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The adrenergic receptors (or adrenoceptors) are a class of G protein-coupled receptors that are targets of the catecholamines, especially noradrenaline (norepinephrine) and adrenaline (epinephrine). Although dopamine is a catecholamine, its receptors are in a different category.

Many cells possess these receptors, and the binding of an agonist will generally cause a sympathetic response (e.g. 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 non-essential organs to skeletal muscle.

Contents

Subtypes

There are two main groups of adrenergic receptors, α and β, with several subtypes.

  • α receptors have the subtypes α1 (a Gq coupled receptor) and α2 (a Gi coupled receptor). Phenylephrine is a selective agonist of the α receptor.
  • β receptors have the subtypes β1, β2 and β3. All three are linked to Gs proteins (although β2 also couples to Gi)[1], which in turn are linked to adenylate cyclase. Agonist binding thus causes a rise in the intracellular concentration of the second messenger cAMP (cyclic adenosine monophosphate). Downstream effectors of cAMP include cAMP-dependent protein kinase (PKA), which mediates some of the intracellular events following hormone binding. Isoprenaline is a selective agonist.
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...)
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 increased intracellular Ca2+ which results in smooth muscle contraction. α2, on the other hand, couples to Gi, which causes a decrease of cAMP activity, resulting in e.g. smooth muscle relaxation. β receptors couple to Gs, and increases intracellular cAMP activity, resulting in e.g. heart muscle contraction, smooth muscle relaxation and glycogenolysis.

Roles in circulation

Adrenaline reacts with both α- and β-adrenoreceptors, causing vasoconstriction and vasodilation, respectively. Although α receptors are less sensitive to epinephrine, when activated, they override the vasodilation mediated by β-adrenoreceptors. The result is that high levels of circulating epinephrine cause vasoconstriction. At lower levels of circulating epinephrine, β-adrenoreceptor stimulation dominates, producing an overall vasodilation.

Comparison

Receptor Agonist potency order Selected action
of agonist
Mechanism Agonists Antagonists
α1:
A, B, D
Norepinephrine > epinephrine >> isoprenaline Smooth muscle contraction Gq: phospholipase C (PLC) activated, IP3 and calcium up

(Alpha-1 agonists)

(Alpha-1 blockers)
α2:
A, B, C
Epinephrinenorepinephrine >> isoprenaline Smooth muscle constriction and neurotransmitter inhibition Gi: adenylate cyclase inactivated, cAMP down

(Alpha-2 agonists)

(Alpha-2 blockers)
β1 Isoprenaline > epinephrine = norepinephrine Heart muscle contraction Gs: adenylate cyclase activated, cAMP up (Beta blockers)
β2 Isoprenaline > epinephrine >> norepinephrine Smooth muscle relaxation Gs: adenylate cyclase activated, cAMP up (also Gi, see β2) (Short/long) (Beta blockers)
β3 Isoprenaline = norepinephrine > epinephrine Enhance lipolysis Gs: adenylate cyclase activated, cAMP up

There is no α1C receptor. At one time, there was a subtype known as C, but was found to be identical to one of the previously discovered subtypes. To avoid confusion, naming was continued with the letter D.

α receptors

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

α1 receptor

Alpha1-adrenergic receptors are members of the G protein-coupled receptor superfamily. Upon activation, a heterotrimeric G protein, Gq, activates phospholipase C (PLC). The PLC cleaves phosphatidylinositol 4,5-bisphosphate (PIP2) which in turn causes an increase in inositol triphosphate (IP3) and diacylglycerol (DAG). The former interacts with calcium channels of endoplasmic and sarcoplasmic retuculum thus changing the calcium content in a cell. This triggers all other effects.

Specific actions of the α1 receptor mainly involves smooth muscle contraction. It causes vasoconstriction in many blood vessels including those of the skin, gastrointestinal system, kidney (renal artery)[6] and brain[7]. Other areas of smooth muscle contraction are:

Further effects include glycogenolysis and gluconeogenesis from adipose tissue[8] and liver, as well as secretion from sweat glands[8] and Na+ reabsorption from kidney.[8]

Antagonists may be used in hypertension.

α2 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

Specific actions of the β1 receptor include:

β2 receptor

Specific actions of the β2 receptor include the following:

β3 receptor

Specific actions of the β3 receptor include:

  • Enhancement of lipolysis in adipose tissue. Beta-3 activating drugs could theoretically be used as weight-loss agents, but are limited by the side effect of tremors.

See also

References

  1. ^ Chen-Izu Y, Xiao RP, Izu LT, Cheng H, Kuschel M, Spurgeon H, Lakatta EG (November 2000). "G(i)-dependent localization of beta(2)-adrenergic receptor signaling to L-type Ca(2+) channels". Biophys. J. 79 (5): 2547–56. doi:10.1016/S0006-3495(00)76495-2. PMID 11053129. 
  2. ^ 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. PMID 8569714. http://molpharm.aspetjournals.org/cgi/content/abstract/49/1/7. 
  3. ^ 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. http://ajpheart.physiology.org/cgi/content/abstract/253/2/H388. 
  4. ^ 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. 
  5. ^ 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. 
  6. ^ 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. PMID 6270306. http://jpet.aspetjournals.org/cgi/content/abstract/219/2/400. 
  7. ^ Circulation & Lung Physiology I M.A.S.T.E.R. Learning Program, UC Davis School of Medicine
  8. ^ a b c d e f g h Fitzpatrick, David; Purves, Dale; Augustine, George (2004). "Table 20:2". Neuroscience (Third ed.). Sunderland, Mass: Sinauer. ISBN 0-87893-725-0. 
  9. ^ Large V, Hellström L, Reynisdottir S, et al. (December 1997). "Human beta-2 adrenoceptor gene polymorphisms are highly frequent in obesity and associate with altered adipocyte beta-2 adrenoceptor function". J. Clin. Invest. 100 (12): 3005–13. doi:10.1172/JCI119854. PMID 9399946. 
  10. ^ Kline WO, Panaro FJ, Yang H, Bodine SC (February 2007). "Rapamycin inhibits the growth and muscle-sparing effects of clenbuterol". J. Appl. Physiol. 102 (2): 740–7. doi:10.1152/japplphysiol.00873.2006. PMID 17068216. 
  11. ^ Kamalakkannan G, Petrilli CM, George I, et al. (April 2008). "Clenbuterol increases lean muscle mass but not endurance in patients with chronic heart failure". J. Heart Lung Transplant. 27 (4): 457–61. doi:10.1016/j.healun.2008.01.013. PMID 18374884. 

Further reading

  • Rang HP, Dale MM, Ritter JM, Moore PK (2003). "Chapter 11: Noradrenergic transmission". Pharmacology (5th ed.). Elsevier Churchill Livingstone. ISBN 0-443-07145-4. 
  • Rang HP, Dale MM, Ritter JM, Flower RJ (2007). "Chapter 11: Noradrenergic transmission". Rang and Dale's Pharmacology (6th ed.). Elsevier Churchill Livingstone. pp. 169–170. ISBN 0-443-06911-5. 

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