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Pharmacological profile of GR117289 in vitro : a novel, potent and specific non‐peptide angiotensin AT 1 receptor antagonist
Author(s) -
Robertson M.J.,
Barnes J.C.,
Drew G.M.,
Clark K.L.,
Marshall F.H.,
Michel A.,
Middlemiss D.,
Ross B.C.,
Scopes D.,
Dowle M.D.
Publication year - 1992
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1992.tb13425.x
Subject(s) - losartan , angiotensin ii , endocrinology , medicine , antagonism , phenylephrine , chemistry , antagonist , receptor antagonist , receptor , competitive antagonist , angiotensin ii receptor type 1 , biology , blood pressure
1 This paper describes the effects of GR117289 (1‐[[3‐bromo‐2‐[2‐(1H‐tetrazol‐5‐yl)phenyl]‐5‐benzofuranyl]methyl]‐2‐butyl‐4‐chloro‐1H‐imidazole‐5‐carboxylic acid) at angiotensin receptors and binding sites in rabbit aorta, rat liver and bovine cerebellum preparations in vitro . 2 In rabbit isolated aortic strips, GR117289 (0.3, 1 and 3 n m ) caused a concentration‐related, insurmountable suppression of the concentration‐response curve to angiotensin II (AII). When the contact time was increased, a greater degree of antagonism of AII was observed, suggesting that GR117289 is slow to reach equilibrium. A p K B of 9.8 ± 0.1 was calculated for GR117289 after 3 h incubation. GR117289 (1 μ m ) did not affect contractile responses to phenylephrine or 5‐hydroxytryptamine (5‐HT) in the rabbit aorta. 3 GR117289 (1 n m ) alone caused a marked suppression and a slight rightward displacement of the AII concentration‐response curve. Co‐incubation with the competitive, surmountable AT 1 receptor antagonist, losartan (10 n m , 100 n m and 1 μ m ), resulted in a concentration‐related upward and rightward displacement of the concentration‐response curve to subsequently administered AII. In separate experiments in which preparations were pre‐incubated with GR117289 (1 n m ), subsequent addition of losartan (1 μ m ) for 2, 15 or 45 min caused a further, but similar, rightward displacement of the concentration‐response curve to subsequently administered AII with a time‐dependent increase in the maximum response. 4 Suppression of AII‐induced contractile responses, caused by superfusion with GR117289 (0.3, 1 or 3 n m ) was not reversed by continuously washing the tissues for 3 h; in fact, the potency of GR117289 was slightly enhanced after this period. 5 In rat liver membranes, GR117289 was a potent competitor with [ 3 H]‐AII for AT 1 binding sites (p K i = 8.7 ± 0.1) but in bovine cerebellum membranes, it was a very weak competitor for AT 2 binding sites (p K i < 6). Pre‐incubation of rat liver membranes with GR117289 had little effect on its affinity (p K i = 9.1 ± 0.21), but increasing the concentration of bovine serum albumen in the assay buffer from 0.001% to 0.1% w/v decreased affinity (p K i = 7.5 ± 0.1). 6 In saturation binding experiments in rat liver membranes, GR117289 (12 n m ) increased the K d of [ 3 H]‐AII from 0.28 ± 0.06 n m to 0.37 ± 0.02 n m , and decreased B max from 10.0 ± 0.1 to 5.6 ± 0.3 fmol mg −1 tissue. In other experiments, GR117289 (1 μ m ) did not alter the rate of dissociation of [ 3 H]‐AII from AT 1 binding sites, following addition of excess unlabelled AII. 7 In rabbit aorta vascular smooth muscle membranes, GR117289 competed with [ 125 I]‐Sar 1 Ile 8 AII for binding to AT 1 binding sites. In the presence of 0.1% w/v bovine serum albumen, a pIC 50 of 7.6 ± 0.1 was calculated. Under the same conditions, but with rat liver membranes, a pIC 50 of 7.8 ± 0.1 was determined. 8 Taken together, these results show that GR117289 is a potent, specific, selective and insurmountable antagonist at angiotensin AT 1 receptors. Its profile in the rabbit aorta is consistent with the proposal that GR117289 is a slowly reversible (pseudo‐irreversible) antagonist at these receptors.