Pharmacological study of atypical beta-adrenoceptors in rat esophageal smooth muscle

The chemical specificity for the beta-adrenoceptor mediated relaxation of rat esophageal smooth muscle was evaluated using selective and non-selective beta-adrenoceptor agonists and antagonists. Pindolol, ICI 89,406, ICI 118551 [erythro-1-(7-methylindan-4-yloxy)-3-(isopropylamine)-butan-2-ol] and the beta-adrenoceptor alkylating agent, pindobind, produced only small rightward shifts in the concentration-response curves of (-)-isoprenaline and (-)-trimetoquinol in this preparation. Rank order potency (pD(2) values) of agonists was: (+/-)-trimetoquinol [1-(3',4',5'-trimethoxybenzyl)-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline] (8.34) = (-)-trimetoquinol (8.26) = BRL 37344 [(R*R*)-(+/-)-4-[2'-{2-hydroxy-2-(3-chlorophenyl)ethylamino}propyl]phenoxyacetic acid] (8.16) = ICI D7114 [(S)-4-(2-hydroxy-3-phenoxy-propylamino-ethoxy)-N-(2-methoyethyl)phenoxyacetamide] (8.03) greater than or equal to (-)-isoprenaline (7.82) > 3',5'-diiodotrimetoquinol [1-(3',5'-diiodo-4'-methoxybenzyl)-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline] (7.28) > 3'-iodotrimetoquinol [1-(3'-iodo-4',5'-dimethoxy-benzyl)-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline] (7.04) > ractopamine (6.84) = 5,8-difluorotrimetoquinol [5,8-difluoro-6,7-dihydroxy-1-(3',4',5'-trimethoxybenzyl)-1,2,3,4-tetrahydroisoquinoline] (6.82) > 8-fluorotrimetoquinol [6,7-dihydroxy-8-fluoro-1-(3',4',5'-trimethoxybenzyl)-1,2,3,4-tetrahydroisoquinoline] (6.56) greater than or equal to (-)-noradrenaline (6.46) greater than or equal to (-)-adrenaline (6.36) > (+/-)-noradrenaline (6.24) > (+/-)-adrenaline (6.00) > clenbuterol (5.83) > (-)-1-benzyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (5.75). Isomeric activity ratios of trimetoquinol isomers [(-)-(S)- >> (+)-(R)-] in esophageal smooth muscle in the presence and absence of 1 mu M pindolol were 1995- and 2951-fold, respectively; and were much greater than those in rat atria (282-fold) and rat trachea (107-fold). The atypical beta/beta(3)-adrenoceptor partial agonist, ICI D7114, produced concentration-dependent rightward shifts of the concentration-response curves of (-)-isoprenaline, (-)-trimetoquinol and the reference atypical beta/beta(3)-adrenoceptor agonist, BRL 37344. Schild plot analysis of ICI D7114 against trimetoquinol gave slope and pA(2) values of 0.91 and of 7.9, respectively. These results clearly demonstrate that the relaxant effects of these agonists in rat esophageal smooth muscle are primarily mediated through the activation of atypical beta/beta(3)-adrenoceptors. (-)-Trimetoquinol was as potent as (-)-isoprenaline and BRL 37344, and was the most stereoselective agonist evaluated in this tissue system.