Ca2+-independent phospholipase A2 contributes to the insulinotropic action of cholecystokinin-8 in rat islets: dissociation from the mechanism of carbachol.
Insulin secretion induced by cholecystokinin-8 (CCK-8) was recently suggested to involve phospholipase A2 (PLA2) activation. In this study, we examined whether CCK-8 stimulates the Ca2+-independent form of PLA2 in isolated rat islets, in comparison with stimulation by the PLA2-activating cholinergic agonist carbachol. We found that CCK-8 (100 nmol/l; 5.6 mmol/l glucose) induces lysophosphatidylcholine accumulation from [3H]palmitate-prelabeled islets (170 +/- 39%; P = 0.003) as well as arachidonic acid (AA) efflux from [3H]AA-prelabeled islets (190 +/- 13%; P < 0.001), and that p-amylcinnamoylantranilic acid (ACA) (50 micromol/l)-mediated PLA2 inhibition reduces CCK-8-induced AA efflux (52 +/- 11%; P = 0.001) and insulin secretion (67 +/- 16%; P < 0.001). Neither the Ca2+ channel antagonist verapamil (100 micromol/l) nor the Ca2+ATPase inhibitor thapsigargin (1 micromol/l) affected CCK-8-induced AA efflux and insulin secretion. Furthermore, despite removal of extracellular Ca2+, CCK-8 still increased AA efflux (48 +/- 14%; P = 0.006) and insulin secretion (105 +/- 46%; P = 0.025). In contrast, carbachol (100 micromol/l)-stimulated AA efflux was reduced by verapamil by 36 +/- 6% (P < 0.001) and abolished by removal of extracellular Ca2+. Overnight protein kinase C (PKC) downregulation by 12-O-tetradecanoyl phorbol-13-acetate (TPA) (500 nmol/l) reduced CCK-8-induced AA efflux (45 +/- 12%; P = 0.003) and insulin secretion (40 +/- 16%; P = 0.020). No additive action regarding either AA formation or insulin secretion was seen by combining TPA overnight and ACA, which implies the involvement of an additional PLA2- and PKC-independent signaling mechanism. The results show that CCK-8, in contrast to carbachol, activates Ca2+-independent PLA2 in islets and that the PLA2-activating capacity of CCK-8 is partly PKC dependent. Hence, Ca2+-independent PLA2 seems important for the insulinotropic effect of CCK-8, but not for that of carbachol.
Simonsson
E
E
Department of Medicine, Lund University, Malmö, Sweden. erik.simonsson@medforsk.mas.lu.se
Karlsson
S
S
Ahrén
B
B
D000818:Animals; D016718:Arachidonic Acid / Q000378:metabolism; D002217:Carbachol / Q000494:pharmacology*; D002478:Cells, Cultured; D002934:Cinnamates / Q000494:pharmacology; D054522:Group VI Phospholipases A2; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000187:drug effects / Q000502:physiology*; D007700:Kinetics; D008244:Lysophosphatidylcholines / Q000096:biosynthesis; D008297:Male; D019308:Palmitic Acid / Q000378:metabolism; D010741:Phospholipases A / Q000378:metabolism*; D054467:Phospholipases A2; D051381:Rats; D017207:Rats, Sprague-Dawley; D012844:Sincalide / Q000494:pharmacology*; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D014700:Verapamil / Q000494:pharmacology; D062367:ortho-Aminobenzoates / Q000494:pharmacology
D002934:Cinnamates; D007328:Insulin; D008244:Lysophosphatidylcholines; D062367:ortho-Aminobenzoates; C085901:4-amylcinnamoylanthranilic acid; D016718:Arachidonic Acid; D019308:Palmitic Acid; D002217:Carbachol; D014700:Verapamil; D010741:Phospholipases A; D054522:Group VI Phospholipases A2; D054467:Phospholipases A2; D012844:Sincalide; D013755:Tetradecanoylphorbol Acetate