Adrenergic receptor Totally Explained

Everything about The Adrenergic Receptors totally explained

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. (Note: Sympathetic activity will result in vasodilation of coronary arteries via the β₂-adrenergic receptors.)

Subtypes

There are several types of adrenergic receptors, but there are two main groups: α-Adrenergic and β-Adrenergic.

α receptors bind noradrenaline (norepinephrine) and adrenaline (epinephrine). Phenylephrine is a selective agonist of the α receptor. They exist as α₁-adrenergic receptors and α₂-adrenergic receptors.
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.

Roles in Circulation: epinephrine 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

Receptor type	Agonist potency order	Selected action of agonist	Mechanism	Agonists	Antagonists
α₁: A, B, D	adrenaline ≥ noradrenaline >> isoprenaline	smooth muscle contraction	G_q: phospholipase C (PLC) activated, IP₃ and calcium up	noradrenaline phenylephrine methoxamine Cirazoline	(Alpha blockers) phenoxybenzamine phentolamine prazosin tamsulosin terazosin
α₂: A, B, C	adrenaline ≥ noradrenaline >> isoprenaline	smooth muscle contraction and neurotransmitter inhibition	G_i: adenylate cyclase inactivated, cAMP down	Clonidine lofexidine xylazine Tizanidine Guanfacine	(Alpha blockers) yohimbine
β₁	isoprenaline > adrenaline = noradrenaline	heart muscle contraction	G_s: adenylate cyclase activated, cAMP up	noradrenaline isoprenaline dobutamine	(Beta blockers) metoprolol atenolol
β₂	isoprenaline > adrenaline >> noradrenaline	smooth muscle relaxation	G_s: adenylate cyclase activated, cAMP up	(Short/long) salbutamol (albuterol in USA) bitolterol mesylate formoterol isoprenaline levalbuterol metaproterenol salmeterol terbutaline ritodrine	(Beta blockers) butoxamine propranolol
β₃	isoprenaline = noradrenaline > adrenaline	Enhance lipolysis	G_s: adenylate cyclase activated, cAMP up	L-796568

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

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

α₁ 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 many blood vessels including those of the skin & gastrointestinal system and to kidney (renal artery) and brain.. Other areas of smooth muscle contraction are for instance:

ureter

vas deferens

hairs (arrector pili muscles)

uterus (when pregnant)

urethral sphincter

bronchioles (although minor to the relaxing effect of β₂ receptor on bronchioles) Further effects include glycogenolysis and gluconeogenesis from adipose tissue Specific actions of the β₂ receptor include:

Smooth muscle relaxation, for example in bronchi.

Relax urinary sphincter and 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

β₃ receptor

Specific actions of the β₃ receptor include:

Enhancement of lipolysis in adipose tissue.Further Information

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