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 A(2) (PLA(2)) activation. In this study, we examined whether CCK-8 stimulates the Ca2+-independent form of PLA(2) in isolated rat islets, in comparison with stimulation by the PLA(2)-activating cholinergic agonist carbachol. We found that CCK-8 (100 nmol/l; 5.6 mmol/l glucose) induces lysophosphatidylcholine accumulation from [H-3]palmitate-prelabeled islets (170 +/- 39%; P = 0.003) as well as arachidonic acid (AA) efflux from [H-3]AA-prelabeled islets (190 +/- 13%; P < 0.001), and that p-amylcinnamoylantranilic acid (ACA) (50 mu mol/l)-mediated PLA(2) 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 mu mol/l) nor the Ca(2+)ATPase inhibitor thapsigargin (1 mu mol/l) affected CCK-8-induced AA efflux and insulin secretion. Furthermore, despite removal of extracellular Ca2+, CCK-8 still increased AB efflux (48 +/- 14%; P = 0.006) and insulin secretion (105 +/- 46%;P = 0.025). In contrast, carbachol (100 mu mol/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 PLA(2)- and PKC-independent signaling mechanism. The results show that CCK-8, in contrast to carbachol, activates Ca2+-independent PLA(2) in islets and that the PLA(2)-activating capacity of CCK-8 is partly PKC dependent. Hence, Ca2+-independent PLA(2) seems important for the insulinotropic effect of CCK-8, but not for that of carbachol.