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

Abstract

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. Non-linear regression analysis, using an equation based on allosteric modulation, gave quantitative estimates of co-operativity (α 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 α 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 α 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.