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)-but an-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 (pD2 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) = ICID7114 [(S)-4-(2-hydroxy- 3-phenoxy-propylamino-ethoxy)-N-(2-methoxyethyl) phenoxyacetamide] (8.03) > or = (-)-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'-dimethoxybenzyl)-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) > or = (-)-noradrenaline (6.46) > or = (-)-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 microM 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 pA2 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.

Isomeric-activity ratios of trimetoquinol enantiomers on beta-adrenergic receptor subtypes: functional and biochemical studies.

Trimetoquinol [1-(3',4',5'-trimethoxybenzyl)-6,7-dihydroxy-1,2,3,4- tetrahydroisoquinoline , TMQ] exists as two enantiomers, and the (-)-(S)-isomer is a potent beta-adrenergic receptor (beta-AR) agonist. Experiments were conducted to examine the functional and biochemical potencies of the (S)-and (R)-enantiomers of TMQ for interaction with beta-AR subtypes in tissues, membrane fractions, and cell systems. The isomeric-activity ratios (IARs) of the TMQ isomers [(S)-isomer > > (R)-isomer] for stimulation of beta 1- and beta 2-AR of guinea pig right atria and trachea were 224 and 1585, respectively; these IARs were similar to those observed on atypical beta-AR systems of rat distal colon (575), rat brown adipocytes (398), but differed from that of rat esophageal smooth muscle (2884) in the presence of pindolol. In the absence of pindolol, the potencies for the TMQ enantiomers were slightly increased; however, the IARs remained unchanged in rat distal colon, rat brown adipocytes, and rat esophageal smooth muscle. Similarly, radioligand binding studies demonstrated that the TMQ isomer beta-AR affinities were stereoselective for the (-)-(S)-isomer in membranes of guinea pig left ventricle (beta 1) and lung (beta 2) giving IARs of 115 and 389, respectively; and in E. coli expressing human beta 1- and beta 2-AR giving IARs of 661 and 724, respectively. Corresponding IARs of receptor affinities and stimulation of cAMP accumulation in Chinese hamster ovary cells expressing human beta 2-AR and rat beta 3-AR were 331 and 282, and 118 and 4678, respectively. These results indicate that the (-)-(S)-isomer of TMQ exhibits high affinity, and is a potent and highly stereoselective agonist for each beta-AR subtype, that the isomers generally fail to differentiate between the beta-AR subtypes, and that, based upon differences in IAR within beta 3-AR containing systems, subtypes of atypical beta (or beta 3)-AR may exist in adipose tissue and smooth muscle.