Role for GTP in glucose-induced phospholipase C activation in pancreatic islets.

We have previously demonstrated a permissive role for GTP in insulin secretion; in the current studies, we examined the effect of GTP on phospholipase C (PLC) activation to explore one possible mechanism for that observation. In rat islets preexposed to the GTP synthesis inhibitors mycophenolic acid (MPA) or mizoribine (MZ), PLC activation induced by 16.7 mM glucose (or by 20 mM alpha-ketoisocaproic acid) was inhibited 63% without altering the labeling of phosphoinositide substrates. Provision of guanine, which normalizes islet GTP content and insulin release, prevented the inhibition of PLC by MPA. Glucose-induced phosphoinositide hydrolysis was blocked by removal of extracellular Ca2+ or by diazoxide. PLC induced directly by Ca2+ influx (i.e., 40 mM K+) was reduced 42% in MPA-pretreated islets but without inhibition of the concomitant 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 Ca(2+)-)-induced PLC activation but not for maximal Ca(2+)-induced exocytosis.