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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 USA), and are activated by these.
Many cells possess these receptors, and the binding of an agonist will generally cause the cell to respond in a fight-or-flight manner. 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 Beta-2 adrenergic receptors.)

Contents

Subtypes of adrenergic receptors

Comparison

Diagrams

See also

References

External links

Subtypes of 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.

★ β receptors are linked to G_s 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.

Comparison

Receptor type	Agonist potency order	Location / Action	Mechanism	Agonists	Antagonists
α1: , ,	noradrenaline≥ adrenaline >> isoprenaline	Smooth muscle. In blood vessels the principal effect is vasoconstriction. Blood vessels with α1 receptors are present in the skin and the gastrointestinal system, and during the fight-or-flight response vasoconstriction results in the decreased blood flow to these organs. This accounts for an individual's skin appearing pale when frightened. In the GI tract, the effect is relaxation.	Gq: phospholipase C (PLC) activated, IP3 and calcium up	noradrenaline phenylephrine Cirazoline	''(Alpha blockers)'' phenoxybenzamine phentolamine prazosin tamsulosin terazosin
α2: , ,	adrenaline > noradrenaline >> isoprenaline	Pre- and postsynaptic nerve terminals. Mediates synaptic transmission.	Gi: adenylate cyclase inactivated, cAMP down	clonidine lofexidine xylazine Tizanidine Guanfacine	''(Alpha blockers)'' yohimbine rauwolscine idazoxan
β1:	isoprenaline > noradrenaline > adrenaline	Heart and cerebral cortex. In heart, agonists enhance myocardial contractility and increase heart rate. Also found on juxtaglomerular cells, activation results in renin release.	Gs: adenylate cyclase activated, cAMP up	noradrenaline isoprenaline dobutamine	''(Beta blockers)'' metoprolol atenolol
β2:	isoprenaline > adrenaline > noradrenaline	Lung, smooth muscle, cerebellum, skeletal muscle. In lung, agonists cause bronchiole dilation. Agonists can be useful in treating asthma. In smooth muscle, relaxes walls. Relaxes uterine muscle and promotes release of insulin.	Gs: adenylate cyclase activated, cAMP up	''(Short/long)'' salbutamol (albuterol in USA) bitolterol mesylate formoterol isoproterenol levalbuterol metaproterenol salmeterol terbutaline	''(Beta blockers)'' butoxamine propranolol
β3:	isoprenaline > noradrenaline = adrenaline	Adipose tissue. Agonists enhance lipolysis.	Gs: adenylate cyclase activated, cAMP up	L-796568 CL 316,243 LY 368842 Ro 40-2148	''(Beta blockers)'' SR 59230A

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.

Diagrams

See also

★ Beta adrenergic receptor kinase

References

★ Pharmacology, , , Rang HP, Dale MM, Ritter JM, Moore PK, Elsevier Churchill Livingstone, 2003, ISBN 0-443-07145-4

★ Rang and Dale's Pharmacology, , , Rang HP, Dale MM, Ritter JM, Flower RJ, Elsevier Churchill Livingstone, 2007, ISBN 0-443-06911-5

External links

★ 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