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Hormone – Human Growth, Replacement Therapy, Growth Hormone Book
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Hormones
Hormone - From Wikipedia, the free encyclopedia
For other uses, see Hormone (disambiguation).
Epinephrine (adrenaline), a catecholamine-type hormoneA 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 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. 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 a multicellular organism.
Endocrine hormone 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 diffusion in a process known as paracrine signalling.
Contents 1 Hierarchical nature of hormonal control 2 Hormone signaling 3 Interactions with receptors 4 Physiology of hormones 5 Hormone effects 6 Chemical classes of hormones 7 Pharmacology 8 Important human hormones 9 References
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 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 acting on input it receives from the central nervous system.[1]
Other hormone secretion occurs in response to local conditions, such as the rate of secretion of parathyroid hormone by the parathyroid cells in response to fluctuations of ionized calcium levels in extracellular fluid.
Hormone signaling Hormonal signaling 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 intracellular receptor protein. Relay and amplification of the received hormonal signal via a signal transduction process. 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-regulation in 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, ovaries or testes). Hormones may exit their cell of origin via exocytosis or another means of membrane transport. However, the hierarchical model is an over simplification of the hormonal signaling process. Typically 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 intracellular or cell membrane associated receptor protein. A cell may have several different receptors that recognize the same hormone and activate different signal transduction pathways, 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 membrane at the surface of the cell. The interaction of hormone and receptor typically triggers a cascade of secondary effects within the cytoplasm of the cell, often involving phosphorylation or dephosphorylation of various other cytoplasmic proteins, changes in ion channel permeability, or increased concentrations of intracellular molecules that may act as secondary messengers (e.g. cyclic AMP). Some protein hormones also interact with intracellular receptors located in the cytoplasm or nucleus by an intracrine mechanism.
For hormones such as steroid or thyroid hormones, their receptors are located intracellularly within the cytoplasm of 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.[2] However, it has been shown that not all steriod receptors are located intracellularly, some are plasma membrane associated.[3]
An important consideration, dictating the level at which cellular signal transduction pathways are activated in response to a hormonal signal is the effective concentration of 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 transduction pathways 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 signalling molecules to other cells, altering their growth, function, or metabolism. The classical hormones produced by 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 feedback control mechanism. Such a mechanism depends on factors which influence the metabolism and excretion of hormones. Thus, higher hormome 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 globulins 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, orexin and 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 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 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 reproductive cycle 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 Vertebrate hormones fall into three chemical classes:
Amine-derived hormones are derivatives of the amino acids tyrosine and 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 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. 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 Tissue Cells Mechanism Target Tissue Effect amine - tryptophan Melatonin (N-acetyl-5-methoxytryptamine) pineal gland pinealocyte makes you sleepy amine - tryptophan Serotonin 5-HT CNS, GI tract enterochromaffin cell amine - tyrosine Thyroxine (thyroid hormone) T4 thyroid gland thyroid epithelial cell direct Increases metabolic rate
amine - tyrosine Triiodothyronine (thyroid hormone) T3 thyroid gland thyroid epithelial cell direct amine - tyrosine (cat) Epinephrine (or adrenaline) EPI adrenal medulla chromaffin cell amine - tyrosine (cat) Norepinephrine (or noradrenaline) NRE adrenal medulla chromaffin cell
amine - tyrosine (cat) Dopamine DPM hypothalamus peptide Antimullerian hormone (or mullerian inhibiting factor or hormone) AMH testes Sertoli cell peptide Adiponectin Acrp30 adipose tissue peptide Adrenocorticotropic hormone (or corticotropin) ACTH anterior pituitary corticotrope 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 parafollicular cell cAMP peptide Cholecystokinin CCK duodenum peptide Corticotropin-releasing hormone CRH hypothalamus cAMP peptide Erythropoietin EPO kidney peptide Follicle-stimulating hormone FSH anterior pituitary gonadotrope cAMP peptide Gastrin GRP stomach, duodenum G cell peptide Ghrelin stomach P/D1 cell peptide Glucagon GCG 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 syncytiotrophoblast cells cAMP peptide Human placental lactogen HPL placenta peptide Growth hormone GH or hGH anterior pituitary somatotropes
peptide Inhibin testes Sertoli cells peptide Insulin INS pancreas beta cells tyrosine kinase peptide Insulin-like growth factor (or somatomedin) IGF liver tyrosine kinase
peptide Leptin LEP adipose tissue peptide Luteinizing hormone LH anterior pituitary gonadotropes cAMP peptide Melanocyte stimulating hormone MSH or α-MSH anterior pituitary/pars intermedia cAMP peptide Oxytocin OXT posterior pituitary IP3 peptide Parathyroid hormone PTH parathyroid gland parathyroid chief cell cAMP peptide Prolactin PRL anterior pituitary lactotrophs peptide Relaxin RLN varies peptide Secretin SCT duodenum S cell peptide Somatostatin SRIF hypothalamus, islets of Langerhans delta cells peptide Thrombopoietin TPO liver, kidney peptide Thyroid-stimulating hormone TSH anterior pituitary thyrotropes cAMP peptide Thyrotropin-releasing hormone TRH hypothalamus IP3 steroid - glu. Cortisol adrenal cortex (zona fasciculata) direct steroid - min. Aldosterone adrenal cortex (zona glomerulosa) direct steroid - sex (and) Testosterone testes Leydig cells 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 E2 ovary granulosa cells direct steroid - sex (est) Estrone ovary granulosa cells direct steroid - sex (est) Estriol placenta syncytiotrophoblast direct steroid - sex (pro) Progesterone ovary, adrenal glands, placenta granulosa cells direct sterol Calcitriol (Vitamin D3) skin/proximal tubule of kidneys direct eicosanoid Prostaglandins PG seminal vesicle eicosanoid Leukotrienes LT white blood cells eicosanoid Prostacyclin PGI2 endothelium eicosanoid Thromboxane TXA2 platelets
References ^ Mathews, CK and van Holde, K. E. (1990). "Integration and control of metabolic processes", in Bowen, D.: Biochemistry. The Benjamin/Cummings publishing group, 790-792. ISBN 0-8053-5015-2. ^ Beato M, Chavez S and Truss M (1996). "Transcriptional regulation by steroid hormones". Steroids 61 (4): 240-251. PMID 8733009. ^ Hammes SR (2003). "The further redefining of steroid-mediated signaling". Proc Natl Acad Sci USA 100 (5): 21680-2170. PMID 12606724.
This page is licensed under the GNU Free Documentation License. It uses content from the Wikipedia article hormone
Follicle-stimulating hormone-receptor complex – (source)
As an important hormonal regulator of reproduction, inhibin has the potential to be involved in reproductive diseases or disorders. – (source)
Disrupting a Delicate Balance: Environmental Effects on the Thyroid - (source)
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