Effects of nutrient secretagogues upon phospholipid metabolism in rat pancreatic islets.

Rat pancreatic islets labelled with [32P]Pi were used to investigate the effects of nutrient secretagogues upon phospholipid metabolism. Stimulatory concentrations of glucose, 4-methyl-2-oxopentanoate, 3-phenylpyruvate and 2-endo-aminonorbornane-2-carboxylic acid caused a marked and specific stimulation of labelling with [32P]Pi of phosphatidylinositol, phosphatidic acid and the polyphosphoinositides. The effects of glucose were concentration-related and were inhibited by mannoheptulose and menadione. Enhanced phospholipid labelling persisted in the absence of added calcium ions but was abolished by excess EDTA. The time-course of labelling of these phospholipids in response to glucose contrasted with that previously observed using carbamylcholine in that the accumulation of radioactivity in phosphatidic acid and in phosphatidylinositol occurred in parallel. We conclude that glucose, in common with other nutrient secretagogues or those which promote the metabolism of endogenous nutrients, causes a marked stimulation of turnover in the phosphatidylinositol cycle. This effect requires the integrity of nutrient catabolism and is probably not dependent upon the stimulation of Ca2+ uptake from the extracellular medium.