INTERACTIONS OF AGONISTS WITH AN ALLOSTERIC ANTAGONIST AT MUSCARINIC ACETYLCHOLINE M(2) RECEPTORS

The interaction of tane-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 shifte d concentration-response curves for the agonists, carbachol, oxotremor ine-M and (+)-cis-dioxolane, to the right in a parallel fashion. Arunl akshana-Schild regressions of the data yielded slopes significantly di fferent to unity, suggesting non-competitive antagonism. mon-linear re gression analysis, using an equation based on allosteric modulation, g ave quantitative estimates of co-operativity (alpha values) and the di ssociation constant of C-7/3'-phth (K-B). In all cases, the K-B estima tes for C-7/3'-phth were not significantly different. Increasing the c arbachol 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 T yrode solution did not significantly alter the K-B for C-7/3'-phth, al though alpha values obtained were consistently lower in Tyrode solutio n, suggesting that the allosteric action may be sensitive to buffer co mposition. A 4-fold higher degree of negative, heterotropic co-operati vity between C-7/3'-phth and agonists than between C-7/3'-phth and com petitive antagonists was also found.