Adrenergic receptor

{{Main|History of catecholamine research}}

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By the turn of the 19th century, it was agreed that the stimulation of sympathetic nerves could cause different effects on body tissues, depending on the conditions of stimulation (such as the presence or absence of some toxin). Over the first half of the 20th century, two main proposals were made to explain this phenomenon:

1. There were (at least) two different types of neurotransmitters released from sympathetic nerve terminals, or
2. There were (at least) two different types of detector mechanisms for a single neurotransmitter.

The first hypothesis was championed by Walter Bradford Cannon and Arturo Rosenblueth,{{cite journal | vauthors = Cannon WB, Rosenbluth A | date= 31 May 1933| title = Studies On Conditions Of Activity In Endocrine Organs XXVI: Sympathin E and Sympathin I | journal = American Journal of Physiology | volume= 104 | issue = 3 | pages = 557–574 | doi= 10.1152/ajplegacy.1933.104.3.557}} who interpreted many experiments to then propose that there were two neurotransmitter substances, which they called sympathin E (for 'excitation') and sympathin I (for 'inhibition').

The second hypothesis found support from 1906 to 1913, when Henry Hallett Dale explored the effects of adrenaline (which he called adrenine at the time), injected into animals, on blood pressure. Usually, adrenaline would increase the blood pressure of these animals. Although, if the animal had been exposed to ergotoxine, the blood pressure decreased.{{cite journal | vauthors = Dale HH | title = On some physiological actions of ergot | journal = The Journal of Physiology | volume = 34 | issue = 3 | pages = 163–206 | date = May 1906 | pmid = 16992821 | doi = 10.1113/jphysiol.1906.sp001148 | pmc=1465771}}{{cite journal | vauthors = Dale HH | title = On the action of ergotoxine; with special reference to the existence of sympathetic vasodilators | journal = The Journal of Physiology | volume = 46 | issue = 3 | pages = 291–300 | date = Jun 1913 | pmid = 16993202 | doi = 10.1113/jphysiol.1913.sp001592 | pmc=1420444}} He proposed that the ergotoxine caused "selective paralysis of motor myoneural junctions" (i.e. those tending to increase the blood pressure) hence revealing that under normal conditions that there was a "mixed response", including a mechanism that would relax smooth muscle and cause a fall in blood pressure. This "mixed response", with the same compound causing either contraction or relaxation, was conceived of as the response of different types of junctions to the same compound.

This line of experiments were developed by several groups, including DT Marsh and colleagues,{{cite journal | vauthors = Marsh DT, Pelletier MH, Rose CA | title = The comparative pharmacology of the N-alkyl-arterenols | journal = The Journal of Pharmacology and Experimental Therapeutics | volume = 92 | issue = 2 | pages = 108–20 | date = Feb 1948 | pmid = 18903395 }} who in February 1948 showed that a series of compounds structurally related to adrenaline could also show either contracting or relaxing effects, depending on whether or not other toxins were present. This again supported the argument that the muscles had two different mechanisms by which they could respond to the same compound. In June of that year, Raymond Ahlquist, Professor of Pharmacology at Medical College of Georgia, published a paper concerning adrenergic nervous transmission.{{cite journal | vauthors = Ahlquist RP | s2cid = 1518772 | title = A study of the adrenotropic receptors | journal = The American Journal of Physiology | volume = 153 | issue = 3 | pages = 586–600 | date = Jun 1948 | pmid = 18882199 | doi = 10.1152/ajplegacy.1948.153.3.586}} In it, he explicitly named the different responses as due to what he called α receptors and β receptors, and that the only sympathetic transmitter was adrenaline. While the latter conclusion was subsequently shown to be incorrect (it is now known to be noradrenaline), his receptor nomenclature and concept of two different types of detector mechanisms for a single neurotransmitter, remains. In 1954, he was able to incorporate his findings in a textbook, Drill's Pharmacology in Medicine,{{cite book|last=Drill|first=Victor Alexander|name-list-style=vanc|date=1954|title=Pharmacology in medicine: a collaborative textbook|url=https://archive.org/details/pharmacologyinme00dril|url-access=registration|location=New York|publisher=McGraw-Hill}} and thereby promulgate the role played by α and β receptor sites in the adrenaline/noradrenaline cellular mechanism. These concepts would revolutionise advances in pharmacotherapeutic research, allowing the selective design of specific molecules to target medical ailments rather than rely upon traditional research into the efficacy of pre-existing herbal medicines.

File:Adrenoreceptor Signal Transduction.png to either α₁, α₂ or β-adrenoreceptors. The α₁ couples to G_q, which results in increased intracellular Ca²⁺ and subsequent smooth muscle contraction. The α₂, on the other hand, couples to G_i, which causes a decrease in neurotransmitter release, as well as a decrease of cAMP activity resulting in smooth muscle contraction. The β receptor couples to G_s and increases intracellular cAMP activity, resulting in e.g. heart muscle contraction, smooth muscle relaxation and glycogenolysis.]]

The mechanism of adrenoreceptors. Adrenaline or noradrenaline are receptor ligands to either α₁, α₂ or β-adrenoreceptors. The α₁ couples to G_q, which results in increased intracellular Ca²⁺ and subsequent smooth muscle contraction. The α₂, on the other hand, couples to G_i, which causes a decrease in neurotransmitter release, as well as a decrease of cAMP activity resulting in smooth muscle contraction. The β receptor couples to G_s and increases intracellular cAMP activity, resulting in e.g. heart muscle contraction, smooth muscle relaxation and glycogenolysis.

There are two main groups of adrenoreceptors, α and β, with 9 subtypes in total:

• α receptors are subdivided into α₁ (a G_q coupled receptor) and α₂ (a G_i coupled receptor)
• α₁ has 3 subtypes: α_1A, α_1B and α_1D{{efn|name=d}}
• α₂ has 3 subtypes: α_2A, α_2B and α_2C
• β receptors are subdivided into β₁, β₂ and β₃. All 3 are coupled to G_s proteins, but β₂ and β₃ also couple to G_i

G_i and G_s are linked to adenylyl cyclase. Agonist binding thus causes a rise in the intracellular concentration of the second messenger (G_i inhibits the production of cAMP) cAMP. Downstream effectors of cAMP include cAMP-dependent protein kinase (PKA), which mediates some of the intracellular events following hormone binding.

Epinephrine (adrenaline) reacts with both α- and β-adrenoreceptors, causing vasoconstriction and vasodilation, respectively. Although α receptors are less sensitive to epinephrine, when activated at pharmacologic doses, they override the vasodilation mediated by β-adrenoreceptors because there are more peripheral α₁ receptors than β-adrenoreceptors. The result is that high levels of circulating epinephrine cause vasoconstriction. However, the opposite is true in the coronary arteries, where β₂ response is greater than that of α₁, resulting in overall dilation with increased sympathetic stimulation. At lower levels of circulating epinephrine (physiologic epinephrine secretion), β-adrenoreceptor stimulation dominates since epinephrine has a higher affinity for the β₂ adrenoreceptor than the α₁ adrenoreceptor, producing vasodilation followed by decrease of peripheral vascular resistance.{{Cite journal|last1=Zwieten|first1=Van|last2=A|first2=P.|date=1986|title=Interaction Between α and β-Adrenoceptor-Mediated Cardiovascular Effects|journal=Journal of Cardiovascular Pharmacology|language=en-US|volume=8|pages=S21-8|doi=10.1097/00005344-198608004-00004|pmid=2427848|issn=0160-2446|doi-access=free}}

Smooth muscle behavior is variable depending on anatomical location. Smooth muscle contraction/relaxation is generalized below. One important note is the differential effects of increased cAMP in smooth muscle compared to cardiac muscle. Increased cAMP will promote relaxation in smooth muscle, while promoting increased contractility and pulse rate in cardiac muscle.

α receptors have actions in common, but also individual effects. Common (or still receptor unspecified) actions include:

• vasoconstriction{{cite journal|vauthors=Elliott J|title=Alpha-adrenoceptors in equine digital veins: evidence for the presence of both alpha1 and alpha2-receptors mediating vasoconstriction|journal=Journal of Veterinary Pharmacology and Therapeutics|volume=20|issue=4|pages=308–17|date=1997|pmid=9280371|doi=10.1046/j.1365-2885.1997.00078.x}}
• decreased flow of smooth muscle in gastrointestinal tract{{cite journal|vauthors=Sagrada A, Fargeas MJ, Bueno L|title=Involvement of alpha-1 and alpha-2 adrenoceptors in the postlaparotomy intestinal motor disturbances in the rat|journal=Gut|volume=28|issue=8|pages=955–9|date=1987|pmid=2889649|pmc=1433140|doi=10.1136/gut.28.8.955}}

Subtype unspecific α agonists (see actions above) can be used to treat rhinitis (they decrease mucus secretion). Subtype unspecific α antagonists can be used to treat pheochromocytoma (they decrease vasoconstriction caused by norepinephrine).

{{Main|Alpha-1 adrenergic receptor}}

α₁-adrenoreceptors are members of the G_q protein-coupled receptor superfamily. Upon activation, a heterotrimeric G protein, G_q, activates phospholipase C (PLC). The PLC cleaves phosphatidylinositol 4,5-bisphosphate (PIP₂), which in turn causes an increase in inositol triphosphate (IP₃) and diacylglycerol (DAG). The former interacts with calcium channels of endoplasmic and sarcoplasmic reticulum, thus changing the calcium content in a cell. This triggers all other effects, including a prominent slow after depolarizing current (sADP) in neurons.{{cite journal | vauthors = Smith RS, Weitz CJ, Araneda RC | title = Excitatory actions of noradrenaline and metabotropic glutamate receptor activation in granule cells of the accessory olfactory bulb | journal = Journal of Neurophysiology | volume = 102 | issue = 2 | pages = 1103–14 | date = Aug 2009 | pmid = 19474170 | doi = 10.1152/jn.91093.2008 | pmc=2724365}}

Actions of the α₁ receptor mainly involve smooth muscle contraction. It causes vasoconstriction in many blood vessels, including those of the skin, gastrointestinal system, kidney (renal artery){{cite journal|vauthors=Schmitz JM, Graham RM, Sagalowsky A, Pettinger WA |title=Renal alpha-1 and alpha-2 adrenergic receptors: biochemical and pharmacological correlations|journal=The Journal of Pharmacology and Experimental Therapeutics|volume=219|issue=2|pages=400–6|date=1981|pmid=6270306|url=http://jpet.aspetjournals.org/cgi/content/abstract/219/2/400 }} and brain.[http://cim.ucdavis.edu/masters/sessions2002/session4_ica.doc Circulation & Lung Physiology I] {{Webarchive|url=https://web.archive.org/web/20110726230145/http://cim.ucdavis.edu/masters/sessions2002/session4_ica.doc |date=2011-07-26 }} M.A.S.T.E.R. Learning Program, UC Davis School of Medicine Other areas of smooth muscle contraction are:

• ureter
• vas deferens
• hair (arrector pili muscles)
• uterus (when pregnant)
• urethral sphincter
• urothelium and lamina propria{{cite journal|vauthors=Moro C, Tajouri L, Chess-Williams R|title=Adrenoceptor function and expression in bladder urothelium and lamina propria|journal=Urology|volume=81|issue=1|pages=211.e1–7|date=2013|pmid=23200975|doi=10.1016/j.urology.2012.09.011}}
• bronchioles (although minor relative to the relaxing effect of β₂ receptor on bronchioles)
• blood vessels of ciliary body and (stimulation of dilator pupillae muscles of iris causes mydriasis)

Actions also include glycogenolysis and gluconeogenesis from adipose tissue and liver; secretion from sweat glands and Na⁺ reabsorption from kidney.

α₁ antagonists can be used to treat:

• hypertension – decrease blood pressure by decreasing peripheral vasoconstriction
• benign prostate hyperplasia – relax smooth muscles within the prostate thus easing urination

{{Main|Alpha-2 adrenergic receptor}}

The α₂ receptor couples to the G_i/o protein.{{cite journal|vauthors=Qin K, Sethi PR, Lambert NA|date=2008|title=Abundance and stability of complexes containing inactive G protein-coupled receptors and G proteins|journal=FASEB Journal|volume=22|issue=8|pages=2920–7|doi=10.1096/fj.08-105775|doi-access=free |pmc=2493464|pmid=18434433}} It is a presynaptic receptor, causing negative feedback on, for example, norepinephrine (NE). When NE is released into the synapse, it feeds back on the α₂ receptor, causing less NE release from the presynaptic neuron. This decreases the effect of NE. There are also α₂ receptors on the nerve terminal membrane of the post-synaptic adrenergic neuron.

Actions of the α₂ receptor include:

• decreased insulin release from the pancreas
• increased glucagon release from the pancreas
• contraction of sphincters of the GI-tract
• negative feedback in the neuronal synapses - presynaptic inhibition of norepinephrine release in CNS
• increased platelet aggregation
• decreases peripheral vascular resistance

α₂ agonists (see actions above) can be used to treat:

• hypertension – decrease blood pressure-raising actions of the sympathetic nervous system

α₂ antagonists can be used to treat:

• impotence – relax penile smooth muscles and ease blood flow
• depression – enhance mood by increasing norepinephrine secretion

Subtype unspecific β agonists can be used to treat:

• heart failure – increase cardiac output acutely in an emergency
• circulatory shock – increase cardiac output thus redistributing blood volume
• anaphylaxis – bronchodilation

Subtype unspecific β antagonists (beta blockers) can be used to treat:

• heart arrhythmia – decrease the output of sinus node thus stabilizing heart function
• coronary artery disease – reduce heart rate and hence increasing oxygen supply
• heart failure – prevent sudden death related to this condition, which is often caused by ischemias or arrhythmias{{cite journal | vauthors = Ørn S, Dickstein K |date=2002-04-01|title=How do heart failure patients die? |journal=European Heart Journal Supplements |volume=4|issue=Suppl D|pages=D59–D65|doi=10.1093/oxfordjournals.ehjsupp.a000770 |doi-access=free}}
• hyperthyroidism – reduce peripheral sympathetic hyper-responsiveness
• migraine – reduce number of attacks
• stage fright – reduce tachycardia and tremor
• glaucoma – reduce intraocular pressure

{{Main|Beta-1 adrenergic receptor}}

Actions of the β₁ receptor include:

• increase cardiac output by increasing heart rate (positive chronotropic effect), conduction velocity (positive dromotropic effect), stroke volume (by enhancing contractility – positive inotropic effect), and rate of relaxation of the myocardium, by increasing calcium ion sequestration rate (positive lusitropic effect), which aids in increasing heart rate
• increase renin secretion from juxtaglomerular cells of the kidney{{cite journal | vauthors = Kim SM, Briggs JP, Schnermann J | title = Convergence of major physiological stimuli for renin release on the Gs-alpha/cyclic adenosine monophosphate signaling pathway | journal = Clinical and Experimental Nephrology | volume = 16 | issue = 1 | pages = 17–24 | date = February 2012 | pmid = 22124804 | pmc = 3482793 | doi = 10.1007/s10157-011-0494-1 }}
• increase ghrelin secretion from the stomach{{cite journal | vauthors = Zhao TJ, Sakata I, Li RL, Liang G, Richardson JA, Brown MS, Goldstein JL, Zigman JM | title = Ghrelin secretion stimulated by {beta}1-adrenergic receptors in cultured ghrelinoma cells and in fasted mice | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 107 | issue = 36 | pages = 15868–73 | date = Sep 2010 | pmid = 20713709 | pmc = 2936616 | doi = 10.1073/pnas.1011116107 | first8 = J. M. | last8 = Zigman | author7 = and others | display-authors = 6 | bibcode = 2010PNAS..10715868Z | doi-access = free }}

{{Main|Beta-2 adrenergic receptor}}Actions of the β₂ receptor include:

• smooth muscle relaxation throughout many areas of the body, e.g. in bronchi (bronchodilation, see salbutamol),{{cite book|title=Neuroscience|last1=Fitzpatrick|first1=David|last2=Purves|first2=Dale|last3=Augustine|first3=George|publisher=Sinauer|year=2004|isbn=978-0-87893-725-7|edition=3rd|location=Sunderland, Mass|chapter=Table 20:2 |name-list-style=vanc}} GI tract (decreased motility), veins (vasodilation of blood vessels), especially those to skeletal muscle (although this vasodilator effect of norepinephrine is relatively minor and overwhelmed by α adrenoceptor-mediated vasoconstriction){{cite web|vauthors = Klabunde R|title=Adrenergic and Cholinergic Receptors in Blood Vessels|url=http://www.cvphysiology.com/Blood%20Pressure/BP010b.htm|website=Cardiovascular Physiology|access-date=5 May 2015}}
• lipolysis in adipose tissue{{cite journal|vauthors=Large V, Hellström L, Reynisdottir S, Lönnqvist F, Eriksson P, Lannfelt L, Arner P|title = Human beta-2 adrenoceptor gene polymorphisms are highly frequent in obesity and associate with altered adipocyte beta-2 adrenoceptor function|journal=The Journal of Clinical Investigation|volume=100|issue=12|pages=3005–13|date=1997|pmid=9399946|pmc=508512|doi=10.1172/JCI119854|display-authors=3}}
• anabolism in skeletal muscle{{cite journal|vauthors=Kline WO, Panaro FJ, Yang H, Bodine SC|s2cid=14292004|title=Rapamycin inhibits the growth and muscle-sparing effects of clenbuterol|journal=Journal of Applied Physiology|volume=102|issue=2|pages=740–7|date=2007|pmid=17068216|doi=10.1152/japplphysiol.00873.2006}}{{cite journal|vauthors=Kamalakkannan G, Petrilli CM, George I, LaManca J, McLaughlin BT, Shane E, Mancini DM, Maybaum S|title=Clenbuterol increases lean muscle mass but not endurance in patients with chronic heart failure|journal=The Journal of Heart and Lung Transplantation|volume=27|issue=4|pages=457–61|date=2008|pmid=18374884|doi=10.1016/j.healun.2008.01.013|display-authors=3}}
• uptake of potassium into cells{{Cite book|title=Basic & Clinical Pharmacology|publisher=MCGraw-Hill Education|year=2018|isbn=978-1-259-64115-2|location=United States of America|pages=148}}
• relax non-pregnant uterus
• relax detrusor urinae muscle of bladder wall
• dilate arteries to skeletal muscle
• glycogenolysis and gluconeogenesis
• stimulates insulin secretion{{cite journal | vauthors = Santulli G, Lombardi A, Sorriento D, Anastasio A, Del Giudice C, Formisano P, Béguinot F, Trimarco B, Miele C, Iaccarino G | title = Age-related impairment in insulin release: the essential role of β(2)-adrenergic receptor | journal = Diabetes | volume = 61 | issue = 3 | pages = 692–701 | date = March 2012 | pmid = 22315324 | pmc = 3282797 | doi = 10.2337/db11-1027 }}
• contract sphincters of GI tract
• thickened secretions from salivary glands
• inhibit histamine-release from mast cells
• involved in brain - immune communication{{cite journal | vauthors = Elenkov IJ, Wilder RL, Chrousos GP, Vizi ES | title = The sympathetic nerve--an integrative interface between two supersystems: the brain and the immune system | journal = Pharmacological Reviews | volume = 52 | issue = 4 | pages = 595–638 | date = December 2000 | pmid = 11121511 }}

β₂ agonists (see actions above) can be used to treat:

• asthma and COPD – reduce bronchial smooth muscle contraction thus dilating the bronchus
• hyperkalemia – increase cellular potassium intake
• preterm birth – reduce uterine smooth muscle contractions{{cite journal | vauthors = Haas DM, Benjamin T, Sawyer R, Quinney SK | title = Short-term tocolytics for preterm delivery - current perspectives | journal = International Journal of Women's Health | volume = 6 | pages = 343–9 | date = 2014 | pmid = 24707187 | pmc = 3971910 | doi = 10.2147/IJWH.S44048 |doi-access=free }}

{{Main|Beta-3 adrenergic receptor}}

Actions of the β₃ receptor include:

• increase of lipolysis in adipose tissue
• relax the bladder

β₃ agonists could theoretically be used as weight-loss drugs, but are limited by the side effect of tremors.

• Beta adrenergic receptor kinase
• Beta adrenergic receptor kinase-2
• Acetylcholine receptor (Cholinergic receptor)

:* Nicotinic acetylcholine receptor

:* Muscarinic acetylcholine receptor

{{notelist|refs={{efn|name=d|There is no α_1C receptor. There was a subtype known as C, but it was found to be identical to one of the previously discovered subtypes. To avoid confusion, naming was continued with the letter D. Before June 1995 α_1A was named α_1C. α_1D was named α_1A, α_1D or α_1A/D.{{cite journal | vauthors = Hieble JP, Bylund DB, Clarke DE, Eikenburg DC, Langer SZ, Lefkowitz RJ, Minneman KP, Ruffolo RR | title = International Union of Pharmacology. X. Recommendation for nomenclature of alpha 1-adrenoceptors: consensus update | journal = Pharmacological Reviews | volume = 47 | issue = 2 | pages = 267–70 | date = June 1995 | pmid = 7568329 }}}}}}

{{Reflist}}

{{refbegin}}

• {{cite book | vauthors =Rang HP, Dale MM, Ritter JM, Flower RJ| year = 2007| title = Rang and Dale's Pharmacology | chapter = Chapter 11: Noradrenergic transmission | edition = 6th | pages = 169–170 | publisher = Elsevier Churchill Livingstone| isbn = 978-0-443-06911-6}}

{{refend}}

• [http://pharmacologycorner.com/alpha-receptors-1-2/ Alpha receptors illustrated]
• [http://macromoleculeinsights.com/adrenergic.php The Adrenergic Receptors]
• [http://www.guidetopharmacology.org/GRAC/FamilyDisplayForward?familyId=4 Adrenoceptors - IUPHAR/BPS guide to pharmacology]
• [https://www.ncbi.nlm.nih.gov/books/NBK28138/ Basic Neurochemistry: α- and β-Adrenergic Receptors]
• [http://cogprints.org/4095/ Theory of receptor activation]
• [http://cogprints.org/4093/ Desensitization of β₁ receptors]

{{Adrenergic agonists}}

{{G protein-coupled receptors}}

{{DEFAULTSORT:Adrenergic Receptor}}

Category:G protein-coupled receptors