Inhibition of glucose-induced insulin secretion by the diacylglycerol lipase inhibitor RHC 80267 and the phospholipase A2 inhibitor ACA through stimulation of K+ permeability without diminution by exogenous arachidonic acid.
The effects of the diacylglycerol lipase inhibitor 1,6-bis-(cyclohexyloximinocarbonyl-amino)-hexane (RHC 80267) and the phospholipase A2 inhibitor N-(p-amylcinnamoyl)anthranilic acid (ACA) on insulin secretion and 86Rb+ efflux in mouse pancreatic islets were studied. RHC 80267 (35 microM) and ACA (100 microM) inhibited glucose (16.7 mM)-induced insulin secretion, but did not inhibit insulin secretion induced by K+ (40 mM) or the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA; 0.16 microM). K+ (40 mM) or TPA (0.16 microM) potentiated glucose (16.7 mM)-induced insulin secretion, and prevented inhibition of glucose (16.7 mM)-induced insulin secretion by RHC 80267 and ACA. In comparison, potentiation of glucose-induced insulin secretion by albumin-bound arachidonic acid (AA; 200 microM total; 10 microM free unbound) failed to counteract inhibition of glucose-induced insulin secretion by RHC 80267 or ACA, suggesting that inhibition of insulin secretion by these agents was not mediated by a decrease in AA accumulation in islets. Determination of 86Rb+ efflux, a marker of K+ channel activity, revealed that both RHC 80267 and ACA stimulated K+ efflux from islets. These effects of RHC 80267 and ACA were observed at both 3.3 and 16.7 mM glucose and persisted in Ca2+-free medium, suggesting that they may represent an opening of ATP-sensitive K+ channels. RHC 80267-mediated stimulation of 86Rb+ efflux was not mimicked by the diacylglycerol analog TPA (0.16 microM) and was not prevented by the diacylglycerol kinase inhibitor R 59022 (50 microM), suggesting that stimulation of 86Rb+ efflux did not reflect a conditional increase in diacylglycerol or in phosphatidic acid upon inhibition of diacylglycerol lipase. In contrast, TPA (0.16 microM) attenuated RHC 80267 and ACA stimulation of 86Rb+ efflux. Addition of AA (200 microM total; 10 microM free unbound) stimulated 86Rb+ efflux, suggesting that stimulation of 86Rb+ efflux by RHC 80267 and ACA was not due to a decrease in AA accumulation. This stimulation by AA was not dependent on AA metabolism because it persisted in the presence of the lipoxygenase inhibitor nordihydroguaiaretic acid (NDGA; 50 microM) or the cyclooxygenase inhibitor indomethacin (50 microM). In contrast to RHC 80267 and ACA, AA stimulation of 86Rb+ efflux was attenuated in Ca2+-free medium, probably implicating Ca2+-sensitive K+ channels in AA regulation of 86Rb+ efflux. Parallel experiments with diazoxide (100 microM) revealed that RHC 80267 and ACA mimicked the effects of diazoxide, a specific activator of ATP-sensitive K+ channels in islets, on both insulin secretion and 86Rb+ efflux. In conclusion, it is suggested that RHC 80267 and ACA, independently of their action on AA release, may inhibit glucose-induced insulin secretion by the opening of ATP-sensitive K+ channels in islets.
Thams
P
P
Department of Medical Biochemistry and Genetics, The Panum Institute, University of Copenhagen, Denmark.
Capito
K
K
D000818:Animals; D016718:Arachidonic Acid / Q000494:pharmacology*; D002934:Cinnamates / Q000494:pharmacology*; D003512:Cyclohexanones / Q000494:pharmacology*; D003981:Diazoxide / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005947:Glucose / Q000037:antagonists & inhibitors*; D066298:In Vitro Techniques; D007328:Insulin / Q000378:metabolism*; D007515:Islets of Langerhans / Q000187:drug effects* / Q000201:enzymology; D008071:Lipoprotein Lipase / Q000037:antagonists & inhibitors*; D008297:Male; D051379:Mice; D010741:Phospholipases A / Q000037:antagonists & inhibitors*; D054467:Phospholipases A2; D015221:Potassium Channels / Q000187:drug effects*; D012414:Rubidium Radioisotopes; D062367:ortho-Aminobenzoates / Q000494:pharmacology*
D002934:Cinnamates; D003512:Cyclohexanones; D004791:Enzyme Inhibitors; D007328:Insulin; D015221:Potassium Channels; D012414:Rubidium Radioisotopes; D062367:ortho-Aminobenzoates; C085901:4-amylcinnamoylanthranilic acid; D016718:Arachidonic Acid; C036535:1,6-bis(cyclohexyloximinocarbonyl)hexane; D010741:Phospholipases A; D008071:Lipoprotein Lipase; D054467:Phospholipases A2; D005947:Glucose; D003981:Diazoxide