Lysophospholipids in Pig Pancreatic Zymogen Granules in Relation to Exocytosis

A hypothesis to explain the stimulatory role of cyclic AMP (adenosine 3′:5′‐monophosphate) in pancreatic enzyme secretion has been tested. In this hypothesis cyclic AMP would activate a phospholipase activity, which would lead to a locally increased lysophospholipid formation, resulting in a fusion between the zymogen granule membrane and the apical plasma membrane. Cyclic AMP added to isolated pig pancreatic zymogen granules leads to an increased lysis of these granules, but the slowness of this effect makes its physiological significance dubious. In pancreatic homogenates or zymogen granules no stimulating effect of cyclic AMP on lipase or phospholipase activity could be demonstrated. Isolated zymogen granules have a high lysophospholipid content (27% of total phospholipids), consisting of the 1‐acyl and 2‐acyl forms of lysophosphatidylcholine and lysophosphatidylethanolamine. Experiments with radioactive phosphatidylcholine indicate that the lysophospholipids are due to the action of endogenous (phospho)lipases during the isolation procedure. It is concluded that these experiments do not lend support to the above hypothesis for the mechanism of action of cyclic AMP in pancreatic enzyme secretion.