Modification of neuronal muscarinic receptor-mediated responses by islet-activating protein

Islet-activating protein (IAP) was used to investigate the role of the guanosine triphosphate binding proteins Gi and/or Go in muscarinic acetylcholine receptor-mediated responses in neuroblastoma cells (clone N1E-115). Incubation of intact cells for 24 h with 20 ng/ml IAP resulted in inhibition of subsequent IAP catalyzed incorporation of [32P]ADP-ribose into a membrane protein doublet of molecular weight 40,000 (Gi alpha and Go alpha). IAP treatment fully blocked muscarinic receptor-mediated inhibition of cAMP accumulation. Incubation of intact cells with carbachol for 8 h resulted in the concentration dependent loss of membrane muscarinic receptor. Pretreatment of cells with IAP prior to carbachol exposure partially blocked the subsequent decrease in receptor number. Pretreatment of cells with IAP had no effect on the ability of carbachol to stimulate phosphoinositide hydrolysis in neuroblastoma cells. Thus, while the guanosine triphosphate binding proteins Gi and/or Go are involved in coupling the muscarinic receptor to some of the physiological responses in these cells, it is clear that activation of phospholipase C by the muscarinic receptor is a Gi/Go independent response.