroid hormones are testosterone and cortisol. Sterol hormones such as calcitriol are a homologous system. The adrenal cortex and the gonads are the main sources of steroid hormones. Examples of eicosanoids are the widely-studied prostaglandins.

Pharmacology

Many hormones are used as medication. The most commonly-prescribed hormones are estrogens and progestagens (in the contraceptive pill and as HRT), thyroxine (as levothyroxine, for hypothyroidism) and steroids (for autoimmune diseases and several respiratory disorders). Insulin is used by many diabetics. Local preparations for use in otolaryngology often contain pharmacologic equivalents of adrenaline, while steroid and vitamin D creams are used extensively in dermatological practice.

A 'pharmacological dose' of a hormone is a dose of a hormone that is much greater than ever occurs naturally in a healthy body. The effects of pharmacological doses can be different from responses to naturally-occurring amounts and can be therapeutically useful. An example is the ability of pharmacological doses of glucocorticoid to suppress inflammation.

References

• Hormones and endocrine system

External links

• The Pituitary Society
• Topical Briefings British Society for Neuroendocrinology