The diacylglycerol kinase inhibitor, R59022, potentiates cholecystokinin‐induced enzyme secretion from rabbit pancreatic acini

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Abstract
The putative inhibitor of diacylglycerol kinase activity, 6-{2-[(4-fluorophenyl)phenylmethylene]-1-piperidinyl}-ethyl-7-methyl-5H-thiazolo[2,1-a]pyrimidin-5-one (R59022), markedly potentiated cholecystokinin-C-terminal-octapeptide(cCK-8-)stimulated enzyme secretion from isolated rabbit pancreatic acini. Maximal potentiation occurred when acini were stimulated in the presence of 5-10 .mu.M R59022. Potentiation depended both on the concentration of R59022 and CCk-8. No potentiation was observed when acini were half-maximally stimulated, whereas the secretory response to maximal and supramaximal concentrations of secretagogue was increased by 50-60%. R59022 alone had no effect on basal enzyme secretion and the drug did not potentiate the secretory response to the Ca2+ ionophore A23187 or to the phorbol ester, 12-O-tetradecanoylphorbol 13-acetate. Moreover no increase in basal secretion was observed when acini were incubated in the presence of both R59022 and forskolin. These observations strongly suggest that receptor-mediated activation of the inositol phospholipid pathway is required for R59022-induced potentiation. R59022 inhibited the CCK-8-stimulated incorporation of 32Pi into phosphatidic acid dose dependently, without affecting the CCK-8-stimulated hydrolysis of 32P-labelled phosphatidylinositol 4,5-bisphosphate. This is consistent with an inhibitory effect of R59022 on acinar cell diacylglycerol kinase activity. The potentiating effect of R59022 was mimicked by 12-O-tetradecanoylphorbol 13-acetate added simultaneously with CCK-8. Therefore, it is concluded that in the presence of 5-10 .mu.M R59022 the receptor-mediated increase in acinar cell diacylglycerol content is enhanced leading to enhanced activation of protein kinase C and to potentiation of the secretory response. The fact that the secretory response to maximal and supramaximal concentrations of CCK-8 is potentiated by R59022 suggests that at these concentrations of secretagogue the diacylglycerol/protein kinase C branch of the signal-transduction route is rate-limiting.

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