Thiopilocarpine

In contrast to other muscarinic agonists, WAL 2014 FU does not induce bronchospasm in laboratory animals. The present investigation was intended to test the hypothesis that this is due to a particular susceptibility of the drug's effect to antagonism by catecholamines, as a result of partial M3-agonism. The tonic activity of the muscarinic agonists, aceclidine, arecoline, carbachol, McN-A-343, RS 86, thiopilocarpine and WAL 2014 FU, was tested in groups of isolated tracheal muscle of the guinea-pig. Susceptibility to functional antagonism by beta-adrenoceptor stimulation was measured by the displacement of the concentration-force curves by 3 microM noradrenaline. Evaluation of the concentration-force relationship revealed differences in potency and intrinsic activity (carbachol-100%) ranging from 114% for arecoline to 36% for thiopilocarpine (WAL 2014 FU-63%). The catecholamine increased the concentration of agonist which induced 5% of the maximum effect achievable (EC05) values fivefold (carbachol) to more than 4,680 fold (thiopilocarpine) (WAL 2014 FU: 2,860 fold). Regression analysis between the intrinsic activity of the seven compounds and the antagonistic effect of noradrenaline revealed a significant correlation (Spearman correlation coefficient (r[s])=-0.79; p=0.036). Inhibition of the effects of endogenous catecholamines by beta-adrenolysis with 50 microM toliprolol increased the maximal contraction induced by 1 mM WAL 2014 FU, but did not affect maximal contraction induced by 30 microM arecoline. Pretreatment with 0.3-1.0 mM dibutyrylcyclic adenosine monophosphate (DBcAMP) shifted the concentration-response curves of arecoline, WAL 2014 FU and thiopilocarpine in a similar manner to noradrenaline. The results exclude an important contribution of adenylate cyclase-coupled M2-receptors to the susceptibility of tracheal contraction by muscarinic agonists to functional antagonism by noradrenaline, but emphasize the importance of intrinsic activity at the M3-receptors. The pronounced susceptibility of WAL 2014 FU-induced contraction to functional antagonism by beta-adrenoceptor activation provides an explanation for the failure of the drug to induce bronchospasm in vivo.

1. The muscarinic receptor subtype mediating contraction of the canine saphenous vein has been characterized using a range of muscarinic agonists and subtype-selective antagonists. 2. Oxotremorine M and (+)-cis-dioxolane behaved as full agonists, while in comparison L-660,863 ((+/-)-3-(3-amino-1,2,4-oxadiazole-5-yl)quinuclidine) acted as a partial agonist. SDZ ENS 163 (thiopilocarpine), pilocarpine and McN-A-343 (0.1 microM-0.3 mM) did not elicit a response. The profile of agonist potencies suggests a low receptor reserve for contraction. 3. The rank order of antagonist apparent affinities was 4-DAMP (4-diphenylacetoxy-N-methylpiperidine methiodide; 8.41) > pirenzepine (8.10) > himbacine (7.34) > or = p-F-HHSiD (para-fluoro-hexahydrosiladifenidol; 7.15) > methoctramine (6.23). This antagonist apparent affinity profile is consistent with the activation of muscarinic M1 receptors.