[H-3] PD 140376 - A NOVEL AND HIGHLY SELECTIVE ANTAGONIST RADIOLIGANDFOR THE CHOLECYSTOKININ-B GASTRIN RECEPTOR IN GUINEA-PIG CEREBRAL-CORTEX AND GASTRIC-MUCOSA

The specific binding characteristics of the novel cholecystokinin (CCK )B/gastrin receptor-selective peptoid antagonist radioligand [H-3]PD 1 40376 were investigated using membrane homogenates prepared from guine a pig cerebral cortex and gastric fundic mucosa. [H-3]PD 140376 (0.01- 10 nm) bound to both cerebral cortex and gastric gland homogenates wit h comparable high affinity (K(d), 0.1-0.2 nm) and to an apparent singl e population of sites with B(max) values of 119 and 296 fmol/mg of pro tein, respectively. The level of specific binding, defined as that dis placed by unlabeled CCK sulfated octapeptide, was routinely between 60 and 70% in the cortex and between 50 and 60% in the fundic mucosa. Ph armacological characterization of the [H-3] PD 140376-labeled binding sites with a series of agonist and antagonist ligands selective for ea ch of the CCK receptor subtypes demonstrated, in both preparations, an affinity profile consistent with that of the CCK(B)/gastrin receptor. However, Hill slopes for the competition curves for the unlabeled ago nist ligands against specific [H-3]PD 140376 binding were significantl y less than unity, whereas those for the antagonist ligands, including unlabeled PD 140376, were close to unity. The affinity and Hill slope for PD 140376 and the related CCK(B)/gastrin antagonist CI-988 were u naffected by the presence of the nonhydrolyzable GTP analogue guanylyl -5'-imidodiphosphate. In contrast, guanylyl-5'-imidodiphosphate caused a characteristic decrease in affinity and an increase in the Hill slo pes towards unity for the agonist ligands CCK sulfated octapeptide and pentagastrin. The binding characteristics of unlabeled PD 140376 were also unaffected by the presence of the monovalent cation sodium. In c onclusion, the present study has demonstrated that [H-3]PD 140376 is t he most potent and selective antagonist radioligand yet described for the characterization of CCK(B)/gastrin receptors in the central and pe ripheral nervous systems.