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The α2 receptor is a type of adrenergic receptor.
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Effect
The α2 receptor has several, general, functions in common with other α-receptors, but also has individual effects.
General
Common (or still unspecified) effects include:
- Vasodilation of arteries (α2A; ADRA2A)[1]
- Vasoconstriction of arteries (α2B; ADRA2B) to heart (coronary artery).[2]
- Vasoconstriction of veins[3]
- Decrease motility of smooth muscle in gastrointestinal tract[4]
- Contraction of male genitalia during ejaculation
Individual
Individual actions of the α2 receptor include:
- Mediates synaptic transmission in pre- and postsynaptic nerve terminals.
- Decrease release of acetylcholine[5]
- Decrease release of noradrenaline[5]
- Inhibit noradrenaline system in brain
- Inhibition[6] of lipolysis in adipose tissue.[7]
- inhibition of insulin release in pancreas.[7]
- induction of glucagon release from pancreas.
- platelet aggregation
- contraction of sphincters of the gastrointestinal tract
- ↓ Secretion from salivary gland[8]
- relax gastrointestinal tract(presynaptic effect)
Mechanism
A G protein - Gi renders adenylate cyclase inactivated, resulting in decrease of cAMP.
The relaxation of gastrointestinal tract motility is by presynaptic inhibition[5], where transmitters inhibit further release by homotropic effects.
Agonists
Adrenaline has higher affinity for the alpha-2 receptor than has noradrenaline, which, in turn, has much higher affinity than has isoprenaline.[5] Other agonists include:
- clonidine*[5] (antihypertensive) -- see below for alternative mechanism --
- dexmedetomidine
- lofexidine (antihypertensive)
- 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.
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- 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. ***
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Antagonists
- yohimbine*[5] (purported aphrodisiac)
- mirtazapine (NaSSA)
- mianserin (tetracyclic antidepressant)
- idazoxan*[5](experimental)[9]
* denotes selective agonists to the receptor.
Types
There are three types of α2 receptors: ADRA2A, ADRA2B, ADRA2C.
See also
- Other adrenergic receptors
References
- ^ Goodman Gilman, Alfred. Goodman & Gilman's The Pharmacological Basis of Therapeutics. Tenth Edition. McGraw-Hill (2001): Page 140.
- ^ Woodman OL, Vatner SF (1987). "Coronary vasoconstriction mediated by α1- and α2-adrenoceptors in conscious dogs". Am. J. Physiol. 253 (2 Pt 2): H388–93. PMID 2887122.
- ^ Elliott J (1997). "Alpha-adrenoceptors in equine digital veins: evidence for the presence of both α1 and α2-receptors mediating vasoconstriction". J. Vet. Pharmacol. Ther. 20 (4): 308–17. doi:. PMID 9280371.
- ^ Sagrada A, Fargeas MJ, Bueno L (1987). "Involvement of α1 and α2 adrenoceptors in the postlaparotomy intestinal motor disturbances in the rat". Gut 28 (8): 955–9. doi:. PMID 2889649.
- ^ a b c d e f g Rang, H. P. (2003). Pharmacology. Edinburgh: Churchill Livingstone. ISBN 0-443-07145-4. Page 163
- ^ Wright EE, Simpson ER (1981). "Inhibition of the lipolytic action of beta-adrenergic agonists in human adipocytes by alpha-adrenergic agonists". J. Lipid Res. 22 (8): 1265–70. PMID 6119348.
- ^ a b Fitzpatrick, David; Purves, Dale; Augustine, George (2004). "Table 20:2", Neuroscience, Third Edition, Sunderland, Mass: Sinauer. ISBN 0-87893-725-0.
- ^ Khan ZP, Ferguson CN, Jones RM (1999). "alpha-2 and imidazoline receptor agonists. Their pharmacology and therapeutic role". Anaesthesia 54 (2): 146–65. doi:. PMID 10215710.
- ^ online-medical-dictionary.org
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