Alpha-2 adrenergic receptor: Difference between revisions

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The α₂ receptor is a type of adrenergic receptor.

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 (α₂_A; ADRA2A)^[1]
Vasoconstriction of arteries (α₂_B; 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 norepinephrine^[5]
  - Inhibit norepinephrine 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 - G_i 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

Epinephrine has higher affinity for the alpha-2 receptor than has norepinephrine, 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.

- - Clonidine is an Alpha 2 Agonist ; initially thought act via postsynaptic 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. ***

Antagonists

(Alpha blockers)

* denotes selective agonists to the receptor.

Types

There are three types of α₂ receptors: ADRA2A, ADRA2B, ADRA2C.

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: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.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^ ^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 ed.). Sunderland, Mass: Sinauer. ISBN 0-87893-725-0. {{cite book}}: |edition= has extra text (help); Cite has empty unknown parameter: |1= (help)CS1 maint: multiple names: authors list (link)
^ 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.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^ online-medical-dictionary.org

This transmembrane receptor-related article is a stub. You can help Wikipedia by expanding it.

External links

"Adrenoceptors". IUPHAR Database of Receptors and Ion Channels. International Union of Basic and Clinical Pharmacology. {{cite web}}: Cite has empty unknown parameter: |coauthors= (help)

[1] Goodman Gilman, Alfred. Goodman & Gilman's The Pharmacological Basis of Therapeutics. Tenth Edition. McGraw-Hill (2001): Page 140.

[pmid2887122-2] 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.

[pmid9280371-3] 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.

[pmid2889649-4] 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.{{cite journal}}: CS1 maint: multiple names: authors list (link)

[Rang163-5] ^ ^a ^b ^c ^d ^e ^f ^g Rang, H. P. (2003). Pharmacology. Edinburgh: Churchill Livingstone. ISBN 0-443-07145-4. Page 163

[pmid6119348-6] 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.

[purves-7] Fitzpatrick, David; Purves, Dale; Augustine, George (2004). "Table 20:2". Neuroscience (Third Edition ed.). Sunderland, Mass: Sinauer. ISBN 0-87893-725-0. {{cite book}}: |edition= has extra text (help); Cite has empty unknown parameter: |1= (help)CS1 maint: multiple names: authors list (link)

[pmid10215710-8] 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.{{cite journal}}: CS1 maint: multiple names: authors list (link)

[9] -medical-dictionary.org

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+==External links==
+* {{cite web | url = http://www.iuphar-db.org/GPCR/ChapterMenuForward?chapterID=1274 | title = Adrenoceptors  | accessdate = | author = | authorlink = | coauthors = | date = | format = | work = IUPHAR Database of Receptors and Ion Channels | publisher = International Union of Basic and Clinical Pharmacology | pages = | language = | archiveurl = | archivedate = | quote = }}