Nutrient and Hormone-Neurotransmitter Stimuli Induce Hydrolysis of Polyphosphoinositides in Rat Pancreatic Islets*

Abstract

Preincubation of rat pancreatic islets with 3H-inositol, and subsequent exposure, in the presence of LiCl, to either glucose or carbamylcholine resulted in a rapid stimulation of ³H-inositol 1,4,5-triphosphate and ³H-myo-inositol 1,4-bis-phosphate formation, the level of which reached a plateau after about 5 min of stimulation. Both stimuli also caused an approximately linear accumulation of ³H-myo-inositol 1-phosphate. The amounts of ³H-inositol phosphates formed were dependent on the concentration of LiCl. Studies of 32P-labeling of islet ATP, phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P₂),and phosphatidylinositol 4-phosphate revealed that these approached isotopic equilibrium after about 240-min incubation, whereas ³²P-labeling of phosphatidylinositol, phosphatidic acid, phosphatidylcholine, and phosphatidylethanolamine proceeded at a lower rate. Carbamylcholine provoked an immediate fall in 32 and, to a lesser extent, ³²P-phosphatidylinos-P-PtdIns(4,5)P₂ itol 4-phosphate. Glucose caused a similar response although, in this case, the most marked decline was in a more polar ³²P-labeled lipid. Cholecystokinin-pancreozymin was also found to induce ³²P-PtdIns(4,5)P₂ hydrolysis, although the ionophore A23187 was without effect. Both carbamylcholine and glucose induced an increase in ³²P-phosphatidic acid. The results provide two independent pieces of evidence suggesting that phospholi-pase C-mediated hydrolysis of polyphosphoinositides occurs as an early response in rat islets to either nutrient or neurotrans-mitter secretagogues. (Endocrinology115: 1814–1820, 1984)