CHARACTERIZATION OF CHOLECYSTOKININ(A) RECEPTOR AGONIST ACTIVITY BY AFAMILY OF CHOLECYSTOKININ(B) RECEPTOR ANTAGONISTS

The dipeptoid cholecystokinin (CCK)(B) antagonists PD136450, Cam-1279 and C1988 stimulated amylase release from isolated rat pancreatic acin i with an efficacy similar to CCK8, but with a much weaker potency (ED (50), 0.6, 0.9 and 1.3 mu M, respectively). In contrast to CCK8, howev er, none of these compounds elicited inhibition of amylase release at supramaximal concentrations. In addition, 10(-4) M PD136450 blocked th e inhibition induced by high concentrations of CCK8. Competitive inhib ition of [I-125]BH-CCK8 binding by PD136450 indicated that this compou nd bound with a single affinity state to all CCK receptors on acini. M aximal stimulation of amylase release by PD136450 was dependent upon o ccupation of virtually the entire complement of CCK receptors. PD13645 0 at all concentrations examined had only a limited ability to stimula te total phosphoinositide hydrolysis and at maximum induced only 20% o f maximal CCK stimulation. Measurement of intracellular calcium ([Ca+](i)) by digital imaging of Fura-2 indicated that 1 mu M PD136450 indu ced repetitive [Ca++](i) oscillations with a magnitude of 346.0 +/- 4. 5 nM and frequency of 1.3 cycles per min. These oscillations were stil l present in Ca++-free medium and were blocked by the phospholipase C inhibitor, U73122. Because the dipeptoid compounds can occupy all avai lable pancreatic CCKA receptors, these compounds must induce a configu ration of the receptor different from either CCK8 or the previously ch aracterized partial agonist CCK-JMV-180, thereby inducing a distinct s ignaling pattern. Because the dipeptoid compounds do not fully mimic C CK actions, it is likely that they interact with only some of the crit ical binding sites within the CCKA receptor normally occupied by CCK8.