- Alpha-2 adrenergic receptor
The α2 receptor is a type of
The α2 receptor has several, general, functions in common with other α-receptors, but also has individual effects.
Common (or still unspecified) effects include:
Vasodilationof arteries(α2A; ADRA2A) [Goodman Gilman, Alfred. "Goodman & Gilman's The Pharmacological Basis of Therapeutics". Tenth Edition. McGraw-Hill (2001): Page 140.]
* Vasoconstriction of arteries (α2B;
ADRA2B) to heart( coronary artery).cite journal | author = Woodman OL, Vatner SF | title = Coronary vasoconstriction mediated by α1- and α2-adrenoceptors in conscious dogs | journal = Am. J. Physiol. | volume = 253 | issue = 2 Pt 2 | pages = H388–93 | year = 1987 | pmid = 2887122 | doi = | issn = | url = http://ajpheart.physiology.org/cgi/content/abstract/253/2/H388 ]
* Vasoconstriction of
veinscite journal | author = Elliott J | title = Alpha-adrenoceptors in equine digital veins: evidence for the presence of both α1 and α2-receptors mediating vasoconstriction | journal = J. Vet. Pharmacol. Ther. | volume = 20 | issue = 4 | pages = 308–17 | year = 1997 | pmid = 9280371 | doi = 10.1046/j.1365-2885.1997.00078.x | issn = ]
* Decrease motility of
smooth musclein gastrointestinal tractcite journal | author = Sagrada A, Fargeas MJ, Bueno L | title = Involvement of α1 and α2 adrenoceptors in the postlaparotomy intestinal motor disturbances in the rat | journal = Gut | volume = 28 | issue = 8 | pages = 955–9 | year = 1987 | pmid = 2889649 | doi = 10.1136/gut.28.8.955 | issn = ]
* Contraction of
male genitaliaduring ejaculation
Individual actions of the α2 receptor include:
Mediates synaptic transmissionin pre- and postsynaptic nerve terminals.
**Decrease release of
**Decrease release of
norepinephrine systemin brain
* Inhibitioncite journal | author = Wright EE, Simpson ER | title = Inhibition of the lipolytic action of beta-adrenergic agonists in human adipocytes by alpha-adrenergic agonists | journal = J. Lipid Res. | volume = 22 | issue = 8 | pages = 1265–70 | year = 1981 | pmid = 6119348 | doi = | issn = | url = http://www.jlr.org/cgi/content/abstract/22/8/1265] of
lipolysisin adipose tissue.
* inhibition of
insulinrelease in pancreas.cite book | author = Fitzpatrick, David; Purves, Dale; Augustine, George | title = Neuroscience | publisher = Sinauer | location = Sunderland, Mass | year = 2004 | | edition = Third Edition | chapter = Table 20:2 | pages = | isbn = 0-87893-725-0 | oclc = | doi = ]
* induction of
glucagonrelease from pancreas.
* contraction of
sphincters of the gastrointestinal tract
* ↓ Secretion from
salivary glandcite journal | author = Khan ZP, Ferguson CN, Jones RM | title = alpha-2 and imidazoline receptor agonists. Their pharmacology and therapeutic role | journal = Anaesthesia | volume = 54 | issue = 2 | pages = 146–65 | year = 1999 | pmid = 10215710 | doi = 10.1046/j.1365-2044.1999.00659.x | issn = ]
gastrointestinal tract(presynaptic effect)
The relaxation of gastrointestinal tract motility is by
presynapticinhibition cite book |author=Rang, H. P. |title=Pharmacology |publisher=Churchill Livingstone |location=Edinburgh |year=2003 |pages= |isbn=0-443-07145-4 |oclc= |doi= Page 163 ] , where transmitters inhibit further release by homotropiceffects.
Epinephrinehas higher affinity for the alpha-2 receptor than has norepinephrine, which, in turn, has much higher affinity than has isoprenaline. Other agonists include:
clonidine* ( antihypertensive) -- see below for alternative mechanism --
xylazine( veterinary; in non-human species this is an immobilizing and anesthetic drug, presumptively also mediated by alpha-2 adrenergic receptors because it is reversed by yohimbine)
tizanidine(in spasms, cramping)
guanfacine(antihypertensive) *denotes selective agonists to the receptor.
***Clonidine is an Alpha 2 Agonist ; initially thought act via presynaptic Alpha 2 receptors --> reduced NE discharge. However! It binds to imidazoline receptors with a much greater affinity than Alpha 2s . Imidazoline Receptors occur in the Nucleus Tractus Solitarius & Ventrolateral Medulla. Clonidine is now thought to decrease BP via this central mechanism. ***
yohimbine* (purported aphrodisiac)
mianserin( tetracyclic antidepressant)
idazoxan*(experimental) [ [http://www.online-medical-dictionary.org/Idazoxan.asp?q=Idazoxan online-medical-dictionary.org] ] *denotes selective agonists to the receptor.
There are three types of α2 receptors:
ADRA2A, ADRA2B, ADRA2C.
Alpha-1 adrenergic receptor
Beta-1 adrenergic receptor
Beta-2 adrenergic receptor
Beta-3 adrenergic receptor
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