Health Dictionary
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Alpha-2 adrenergic receptor
Introduction
The α2 receptor is a type of adrenergic receptor.
Effect
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:
- Vasoconstriction of arteries to heart (coronary artery).[1]
- Vasoconstriction of veins[2]
- Decrease motility of smooth muscle in gastrointestinal tract[3]
- 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[4]
- Decrease release of noradrenaline[4]
- Inhibit noradrenaline system in brain
- Inhibition[5] of lipolysis in adipose tissue.[6]
- inhibition of insulin release in pancreas.[6]
- induction of glucagon release from pancreas.
- platelet aggregation
- contraction of sphincters of the gastrointestinal tract
- ↓ Secretion from salivary gland[7]
Mechanism
Mechanism
A G protein – Gi renders adenylate cyclase inactivated, resulting in decrease of cAMP.
The relaxation of gastrointestinal tract motility is by presynaptic inhibition[4], where transmitters inhibit further release by homotropic effects.
Agonists
Agonists
adrenaline has higher affinity for the alpha-2 receptor than has noradrenaline, which, in turn, has much higher affinity than has isoprenaline.[4] Other agonists include:
- clonidine*[4] (antihypertensive)
- lofexidine (antihypertensive)
- xylazine (veterinary)
- tizanidine (in spasms, cramping)
- guanfacine (antihypertensive)
- clenbuterol*[4] (decongestant and bronchodilator)
* denotes selective agonists to the receptor.
Antagonists
Antagonists
- yohimbine*[4] (purported aphrodisiac)
- mirtazapine (NaSSA)
- mianserin (tetracyclic antidepressant)
- idazoxan*[4](experimental)[8]
* denotes selective agonists to the receptor.
See also
See also
References
References
- ↑ 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:10.1046/j.1365-2885.1997.00078.x. 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:10.1136/gut.28.8.955. PMID 2889649.
- ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 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.
- ↑ 6.0 6.1 Fitzpatrick, David; Purves, Dale; Augustine, George (2004). “Table 20:2”. Neuroscience (Third Edition ed.). 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:10.1046/j.1365-2044.1999.00659.x. PMID 10215710.
- ↑ online-medical-dictionary.org