Identification and metabolism of polyphosphoinositides in isolated islets of Langerhans

Isolated islets were incubated with [³²P]P(i) 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 is an early metabolic event during the initiation of insulin release.