Identification and metabolism of polyphosphoinositides in isolated islets of Langerhans

Isolated islets were incubated with [32P]P1 and radiolabelling of polyphosphoinositides were determined. Labelling equilibrium was approached after 45 min, with a half-time of 15 min. D-Glucose decreased the amount of [32P]PO4 in phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] and phosphatidylinositol 4-phosphate (PtdIns4P) within 0.5 min, and loss of radiolabel was still evident at 1 min. [32P]PO4 levels in polyphosphoinositides returned to basal levels within 5 min. Neither D-galactose nor D-glucose after pretreatment of islets with mannoheptulose elicited the polyphosphoinositide effect. The glucose-stimulated breakdown of polyphosphoinositides was inhibited by EGTA; re-addition of Ca2+ partially restored the glucose effect. Ionomycin and tolbutamide promoted the rapid breakdown of PtdIns(4,5)P2, whereas the breakdown of PtdIns4P was less rapid and of a lesser magnitude. The results suggest that the Ca2+-dependent breakdown of polyphosphoinositides in an early metabolic event during the initiation of insulin release.