A functional evaluation of the recently developed cholecystokinin type-B (CCK-B) receptor antagonist YM022 [(R)-1-[2,3-dihydro-1-(2'-methylphenacyl)-2-oxo-5-phenyl-1H-1,4-++ +benzodiazepin-3-yl]-3-(3-methylphenyl)urea] was undertaken in Chinese hamster ovary cells stably expressing the human CCK-B receptor gene (hCCK-B.CHO). YM022 exhibited high affinity and selectivity for the CCK-B receptor subtype as estimated from [125I]CCK8S displacement studies using membranes derived from hCCK-B.CHO and hCCK-A.CHO cells. Functional antagonist activity of YM022 was demonstrated employing CCK-4-stimulated Ca2+ mobilization in hCCK-B.CHO cells. In the presence of 30 nM YM022, the maximum effect of CCK-4 was suppressed to 48 +/- 11% of control, an effect that was accompanied by a modest rightward shift in the CCK-4 concentration-response curve. In contrast, the structurally similar CCK-B receptor antagonist L-365,260 [3R(+)-N-[2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzodiazepin -3-yl]-N'-(methylphenyl)urea; 30 nM-10 microM] produced progressive rightward shifts in the CCK-4 concentration-response curve, with no effect observed on the CCK-4 maximum response. Further characterization using the technique of microphysiometry revealed that the agonist activity of CCK-4 was not restored following washout after exposure to YM022. The antagonist activity of L-365,260, however, was found to be fully reversible in this system. Thus, YM022 behaves as an irreversible antagonist, whilst its structural analogue L-365,260 exhibits properties consistent with a competitive antagonist.