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 non-essential organs to skeletal muscle.
Subtypes
There are two main groups of adrenergic receptors, α and β, with several subtypes.
- α receptors have the subtypes α 1 (a G q coupled receptor) and α 2 (a G i coupled receptor). Phenylephrine is a selective agonist of the α receptor.
- β receptors have the subtypes β 1 , β 2 and β 3 . All three are linked to G s proteins, which in turn are linked to adenylate 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. Isoprenaline is a selective agonist.
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
† There is no α 1C 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:
- Vasoconstriction of arteries to heart (coronary artery).
- Vasoconstriction of veins
- Decrease motility of smooth muscle in gastrointestinal tract
α 1 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 α 1 receptor mainly involves smooth muscle contraction. It causes vasoconstriction in many blood vessels including those of the skin & gastrointestinal system and to kidney (renal artery) and brain. Other areas of smooth muscle contraction are:
- ureter
- vas deferens
- hairs (erector pili muscles)
- uterus (when pregnant)
- urethral sphincter
- bronchioles (although minor to the relaxing effect of β 2 receptor on bronchioles)
- blood vessels of ciliary body (stimulation causes mydriasis)
Further effects include glycogenolysis and gluconeogenesis from adipose tissue and liver, as well as secretion from sweat glands and Na + reabsorption from kidney.
Antagonists may be used in hypertension.
α 2 receptor
There are 3 highly homologous subtypes of α 2 receptors: α2A, α2Β, and α2C.
Specific actions of the α 2 receptor include:
- inhibition of insulin release in pancreas.
- induction of glucagon release from pancreas.
- contraction of sphincters of the gastrointestinal tract
- negative feedback in the neuronal synapses
β receptors
β 1 receptor
Specific actions of the β 1 receptor include:
- Increase cardiac output, both by raising heart rate and increasing the volume expelled with each beat (increased ejection fraction).
- Renin release from juxtaglomerular cells.
- Lipolysis in adipose tissue.
β 2 receptor
The 3D crystallographic structure of the β 2 -adrenergic receptor has been determined (PDB 2R4R, 2R4S , 2RH1 ).
Specific actions of the β 2 receptor include:
- Smooth muscle relaxation, e.g. in bronchi.
- Lipolysis in adipose tissue.
- Anabolism in skeletal muscle.
- Relax non-pregnant uterus.
- 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
β 3 receptor
Specific actions of the β 3 receptor include:
- Enhancement of lipolysis in adipose tissue. However, acitvation of Beta-3 receptors often causes tremors, which is the paramount reason that Beta-3 activating pharmacological agents are not utilized as weight loss agents Clarify
See also
- Beta adrenergic receptor kinase
- Beta adrenergic receptor kinase-2
References
- ^ Nisoli E, Tonello C, Landi M, Carruba MO (1996). "Functional studies of the first selective β 3 -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 .
- ^ Woodman OL, Vatner SF (1987). "Coronary vasoconstriction mediated by α 1 - and α 2 -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 .
- ^ Elliott J (1997). "Alpha-adrenoceptors in equine digital veins: evidence for the presence of both α 1 - and α 2 -receptors mediating vasoconstriction". J. Vet. Pharmacol. Ther. 20 (4): 308–17. doi: 10.1046/j.1365-2885.1997.00078.x . PMID 9280371.
- ^ Sagrada A, Fargeas MJ, Bueno L (1987). "Involvement of α 1 and α 2 adrenoceptors in the postlaparotomy intestinal motor disturbances in the rat". Gut 28 (8): 955–9. PMID 2889649. ...
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