Regulation of Muscarinic Agonist‐Induced Activation of Phosphoinositidase C in Electrically Permeabilized SH‐SY5Y Human Neuroblastoma Cells by Guanine Nucleotides

myo ‐[ 3 H]Inositol‐labelled SH‐SY5Y cells were permeabilized with electrical discharges. 3 H‐Inositol phosphate formation in cells shown to be fully permeable was stimulated by the muscarinic agonist carbachol, by guanosine 5′‐( γ ‐thio)triphosphate [GTP(S)], and by guanosine 5′‐( β ‐thio)diphosphate (GppNHp). Synergism was observed on coincubation of these GTP analogues with carbachol. GTP was also stimulatory and guanosine 5′‐( β ‐thio)diphosphate was inhibitory in the presence of agonist. Atropine blocked the effects of carbachol. Stimulation by GTP(S) (0.1 m M ) occurred after a 1‐2‐min lag, whereas Ca 2+ (0.5 m M ), carbachol (1 m M ), and carbachol plus GTP(S) stimulated without delay. The effects of carbachol plus GTP(S) but not those of Ca 2+ were inhibited by spermine (4 m M ). Accumulation of 3 H‐inositol phosphates was enhanced by Li + (4 m M ) only in intact cells. In intact or permeabilized cells, the “partial” agonist arecoline was maximally 40–50% as efficacious as carbachol. In permeabilized cells, the maximal effects of carbachol and arecoline were enhanced 2.8‐ and 5.3‐fold, respectively, by 0.1 m M GTP(S), but only the EC 50 for carbachol was substantially reduced. The binding affinity of carbachol but not that of arecoline in permeabilized cells was significantly reduced by 0.1 m M GppNHp. These data indicate that a guanine nucleotide‐binding regulatory protein is involved in coupling muscarinic receptors to phosphoinositidase C in SH‐SY5Y cells and that the activity of this protein influences the relationship between receptor occupation and phosphoinositide response.