Hormones (from Greek "Polytonic|ὁρμή" - "impetus") are chemicals released by cells that affect cells in other parts of the body. Only a small amount of hormone is required to alter cell metabolism. It is also a chemical messenger that transports a signal from one cell to another. All
multicellular organisms produce hormones; planthormones are also called phytohormones. Hormones in animals are often transported in the blood. Cells respond to a hormone when they express a specific receptor for that hormone. The hormone binds to the receptor protein, resulting in the activation of a signal transductionmechanism that ultimately leads to cell type-specific responses.
molecules are secreted (released) directly into the bloodstream, while exocrine hormones (or ectohormones) are secreted directly into a duct, and from the duct they either flow into the bloodstream or they flow from cell to cell by diffusionin a process known as paracrine signalling.
Hierarchical nature of hormonal control
Hormonal regulation of some physiological activities involves a hierarchy of cell types acting on each other either to stimulate or to modulate the release and action of a particular hormone. The secretion of hormones from successive levels of endocrine cells is stimulated by chemical signals originating from cells higher up the hierarchical system. The master coordinator of hormonal activity in
mammals is the hypothalamus, which acts on input that it receives from the central nervous system. [cite book | title=Biochemistry | author=Mathews, CK and van Holde, K. E. | chapter=Integration and control of metabolic processes | editor=Bowen, D. | year=1990 | publisher=The Benjamin/Cummings publishing group | pages=790–792 | isbn=0-8053-5015-2 | oclc=20352437]
Other hormone secretion occurs in response to local conditions, such as the rate of secretion of
parathyroid hormoneby the parathyroidcells in response to fluctuations of ionized calciumlevels in extracellular fluid.
Hormonal signalling across this hierarchy involves the following:
#Biosynthesis of a particular hormone in a particular tissue
#Storage and secretion of the hormone
#Transport of the hormone to the target cell(s)
#Recognition of the hormone by an associated cell membrane or
#Relay and amplification of the received hormonal signal via a
signal transductionprocess: This then leads to a cellular response. The reaction of the target cells may then be recognized by the original hormone-producing cells, leading to a down-regulationin hormone production. This is an example of a homeostatic negative feedback loop.
#Degradation of the hormone.
As can be inferred from the hierarchical diagram, hormone biosynthetic cells are typically of a specialized cell type, residing within a particular endocrine gland (e.g., the
thyroid gland, the ovaries, or the testes). Hormones may exit their cell of origin via exocytosisor another means of membrane transport. However, the hierarchical model is an oversimplification of the hormonal signaling process. Cellular recipients of a particular hormonal signal may be one of several cell types that reside within a number of different tissues, as is the case for insulin, which triggers a diverse range of systemic physiological effects. Different tissue types may also respond differently to the same hormonal signal. Because of this, hormonal signaling is elaborate and hard to dissect.
Interactions with receptors
Most hormones initiate a cellular response by initially combining with either a specific
intracellularor cell membrane associated receptor protein. A cell may have several different receptors that recognize the same hormone and activate different signal transductionpathways, or alternatively different hormones and their receptors may invoke the same biochemical pathway. For many hormones, including most protein hormones, the receptor is membrane associated and embedded in the plasma membraneat the surface of the cell. The interaction of hormone and receptor typically triggers a cascade of secondary effects within the cytoplasmof the cell, often involving phosphorylationor dephosphorylation of various other cytoplasmic proteins, changes in ion channelpermeability, or increased concentrations of intracellular molecules that may act as secondary messengers (e.g. cyclic AMP). Some protein hormones also interact with intracellularreceptors located in the cytoplasmor nucleus by an intracrinemechanism.
For hormones such as steroid or thyroid hormones, their receptors are located intracellularly within the
cytoplasmof their target cell. In order to bind their receptors these hormones must cross the cell membrane. The combined hormone-receptor complex then moves across the nuclear membrane into the nucleus of the cell, where it binds to specific DNA sequences, effectively amplifying or suppressing the action of certain genes, and affecting protein synthesis.cite journal | author =Beato M, Chavez S and Truss M | title = Transcriptional regulation by steroid hormones | journal = Steroids | year=1996 | volume=61 | issue=4 | pages=240–251 | pmid = 8733009 | doi = 10.1016/0039-128X(96)00030-X ] However, it has been shown that not all steroid receptors are located intracellularly, some are plasma membraneassociated.cite journal | author =Hammes SR | title = The further redefining of steroid-mediated signaling | journal = Proc Natl Acad Sci USA | year=2003 | volume=100 | issue=5 | pages=21680–2170 | pmid = 12606724 | doi = 10.1073/pnas.0530224100 ]
An important consideration, dictating the level at which cellular
signal transductionpathways are activated in response to a hormonal signal is the effective concentrationof hormone-receptor complexes that are formed. Hormone-receptor complex concentrations are effectively determined by three factors:
#The number of hormone molecules available for complex formation
#The number of receptor molecules available for complex formation and
#The binding affinity between hormone and receptor. The number of hormone molecules available for complex formation is usually the key factor in determining the level at which
signal transductionpathways are activated. The number of hormone molecules available being determined by the concentration of circulating hormone, which is in turn influenced by the level and rate at which they are secreted by biosynthetic cells. The number of receptors at the cell surface of the receiving cell can also be varied as can the affinity between the hormone and its receptor.
Physiology of hormones
Most cells are capable of producing one or more molecules, which act as signaling molecules to other cells, altering their growth, function, or metabolism. The classical hormones produced by cells in the
endocrine glands mentioned so far in this article are cellular products, specialized to serve as regulators at the overall organism level. However they may also exert their effects solely within the tissue in which they are produced and originally released.
The rate of hormone biosynthesis and secretion is often regulated by a homeostatic
negative feedbackcontrol mechanism. Such a mechanism depends on factors which influence the metabolismand excretionof hormones. Thus, higher hormone concentration alone can not trigger the negative feedback mechanism. Negative feedback must be triggered by overproduction of an "effect" of the hormone.
Hormone secretion can be stimulated and inhibited by:
*Other hormones ("stimulating"- or "releasing"-hormones)
*Plasma concentrations of ions or nutrients, as well as binding
Neurons and mental activity
*Environmental changes, e.g., of light or temperature
One special group of hormones is the
tropic 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, orexinand PYY 3-36- and "satiety hormones" - e.g., leptin, obestatin, nesfatin-1.
In order to release active hormones quickly into the
circulation, hormone biosynthetic cells may produce and store biologically inactive hormones in the form of pre- or prohormones. These can then be quickly converted into their active hormone form in response to a particular stimulus.
Hormone effects vary widely, but can include:
* stimulation or inhibition of growth,
* In puberty hormones can affect
* induction or suppression of
apoptosis(programmed cell death)
* activation or inhibition of the
* preparation for a new activity (e.g.,
fighting, fleeing, mating)
* preparation for a new phase of life (e.g.,
puberty, caring for offspring, menopause)
* controlling the
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.
Chemical classes of hormones
Vertebratehormones fall into three chemical classes:
Amine-derived hormones are derivatives of the amino acids tyrosineand tryptophan. Examples are catecholamines and thyroxine.
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 insulinand growth hormone. More complex protein hormones bear carbohydrateside chains and are called glycoproteinhormones. Luteinizing hormone, follicle-stimulating hormoneand thyroid-stimulating hormoneare glycoprotein hormones.
Lipidand phospholipid-derived hormones derive from lipids such as linoleic acidand arachidonic acidand phospholipids. The main classes are the steroid hormonesthat derive from cholesteroland the eicosanoids. Examples of steroid hormonesare testosteroneand cortisol. Sterolhormones such as calcitriolare a homologous system. The adrenal cortexand the gonads are primary sources of steroid hormones. Examples of eicosanoids are the widely studied prostaglandins.
Many hormones and their analogues are used as
medication. The most commonly-prescribed hormones are estrogens and progestagens (as methods of hormonal contraceptionand as HRT), thyroxine(as levothyroxine, for hypothyroidism) and steroids (for autoimmune diseases and several respiratory disorders). Insulinis used by many diabetics. Local preparations for use in otolaryngologyoften contain pharmacologic equivalents of adrenaline, while steroidand vitamin Dcreams 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
glucocorticoidto 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
diphthongin oestrogen and favors the earlier spelling gonadotrophin (from "trophē" ‘nourishment, sustenance’ rather than "tropē" ‘turning, change’.
Plant hormonesor plant growth regulators
* [http://www.hormone.org The Hormone Foundation]
* [http://video.google.com/videoplay?docid=2730429945740914542&q=living+smart%3A+Hormones+and+Nutrition&hl=en Hormones and Nutrition]
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