The binding characteristics of 125I-gastrin and 125I-CCK8 to guinea pig fundic gastric glands differ: is there more than one binding site for peptides of the CCK-gastrin family?

The binding of biologically active 125I-labeled derivatives of the C-terminal octapeptide of cholecystokinin (125I-CCK8) and gastrin (125I-G) to dispersed guinea pig fundic glands were compared at 24 degrees C. Although both peptides share the same C-terminal pentapeptide sequence, differences were found in the amount of each radioligand bound to fundic glands, their dissociation behavior, and their Scatchard plots. However, each peptide was able to displace the other radioligand from the glands at nM concentrations which indicated that both peptides bound to the same site. The different binding characteristics observed for 125I-G and 125I-CCK8 most likely resulted from the different dissociation rates of each peptide.