Pseudo-noncompetitive antagonism of M1, M3, and M5, muscarinic receptor-mediated Ca2- mobilization by muscarinic antagonists

Muscarinic receptors M-1, M-3 and M-5 were expressed in Sf9 cells, Three different patterns of inhibition of Ca2+ elevations could be resolved for the subtype non-selective muscarinic receptor antagonists: (i) a right shift of the agonist dose-response curve, (ii) a right shift of the agonist dose-response curve and a depression of the maximum signal, and (iii) an intermediate pattern where the antagonist apparently behaved more competitively at higher concentrations, A simulation performed assuming that these differences are due to differences in the dissociation rates of the antagonists reproduced all three different modes of inhibition; the novel intermediate pattern (iii) is suggested to be caused by an intermediate antagonist dissociation rate, A direct correlation between the type of inhibition and the measured dissociation rate of the antagonists was also observed, Functional selectivity between receptor subtypes based on the dissociation constants is suggested based on the results, (C) 1998 Academic Press.