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The adrenergic receptors (or adrenoceptors) are a class of G protein-coupled receptors that are targets of the catecholamines. Adrenergic receptors specifically bind their endogenous ligands, the catecholamines adrenaline and noradrenaline (called epinephrine and norepinephrine in the United States), and are activated by these.

Many cells possess these receptors, and the binding of an agonist will generally cause a sympathetic response (ie the fight-or-flight response). For instance, the heart rate will increase and the pupils will dilate, energy will be mobilized, and blood flow diverted from other organs to skeletal muscle. (Note: Sympathetic activity will result in vasodilation of coronary arteries via the β2-adrenergic receptors.)


Epinephrine binds its receptor, that associates with an heterotrimeric G protein. The G protein associates with adenylate cyclase that converts ATP to cAMP, spreading the signal (more details...)

File:Adrenoceptor-Signal transduktion.PNG

The mechanism of adrenergic receptors. Adrenaline or noradrenaline are receptor ligands to either α1, α2 or β-adrenergic receptors. α1 couples to Gq, which results in incerased intracellular Ca2+ which results in e.g. smooth muscle contraction. α2, on the other hand, couples to Gi, which causes a decrease of cAMP activity, resulting in e.g. smooth muscle contraction. β receptors couple to Gs, and increases intracellular cAMP activity, resulting in e.g. heart muscle contraction, smooth muscle relaxation and glycogenolysis.

There are several types of adrenergic receptors, but there are two main groups: α-Adrenergic and β-Adrenergic.


Receptor type Agonist potency order Selected action
of agonist
Mechanism Agonists Antagonists
A, B, D
noradrenaline≥ adrenaline >> isoprenaline smooth muscle contraction Gq: phospholipase C (PLC) activated, IP3 and calcium up (Alpha blockers)
A, B, C
adrenaline > noradrenaline >> isoprenaline smooth muscle contraction Gi: adenylate cyclase inactivated, cAMP down
  • clonidine
  • lofexidine
  • xylazine
  • Tizanidine
  • Guanfacine
(Alpha blockers)
β1 isoprenaline > noradrenaline > adrenaline heart muscle contraction Gs: adenylate cyclase activated, cAMP up (Beta blockers)
  • metoprolol
  • atenolol
β2 isoprenaline > adrenaline > noradrenaline smooth muscle relaxation Gs: adenylate cyclase activated, cAMP up (Short/long) (Beta blockers)
β3 isoprenaline > noradrenaline = adrenaline Enhance lipolysis Gs: adenylate cyclase activated, cAMP up

The absence of "ADRA1C" is intentional. At one time, there was a subtype known as C, but was found to be one of the previously discovered subtypes. To avoid confusion, it was decided that there would never be a C subtype again and so if any new subtypes were discovered, naming would start with D.

α receptors

α receptors have several functions in common, but also individual effects. Common (or still unspecified) effects include:

α1 receptor

Main article: Alpha-1 adrenergic receptor

Alpha1-adrenergic receptors are members of the G protein-coupled receptor superfamily. Upon activation, a heterotrimeric G protein, Gq, activates phospholipase C (PLC), which causes an increase in IP3 and calcium. This triggers all other effects.

Specific actions of the α1 receptor mainly involves smooth muscle contraction. It causes vasoconstriction in blood vessels of skin & gastrointestinal system and to kidney (renal artery)[5] and brain.[6]. Other areas of smooth muscle contraction are for instance:

  • ureter
  • hairs (arrector pili muscles)
  • uterus (when pregnant)
  • urethral sphincter
  • bronchioles (although minor to the relaxing effect of β2 receptor on bronchioles)

Further effects include glycogenolysis and gluconeogenesis from adipose tissue[7] and liver, as well as secretion from sweat glands[7] and Na+ reabsorption from kidney.[7]

Antagonists may be used in hypertension.

α2 receptor

Main article: Alpha-2 adrenergic receptor

There are 3 highly homologous subtypes of α2 receptors: α2A, α2Β, and α2C.

Specific actions of the α2 receptor include:

β receptors

β1 receptor

Main article: Beta-1 adrenergic receptor

Specific actions of the β1 receptor include:

β2 receptor

Main article: Beta-2 adrenergic receptor

The 3D crystallographic structure of the β2-adrenergic receptor has been determined (PDB 2R4R, Template:PDB2, Template:PDB2).[8][9][10]

Specific actions of the β2 receptor include:

β3 receptor

Main article: Beta-3 adrenergic receptor

Specific actions of the β3 receptor include:

See also


  1. Nisoli E, Tonello C, Landi M, Carruba MO (1996). Functional studies of the first selective β3-adrenergic receptor antagonist SR 59230A in rat brown adipocytes. Mol. Pharmacol. 49 (1): 7–14.
  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.
  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.
  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.
  5. Schmitz JM, Graham RM, Sagalowsky A, Pettinger WA (1981). Renal α1 and α2 adrenergic receptors: biochemical and pharmacological correlations. J. Pharmacol. Exp. Ther. 219 (2): 400–6.
  6. Circulation & Lung Physiology I M.A.S.T.E.R. Learning Program, UC Davis School of Medicine
  7. 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7 Fitzpatrick, David; Purves, Dale; Augustine, George (2004). "Table 20:2" Neuroscience, Third Edition, Sunderland, Mass: Sinauer.
  8. Rasmussen SG, Choi HJ, Rosenbaum DM, Kobilka TS, Thian FS, Edwards PC, Burghammer M, Ratnala VR, Sanishvili R, Fischetti RF, Schertler GF, Weis WI, Kobilka BK (2007). Crystal structure of the human β2-adrenergic G-protein-coupled receptor. Nature 450 (7168): 383–7.
  9. Cherezov V, Rosenbaum DM, Hanson MA, Rasmussen SG, Thian FS, Kobilka TS, Choi HJ, Kuhn P, Weis WI, Kobilka BK, Stevens RC (2007). High-resolution crystal structure of an engineered human β2-adrenergic G protein-coupled receptor. Science 318 (5854): 1258–65.
  10. Rosenbaum DM, Cherezov V, Hanson MA, Rasmussen SG, Thian FS, Kobilka TS, Choi HJ, Yao XJ, Weis WI, Stevens RC, Kobilka BK (2007). GPCR engineering yields high-resolution structural insights into β2-adrenergic receptor function. Science 318 (5854): 1266–73.

Further reading

  • Rang HP, Dale MM, Ritter JM, Moore PK (2003). "Ch. 11" Pharmacology, Elsevier Churchill Livingstone. ISBN 0-443-07145-4.
  • Rang HP, Dale MM, Ritter JM, Flower RJ (2007). "Ch. 11" Rang and Dale's Pharmacology, 169-170, Elsevier Churchill Livingstone. ISBN 0-443-06911-5.

External links

- 3D structure of beta-2 adrenergic receptor in membrane

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