The cholecystokinin analogues JMV-180 and CCK-8 stimulate phospholipase C through the same binding site of CCKA receptor in rat pancreatic acini

1 This study was designed to address the controversy related to the involve ment of phospholipase C in the signalling pathway linked to CCKA receptor s timulation by the cholecystokinin analogue JMV-180, a full agonist for amyl ase release, in rat pancreatic acini. 2 JMV-180 was shown to stimulate phospholipase C by measuring the incorpora tion of [P-32]orthophosphoric acid ([P-32]-Pi) into phosphatidic acid (PtdO H) and phosphatidylinositol (PtdIns). Both responses elicited by JMV-180 we re time and concentration dependent. Maximal effects elicited by JMV-180 we re 39.08+/-0.72 and 8.02+/-0.40% for the labelling of [P-32]-PtdIns and [P- 32]-PtdOH, respectively, as compared to the maximal effects of CCK-8, a ful l agonist of the CCKA receptor. 3 [P-32]-Pi incorporation into PtdOH and PtdIns was sensitive to lithium, d emonstrating that both responses are a consequence of phospholipase C activ ation. However, since lithium blocks the phosphoinositide cycle by an uncom petitive mechanism, its effect was only apparent at high concentrations of ( CK-8 (> 10 pM), which elicited stimuli above 20 and 60% of the maximal [P -32]-PtdOH and [P-32]-PtdIns labelling, respectively. 4 JMV-180 inhibited the incorporation of [P-32]-Pi into PtdOH and PtdIns as stimulated by CCK-8, down to its own maximal effect. The estimated IC50 va lues for the inhibition curves were not significantly different from those calculated assuming the same single binding site for both agonists. These r esults indicated that the well established role of JMV-180 as a partial ago nist for CCKA receptor-linked signalling responses, also applies for the st imulation of phospholipase C. 5 The comparison of CCK-8 and JMV-180 dose-response curves of amylase relea se to those of PtdIns and PtdOH labelling with [P-32]-Pi showed the existen ce of an amplification mechanism between phospholipase C and amylase releas e for both agonists. 6 In conclusion, we show that JMV-180, as well as CCK-8, stimulate phosphol ipase C upon interaction with the same binding site at the CCKA receptor in rat pancreatic acini.