Ca 2+-Dependent and Ca 2+-independent effects of pancreatic secretagogues on phosphatidylinositol metabolism

Abstract

To gain further insight into the nature and importance of the phosphatidylinositol hydrolysis response, we tested the effects of a variety of secretagogues on [ 32P]phosphate incorporation into, and net changes in the concentrations of, phosphatidic acid and phosphatidylinositol during incubation of pancreatic tissue fragments in vitro. All tested secretagogues, including carbachol, cholecystokinin, secretin, vasoactive intestinal peptide, gastric inhibitory polypeptide, insulin, dibutyryl cyclic AMP, ionophore A23187, prostaglandins E 1 and E, caused a 30–40% decrease in the concentration of phosphatidylinositol and a 50–100% increase in the concentration of phoshatidic acid. The effects of prostaglindins E 1 and E 2 appeared to be due to enhanced insulin secretion, since blockade of the latter by epinephrine selectively abolished the effects of prostaglandins on phospholipid metabolism. Secretagogue-induced amylase secretion and changes in the concentrations of phosphatidic acid and phosphatidylinositol were abolished by Ca 2+ deficiency. The increases in phosphatidic acid concentration were inhibited by cycloheximide. Decreases in phosphatidylinositol concentration correlated reasonably well with increases in amylase secretion in all tested experimental conditions. In contrast to the above changes in phospholipid concentrations, the increases in [ 32P]phosphate incorporation into phosphatidic acid and phosphatidylinositol provoked by carbachol and cholecystokinin (but not dibutyryl cyclic AMP) were not influenced by either Ca 2+ deficiency or cycloheximide treatment. This Ca 2+-independent effect on [ 32 P]phosphate incorporation may reflect the enhanced rapid turnover of a relatively small pool of phosphatidylinositol occurring prior to increases in cytosolic Ca 2+in the action of carbachol, cholecystokinin and, presumably, other related peptides. The large-scale, Ca 2+ -dependent decrease in phosphatidylinositol, however, appears to be a ‘post-second messenger’ event. The latter, since it occurs with all tested secretagogues, may contribute to exocytosis.