Effects of an agonist, allosteric modulator, and antagonist on guanosine-γ-[35S]thiotriphosphate binding to liposomes with varying muscarinic receptor/Go protein stoichiometry

We investigated whether alcuronium, an allosteric modulator of muscarinic acetylcholine receptors, can induce receptor-mediated activation of G(o) proteins in liposomal membranes incorporating purified M-2 receptors and G(o) proteins and whether its action is affected by the receptor/G(o) protein (R/G(o)) ratio. The binding of guanosine-gamma-[S-35]thiotriphosphate ([S-35]GTP gamma S) served as the indicator of G protein activation. It was stimulated by empty receptors at high receptor densities, and the dose-response curve was shifted to the left by the agonist carbachol and to the right by the antagonist atropine. At an R/G(o) ratio of 300:100, the rate of [S-35]GTP gamma S binding was the same in the presence or absence of 0.1 mM carbachol. Alcuronium increased the binding of [S-35]GTP gamma S at R/G(o) ratios of <3:100 and diminished it at R/G(o) ratios of >10:100, similar to previous observations on intact cells expressing muscarinic receptors at different densities. The apparent biphasicity of alcuronium action indicates that the allosteric modulator has at least two effects on muscarinic receptor/G protein interaction but its mechanistic basis is unclear. The "active state" of muscarinic receptors induced by alcuronium probably is different from that induced by carbachol. Changes in the densities of receptors and G(o) proteins had little effect on the kinetics of [S-35]GTP gamma S binding and on receptor affinity for carbachol, provided the R/G(o) ratio was kept constant. This suggests that the receptors and G proteins are located in microdomains in which their concentrations remain constant, despite variations in the amounts of lipidic membranes in the system.