Adrenergic receptor

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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 (also 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.

1 Subtypes of adrenergic receptors
2 Comparison
3 Diagrams
4 See also
5 References
6 External links

[edit] 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.

[edit] Comparison

Receptor type	Agonist potency order	Location / Action	Mechanism	Agonists	Antagonists
α₁: ADRA1A, ADRA1B, ADRA1D	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.	G_q: phospholipase C (PLC) activated, IP₃ and calcium up	noradrenaline phenylephrine	(Alpha blockers) phenoxybenzamine phentolamine prazosin tamsulosin terazosin
α₂: ADRA2A, ADRA2B, ADRA2C	adrenaline > noradrenaline >> isoprenaline	Pre- and postsynaptic nerve terminals. Mediates synaptic transmission.	G_i: adenylate cyclase inactived, cAMP down	clonidine lofexidine xylazine	(Alpha blockers) yohimbine
β₁: ADRB1	isoprenaline > noradrenaline > adrenaline	Heart and cerebral cortex. In heart, agonists enhance myocardial contractility and increase heart rate.	G_s: adenylate cyclase actived, cAMP up	noradrenaline isoproterenol dobutamine	(Beta blockers) metoprolol atenolol
β₂: ADRB2	isoprenaline > adrenaline > noradrenaline	Lung, smooth muscle, cerebellum. In lung, agonists cause bronchiole dilation. Agonists can be useful in treating asthma. In smooth muscle, relaxes walls.	G_s: adenylate cyclase actived, cAMP up	(Short/long) albuterol bitolterol mesylate formoterol isoproterenol levalbuterol metaproterenol salmeterol terbutaline	(Beta blockers) butoxamine propranolol
β₃: ADRB3	isoprenaline > noradrenaline > adrenaline	Adipose tissue. Agonists enhance lipolysis.	G_s: adenylate cyclase actived, cAMP up	L-796568	(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 E.

[edit] Diagrams

Labels in German, but most ideas can be understood. "Glatter" = "smooth", "hemmung" = "inhibition", "herzmuskels" = "heart muscle".

Epinephrin 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...)