In exocrine pancreas there are two distinct intracellular transduction pathways that mediate secretagogue-stimulated digestive enzyme secretion. Agents such as vasoactive intestinal peptide and secretin act to increase secretion by increasing cellular cyclic AMP while the mechanism involved in secretion caused by agents such as cholecystokinin (CCK), carbachol and bombesin includes metabolism of membrane phosphoinositides and alterations in cellular Ca2+. The long-term goals of the project are to determine the mechanisms of CCK-, carbachol- and bombesin-induced phosphoinositide and Ca2+ metabolism and the roles of the various metabolic products in stimulus-secretion coupling. Previous studies indicate agents cause hydrolysis of membrane phosphoinositides to 1,2 diacylglycerol (1,2 DAG) and inositol phosphates. One of the inositol phosphates, inositol 1,4,5-trisphosphate, mediates initial secretion by transiently increasing cytosolic free Ca2+ ([Ca2+]i). However, the mediator of sustained secretion has not been identified. Recent studies suggest that 1,2 DAG does not mediate sustained secretion. However, because the predominant fatty acid in the sn-2 position of 1,2 DAG is arachidonic acid (AA) and because AA is released by cellular diglyceride lipase, we plan to explore the possibility that AA release is involved in sustained secretion. These studies involve loading of acinar cell membranes with [3H]AA and determining release of [3H]AA into both the cytosol and extracellular medium stimulated by the various secretagogues. They will also involve determining the effect of diglyceride lipase inhibitors and phospholipase A2 inhibitors on both [3H]AA release and enzyme secretion. Studies will be performed to determine the source of released [3H]AA by thin layer chromatography of the various phospholipids. In addition, the metabolism of the released [3H]AA will be investigated by analyzing the [3H]AA released by both thin layer chromatography and high performance liquid chromatography. The immediate benefit resulting from findings of this project is an improved understanding of the cellular physiology underlying stimulus-secretion coupling in the exocrine pancreas as well as other secretory tissues. Application of this knowledge may help in determining the pathophysiology of certain disease states where abnormalities in the phosphoinositide-Ca2+ transduction system exist including diseases such as pancreatitis and pancreatic insufficiency.
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