Adrenergic Receptors

Adrenergic receptors (adrenoceptors) are classified into two main groups, α and β. The α group has been further divided into α₁ and α₂ which are, in turn, subdivided into α_1A, α_1B, α_1D and α_2A, α_2B and α_2C respectively. β-Adrenoceptors are currently classified into β₁, β₂ and β₃ subgroups with a putative β₄ receptor, as yet uncloned but exhibiting a distinct pharmacological profile.

Receptor Subtype	α_1A	α_1B	α_1D

Transduction Mechanism	Activates G_p/q, ↑ PI turnover, ↑[Ca²⁺]_i.c.) and activates voltage-gated Ca²⁺ channels
Localization	CNS, heart, liver, prostate, urethra	CNS, somatic arteries and veins, spleen, kidney	CNS, aorta, bladder, prostate
Tissue Function	Smooth muscle contraction, myocyte hypertrophy, activation of sarcolemmal Na⁺-H⁺ exchanger	Smooth muscle contraction, cardiac growth and contractile function	Smooth muscle contraction
Selective Agonists	A 61603 (1052) Oxymetazoline (1142)	Unknown	Unknown
Non-subtype Selective Agonists	Cirazoline (0888) (R)-(-)-Phenylephrine hydrochloride (2838)	Cirazoline (0888) (R)-(-)-Phenylephrine hydrochloride (2838)	Cirazoline (0888) (R)-(-)-Phenylephrine hydrochloride (2838)
Selective Antagonists	RS 17053 (0985) RS 100329 (1325) SNAP 5089 (2398) WB 4101 (0946) Tamsulosin (3050)	Rec 15/2615 (3284)	BMY 7378 (1006)
Non-subtype Selective Antagonists	Prazosin (0623) Doxazosin (2964) Alfuzosin (3305)	Prazosin (0623) Doxazosin (2964) Alfuzosin (3305)	Prazosin (0623) Doxazosin (2964) Alfuzosin (3305)

α₂-Adrenergic Receptor Data

Receptor Subtype	α_2A	α_2B	α_2C

Transduction Mechanism	Activates G_i/o, ↓ PI turnover, inhibits voltage-gated Ca²⁺ channels, activates voltage-gated Ca²⁺ dependent K⁺ channels
Localization	Brain, spleen, kidney, aorta, lung, skeletal muscle, heart, liver	Spleen, kidney, aorta, lung, skeletal muscle, heart, liver	Brain, kidney, aorta, lung, skeletal muscle, heart, spleen
Tissue Function	Hypotension, sedation, analgesia, hypothermia, anesthesia, inhibition of noradrenalin release	Vasoconstriction	Presynaptic inhibition of noradrenalin release
Selective Agonists	Oxymetazoline* (1142) Guanfacine (1030)	Unknown	(R)-(+)-m-Nitrobiphenyline oxalate (2948) ST 91 (2638)
Non-subtype Selective Agonists	Clonidine (0690) UK 14,304 tartrate (2466)	Clonidine (0690) UK 14,304 tartrate (2466)	Clonidine (0690) UK 14,304 tartrate (2466)
Selective Antagonists	BRL 44408 (1133)	ARC (0928) Imiloxan (0986) Prazosin (0623)	JP 1302 (2666) Rauwolscine† (0891)
Non-subtype Selective Antagonists	RS 79948 (0987) Yohimbine (1127) Efaroxan (0792) Idazoxan (0793)	RS 79948 (0987) Yohimbine (1127) Efaroxan (0792) Idazoxan (0793)	RS 79948 (0987) Yohimbine (1127) Efaroxan (0792) Idazoxan (0793)

*partial agonist
†10-20 fold selective for α_2C

β-Adrenergic Receptor Data

Receptor Subtype	β₁	β₂	β₃

Transduction Mechanism	↑ Adenylyl cyclase (via G_s)
Localization	CNS, heart, coronary artery, lung, spleen, kidney, liver, muscle	CNS, heart, lung, spleen, kidney, liver, skeletal muscle	Adipose tissue, gall bladder, small intestines, stomach, prostate, left atrium, bladder, vascular endothelium
Tissue Function	Tachycardia, coronary vasodilation, increase heart contractile force, apoptosis, relaxation of colon and esophagus	Hypotension, smooth muscle relaxation e.g. bronchodilation, inhibition of apoptosis	Lipolysis, glucose uptake, cardioinhibition, relaxation of colon, esophagus and bladder

Key Compounds	K_i values (nM)

Agonists	BRL 37344 (0948) CGP 12177* (1134) Cimaterol (0435) Pindolol (0994) Salbutamol (0634)	1750 0.9 - 3.4 -	1120 4 - 2.3 -	287 88 4700 11 53000
Antagonists	ICI 118,551 (0821) L-748,337 (2760)	120 390	1.2 204	257 4

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