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Pertussis toxin was used to examine the role of the inhibitory guanine nucleotide regulatory protein, Ni, in muscarinic-receptor-mediated stimulation of phosphoinositide turnover and calcium mobilization. In cultured chick heart cells, pertussis-toxin treatment inhibited muscarinic-receptor-mediated attenuation of isoprenaline-stimulated cyclic AMP accumulation. This finding is consistent with the proposal that pertussis toxin blocks the capacity of Ni to couple muscarinic receptors to adenylate cyclase. In contrast, treatment of chick heart cells or 1321N1 human astrocytoma cells with pertussis toxin did not block muscarinic-receptor-mediated stimulation of phosphoinositide hydrolysis, as measured by 3Hinositol phosphate accumulation in the presence of Li+. Pertussis-toxin treatment also had little effect on basal and muscarinic-receptor-stimulated phosphatidylinositol synthesis, as measured by the incorporation of 3Hinositol into phosphatidylinositol. Activation of muscarinic receptors also enhances the rate of unidirectional 45Ca2+ efflux in 1321N1 cells; this response, like phosphoinositide hydrolysis, was not prevented by pertussis-toxin treatment. Our data suggest that muscarinic receptors are not coupled to phosphoinositide hydrolysis or calcium mobilization through Ni.
Masters et al. (Wed,) studied this question.