Characterization of cholecystokininA receptor agonist activity by a family of cholecystokininB receptor antagonists.

The dipeptoid cholecystokinin (CCK)B antagonists PD136450, Cam-1279 and CI988 stimulated amylase release from isolated rat pancreatic acini with an efficacy similar to CCK8, but with a much weaker potency (ED50, 0.6, 0.9 and 1.3 microM, respectively). In contrast to CCK8, however, none of these compounds elicited inhibition of amylase release at supramaximal concentrations. In addition, 10(-4) M PD136450 blocked the inhibition induced by high concentrations of CCK8. Competitive inhibition of [125I]BH-CCK8 binding by PD136450 indicated that this compound bound with a single affinity state to all CCK receptors on acini. Maximal stimulation of amylase release by PD136450 was dependent upon occupation of virtually the entire complement of CCK receptors. PD136450 at all concentrations examined had only a limited ability to stimulate total phosphoinositide hydrolysis and at maximum induced only 20% of maximal CCK stimulation. Measurement of intracellular calcium ([Ca++]i) by digital imaging of Fura-2 indicated that 1 microM PD136450 induced repetitive [Ca++]i oscillations with a magnitude of 346.0 +/- 4.5 nM and frequency of 1.3 cycles per min. These oscillations were still present in Ca(++)-free medium and were blocked by the phospholipase C inhibitor, U73122. Because the dipeptoid compounds can occupy all available pancreatic CCKA receptors, these compounds must induce a configuration of the receptor different from either CCK8 or the previously characterized partial agonist CCK-JMV-180, thereby inducing a distinct signaling pattern. Because the dipeptoid compounds do not fully mimic CCK actions, it is likely that they interact with only some of the critical binding sites within the CCKA receptor normally occupied by CCK8.