The role of calcium in phospholipid turnover following glucose stimulation in neonatal rat cultured islets.

Phospholipid turnover was studied in cultured neonatal rat pancreatic islets. In islets prelabeled with [32P]Pi, 15-min stimulation with glucose (16.7 mM) caused increased labeling of phosphatidic acid (93%) and phosphatidylinositol (94%) and decreased labeling of the polyphosphoinositides (20%). Omission of calcium ion during the period of glucose stimulation did not modify the changes in inositol phospholipids. In islets equilibrated with [32P]Pi in the presence and absence of stimulatory glucose concentrations (11.1 and 1.7 mM, respectively), chelation of calcium by ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid prevented the increase in phosphatidic acid and phosphatidylinositol labeling. However, the decrease in polyphosphoinositide labeling was inhibited by the chelator only in islets labeled in the absence of stimulatory glucose concentrations, the decrease persisting in islets labeled in the presence of glucose. This suggests that a specific pool of polyphosphoinositides is labeled in the presence of agonist and decreases in response to acute glucose stimulation irrespective of availability of external calcium. In the absence of calcium, the addition of [gamma-32Pi]ATP to a membrane preparation of cultured islets yielded three lipid phosphorylation products (phosphatidic acid, phosphatidylinositol 4-monophosphate, and phosphatidylinositol 4,5-bisphosphate). In broken cell preparations, [32P]Pi-labeled phosphatidylinositol was also detected. The extent of all these phosphorylations was decreased by the presence of free calcium ion (40 microM). These data indicate that polyphosphoinositide turnover takes place after glucose stimulation independent of extracellular calcium and support the possibility that this may play a primary role in altering cell calcium availability.