Different receptors/binding sites have been identified for the biologically active angiotensin (Ang) peptides, i.e. Ang II (1-8), Ang III (2-8), Ang IV (3-8) and Ang (1-7), based on the availability of selective agonists and antagonists, signal transduction mechanisms and structure of the receptor proteins. The primary receptors for Ang II (and Ang III) are designated AT1 and AT2. AT1 receptors exhibit high affinity for the biphenyltetrazole class of non-peptide antagonists. These clinically used angiotensin receptor blockers (ARB’s) are generically known as sartans. Tetrahydroimidazolepyridines behave as selective non-peptide AT2 receptor antagonists. The octapeptide Ang II receptor antagonist, saralasin, [Sar1,Ala8]-Ang II and other 8 position aliphatic amino acid substituted Ang II analogs, do not discriminate between AT1 and AT2 receptors. On the other hand, the antagonist [Sar1,Gly8] Ang II shows moderate selectivity for the AT1 receptor. The peptides CGP42112A and p-aminophenylalanine6 Ang II bind selectively to the AT2 receptor, and exhibit agonist activity.
The sequence identity between the AT1 (359 aa) and AT2 (363 aa) receptors is only 34%, but both receptors belong to the G protein-coupled receptor superfamily. In humans, the single gene encoding the AT1 receptor protein is found on chromosome 3. Two subtypes, AT1A and AT1B, exhibiting a 94% overall sequence identity, are found in rodents. In the rat, the AT1A and AT1B receptor genes are located on chromosomes 17 and 2, respectively. In the mouse the genes are located on chromosomes 13 and 3, respectively. Upon stimulation, the AT1 receptor couples via G protein- (predominantly Gq/11) dependent and independent mechanisms, modulating several intracellular signaling mechanisms involving phospholipases C, D and A2, adenylyl cyclase, Erk MAP kinase, c-Jun N-Terminal kinase (JNK) and the Jak/STAT pathway, some of which involve transactivation of the EGF receptor. Mouse, rat and human AT2 receptor genes have been mapped to the X-chromosome and no subtypes or splice variants have been described. The AT2 receptor also couples via G protein- (Gi) dependent and independent mechanisms. These include activating tyrosine phosphatases (MKP1 and SHP-1) and serine/threonine phosphatases to decrease in MAP kinase activity, activation of PLA2, opening of delayed-rectifier potassium channels and closing of T-type calcium channels, and stimulation of ceramide production.
The predominant role of the AT1 receptor in mediating the pathophysiological actions of Ang II underlies the effectiveness of AT1 receptor antagonists to lower arterial blood pressure, reduce cardiac pre- and afterload, inhibit sympathetic activity, promote salt and water excretion, and prevent cardiovascular hypertrophy, cardiac failure and atherosclerosis mediated by activation of the renin-angiotensin system. The functional role(s) of the AT2 receptor remain(s) incompletely understood, but reports indicate vasodilatory, natriuretic, antiproliferative/antihypertrophic, apoptotic, cardioprotective and possibly cerebroprotective actions of Ang II via this site. A consistent theme is that stimulation of AT2 receptors produces opposite effects to those mediated by AT1 receptors. Since AT1 receptors exert feedback inhibition on Ang II formation, selective AT1 receptor blockade will increase circulating Ang II in the bloodstream leading to increased AT2 receptor stimulation, which (via a vasodilatory action) may contribute to the beneficial actions of ARBs.
The 3-8 hexapeptide Ang IV binds to a site that is distinct from the AT1 and AT2 receptors , designated as the AT4 receptor. This site has high affinity for Ang IV and the structurally unrelated decapeptide LVV-Hemorphin-7 (LVV-H7), and much lower affinity for Ang II and AT1/AT2 receptor-selective ligands. The AT4 receptor has been identified as the transmembrane enzyme insulin-regulated membrane aminopeptidase (IRAP), and Ang IV, LVV-H7 and other AT4 receptor-selective ligands inhibit IRAP catalytic activity. The major described function of Ang IV is facilitation of memory retention and retrieval.
Ang (1-7) can be formed directly from Ang I by the action of neutral- or prolyl-endopeptidases, or from Ang II via angiotensin converting enzyme-2 (ACE-2). The major described actions of Ang (1-7) are vasodilation, via stimulation of nitric oxide, prostaglandins and potentiation of bradykinin actions, and antidiuresis. These effects are abolished by the specific Ang (1-7) antagonist [D-Ala7]-Ang (1-7), which has little effect at AT1 or AT2 receptors. While an Ang (1-7) receptor has not been cloned, evidence indicates that it is an endogenous ligand for the G protein coupled receptor Mas.
The Table below contains accepted modulators and additional information. For a list of additional products, see the Materials section below.
a) Further subtypes of rat and mouse AT1 receptors, designated AT1A and AT1B, have been cloned and sequenced. In the human, only one AT1 receptor gene has been identified.
b) “Atypical” AT receptors: 362/363 aa (amphibian), 359 aa (chicken), 359 aa (turkey) and 359 aa ( gerbil) have also been cloned.
BIBR277: 4'-[(1,4'-Dimethyl-2'-propyl[2,6'-bi-1H-benzimidazol]-1'-yl)methyl]-[1,1'-biphenyl]-2-carboxylic acid
CGP42112A: Nicotinic acid-Tyr-N-benzoxyl-carbonyl-Arg-Lys-His-Pro-Ile-OH
CS-866: 4-(1-Hydroxy-1-methylethyl)-2-propyl-1-[[2'-(1H-tetrazol-5-yl)[1,1'-biphenyl]-4-yl]methyl]-1H-imidazole-5-carboxylic acid (5-methyl-2-oxo-1,3-dioxol-4-yl)methyl ester
E3174: 2-Butyl-4-chloro-1-[[2'-(1H-tetrazol-5-yl)[1,1'-biphenyl]-4-yl]methyl]-1H-imidazole-5-carboxylic acid
EXP801: 2-(Diphenylacetyl)-6-phenyl-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid
L-159,686: 1,4-Bis-diphenylcarbamoyl-piperazine-2-carboxylic acid
PD 123,177: S(+)-1-[(4-Amino-3-methylphenyl)methyl]-5-(diphenylacetyl)-4,5,6,7-tetrahydro-1H-imidazo-(4,5-c]pyridine-6-carboxylic acid
PD 123,319: S(+)-1-[[4-Dimethylamino)-3-methylphenyl]methyl]-5-(diphenylacetyl)-4,5,6,7-tetrahydro-1H-imidazo-[4,5-c]pyridine-6-carboxylic acid
PD 126,055: 2-(Diphenylacetyl)-5-benzyloxy-6-methoxy-1,2,3,4-tetrahydro-isoquinoline-3-carboxylic acid
SKF 108566: (aE)-a-[[2-Butyl-1-[(4-carboxyphenyl)methyl]-1H-imidazol-5-yl]methylene]-2-thiophenepropanoic acid
SR 47436: 2-Butyl-3-[[2'-(1H-tetrazol-5-yl)[1,1'-biphenyl]-4-yl]methyl]-1,3-diazaspiro[4.4]non-1-en-4-one
TCV 116: 2-Ethoxy-1-[[2'-(1H-tetrazol-5-yl)[1,1'-biphenyl]-4-yl]methyl]-1H-benzimidazole-7-carboxylic acid 1-[[(cyclohexyloxy)carbonyl]oxy]ethyl ester
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