Hormone

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A hormone (from Greek όρμή - "to set in motion") is a chemical messenger from one cell (or group of cells) to another. All multicellular organisms produce hormones (including plants - see phytohormone).

The best-known animal hormones are those produced by endocrine glands of vertebrate animals, but hormones are produced by nearly every organ system and tissue type in an animal body. Hormone molecules are secreted (released) directly into the bloodstream; some hormones, called ectohormones, are not secreted into the blood stream, they move by circulation or 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. The function of hormones is to serve as a signal to the target cells; the action of hormones is determined by the pattern of secretion and the signal transduction of the receiving tissue.

Most hormones signal a cell change by combining with a receptor. For many hormones, including most protein hormones, the receptor is embedded in the membrane on the surface of the cell. The interaction of the hormone and the receptor typically triggers a cascade of secondary effects within the cytoplasm of the cell, often involving phosphorylation or dephosphorylation of proteins, changes in ion channels, or increased amounts of an intracellular molecule that serves as a second messenger (e.g., cyclic AMP). The second common type of mechanism, typically involving smaller-sized hormones such as steroid or thyroid hormones, begins with entry of the hormone molecule into the cytoplasm of the cell where it combines with a loose and mobile receptor. The combined hormone-receptor ligand then moves across the nuclear membrane into the nucleus of the cell and binds to the DNA, effectively amplifying or suppressing the action of certain genes, thereby affecting protein synthesis.

Hormone effects 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 of the responses to hormone signals can be described as serving to regulate metabolic activity of an organ or tissue. Hormones also control the reproductive cycle of virtually all multicellular organisms.

Contents

Physiology of hormones

Most cells are capable of producing one or more, sometimes many, molecules which 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, generally 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 the trophic hormones that stimulate the hormone production of other endocrine glands. For example, thyroid-stimulating hormone (TSH) causes growth and increased activity of another endocrine gland, the thyroid, which increases output of thyroid hormones.

A recently-identified class of hormones is that of the "hunger hormones" - ghrelin, orexin and PYY 3-36 - and "satiety hormones" - e.g., leptin, obestatin, nesfatin-1.

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 catecholamines and thyroxine.
  2. Peptide hormones consist of chains of amino acids. Examples of small peptide hormones are TRH and vasopressin. Peptides composed of scores or hundreds of amino acids are referred to as proteins. Examples of protein hormones include insulin and growth hormone. More complex protein hormones bear carbohydrate side chains and are called glycoprotein hormones. Luteinizing hormone, follicle-stimulating hormone and thyroid-stimulating hormone are glycoprotein hormones.
  3. Lipid and phospholipid-derived hormones derive from lipids such as linoleic acid and arachidonic acid and phospholipids. 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 primary sources of steroid hormones. Examples of eicosanoids are the widely studied prostaglandins.

Pharmacology

Many hormones and their analogues are used as medication. The most commonly-prescribed hormones are estrogens and progestagens (as methods of hormonal contraception 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 "pharmacologic dose" of a hormone is a medical usage referring to an amount of a hormone far greater than naturally occurs in a healthy body. The effects of pharmacologic doses of hormones may be different from responses to naturally-occurring amounts and may be therapeutically useful. An example is the ability of pharmacologic doses of glucocorticoid to suppress inflammation.

Important human hormones

Spelling is not uniform for many hormones. Current North American and international usage is estrogen, gonadotropin, while British usage retains the Greek diphthong in oestrogen and the unvoiced aspirant h in gonadotrophin.

Structure Name Abbreviation Origin Mechanism
amine - tryptophan Melatonin (N-acetyl-5-methoxytryptamine) pinealocyte
amine - tryptophan Serotonin 5-HT enterochromaffin cell
amine - tyrosine Thyroxine T4 thyroid gland direct
amine - tyrosine Triiodothyronine T3 thyroid gland direct
amine - tyrosine (cat) Epinephrine (or adrenaline) EPI adrenal medulla
amine - tyrosine (cat) Norepinephrine (or noradrenaline) NRE adrenal medulla
amine - tyrosine (cat) Dopamine DPM hypothalamus
peptide Antimullerian hormone (or mullerian inhibiting factor or hormone) AMH Sertoli cell
peptide Adiponectin Acrp30 adipose tissue
peptide Adrenocorticotropic hormone (or corticotropin) ACTH anterior pituitary cAMP
peptide Angiotensinogen and angiotensin AGT liver IP3
peptide Antidiuretic hormone (or vasopressin, arginine vasopressin) ADH posterior pituitary varies
peptide Atrial-natriuretic peptide (or atriopeptin) ANP heart cGMP
peptide Calcitonin CT thyroid gland cAMP
peptide Cholecystokinin CCK duodenum
peptide Corticotropin-releasing hormone CRH hypothalamus cAMP
peptide Erythropoietin EPO kidney
peptide Follicle-stimulating hormone FSH anterior pituitary cAMP
peptide Gastrin GRP G cell
peptide Ghrelin P/D1 cell
peptide Glucagon pancreas (alpha cells) cAMP
peptide Gonadotropin-releasing hormone GnRH hypothalamus IP3
peptide Growth hormone-releasing hormone GHRH hypothalamus IP3
peptide Human chorionic gonadotropin hCG placenta cAMP
peptide Human placental lactogen HPL placenta
peptide Growth hormone GH or hGH anterior pituitary
peptide Inhibin
peptide Insulin INS pancreas (beta cells) tyrosine kinase
peptide Insulin-like growth factor (or somatomedin) IGF liver tyrosine kinase
peptide Leptin
peptide Luteinizing hormone LH anterior pituitary cAMP
peptide Melanocyte stimulating hormone MSH or α-MSH anterior pituitary cAMP
peptide Neuropeptide Y
peptide Oxytocin posterior pituitary IP3
peptide Parathyroid hormone PTH parathyroid gland cAMP
peptide Prolactin PRL lactotroph, others
peptide Relaxin varies
peptide Secretin S cell
peptide Somatostatin SRIF hypothalamus
peptide Thrombopoietin liver, kidney
peptide Thyroid-stimulating hormone TSH anterior pituitary cAMP
peptide Thyrotropin-releasing hormone TRH hypothalamus IP3
steroid - glu. Cortisol adrenal cortex direct
steroid - min. Aldosterone adrenal cortex direct
steroid - sex (and) Testosterone testes direct
steroid - sex (and) Dehydroepiandrosterone DHEA multiple direct
steroid - sex (and) Androstenedione adrenal glands, gonads direct
steroid - sex (and) Dihydrotestosterone DHT multiple direct
steroid - sex (est) Estradiol ovary direct
steroid - sex (est) Estrone ovary direct
steroid - sex (est) Estriol placenta direct
steroid - sex (pro) Progesterone ovary direct
sterol Calcitriol kidneys direct
eicosanoid Prostaglandins seminal vesicle
eicosanoid Leukotrienes white blood cells
eicosanoid Prostacyclin endothelium
eicosanoid Thromboxane platelets

See also

References

External Links


Template:Hormonesar:هرمون bs:Hormon bg:Хормон ca:Hormona cs:Hormon da:Hormon de:Hormon es:Hormona eo:Hormono fr:Hormone ko:호르몬 io:Hormono id:Hormon it:Ormone he:הורמון lt:Hormonas hu:Hormon mk:Хормони nl:Hormoon ja:ホルモン no:Hormon nn:Hormon pl:Hormon pt:Hormona ro:Hormon ru:Гормоны simple:Hormone sk:Hormón sl:Hormon sr:Хормони su:Hormon fi:Hormoni sv:Hormoner th:ฮอร์โมน tr:Hormon uk:Гормон yi:הארמאון zh:激素

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