MOLECULAR RIGIDITY AND POTENCY OF BISPYRIDINIUM TYPE ALLOSTERIC MODULATORS AT MUSCARINIC M-2-RECEPTORS

Several bispyridinium compounds have been shown to be potent allosteri c modulators of ligand binding to muscarinic M-2-receptors. ''Uno comp ounds'' are benzyl derivatives of the bispyridinium ''TMB4'' imethylen e-bis-[4-hydroxy-iminomethylpyridinium]). To gain more insight into st ructure activity relationships, eleven derivatives with varying struct ure of the oxime-linked aromatic substituent were tested for their abi lity to inhibit the equilibrium-binding of [H-3]N-methylscopolamine ([ H-3]NMS) in guinea pig cardiac membranes and to retard [H-3]NMS-dissoc iation allosterically. At a concentration of 3 mu M, all compounds red uced [H-3]NMS-binding to about 40 % of the control level, indicating a similar potency to inhibit the association of [H-3]NMS. Allosteric re tardation of [H-3]NMS-dissociation required higher concentrations. Com paring the effects of the compounds at 30 and 300 mu M, respectively, revealed considerable differences in potency. Therefore, the concentra tion-dependency of the delay of [H-3]NMS-dissociation was determined f or selected compounds. The results indicate that introduction of a ben zyl-moiety into TMB4 leads to a 20-fold increase in allosteric potency . A further increment by a factor of 10 is obtained with the 2,6-dichl orobenzyl-substitution and with the naphthyl-derivative. The other com pounds were less potent. An inverse correlation was found between the rotational freedom of the aromatic substituent and the allosteric pote ncy. In conclusion, the aromatic moiety of non-symmetric bispyridinium -type modulators does not seem to be part of the pharmacophore involve d in the inhibitory effect on the association of [H-3]NMS. In contrast , a rigid aromatic lateral moiety appears to be essential for the inte raction with the recognition site mediating the allosteric delay of [H -3]NMS dissociation from muscarinic M-2-receptors.