THE INFLUENCE OF GUANYL NUCLEOTIDE ON AGONIST AND ANTAGONIST AFFINITYAT GUINEA-PIG CCK-B GASTRIN RECEPTORS - BINDING-STUDIES USING [H-3] PD140376/

The novel radioligand [H-3]PD140376 was used to label receptors that b ind cholecystokinin (CCK) and related peptides in membranes prepared f rom guinea-pig brain and gastric glands. Under control conditions, mea surements of the apparent affinity of 11 agonist and 16 antagonist lig ands in both tissues revealed a strong positive relationship between t he affinity of a compound in either tissue (slope of the regression li ne = 0.89, r(2) = 0.908). Agonists consistently showed higher affinity for sites in gastric glands compared to brain. If agonists were exclu ded from the analysis, the degree of correspondence between affinities measured in each tissue was almost perfect (slope = 0.93, r(2) = 0.98 6). In the presence of the guanyl nucleotide 5'-guanylimidodiphosphate (GppNHp), agonist affinity in gastric glands, but not brain, was redu ced such that there was a direct relationship between binding affinity in each tissue. These data are consistent with the notion that the re ceptor sites in brain and gastric glands, which recognise CCK and gast rin related compounds, are the same and of the CCK-B/gastrin subtype. The receptors in the two respective tissues, however, do appear to dif fer in the degree of post-receptor coupling. These findings may explai n previously reported differences between gastrin and CCK-B receptors that were based upon binding studies using agonist ligands.