AbstractThe regulation of phosphoinositidase C (PIC) activity by guanosine‐5′‐(3‐O‐thio)triphosphate (GTPγgMS) was characterized in a cholate‐solubilized peripheral myelin‐enriched fraction from rat sciatic nerve. The GTP analog maximally enhanced PIC‐catalyzed hydrolysis of exogenous phosphatidylinositol‐4,‐5‐bisphosphate (PIP2) in a dose‐dependent manner only within a narrow range of cholate concentrations. Maximal stimulation was attained at 0.6 μM GTPγS and could be completely prevented by 1 μM guanosine‐5′‐(2‐O‐thio)diphosphate. Neither adenylyl‐imidodiphosphate nor adenosine triphosphate (ATP) enhanced PIC activity. Carbamoylcholine (1 mM)added together with GTPγS increased the extent of PIP2hydrolysis over that elicited by GTPγ increased the extent of PIP2hydrolysis over that elicited by GTPγS alone and this stimulation was blocked by the muscarinic receptor antagonist, atropine (50 μM). In detergent solubilized myelin preparations from streptozotocin induced diabetic rats, a higher concentration of the guanine nucleotide analogn was required to achieve stimulation comparable to that obtained with corresponding preparations from normal animals. These results suggest that sciatic nerve myelin possesses muscarinic receptors coupled via a GTP‐binding protein to PIC and that this system can be reconstituted in detergent‐solubilized extracts. It is possible that the function of G Proteins in cell signaling is impaired in experimen