Receptor antagonists for gastrointestinal peptides

Analysis of the behaviour of selected CCKB/gastrin receptor antagonists in radioligand binding assays performed in mouse and rat cerebral cortex

1. The previously described complex behaviour of the CCKB/gastrin receptor antagonist, L-365,260, in radioligand binding assays could be explained by a variable population of two binding sites. We have investigated whether other CCKB/gastrin receptor ligands (PD134,308, PD140,376, YM022 and JB93182) can distinguish between these sites. 2. In the mouse cortex assay, Hill slopes were not different from unity and the ligand pKI values did not differ when either [125I]-BH-CCK-8S or [3H]-PD140,376 was used as label as expected for a single site (G2). 3. In the rat cortex, where previous analysis of replicate (n=48) L-365,260 data indicated the presence of two CCKB/gastrin sites (G1 and G2), the competition data for PD134,308, PD140,376, YM022 and JB93182 could be explained by a homogeneous population of CCKB/gastrin sites because the Hill slope estimates were not significantly different from unity. However, the estimated affinity values for JB93182 and YM022 were significantly higher and that for PD134,308 was significantly lower than those obtained in the mouse cortex when the same radioligand was used. In view of our previous data obtained with L-365,260, the rat cortex data were also interpreted using a two-site model. In this analysis, SR27897 expressed approximately 9 fold, PD134,308 approximately 13 fold and PD140,376 approximately 11 fold selectivity for the G2 site. In contrast, JB93182 expressed approximately 23 fold and YM022 approximately 4 fold selectivity for the G1 site. If the two-site interpretation of the data is valid then, because of its reverse selectivity to L-365,260, JB93182 has been identified as a compound which if radiolabelled could provide a test of this receptor subdivision.

Pharmacological analysis of the CCKB/gastrin receptors mediating pentagastrin-stimulated gastric acid secretion in the isolated stomach of the immature rat

1. The CCKB/gastrin receptors mediating pentagastrin stimulation of gastric acid secretion by histamine release and by direct stimulation of oxyntic cells have been characterized in the immature rat isolated stomach assay. This was achieved by estimating antagonist affinity values for competitive antagonists from three distinct chemical classes (L-365,260, PD134,308 and JB93190) in the absence and presence of a high concentration of the histamine H2-receptor antagonist, famotidine (30 microM). 2. Pentagastrin produced concentration-dependent stimulation of gastric acid secretion in the absence and presence of famotidine. Famotidine depressed the maximum secretory response to pentagastrin although the degree of depression varied between experimental replicates (25-60%). This variation was attributed to the histamine-release mediated component of acid secretion, as judged by the consistency of the maximum responses obtained in the presence, but not absence, of famotidine. 3. All three CCKB/gastrin receptor antagonists behaved as surmountable antagonists in the absence and presence of famotidine. JB93190 (pKB approximately 9.1, approximately 8.9, in the absence and presence of famotidine, respectively) was approximately 30 fold more potent than either L-365,260 (pKB approximately 7.4, approximately 7.1) or PD134,308 (pKB approximately 7.6, approximately 7.4). 4. It was assumed that the famotidine treatment converted pentagastrin-stimulated acid secretion from a combination of an indirect action due to the release of histamine and a direct action on the oxyntic cell to solely a direct action on the oxyntic cell. A simple mathematical model of this two-receptor system was developed. The direct and indirect components were assumed to sum to produce the total response to pentagastrin obtained in the absence of famotidine. It was found that this model could account quantitatively for the behaviour of the three antagonists without invoking a difference in antagonist affinity for the CCKB/gastrin receptors mediating the direct and indirect actions of pentagastrin. However, a conclusion of receptor homogeneity has to be qualified because the model was also used to generate simulations which indicated that the analysis could only detect antagonist affinity differences of greater than one log-unit between enterochromaffin-like (ECL) and oxyntic cell CCKB/gastrin receptor populations.

Direct demonstration of three different states of the pancreatic cholecystokinin receptor

We used rat pancreatic acini as well as COS-7 cells transfected with the cloned pancreatic cholecystokinin (CCK) receptor and measured the abilities of CCK octapeptide (CCK-8) and L-364,718 (a CCK receptor antagonist) to inhibit binding of 125I-labeled CCK-8 (125I-CCK-8) and [3H]L-364,718. With pancreatic acini 125I-CCK-8 bound to two different states of the CCK receptor. The high-affinity state (1% of the receptors) had a Kd for CCK-8 of 985 pM and the low-affinity state (19% of the receptors) had a Kd for CCK-8 of 30 nM. [3H]L-364,718 bound to low-affinity receptors and to a previously unrecognized very-low-affinity state (80% of the receptors) having a Kd for CCK-8 of 13 microM. L-364,718 had the same affinity (Kd 3 nM) for each of the three different states of the CCK receptor. Similar measurements using transfected COS cells also identified three different states of the CCK receptor, with the very-low-affinity state being the most abundant. Thus, the ability of the CCK receptor to exist in three different states is an intrinsic property of the CCK receptor molecule itself.

The human gastrin/cholecystokinin type B receptor gene: alternative splice donor site in exon 4 generates two variant mRNAs

Gastrin and its carboxyl-terminal homolog cholecystokinin (CCK) exert a variety of biological actions in the brain and gastrointestinal tract that are mediated in part through one or more G protein-coupled receptors which exhibit similar affinity for both peptides. Genomic clones encoding a human gastrin/CCKB receptor were isolated by screening a human EMBL phage library with a partial-length DNA fragment which was based on the nucleotide sequence of the canine gastrin receptor. The gene contained a 1356-bp open reading frame consisting of five exons interrupted by 4 introns and was assigned to human chromosome 11p15.4. A region of exon 4, which encodes a portion of the putative third intracellular loop, appears to be alternatively spliced to yield two different mRNAs, one containing (452 amino acids; long isoform) and the other lacking (447 amino acids; short isoform) the pentapeptide sequence Gly-Gly-Ala-Gly-Pro. The two receptor isoforms may contribute to functional differences in gastrin- and CCK-mediated signal transduction.