Multiple sources of 1,2-diacylglycerol in isolated rat pancreatic acini stimulated by cholecystokinin. Involvement of phosphatidylinositol bisphosphate and phosphatidylcholine hydrolysis

Abstract

Changes in the cellular content of 1,2-diacylglycerol (DAG) in isolated rat pancreatic acini in response to agonist stimulation were studied using a sensitive mass assay. When acini were stimulated by 10 nM COOH-terminal cholecystokinin-octapeptide (CCK8), the increase in DAG was biphasic, consisting of an early peak at 5 s and a second, larger, gradual increase that was maximal by 15 min. The basal level of DAG in acini was 1.04 nmol/mg of protein, which was increased to 1.24 nmol/mg of protein at 5 s and 2.76 nmol/mg of protein at 30 min. In comparison, the increase in DAG stimulated by 30 pM CCK8, a submaximal concentration for amylase release, was monophasic, increasing without an early peak but sustained to 60 min. Other Ca2+-mobilizing secretagogues such as carbamylcholine and bombesin increased DAG in acini, whereas vasoactive intestinal peptide, which acts to increase cAMP, had no effect. Phorbol ester and Ca2+ ionophore also stimulated DAG production. Analysis of the mass level of inositol 1,4,5-trisphosphate (1,4,5-IP3) showed that the generation of 1,4,5-IP3 stimulated by 10 nM CCK8 peaked at 5 s, a finding consistent with the early peak of DAG. The basal level was 4.7 pmol/mg of protein, which was increased to 144.6 pmol/mg of protein at 5 s by 10 nM CCK8. The levels of 1,4,5-IP3 then returned toward basal in contrast to the gradual and sustained increase of DAG. The dose dependencies of 1,4,5-IP3 and DAG formation at 5 s with respect to CCK8 were almost identical. This suggests that phosphatidylinositol 4,5-bisphosphate hydrolysis is a major source of the early increase in DAG but not of the sustained increase in DAG. Therefore, a possible contribution of phosphatidylcholine hydrolysis to DAG formation was examined utilizing acini prelabeled with [3H]choline. CCK8 (1 nM) maximally increased [3H]choline metabolite release by 133% of control at 30 min. Separation of these metabolites by thin layer chromatography showed that the products of CCK8-stimulated release were almost entirely phosphorylcholine, indicating the activation of a phospholipase C specific for phosphatidylcholine. By comparison, 1 nM CCK8 stimulated [3H]ethanolamine metabolite release from [3H]ethanolamine-labeled acini by only 22% of control. These data suggest that CCK stimulates both phosphatidylinositol 4,5-bisphosphate and phosphatidylcholine hydrolysis; the latter may contribute to the sustained generation of DAG and hence the maintained activation of protein kinase C.