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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.)
There are several types of adrenergic receptors, but there are two main groups: α-Adrenergic and β-Adrenergic.
- α receptors bind norepinephrine and epinephrine, though norepinephrine has higher affinity. Phenylephrine is a selective agonist of the α receptor. They exist as α1-adrenergic receptors and α2-adrenergic receptors.
- β receptors are linked to Gs proteins, which in turn are linked to adenylyl cyclase. Agonist binding thus causes a rise in the intracellular concentration of the second messenger cAMP. Downstream effectors of cAMP include cAMP-dependent protein kinase (PKA), which mediates some of the intracellular events following hormone binding.
|Receptor type||Agonist potency order||Selected action
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||
|β1||isoprenaline > noradrenaline > adrenaline||heart muscle contraction||Gs: adenylate cyclase activated, cAMP up||(Beta blockers)
|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 have several functions in common, but also individual effects. Common (or still unspecified) effects include:
- Vasoconstriction of arteries to heart (coronary artery).
- Vasoconstriction of veins
- Decrease motility of smooth muscle in gastrointestinal tract
- 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) and brain.. Other areas of smooth muscle contraction are for instance:
- hairs (arrector pili muscles)
- uterus (when pregnant)
- urethral sphincter
- bronchioles (although minor to the relaxing effect of β2 receptor on bronchioles)
Antagonists may be used in hypertension.
- 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:
- inhibition of insulin release in pancreas.
- induction of glucagon release from pancreas.
- contraction of sphincters of the gastrointestinal tract
- Main article: Beta-1 adrenergic receptor
Specific actions of the β1 receptor include:
- Increase cardiac output
- Renin release from juxtaglomerular cells.
- Lipolysis in adipose tissue.
- Main article: Beta-2 adrenergic receptor
Specific actions of the β2 receptor include:
- Smooth muscle relaxation, e.g. in bronchi.
- Relax urinary sphincter.
- Relax detrusor urinae muscle of bladder wall
- Dilate arteries to skeletal muscle
- Glycogenolysis and gluconeogenesis
- Contract sphincters of GI tract
- Thickened secretions from salivary glands.
- Inhibit histamine-release from mast cells
- Increase renin secretion from kidney
- Main article: Beta-3 adrenergic receptor
Specific actions of the β3 receptor include:
- Enhancement of lipolysis in adipose tissue.
- adrenergic blocking drugs
- Adrenergic drugs
- Adrenergic nerves
- Beta adrenergic receptor kinase
- Beta adrenergic receptor kinase-2
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- 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.
- 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.
- 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.
- Circulation & Lung Physiology I M.A.S.T.E.R. Learning Program, UC Davis School of Medicine
- Fitzpatrick, David; Purves, Dale; Augustine, George (2004). "Table 20:2" Neuroscience, Third Edition, Sunderland, Mass: Sinauer.
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- 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.
- The Adrenergic Receptors
- IUPHAR GPCR Database - Adrenoceptors
- Basic Neurochemistry: α- and β-Adrenergic Receptors
- Brief overview of functions of the beta-3 receptor
- Theory of receptor activation
- Desensitization of beta-1-receptors
- UMich Orientation of Proteins in Membranes protein/pdbid-2rh1
- 3D structure of beta-2 adrenergic receptor in membrane
|Direct Acting||Albuterol – Clonidine – Dobutamine – Dopamine - Epinephrine - Formoterol - Isoproterenol - Metaproterenol - Methoxamine - Norepinephrine - Phenylephrine - Piruterol - Salmeterol - Tamsulosine - Tamsulosine - Terbutaline|
|Indirect Acting||Amphetamine - Tyramine|
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