Interactions of agonists with an allosteric antagonist at muscarinic acetylcholine M(2) receptors

The interaction of heptane-1,7-bis(dimethyl-3'-phthalimidopropylammonium bromide) (C-7/3'-phth), with several agonists, was investigated at the muscarinic M(2) receptor in guinea-pig left atria. C-7/3'-phth shifted concentration-response curves for the agonists, carbachol, oxotremorine-M and (+)-cis-dioxolane, to the right in a parallel fashion. Arunlakshana-Schild regressions of the data yielded slopes significantly different to unity, suggesting non-competitive antagonism. mon-linear regression analysis, using an equation based on allosteric modulation, gave quantitative estimates of co-operativity (alpha values) and the dissociation constant of C-7/3'-phth (K-B). In all cases, the K-B estimates for C-7/3'-phth were not significantly different. Increasing the carbachol contact time 10-fold did not significantly influence the K-B or the alpha value obtained with C-7/3'-phth. Changing from Krebs to Tyrode solution did not significantly alter the K-B for C-7/3'-phth, although alpha values obtained were consistently lower in Tyrode solution, suggesting that the allosteric action may be sensitive to buffer composition. A 4-fold higher degree of negative, heterotropic co-operativity between C-7/3'-phth and agonists than between C-7/3'-phth and competitive antagonists was also found.