Receptors for cholecystokinin and gastrin peptides display specific binding properties and are structurally different in guinea-pig and dog pancreas

In the light of the strong potency of gastrin-related peptides on pancreatic exocrine secretion in dog, we analyzed the binding properties of peptides related to cholecystokinin (CCK) and gastrin on dog pancreatic acini compared to guinea-pig acini. Moreover, we determined apparent molecular masses of photoaffinity labelled CCK/gastrin receptors in the two models. Using the CCK radioligand, receptor selectivity towards CCK/gastrin agonists and antagonists was found to be lower in dog acini than in guinea-pig acini. Performing the binding with CCK and gastrin radioligands in combination with N2,O2'-dibutyryl-guanosine 3',5'-monophosphate, revealed that in dog acini there exist two different sub-classes of CCK/gastrin receptors having high and low selectivity, the latter ones being able to bind gastrin with high affinity (Kd = 2.1 nM). SDS-PAGE analysis of covalently cross-linked receptors using several photosensitive CCK and gastrin probes of different peptide chain lengths demonstrated that in guinea-pig, CCK peptides bound to a 84-kDa component whereas in dog pancreas, CCK and gastrin peptides bound to three distinct molecular species (Mr approximately equal to 78,000, 45,000, 28,000). Performing cross-linking in the presence of 1 microM CCK indicated that a 45-kDa protein is the putative CCK/gastrin receptor in dog pancreas. Our results support the concept of heterogeneity of CCK/gastrin receptors.