ROLE FOR GTP IN GLUCOSE-INDUCED PHOSPHOLIPASE-C ACTIVATION IN PANCREATIC-ISLETS

We have previously demonstrated a permissive role for GTP in insulin s ecretion; in the current studies, we examined the effect of GTP on pho spholipase C (PLC) activation to explore one possible mechanism for th at observation. In rat islets preexposed to the GTP synthesis inhibito rs mycophenolic acid (MPA) or mizoribine (MZ), PLC activation induced by 16.7 mM glucose (or by 20 mM alpha-ketoisocaproic acid) was inhibit ed 63% without altering the labeling of phosphoinositide substrates. P rovision of guanine, which normalizes islet GTP content and insulin re lease, prevented the inhibition of PLC by MPA. Glucose-induced phospho inositide hydrolysis was blocked by removal of extracellular Ca2+ or b y diazoxide. PLC induced directly by Ca2+ influx (i.e., 40 mM K+) was reduced 42% in MPA-pretreated islets but without inhibition of the con comitant insulin release. These data indicate that glucose-induced PLC activation largely reflects Ca2+ entry and demonstrate (for the first time in intact cells) that adequate GTP is necessary for glucose (and Ca2+-)-induced PLC activation but not for maximal Ca2+-induced exocyt osis.