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14-September-2008 18:02:41 - receptor The adenosine receptors or P1 receptors1 are a class of purinergic receptors, G-protein coupled receptors with adenosine as endogenous ligand.2 In humans, there are four adenosine receptors. Each is encoded by a separate gene and has different functions, although also overlapping. For instance, both A1 receptors and A2A play roles in the heart, regulating myocardial oxygen consumption and coronary blood flow. Contents 1 Comparison of subtypes 2 A1 adenosine receptor 2.1 Mechanism 2.2 Antagonism and agonism 2.3 In the heart 2.4 In neonatal medicine 3 A2A adenosine receptor 3.1 Mechanism 3.2 Function 3.3 Agonists and antagonists 4 A2B adenosine receptor 5 A3 adenosine receptor 6 References 7 External links Comparison of subtypes Adenosine receptors Receptor Gene Mechanism 3 Effects Agonists Antagonists A1 ADORA1 Gi/o -- cAMP↑/↓ Inhibition ↓ vesicle release ↓ NMDA receptor activity decrease heart rate CCPA caffeine theophylline DPCPX CPT CPX A2A ADORA2A Gs -- cAMP↑ coronary artery vasodilatation CGS21680 ATL-146e caffeine theophylline KW6002 SCH-58261 A2b ADORA2B Gs -- cAMP↑ theophylline A3 ADORA3 Gi/o -- cardioprotective in cardiac ischemia inhibition of neutrophil degranulation Cl-IB-MECA MRS3558 theophylline MRS1191 MRS1523 MRE3008F20 A1 adenosine receptor Main article: Adenosine A1 receptor The adenosine A1 receptor has been found to be ubiquitous throughout the entire body. Mechanism This receptor has an inhibitory function on most of the tissues in which it rests. In the brain, it slows metabolic activity by a combination of actions. Presynaptically, it reduces synaptic vesicle release while post synaptically it has been found to stabilize the magnesium on the NMDA receptor. Antagonism and agonism Caffeine, along with theophylline have been found to antagonize both A1 and A2A receptors in the brain. Specific antagonists include 8-Cyclopentyl-1,3-dipropylxanthine DPCPX, and Cyclopentyltheophylline CPT or 8-cyclopentyl-1,3-dipropylxanthine CPX, while specific agonists include 2-chloro-N6-cyclopentyladenosine CCPA. In the heart The A1, together with A2A receptors, of endogenous adenosine are believed to play a role in regulating myocardial oxygen consumption and coronary blood flow. Stimulation of the A1 receptor has a myocardial depressant effect by decreasing the conduction of electrical impulses and suppressing pacemaker cell function, resulting in a decrease in heart rate. This makes adenosine a useful medication for treating and diagnosing tachyarrhythmias, or excessively fast heart rates. This effect on the A1 receptor also explains why there is a brief moment of cardiac standstill when adenosine is administered as a rapid IV push during cardiac resuscitation. The rapid infusion causes a momentary myocardial stunning effect. In normal physiological states, this serves as protective mechanisms. However, in altered cardiac function, such as hypoperfusion caused by hypotension, heart attack or cardiac arrest caused by nonperfusing bradycardias, adenosine has a negative effect on physiological functioning by preventing necessary compensatory increases in heart rate and blood pressure that attempt to maintain cerebral perfusion. In neonatal medicine Adenosine antagonists are widely used in neonatal medicine; Because a reduction in A1 expression appears to prevent hypoxia-induced ventriculomegaly and loss of white matter and therefore raise the possibility that pharmacological blockade of A1 may have clinical utility. Theophylline and caffeine are nonselective adenosine antagonists that are used to stimulate respiration in premature infants. However, we are unaware of clinical studies that have examined the incidence of periventricular leukomalacia PVL as related to neonatal caffeine use. Caffeine may reduce cerebral blood flow in premature infants, possibly by blocking vascular A2 ARs. Thus, it may prove more advantageous to use selective A1 antagonists to help reduce adenosine-induced brain injury. A2A adenosine receptor Main article: Adenosine A2A receptor As with the A1, the A2A receptors are believed to play a role in regulating myocardial oxygen consumption and coronary blood flow. Mechanism The activity of A2A adenosine receptor, a G-protein coupled receptor family member, is mediated by G proteins which activate adenylyl cyclase. It is abundant in basal ganglia, vasculature and platelets and it is a major target of caffeine.4 Function The A2A receptor is responsible for regulating myocardial blood flow by vasodilating the coronary arteries, which increases blood flow to the myocardium, but may lead to hypotension. Just as in A1 receptors, this normally serves as a protective mechanism, but may be destructive in altered cardiac function. Agonists and antagonists Specific antagonists include KW6002 and SCH-58261, while specific agonists include CGS21680 and ATL-146e.5 A2B adenosine receptor Main article: Adenosine A2b receptor This integral membrane protein stimulates adenylate cyclase activity in the presence of adenosine. This protein also interacts with netrin-1, which is involved in axon elongation. A3 adenosine receptor Main article: Adenosine A3 receptor It has been shown in studies to inhibit some specific signal pathways of adenosine. It allows for the inhibition of growth in human melanoma cells. Specific antagonists include MRS1191, MRS1523 and MRE3008F20, while specific agonists include Cl-IB-MECA and MRS3558.5 References ^ Fredholm BB, Abbracchio MP, Burnstock G, Dubyak GR, Harden TK, Jacobson KA, Schwabe U, Williams M 1997. Towards a revised nomenclature for P1 and P2 receptors. Trends Pharmacol. Sci. 18 3: 79-82. doi:10.1016/S0165-61479601038-3. PMID 9133776. ^ Fredholm BB, IJzerman AP, Jacobson KA, Klotz KN, Linden J 2001. International Union of Pharmacology. XXV. Nomenclature and classification of adenosine receptors. Pharmacol. Rev. 53 4: 527-52. PMID 11734617. ^ Unless else specified in boxes, then ref is:senselab ^ Entrez Gene: ADORA2A adenosine A2A receptor. ^ a b Jacobson KA, Gao ZG 2006. Adenosine receptors as therapeutic targets. Nature reviews. Drug discovery 5 3: 247-64. doi:10.1038/nrd1983. PMID 16518376. External links IUPHAR GPCR Database - Adenosine Receptors MeSH Adenosine+Receptors v d e Transmembrane receptor: G protein-coupled receptors Class A: Rhodopsin like Adrenergic α1 A, B, D, α2 A, B, C, β1, β2, β3 Eicosanoid CysLT 1, 2, LTB4 1, 2, FPRL1, OXE, Prostaglandin DP, EP 1, 2, 3, 4, PGF, Prostacyclin, Thromboxane Neuropeptide B/W 1, 2, FF 1, 2, S, Y 1, 2, 4, 5 Orphan GPR 1, 3, 4, 6, 12, 15, 17, 18, 19, 20, 21, 22, 23, 25, 26, 27, 31, 32, 33, 34, 35, 37, 39, 42, 44, 45, 50, 52, 55, 61, 62, 63, 65, 68, 75, 77, 78, 79, 82, 83, 84, 85, 87, 88, 92, 101, 103, 119, 120, 132, 135, 139, 141, 142, 146, 148, 149, 150, 151, 152, 153, 160, 161, 162, 171, 172, 173, 174, 176, 177, 182 Purinergics Adenosine A1, A2a, A2b, A3, P2Y, 1, 2, 4, 5, 6, 8, 9, 10, 11, 12, 13, 14 Serotonin all but 5-HT3 5-HT1 A, B, D, E, F, 5-HT2 A, B, C, 5-HT 4, 5A, 6, 7 Other Acetylcholine M1, M2, M3, M4, M5 - Adrenomedullin - Anaphylatoxin C3a, C5a - Angiotensin 1, 2 - Apelin - Bile acid - Bombesin BRS3, GRPR, NMBR - Bradykinin B1, B2 - Cannabinoid CB1, CB2 - Chemokine - Cholecystokinin A, B - Dopamine D1, D2, D3, D4, D5 - EBI2 - Endothelin A, B - Estrogen - Formyl peptide 1, L1, L2 - Free fatty acid 1, 2, 3, 4 - FSH - Galanin 1, 2, 3 - Gonadotropin-releasing hormone 1, 2 - Ghrelin - Histamine H1, H2, H3, H4 - Kisspeptin - Luteinizing hormone/choriogonadotropin - Lysophospholipid 1, 2, 3, 4, 5, 6, 7, 8 - MAS 1, 1L, D, E, F, G, X1, X2, X3, X4 - Melanocortin 1, 2, 3, 4, 5 - MCHR 1, 2 - Melatonin 1A, 1B- Motilin - neuromedin B, U 1, 2 - Neurotensin 1, 2 - Opioid Delta, Kappa, Mu, Nociceptin, but not Sigma - Olfactory - Opsin 3, 4, 5, 1LW, 1MW, 1SW, RGR, RRH - Orexin 1, 2 - Oxytocin - Oxoglutarate - PAF - Prokineticin 1, 2 - Prolactin-releasing peptide - Protease-activated 1, 2, 3, 4 - Relaxin 1, 2, 3, 4 - Somatostatin 1, 2, 3, 4, 5 - SREB - Succinate - TAAR 1, 2, 3, 5, 6, 8, 9 - Tachykinin 1, 2, 3 - Thyrotropin - Thyrotropin-releasing hormone - Urotensin-II - Vasopressin 1A, 1B, 2 Class B: Secretin like Brain-specific angiogenesis inhibitor 1, 2, 3 - Cadherin 1, 2, 3 - Calcitonin - CD97 - Corticotropin-releasing hormone 1, 2 - EMR 1, 2, 3 - Glucagon GR, GIPR, GLP1R, GLP2R - Growth hormone releasing hormone - PACAPR1- GPR 56, 64, 97, 98, 110, 111, 112, 113, 114, 115, 116, 123, 124, 125, 126, 128, 133, 143, 144, 157 - Latrophilin 1, 2, 3, ELTD1 - Parathyroid hormone 1, 2 - Secretin - Vasoactive intestinal peptide 1, 2 Class C: Metabotropic glutamate / pheromone Calcium-sensing receptor - GABA B 1, 2 - Glutamate receptor Metabotropic glutamate 1, 2, 3, 4, 5, 6, 7, 8 - GPRC6A - GPR 156, 158, 179 - RAIG 1, 2, 3, 4 - Taste receptors TAS1R 1, 2, 3 TAS2R 1, 3, 4, 5, 8, 9, 10, 12, 13, 14, 16, 38, 39, 40, 41, 43, 44, 45, 46, 47, 48, 49, 50 Frizzled / Smoothened Frizzled 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 - Smoothened Retrieved from http://en..org/wiki/Adenosine_receptor Categories: G protein coupled receptors Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page This page was last modified on 12 September 2008, at 20:02
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