Involvement of phospholipase D in caerulein-induced phosphatidylcholine hydrolysis in rat pancreatic acini.

Phosphatidylcholine (PC) metabolism stimulated by caerulein (Cae), a cholecystokinin analogue, was investigated in rat pancreatic acini prelabeled with [3H]choline or [3H]-myristic acid. Both labels were incorporated mostly into PC. An inhibition of choline incorporation into PC was first observed in response to Cae (100 and 500 pM) stimulation, as indicated by reduced [3H]choline incorporation into trichloroacetic acid-precipitable material. Whereas choline incorporation was reduced in PC, Cae (500 pM) significantly increased [3H]choline metabolites release in the incubation medium. Separation of these metabolites by thin-layer chromatography showed that approximately 90% of the labeled products released into the medium was phosphocholine; however, Cae caused significant increases of [3H]choline release after 5, 15, and 30 min. In response to Cae, manoalide, a phospholipase C (PLC) inhibitor, totally prevented phosphocholine release into the medium but did not affect choline release. Staurosporine, a protein kinase C inhibitor, did not influence basal and Cae-induced choline release. In cells prelabeled with [3H]myristic acid, Cae stimulated within 5 min a rapid increase in intracellular [3H]phosphatidic acid (PA) levels in the presence of the PA phosphohydrolase inhibitor, propranolol; this PA production was further increased after 15 and 30 min of stimulation. The time course of [3H]PA formation in the presence of propranolol was similar to that of choline release in the medium. Staurosporine partially blocked PA accumulation stimulated by Cae after 30 min. In contrast, manoalide significantly reduced basal PA accumulation but did not prevent its production in response to Cae. In the presence of ethanol, Cae also significantly stimulated above control values the formation of [3H]phosphatidylethanol. These data indicate that Cae-induced PC hydrolysis in rat pancreatic acini is mediated mostly by phospholipase D (PLD) to produce PA and choline; they suggest a direct action of Cae on PLD activation, an effect independent of PLC activation.