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Andrenergic (Epi & NE) Receptors




ANDRENERGIC RECEPTORS
--aka adrenoceptors
--targets of catecholamines: epi and NE (epinephrine/adrenaline and norepi/noradrenaline)
--(dopamine is a catecholamine but has a different category of receptors, epi and NE are metabolites of it)
--2 classes, α and β
--G-protein coupled receptors
--general response: sympathetic aka "flight or fight"
--increase HR, dilate pupils, mobilize sugar, blood flow to skeletal muscle


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. (from http://en.wikipedia.org/wiki/Adrenergic_receptor)

ALPHA RECEPTORS
--excitatory except in the GI tract
--α receptors subtypes: α1 (a Gq coupled receptor) and α2 (a Gi coupled receptor)
--phenylephrine is a selective agonist of the α receptor

ALPHA 1 RECEPTORS
--postsynaptic sympathetic
--generally excitatory (vasoconstriction)
--smooth muscle contraction
--inhibitory in GI tract
--Gq-->phosphlipase C-->IP3, DAG

ALPHA 2 RECEPTORS
--presynaptic sympathetic (decrease catecholamine release)
--(decrease sympathetic tone)
--smooth muscle contraction and neurotransmitter inhibition
--Gi-->inhibit adenylate cyclase-->decrease cAMP
--blockers may help treat raynaud's phenomenon



BETA RECEPTORS
--inhibitory except at the heart
--β receptor subtypes: β1, β2 and β3
--all linked to Gs proteins, which in turn are linked to adenylate cyclase
--agonist binding-->increased intracellular cyclic adenosine monophosphate
--downstream effectors of cAMP: cAMP-dependent protein kinase (PKA)
--PKA mediates some intracellular events following hormone binding
--isoprenaline is a selective agonist



BETA 1 RECEPTORS
--postsynaptic sympathetic (cardiac)
--causes heart muscle contraction
--excitatory (chonotrope, dromotrope, inotrope)
--Gs-->adenylate cyclase-->increase cAMP

BETA 2 RECEPTORS
--postsynaptic sympathetic (all non-cardiac)
--smooth muscle relaxation
--inhibitory (vasodilation, bronchodilation)
--Gs-->adenylate cyclase-->increase cAMP

BETA 3
--enhance lipolysis in adipose
--activation may cause tremors, this is the reason that Beta-3 activating pharmacological agents are not utilized as weight loss agents


WIKI TEXT TO SORT THRU

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-biphosphate (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 and to kidney (renal artery)[5] and brain[6]. Other areas of smooth muscle contraction are:

* ureter
* vas deferens
* hair (erector pili muscles)
* uterus (when pregnant)
* urethral sphincter
* bronchioles (although minor to the relaxing effect of β2 receptor on bronchioles)
* blood vessels of ciliary body (stimulation causes mydriasis)

Further effects include glycogenolysis and gluconeogenesis from adipose tissue[7] and liver, as well as secretion from sweat glands[7] and Na+ reabsorption from kidney.[7]

Antagonists may be used in hypertension.

[edit] α2 receptor
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.[7]
* induction of glucagon release from pancreas.
* contraction of sphincters of the gastrointestinal tract
* negative feedback in the neuronal synapses
* platelet aggregation

[edit] β receptors

[edit] β1 receptor
Main article: Beta-1 adrenergic receptor

Specific actions of the β1 receptor include:

* Increase cardiac output, by raising heart rate (positive chronotropic effect) and increasing impulse conduction and increasing contraction thus increasing the volume expelled with each beat (increased ejection fraction).
* Renin release from juxtaglomerular cells.[7]
* Lipolysis in adipose tissue.[7]

[edit] β2 receptor
Main article: Beta-2 adrenergic receptor

The 3D crystallographic structure of the β2-adrenergic receptor has been determined (PDB 2R4R, 2R4S, 2RH1).[8][9][10]

Specific actions of the β2 receptor include:

* Smooth muscle relaxation, e.g. in bronchi.[7]
* Lipolysis in adipose tissue.[11]
* Anabolism in skeletal muscle.[12][13]
* 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.[7]
* Inhibit histamine-release from mast cells
* Increase renin secretion from kidney
* Promotes insulin release from pancreatic beta cells

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