GUANOSINE-5'-(3-O-THIO)TRIPHOSPHATE-MEDIATED STIMULATION OF PHOSPHOINOSITIDASE-C IN SOLUBILIZED RAT PERIPHERAL-NERVE MYELIN AND ITS ALTERATION IN STREPTOZOTOCIN-INDUCED DIABETES

The regulation of phosphoinositidase C (PIC) activity by guanosine-5'- (3-O-thio)triphosphate (GTP gamma S) was characterized in a cholate-so lubilized peripheral myelin-enriched fraction from rat sciatic nerve. The GTP analog maximally enhanced PIC-catalyzed hydrolysis of exogenou s phosphatidylinositol-4,5-bisphosphate (PIP2) in a dose-dependent man ner only within a narrow range of cholate concentrations. Maximal stim ulation was attained at 0.6 mu M GTP gamma S and could be completely p revented by 1 mu M guanosine-5'-(2-O-thio)diphosphate. Neither adenyly l-imidodiphosphate nor adenosine triphosphate (ATP) enhanced PIC activ ity. Carbamoylcholine (1 mM) added together with GTP gamma S increased the extent of PIP2 hydrolysis over that elicited by GTP gamma S alone and this stimulation was blocked by the muscarinic receptor antagonis t, atropine (50 mu M) In detergent-solubilized myelin preparations fro m streptozotocin-induced diabetic rats, a higher concentration of the guanine nucleotide analog was required to achieve stimulation comparab le to that obtained with corresponding preparations from normal animal s. These results suggest that sciatic nerve myelin possesses muscarini c receptors coupled via a GTP-binding protein to PIC and that this sys tem can be reconstituted in detergent-solubilized extracts. It is poss ible that the function of G proteins in cell signaling is impaired in experimental diabetic neuropathy.