Cholecystokinin-Induced Duodenogastric Bile Reflux Increases the Severity of Indomethacin-Induced Gastric Antral Ulcers in Re-fed Mice

BACKGROUND/AIMS

We examined the involvement of cholecystokinin (CCK) in the exacerbation of indomethacin (IND)-induced gastric antral ulcers by gastroparesis caused by atropine or dopamine in mice.

METHODS

Male mice were fed for 2 h (re-feeding) following a 22-h fast. Indomethacin (IND; 10 mg/kg, s.c.) was administered after re-feeding; gastric lesions were examined 24 h after IND treatment. In another experiment, mice were fed for 2 h after a 22-h fast, after which the stomachs were removed 1.5 h after the end of the feeding period. Antral lesions, the amount of gastric contents, and the gastric luminal bile acids concentration were measured with or without the administration of the pro- and antimotility drugs CCK-octapeptide (CCK-8), atropine, dopamine, SR57227 (5-HT3 receptor agonist), apomorphine, lorglumide (CCK1 receptor antagonist), ondansetron, and haloperidol alone and in combination.

RESULTS

IND produced severe lesions only in the gastric antrum in re-fed mice. CCK-8, atropine, dopamine, SR57227 and apomorphine administered just after re-feeding increased bile reflux and worsened IND-induced antral lesions. These effects were significantly prevented by pretreatment with lorglumide. Although atropine and dopamine also increased the amount of gastric content, lorglumide had no effect on the delayed gastric emptying provoked by atropine and dopamine. Both ondansetron and haloperidol significantly inhibited the increase of bile reflux and the exacerbation of antral lesions induced by atropine and dopamine, respectively, but did not affect the effects of CCK-8.

CONCLUSIONS

These results suggest that CCK-CCK1 receptor signal increases bile reflux during gastroparesis induced by atropine and dopamine, exacerbating IND-induced antral ulcers.

Cholecystokinin and cholecystokinin-A receptor: An attractive treatment strategy for biliary dyskinesia?

Biliary dyskinesia is a relatively common gastrointestinal disease that is increasing in incidence as living standards improve. However, its underlying pathogenesis remains unclear, hindering the development of therapeutic drugs. Recently, "Expression and functional study of cholecystokinin-A receptors on the interstitial Cajal-like cells of the guinea pig common bile duct" demonstrated that cholecystokinin (CCK) regulates the contractile function of the common bile duct through interaction with the CCK-A receptor in interstitial Cajal-like cells, contributing to improving the academic understanding of biliary tract dynamics and providing emerging directions for the pathogenesis and clinical management of biliary dyskinesia. This letter provides a brief overview of the role of CCK and CCK-A receptors in biliary dyskinesia from the perspective of animal experiments and clinical studies, and discusses prospects and challenges for the clinical application of CCK and CCK-A receptors as potential therapeutic targets.

Elevated Serum Gastrin Is Associated with Melanoma Progression: Putative Role in Increased Migration and Invasion of Melanoma Cells

Micro-environmental factors, including stromal and immune cells, cytokines, and circulating hormones are well recognized to determine cancer progression. Melanoma cell growth was recently shown to be suppressed by cholecystokinin/gastrin (CCK) receptor antagonists, and our preliminary data suggested that melanoma patients with Helicobacter gastritis (which is associated with elevated serum gastrin) might have an increased risk of cancer progression. Therefore, in the present study, we examined how gastrin may act on melanoma cells. In 89 melanoma patients, we found a statistically significant association between circulating gastrin concentrations and melanoma thickness and metastasis, which are known risk factors of melanoma progression and prognosis. Immunocytochemistry using a validated antibody confirmed weak to moderate CCK2R expression in both primary malignant melanoma cells and the melanoma cell lines SK-MEL-2 and G361. Furthermore, among the 219 tumors in the Skin Cutaneous Melanoma TCGA Pan-Cancer dataset showing gastrin receptor (CCKBR) expression, significantly higher CCKBR mRNA levels were linked to stage III-IV than stage I-II melanomas. In both cell lines, gastrin increased intracellular calcium levels and stimulated cell migration and invasion through mechanisms inhibited by a CCK2 receptor antagonist. Proteomic studies identified increased MMP-2 and reduced TIMP-3 levels in response to gastrin that were likely to contribute to the increased migration of both cell lines. However, the effects of gastrin on tumor cell invasion were relatively weak in the presence of the extracellular matrix. Nevertheless, dermal fibroblasts/myofibroblasts, known also to express CCK2R, increased gastrin-induced cancer cell invasion. Our data suggest that in a subset of melanoma patients, an elevated serum gastrin concentration is a risk factor for melanoma tumor progression, and that gastrin may act on both melanoma and adjacent stromal cells through CCK2 receptors to promote mechanisms of tumor migration and invasion.

Effect of cholecystokinin on small intestinal motility in suncus murinus

In a fasting gastrointestinal tract, a characteristic cyclical rhythmic migrating motor complex (MMC) occur that comprises of three phases: I, II, and III. Among these, phase III contractions propagate from the stomach to the lower intestine in mammals, including humans, dogs, and Suncus murinus (suncus). Apart from the phase III of MMC propagating from the stomach, during the gastric phase II, small intestine-originated strong contractions propagate to the lower small intestine; however, the mechanism of contractions originating in the small intestine has not been clarified. In this study, we aimed to elucidate the role of cholecystokinin (CCK) in small intestinal motility. Administration of sulfated CCK-8 in phase I induced phase II-like contractions in the small intestine, which lasted for approximately 10-20 min and then returned to the baseline, while no change was observed in the stomach. Contractions of small intestine induced by CCK-8 were abolished by lorglumide, a CCK1 receptor antagonist. Gastrin, a ligand for the CCK2 receptor, evoked strong contractions in the stomach, but did not induce contractions in the small intestine. To examine the effect of endogenous CCK on contractions of small intestinal origin, lorglumide was administered during phase II. However, there was no change in the duodenal motility pattern, and strong contractions of small intestinal origin were not abolished by treatment with lorglumide. These results suggest that exogenous CCK stimulates contractions of small intestine via CCK1 receptors, whereas endogenous CCK is not involved in the strong contractions of small intestinal origin.

Prediction of Cholecystokinin-Secretory Peptides Using Bidirectional Long Short-term Memory Model Based on Transfer Learning and Hierarchical Attention Network Mechanism

Cholecystokinin (CCK) can make the human body feel full and has neurotrophic and anti-inflammatory effects. It is beneficial in treating obesity, Parkinson's disease, pancreatic cancer, and cholangiocarcinoma. Traditional biological experiments are costly and time-consuming when it comes to finding and identifying novel CCK-secretory peptides, and there is an urgent need to develop a new computational method to predict new CCK-secretory peptides. This study combines the transfer learning method with the SMILES enumeration data augmentation strategy to solve the data scarcity problem. It establishes a fusion model of the hierarchical attention network (HAN) and bidirectional long short-term memory (BiLSTM), which fully extracts peptide chain features to predict CCK-secretory peptides efficiently. The average accuracy of the proposed method in this study is 95.99%, with an AUC of 98.07%. The experimental results show that the proposed method is significantly superior to other comparative methods in accuracy and robustness. Therefore, this method is expected to be applied to the preliminary screening of CCK-secretory peptides.

Serum cholecystokinin levels can be a predictive factor for difficult cholecystectomy: Decreased cholecystokinin receptor levels

BACKGROUND

Laparoscopic cholecystectomy (LC) is being performed frequently in general surgery practice. Estimation of difficult cholecystectomy is very important to take precautions against complications. Cholecystokinin (CCK) is an important enzyme for gall-bladder motility. CCK receptor is the target for CCK. Fibrosis and emptying problems of gallbladder are related with difficult cholecys-tectomies. We aimed to evaluate the association between plasma CCK and difficult cholecystectomy and try to explain the mechanism.

METHODS

Prospective cross-sectional study was conducted on a group of patients with cholelithiasis Patients who underwent elective cholecystectomy were classified into easy, difficult and very difficult preoperatively using LC difficulty scores. Pre-operative gallbladder empting ratios were measured by ultrasonography. Serum C-reactive protein, and postprandial serum CCK and pancreas polypeptide levels were measured before the operation. Operation data including operation times, adhesion scores, and complications were collected. Tissue CCK receptor levels and tissue fibrosis scores were obtained.

RESULTS

Easy, difficult, and very difficult LC (DLC) groups were consisted of 34, 28, and 8 patients, respectively. Gallbladder emp-tying was 60% in easy LC group, but 15% in very DLC group. Plasma CCK levels in easy group (37.4 pg/ml) were significantly lower than plasma CCK levels of difficult (58.6 pg/ml), and very difficult groups (66.23 pg/ml). Tissue CCK receptor levels of easy, difficult, and very difficult were 372.4, 178.3, and 144.1 ng/100 mg, respectively. Adhesion scores and fibrosis scores of very difficult group were significantly higher than other groups. Operation times were significantly longer in very difficult group. There were two conversions to open in very DLC group (25%).

CONCLUSION

CCK is a reliable parameter for determining the difficulty of LC. Decreased CCK receptor levels with fibrosis of gallbladder are the probably responsible mechanism.

Vagal afferent cholecystokinin receptor activation is required for glucagon-like peptide-1-induced satiation

Peripheral glucagon-like peptide-1 (GLP-1) and cholecystokinin (CCK) are secreted from enteroendocrine cells, and their plasma concentrations increase in response to eating. While the satiating effect of gut-derived CCK on food-intake control is well documented, the effect of peripheral GLP-1 is less clear. There is evidence that native GLP-1 can inhibit food intake only in the fed state but not in the fasting state. We therefore hypothesized that other gut peptides released during a meal might influence the subsequent effect of endogenous GLP-1 and investigated whether CCK could do so. We found that intraperitoneal injection of CCK in food-restricted mice inhibited food intake during the first 30-minute segment of a 1-hour session of ad libitum chow intake and that mice compensated by increasing their intake during the second half of the session. Importantly, this compensatory behaviour was abolished by an intraperitoneal injection of GLP-1 administered following an intraperitoneal injection of CCK and prior to the 1-hour session. In vivo activation of the free fatty acid 1 (FFA1) receptor with orally administered TAK875 increased plasma CCK concentration and, consistent with the effect of exogenous CCK, we found that prior oral administration of TAK875 increased the eating inhibitory effect of peripherally administered GLP-1. To examine the role of the vagus nerve in this effect, we utilized a saporin-based lesioning procedure to selectively ablate the CCK receptor-expressing gastrointestinal vagal afferent neurones (VANs). We found that the combined anorectic effect of TAK875 and GLP-1 was significantly attenuated in the absence of CCK receptor expressing VANs. Taken together, our results indicate that endogenous CCK interacts with GLP-1 to promote satiation and that activation of the FFA1 receptor can initiate this interaction by stimulating the release of CCK.

Cholecystokinin-Mediated Neuromodulation of Anxiety and Schizophrenia: A "Dimmer-Switch" Hypothesis

Cholecystokinin (CCK), the most abundant brain neuropeptide, is involved in relevant behavioral functions like memory, cognition, and reward through its interactions with the opioid and dopaminergic systems in the limbic system. CCK excites neurons by binding two receptors, CCK1 and CCK2, expressed at low and high levels in the brain, respectively. Historically, CCK2 receptors have been related to the induction of panic attacks in humans. Disturbances in brain CCK expression also underlie the physiopathology of schizophrenia, which is attributed to the modulation by CCK1 receptors of the dopamine flux in the basal striatum. Despite this evidence, neither CCK2 receptor antagonists ameliorate human anxiety nor CCK agonists have consistently shown neuroleptic effects in clinical trials. A neglected aspect of the function of brain CCK is its neuromodulatory role in mental disorders. Interestingly, CCK is expressed in pivotal inhibitory interneurons that sculpt cortical dynamics and the flux of nerve impulses across corticolimbic areas and the excitatory projections to mesolimbic pathways. At the basal striatum, CCK modulates the excitability of glutamate, the release of inhibitory GABA, and the discharge of dopamine. Here we focus on how CCK may reduce rather than trigger anxiety by regulating its cognitive component. Adequate levels of CCK release in the basal striatum may control the interplay between cognition and reward circuitry, which is critical in schizophrenia. Hence, it is proposed that disturbances in the excitatory/ inhibitory interplay modulated by CCK may contribute to the imbalanced interaction between corticolimbic and mesolimbic neural activity found in anxiety and schizophrenia.

[Photodynamic modulation of cellular functions]

Photodynamic action, due to the rather limited lifetime (1 μs) and effective reactive distance of singlet oxygen (< 10 nm), could subcellular-specifically regulate different cellular functions. Photodynamic action could activate permanently cholecystokinin (CCK) 1 receptors, and sensitize or desensitize other G protein-coupled receptors. The emergence in recent years of genetically- encoded protein photosensitisers has enabled more precisely-targeted photodynamic modulation of subcellular organelles and functional proteins. Protein photosensitisers (such as KillerRed, miniSOG or SOPP) expressed on the plasma membrane, mitochondria, lysosomes or endoplasmic reticulum can modulate photodynamically subcellular functions and fine-tune protein activity by targeted photooxidation. With the newly emerged active illumination technique, simultaneous photodynamic action localized at multiple sites is now possible, and the contribution of subcellular regions to the whole cell or individual cells to a cell cluster could be quantitated. Photodynamic action with protein photosensitiser will be a powerful tool for nano-manipulation in cell physiology research.

Cloning and functional expression of the first Drosophila melanogaster sulfakinin receptor DSK-R1

Described in this report is a successful cloning and characterization of a functionally active Drosophila sulfakinin receptor designated DSK-R1. When expressed in mammalian cells, DSK-R1 was activated by a sulfated, Met(7-->Leu(7)-substituted analog of drosulfakinin-1, FDDY(SO(3)H)GHLRF-NH(2) ([Leu(7)]-DSK-1S). The interaction of [Leu(7)]-DSK-1S with DSK-R1 led to a dose-dependent intracellular calcium increase with an EC(50) in the low nanomolar range. The observed Ca(2+) signal predominantly resulted from activation of pertussis toxin (PTX)-insensitive signaling pathways pointing most likely to G(q/11) involvement in coupling to the activated receptor. The unsulfated [Leu(7)]-DSK-1 was ca. 3000-fold less potent than its sulfated counterpart which stresses the importance of the sulfate moiety for the biological activity of drosulfakinin. The DSK-R1 was specific for the insect sulfakinin since two related vertebrate sulfated peptides, human CCK-8 and gastrin-II, were found inactive when tested at concentrations up to 10(-5) M. To our knowledge, the cloned DSK-R1 receptor is the first functionally active Drosophila sulfakinin receptor reported to date.

Endogenous gastrin stimulates regeneration of remnant pancreas after partial pancreatectomy

The purpose of this study was to evaluate the effect of endogenous gastrin on pancreatic regeneration after a partial pancreatectomy in the rat. Sixty rats were divided into 6 groups. Groups I and II received sham operation (splenectomy only), and groups III, IV, V, and VI received both 66% partial pancreatectomy (PPx) and splenectomy. Endogenous hypergastrinemia was induced in groups II, IV, and VI by gavage of Lansoprozole (LSP) for three weeks. In groups V and VI, gastrin receptor blocker, L365,260, was given continuously using an osmotic minipump. Following three weeks of treatment, PPx alone increased the pancreatic weight and protein content, but not RNA or DNA content (Group III). In the PPx groups, the pancreatic weight, and contents of protein, RNA, and DNA were significantly increased in the LSP treated rats (Group IV). This effect was abolished by L365,260 (Group VI). This results suggest that endogenous gastrin specifically stimulates regeneration of the remnant pancreas after partial pancreatectomy in rat. Induction of endogenous hypergastrinemia may be useful in patients following pancreatic resection for the prevention and treatment of pancreatic insufficiency.

Promiscuous coupling of receptors to Gq class alpha subunits and effector proteins in pancreatic and submandibular gland cells

Mice with deficiencies in one or more Gq class alpha subunit genes were used to examine the role of the alpha subunit in regulating Ca2+ signaling in pancreatic and submandibular gland cells. Western blot analysis showed that these cells express three of the four Gq class subunits, Galphaq, Galpha11, and Galpha14 but not Galpha15. Surprisingly, all parameters of Ca2+ signaling were identical in cells from wild type and four lines of mutant mice: 1) Galpha11-/-, 2) Galpha11-/-/Galpha14-/-, 3) Galpha14-/-/Galpha15-/-, and 4) Galphaq-/-/Galpha15-/-. These parameters included the Kapp for several Gq class coupled receptors, induction of [Ca2+]i oscillations by weak stimulation, and a biphasic [Ca2+]i response by strong stimulation. Furthermore, Ca2+ release from internal stores and Ca2+ entry were not affected in cells from any of the mutant mice. We conclude that Galphaq, Galpha11, and Galpha14 promiscuously couple several receptors (m3 muscarinic, bombesin, cholecystokinin, and alpha1 adrenergic) to effector proteins that activate both Ca2+ release from internal stores and Ca2+ entry.

Targeting of a cholecystokinin-DNA complex to pancreatic cells in vitro and in vivo

The carboxy terminal octapeptide of cholecystokinin (CCK8) is a hormone that binds high affinity receptors in a number of tissues including pancreas and pancreatic tumours. As part of our studies to develop effective gene therapy for the treatment of pancreatic cancers, we have investigated various gene delivery systems that depend on CCK8 receptor targeting. In this paper, we describe the synthesis of a CCK8-DNA complex designed to deliver foreign DNA to cholecystokinin receptor-positive cells. CCK8 was ligated to avidin and then complexed to linearised biotinylated DNA (pSV-CAT). The uptake of 32P-labelled CCK8-DNA complex by rat pancreatic acini was linear with time over 4 h with 65-70% of uptake inhibited by 100 nM CCK8. The complex appeared to be internalised since it could not be removed by acid wash. When administered intra-arterially, the complex was rapidly removed from the circulation with no evidence of targeted delivery to the pancreas. However, following a single intraperitoneal dose, the pancreas accumulated 5-8% of the total administered complex by 24 h. These results suggest that peptide-dependent gene delivery to CCK receptor positive cells in vivo is feasible but, when administered directly into the circulation, diffusional barriers across the endothelium may limit distribution to peripheral tissues. Intraperitoneal administration therefore may be a useful alternative for targeting the pancreas.

Phylogeny of the cholecystokinin/gastrin family

The neuroendocrine peptides cholecystokinin (CCK) and gastrin, originally identified in mammals, are characterized by a common amidated C-terminal tetrapeptide sequence, Trp-Met-Asp-Phe.NH2, which also constitutes the minimal structure necessary for biological activity of both. Hence, it has been proposed that CCK and gastrin have evolved from a common ancestor. Although the occurrence of CCK/gastrin-related peptides has been suggested in representatives of several invertebrate phyla, the evidence, mostly based on immunoreactivity, has not been substantiated by peptide identification. Instead, CCK/gastrin-specific antibodies might be cross-reacting with Asp-Phe-amides, like the lymnaDFamides, isolated from the freshwater snail Lymnaea stagnalis. Cionin, isolated from Ciona intestinalis, a representative of the protochordates that occupy a key position at the transition to vertebrates, so far represents the oldest genuine member of the CCK/gastrin family, dating the emergence of these peptides back to at least 500 million years ago. The CCK/gastrin family appears to be represented in the whole chordate phylum, and in addition to mammals, CCK and gastrin have recently been identified in a number of nonmammalian species representing the major vertebrate classes, including fishes, amphibians, reptiles, and birds. This now makes it possible to consider the CCK/gastrin phylogeny based on structural information. A duplication of the ancestral gene appears to have already occurred before or during the appearance of cartilaginous fish, giving rise to two peptides most likely homologous to mammalian CCK and gastrin. Indicative of a function of gastrin, the acid secretory system appears to have developed concomitantly in sharks. The segregation of CCK and gastrin early in vertebrate evolution resembles the situation in other peptide families, in accordance with a suggested widespread pattern of multiplication within vertebrate peptide and protein families around 400 million years ago. At the amphibian level, two separate peptide systems, resembling mammalian CCK and gastrin, have been characterized by identification of the mature bioactive peptides, cDNAs, gene structures, primary and secondary sites of gene expression, and their physiological actions. The overall gene structure, including exon/intron organization, is similar in all mammalian and nonmammalian CCK/gastrin genes. CCK is well conserved in all vertebrate species investigated, while the mammalian gastrins at first sight appear as a distinct group with little similarity to the nonmammalian gastrins outside the invariant C-terminal tetrapeptide and the C-terminal flanking peptide of the prohormone. However, evidence indicates that the transition from nonmammalian to mammalian gastrin may not be as dramatic as first anticipated. In conclusion, the CCK/gastrin family appears to be represented in most, if not all, chordates, to which group it may also be limited. The two major classes, CCK and gastrin, probably arose as distinct peptide systems early in vertebrate history. While CCK is well conserved in all vertebrates, a major structural change of gastrin accompanied the transition to mammals.

Vagal sensors in the rat duodenal mucosa: distribution and structure as revealed by in vivo DiI-tracing

Results from functional studies point to the importance of chemoreceptive endings in the duodenum innervated by vagal afferents in the regulation of gastrointestinal functions such as gastric emptying and acid secretion, as well as in the process of satiation. In order to visualize the vagal sensory innervation of this gut segment, vagal afferents were selectively labeled in vivo by injecting the lipophilic carbocyanine dye DiI into either the left or the right nodose ganglion of young adult rats. Thick cryostat sections or whole-mounted peels of muscularis externa or submucosa of formalin-fixed tissue were analyzed with conventional and/or confocal microscopy. In the mucosa, many DiI-labeled vagal afferent fibers were found with terminal arborizations mainly between the crypts and the villous lamina propria. In both areas, vagal terminal branches came in close contact with the basal lamina, but did not appear to penetrate it so as to make direct contact with epithelial cells. Labeled vagal afferent fibers in the villous and cryptic lamina propria were found to be in intimate anatomical contact with fibrocyte-like cells that may belong to the class of interstitial cells of Cajal, and with small granular cells that might be granulocytes or histiocytes. Although our analysis was not quantitative, and considering that labeling was unilateral and not complete, it appears that the overall density of vagal afferent mucosal innervation was variable; many villi showed no evidence for innervation while other areas had quite dense networks of arborizing terminal fibers in several neighboring villi. Analysis of separate whole-mounted muscularis externa and submucosa peels revealed the presence of large bundles of labeled afferent fibers running within the myenteric plexus along the mesenteric attachment primarily in an aboral direction, with individual fibers turning towards the antimesenteric pole, and either penetrating into the submucosa or forming the characteristic intraganglionic laminar endings (IGLEs). Although the possibility of individual fibers issuing collaterals to myenteric IGLEs and at the same time to mucosal terminals was not demonstrated, it cannot be ruled out. These anatomical findings are discussed in the context of absorptive mechanisms for the different macronutrients and the implication of enteroendocrine cells such as CCK-containing cells that may function as intestinal "taste cells".

Identification of a gastrin binding protein in porcine gastric mucosal membranes by covalent cross-linking with iodinated gastrin

A gastrin binding protein (GBP) has been identified in detergent extracts of porcine gastric mucosal membranes by covalent cross-linking to 125I-[Nle15]gastrin with disuccinimidyl suberate. The apparent molecular weight of the cross-linked complex (80,000) is uneffected by reduction suggesting that the GBP is not composed of disulfide-bonded subunits. Subtraction of the molecular weight of 125I-gastrin indicates that the molecular weight of the GBP is 78,000. A similar molecular weight has been observed previously for the gastrin receptor (74,000) on intact canine parietal cells and plasma membranes therefrom, and for the receptor for the related hormone cholecystokinin (76,000-85,000) on pancreatic acinar membranes under reducing conditions. The similarity in molecular weight between the gastrin receptor and the solubilized GBP suggests that the latter protein is probably the gastrin receptor. However, the concentration (2 microM) of [Nle15]gastrin required for 50% inhibition of cross-linking of gastrin to the GBP solubilized in 0.1% Triton X-100 is 200-fold greater than the value (10 nM) observed for the gastrin receptor on isolated canine gastric parietal cells. A lower concentration (0.3 microM) of [Nle15]gastrin was required to inhibit cross-linking in a milder detergent (0.4% digitonin, 0.08% cholate). Thus, the reduced affinity for gastrin of the putative solubilized form of the gastrin receptor appears to be a result of detergent extraction.

Characterization of the binding of [3H]-(+/-)-L-364,718: a new potent, nonpeptide cholecystokinin antagonist radioligand selective for peripheral receptors

[3H]-(+/-)-L-364,718 a new, potent and selective nonpeptide peripheral cholecystokinin (CCK) antagonist bound saturably and reversibly to rat pancreatic membranes. The radioligand recognized a single class of binding sites with a high affinity (Kd = 0.23 nM). The binding of [3H]-(+/-)-L-364,718 was stereospecific in that the more biologically active (-)-enantiomer demonstrated greater potency than the (+)-enantiomer. The rank order of potency of various CCK agonists and antagonists in displacing [3H]-(+/-)-L-364,718 correlated with their ability to displace [125I]CCK-8 and their known pharmacological activities in peripheral tissues. However, the absolute potencies of agonists were greater in displacing [125I]CCK-8 than [3H]-(+/-)-L-364,718. As described for other physiologically relevant receptor systems, the potency for displacement of [3H]-(+/-)-L-364,718 binding by CCK agonists, but not antagonists, was reduced by guanosine 5'-(beta, gamma-imido)triphosphate and NaCl and enhanced by MgCl2. [3H]-(+/-)-L-364,718 also demonstrated specific binding to bovine gall bladder tissue but not guinea pig brain or gastric glands, consistent with its selectivity as a peripheral CCK antagonist. [3H]-(+/-)-L-364,718 binding to pancreatic membranes was not affected by various pharmacological agents known to interact with other common peptide and nonpeptide receptor systems. These data indicate that [3H]-(+/-)-L-364,718 represents a new potent nonpeptide antagonist radioligand for the study of peripheral CCK receptors which may allow differentiation of agonist and antagonist interactions.

A reduced pancreatic protein secretion in response to cholecystokinin (CCK) in the obese Zucker rat correlates with a reduced receptor capacity for CCK

Pancreatic membrane receptors for cholecystokinin (CCK) in obese and nonobese Zucker rats were compared with the use of a biologically active [125I]iodo-CCK-8 radioprobe. Membrane homogenates from obese rats bound half the amount of radioligand in 2 h as did membranes from lean rats (specifically bound, 7.0% vs. 14.0%; P less than 0.001). The reduced binding in membranes from obese rats did not result from kinetic effects or radioligand degradation; similar rates of association and dissociation of [125I]iodo-CCK-8 were obtained in membrane preparations from both, and no differences were found in the extent of radioligand degradation in the two membrane preparations. These differences also did not reflect an effect of cell size, as pancreatic acinar cells from obese and nonobese rats had about the same perimeters (24.6 and 26.3 micron, respectively) and areas (30.1 and 34.2 micron 2, respectively). Scatchard-type plots of competitive displacement data for CCK-binding sites on pancreatic membranes from both genotypes were curvilinear and were analyzed by a two-site binding model. The Kd values for both the high (0.56 vs. 0.45 nM) and low (9.0 vs. 14 nM) affinity sites on membranes from nonobese and obese rats, respectively, were the same (P greater than 0.1), whereas the capacities for CCK in the high (365 vs. 165 fmol/mg protein) and low (1020 vs. 360 fmol/mg protein) affinity regions were significantly different (P less than 0.025). This difference in CCK receptor capacity was reflected by a reduced pancreatic protein secretory response in the obese rat. After injections of 40, 80, 160, and 320 ng CCK/kg BW, total pancreatic protein secretion in nonobese rats increased 5, 12, 19, and 21 times above basal levels, whereas the same doses caused 2-, 6-, 12-, and 13-fold increases in obese rats. Whereas the reduced secretion in the obese rat may reflect a difference in the intracellular machinery leading to protein secretion between the two genotypes, these data are more consistent with a direct mechanism, whereby reduced numbers of pancreatic receptors for CCK are responsible for reduced protein secretion.

Design of potent, orally effective, nonpeptidal antagonists of the peptide hormone cholecystokinin

We describe the design and synthesis of nonpeptidal antagonists of the peptide hormone cholecystokinin. Several of these compounds have high specificity and nanomolar binding affinity and are active after oral administration. To our knowledge, the design of such agents has not previously been accomplished for any peptide hormone. The structural similarities between these synthetic compounds and the anxiolytic 1,4-benzodiazepines are noted, and the potential of this structural feature for future design of ligands for other peptide hormone receptors is discussed.

Biochemical and pharmacological characterization of an extremely potent and selective nonpeptide cholecystokinin antagonist

3S(-)-N-(2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4- benzodiazepine-3-yl)-1H-indole-2-carboxamide (L-364,718) interacted in a competitive manner with rat pancreatic cholecystokinin (CCK) receptors as determined by Scatchard analysis of the specific binding of 125I-labeled CCK. The affinity of L-364,718 for both pancreatic (IC50, 81 pM) and gallbladder (IC50, 45 pM) CCK receptors in radioligand binding assays greatly exceeded that of other reported nonpeptide CCK antagonists and was similar to that of CCK itself. In vitro functional studies utilizing CCK-induced contractions of the isolated guinea pig ileum and colon further demonstrated that L-364,718 acts as a competitive CCK antagonist, which lacks agonist activity and has a similar high affinity in these tissues (pA2, 9.9). L-364,718 exhibited a very high selectivity for peripheral CCK receptors relative to brain CCK, gastrin, and various other peptide and nonpeptide receptors in both in vitro radioligand and isolated tissue assays. In vivo, low intravenous doses of L-364,718 (0.1 mg/kg) markedly antagonized the contractions of the guinea pig gallbladder produced by intravenous administration of CCK for at least 2 hr. Administered orally, L-364,718 (ED50, 0.04 mg/kg) was highly effective as an antagonist of CCK-induced inhibition of gastric emptying in mice. The biochemical and pharmacological properties of L-364,718--namely, very high affinity and selectivity for peripheral CCK receptors, long-lasting in vivo efficacy, and oral bioavailability--makes this compound a powerful tool for investigating the physiological and pharmacological actions of CCK, and possibly its role in gastrointestinal disorders.

Occurrence of two cholecystokinin binding sites in guinea-pig brain cortex

Saturation experiments of the highly potent cholecystokinin analogue [3H]Boc(diNle28,31)CCK27-33 ([3H]BNDL-CCK7, 100 Ci/mmol) with guinea pig brain cortex in a large concentration range (0.05 nM to 30 nM) show the presence of two different binding sites (A site: KD = 0.13 nM, Bmax = 35 fmol/mg; B site: KD = 6.4 nM, Bmax = 92 fmol/mg). Both sites exhibit different sensitivity to sodium ions and therefore can be selectively investigated at [3H]BDNL-CCK7 concentration lower than 1 nM for the A site in Tris buffer and in Krebs buffer for the B site. The selectivity factors KIB/KIA of various CCK related peptides vary from 58 for CCK4 to 26 for CCK8 and 4 for the antagonist (Nle28,31) CCK27-32-NH2. The occurrence of two different CCK binding sites in the brain could explain biphasic pharmacological effects of CCK8.

Laminar distribution of receptors in monkey (Macaca fascicularis) geniculostriate system

We have examined the laminar distributions of eight types of receptor in the primary visual cortex (area 17) and the lateral geniculate nucleus (LGN) of the macaque monkey. The receptor populations and subpopulations examined included those selective for gamma-aminobutyric acid (GABA) (using [3H]-muscimol as ligand), L-glutamate-related receptors (using [3H]-L-glutamate and [3H]-AMPA), muscarinic acetylcholine (using [3H]-quinuclidinyl benzilate--QNB and [3H]-N-methyl scopolamine--NMS), cholecystokinin (CCK) (using [3H] pentagastrin), benzodiazepine (using [3H]-flunitrazepam), and adenosine (using [3H]-cyclohexyladenosine--CHA). Each of the receptors examined exhibited characteristic and differing laminar patterns of binding in the striate cortex. Perhaps reflecting the high density of cell bodies and synapses in layer 4C, most receptors, except those labelled by [3H]-L-glutamate or [3H]-AMPA, showed dense concentrations in this layer. Layers 4B and 5, which contain relatively few cell bodies and heavy myelin concentrations, were in general lightly labelled. Layer 6 showed relatively heavy labelling when [3H]-AMPA (quisqualate) or [3H]-pentagastrin (CCK) were used as ligands. The superficial layers of the cortex were zones of relative concentration of GABA, benzodiazepine, acetylcholine, glutamate-related, and adenosine receptors. In general, the binding patterns resembled those previously described for cat visual cortex, but there were also some clear differences. The distributions of all of these receptors likely reflect the differential input substances to different laminae of the visual cortex. Of the receptors examined, only those for GABA, benzodiazepine, and acetylcholine were found in substantial concentration in the LGN. Of these, GABA and benzodiazepine receptors showed especially dense binding in the magnocellular layers of the LGN compared to the parvicellular layers.

[Regulation of acinar cell receptors of the pancreas by peptides]

Peptides may act on the same receptor they regulate or on another receptor by causing regulations via receptor interactions. These receptor regulations include changes of receptor affinity and capacity. Receptor capacity is regulated by internalization, recycling, degradation, synthesis, and modification of bioavailability without migration of the receptor. Examples for those regulations, mostly based on experiments with isolated pancreatic acini from the rat, mouse, or guinea pig, are given. For the CCK receptor these examples include complex regulations of this receptor by CCK itself, bringing into discussion the hypothesis of negative cooperativity and the two-site receptor model, desensitization of the receptor by CCK, in vivo CCK influences on its receptor, and insulin receptor/CCK receptor interactions. For the insulin receptor the physiological significance of "up and down regulation" of this receptor by insulin itself is discussed. For the IGF receptors and the EGF receptor CCK-induced, Ca2+-mediated regulation of receptor internalization are another type of regulation with unknown physiological and pathophysiological significance. Finally CCK-induced, Ca2+-mediated regulation of somatostatin receptor capacity and affinity are mentioned. It is postulated that those regulations play an important role in influencing the biological effect of hormones and that knowledge about them may improve our understanding of pathophysiology.

New proglumide-analogue CCK receptor antagonists: very potent and selective for peripheral tissues

The present study evaluates the ability of two recently synthesized analogues of proglumide, both 4-benzamido-N,N-di-alkyl-glutaramic acid derivatives, to act as cholecystokinin receptor antagonists. Both new antagonists inhibited cholecystokinin-stimulated amylase release and, similarly, binding of 125I-cholecystokinin to isolated rat pancreatic acini. These effects displayed competitive kinetics; both antagonists showed no agonist activity and were specific in that only those secretagogues were inhibited that interact with the cholecystokinin receptor. Both antagonists also inhibited binding of 125I-cholecystokinin to mouse pancreatic membrane particles similarly to results with rat pancreatic acini. With the more potent of the two new antagonists, half-maximal inhibition of action and binding of cholecystokinin was observed with low concentrations of approximately 10(-7) M; compared with proglumide, the new antagonists were as much as 4,000 times more potent. Unlike proglumide, which inhibits binding of cholecystokinin to pancreas and brain tissue similarly, both antagonists inhibited binding of cholecystokinin to the pancreas at much lower concentrations compared with brain. The more potent of the inhibitors was 300 times more potent in inhibiting binding of cholecystokinin to pancreatic tissues compared with brain.

Evidence for a common evolutionary origin of brain and pancreas cholecystokinin receptors

An evolutionary basis for the distinct forms of cholecystokinin (CCK) receptors in the mammalian brain and pancreas was examined. The brains and pancreases of ratfish, frog, snake, and chicken contained saturable, high-affinity binding sites for iodinated porcine CCK-33. In the ectothermic species, the brain and pancreas CCK receptors exhibited nearly the same relative specificities for various CCKs and gastrins. Sulfated CCK-8 and sulfated gastrin-17 were the most potent while their nonsulfated analogs and gastrin-4 were less potent. By contrast, in the chicken, the specificities of brain and pancreas CCK receptors closely resembled their mammalian counterparts. We conclude that brain and pancreas CCK receptors with new specificities for binding CCKs and gastrins evolved at the level of the divergence of endotherms (birds and mammals) from reptiles. We propose that the prior evolution of gastrin provided the selection pressure for these changes. The endotherm pancreas receptor arose in evolution by narrowing its requirement for the position of a sulfated tyrosine residue in its ligands from either the sixth or seventh position from the carboxyl terminus to the seventh position only. The endotherm brain receptor arose in evolution by losing its requirement for a sulfated ligand and by transferring its high-affinity binding domain from the tyrosine residue in the carboxyl termini of CCK and gastrin to the carboxyl-terminal tetrapeptide active site common to CCKs and gastrins.

Preparation and characterization of a new cholecystokinin receptor probe that can be oxidatively radioiodinated

Oxidative iodination is the simplest, most efficient, and least expensive method for radiolabeling peptide hormones. It has not been applied to cholecystokinin (CCK), however, because this peptide lacks an unsubstituted tyrosine residue and oxidation abolishes its biologic activity. We report the synthesis, purification, and characterization of the first derivative of CCK that can be radioiodinated with an oxidative method while maintaining full biologic activity and receptor binding affinity. Boc-Tyr-[(Thr28, Nle31)CCK-25-33] was synthesized, iodinated using a solid-phase oxidant, and purified on reverse-phase high-pressure liquid chromatography to a specific activity of 2125 Ci/mmol. Both native and iodinated Boc-Tyr-[(Thr28, Nle31)CCK-25-33] exhibited efficacy and potency for stimulation of in vitro pancreatic enzyme secretion that were identical to CCK-8. Binding of Boc-125I-Tyr-[(Thr28, Nle31)CCK-25-33] to rat pancreatic plasma membranes was rapid, reversible, temperature-dependent, saturable, and specific. The abilities of various molecular forms of CCK to compete for this binding paralleled their potencies for stimulation of pancreatic secretion (Kd: CCK-8, 0.8 nM; CCK-33, 3 nM; CCK-8DS, 1 microM; CCK-4, 50 microM), whereas structurally unrelated pancreatic ligands (1 microM) demonstrated no significant competition. These findings suggest that Boc-125I-Tyr-[(Thr28, Nle31)CCK-25-33] interacts with the same high-affinity pancreatic receptor as the CCK receptor probes previously reported, i.e., the Bolton-Hunter-labeled CCK-33 and CCK-8. This probe also possesses the major advantages of simplicity, efficiency, and cost of the labeling procedure, and the stability and relative oxidation-resistance of the product. Boc-125I-Tyr-[(Thr28, Nle31)CCK-25-33] will be useful for the characterization of binding interactions between CCK and its target tissues.

Binding and degradation of 125I-gastrin by plasma membranes from homogenized rat gastric mucosa

Binding of 125I-gastrin to the 270-30,000 g fraction from homogenized rat oxyntic mucosa was studied. 'Specific' binding was calculated by subtracting the binding at excess cold gastrin from the binding with labelled gastrin (250 pM) only. At 30 degrees C specific binding rose rapidly to a short-lived maximum before falling gradually, whereas at 15 degrees C and 0 degree C specific binding rose gradually to a higher plateau level. The reduced binding at 30 degrees C could be caused by degradation of either the tracer or the binding site or by a combination of these two events. Degradation of 125I-gastrin was evaluated by trichloroacetic acid (TCA) precipitation, fast protein liquid chromatography, and binding to a gastrin antibody (immunoreactivity). The effect of incubation on the binding site was evaluated by preincubation of the homogenate fraction before adding gastrin. In separate studies, the proteolytic activity of the homogenate fraction was studied by TCA precipitation of radioactive casein. Different enzyme inhibitors tested were virtually ineffective in preventing gastrin and casein degradation. Only lowering the incubation temperature to 15 degrees C or lower could prevent this degradation. The reduced and transient binding of 125I-gastrin at 30 degrees C most probably reflects tracer degradation. Accordingly, the gastrin binding experiments were performed at 15 degrees C. Only homogenates from the oxyntic area of the stomach bound 125I-gastrin specifically and with a Kd of 0.8 nM (Scatchard analysis). However, micromolar concentrations of unlabelled gastrin were required to inhibit half maximal binding of the tracer. The tracer binding was unaffected by secretin, slightly reduced by a CCK-9 analogue, and more markedly reduced by pentagastrin.

A substance P antagonist also inhibits specific binding and mitogenic effects of vasopressin and bombesin-related peptides in Swiss 3T3 cells

While vasopressin and peptides of the bombesin family bind to different receptors in quiescent Swiss 3T3 cells, the antagonist [D-Arg1,D-pro2,D-Trp7,9,Leu11] substance P blocks the specific binding of both (3H) vasopressin and 125I-gastrin-releasing peptide to these cells. In addition, the antagonist inhibits the mobilization of Ca2+ and induction of DNA synthesis by vasopressin. These results indicate that [D-Arg1,D-Pro2,D-Trp7,9,Leu11] substance P has the ability to interact with the receptors for three structurally unrelated peptide hormones.

A study of the cerebral cortex cholecystokinin receptor using two radiolabelled probes: evidence for a common CCK 8 and CCK 4 cholecystokinin receptor binding site

This study was directed at the issue of whether or not subpopulations of cholecystokinin (CCK) receptors exist within the CNS. This was achieved through the use of two radiolabelled probes, namely [125I] Bolton-Hunter (BH) CCK 8 and [3H]pentagastrin (Boc-beta-Ala CCK 4), in comparative studies under identical conditions. Both probes bound with high affinity to the mouse cerebral cortical CCK receptor binding site with apparent equilibrium dissociation constants (KD) of 1.9 nM and 1.4 nM for [3H]pentagastrin and [125I]BH CCK 8, respectively. The maximal binding capacity was 1.05 and 1.15 pmol/g weight for the tritium and iodinated probes, respectively. Hill analysis yielded Hill numbers close to unity, suggesting the absence of more than one binding site and the lack of cooperativity of CCK receptor binding. Kinetic studies revealed binding site homogeneity in that no evidence of multiphasic dissociation curves was seen. Computerised analysis of displacement binding data using LIGAND established that both radiolabelled probes bound to a single site, with the one-site model providing the best fit of the data. Similar rank orders of potency were obtained for various fragments of CCK 8 in competing for the CCK receptor, labelled with either probe. Both CCK 8 and CCK 4 bound with roughly equinanomolar affinity. These studies demonstrate that both CCK 8 and its shorter C-terminal fragment CCK 4 bind to a single class of high-affinity binding site, with as yet no evidence of CNS CCK receptor multiplicity.

Role of gastrin and gastrin receptors on the growth of a transplantable mouse colon carcinoma (MC-26) in BALB/c mice

We recently reported trophic response of transplantable mouse colon cancer cells (MC-26) to pentagastrin, in vivo, and demonstrated gastrin receptors on MC-26 cells, in vitro. In the present study, growth of MC-26 cells in mice, in response to pentagastrin, was studied in relation to binding kinetics and capacity of gastrin receptor. Gastrin receptor levels on mouse fundic and colonic membranes and on MC-26 cellular membranes were determined before MC-26 cell inoculation and designated as Day 0 levels. Four groups of mice were next inoculated with MC-26 cells and given injections of either pentagastrin (treated) or normal saline (control) for 10 or 15 days. At the end of the treatment periods, body, tumor, fundic, and colon weights were noted and gastrin receptor measured. tumor and fundic weights increased significantly within 15 days of pentagastrin treatment, compared to control values. In control (non-pentagastrin treated) mice, the binding affinity of gastrin receptor on tumor membranes was significantly decreased and associated with the complete loss of high-affinity gastrin receptor (Kd = less than 0.5 nM) by Day 15 of tumor growth. On the other hand, both the binding affinity and gastrin receptor levels of tumor membranes were maintained at Day 0 values by pentagastrin treatment. Endogenous gastrin was therefore ineffective in maintaining high-affinity gastrin receptor on control tumors. A significant number of low-affinity gastrin-binding sites (Kd = less than 2 nM) appeared in control tumors by Day 15, which could reflect rapid dedifferentiation or conformational changes of gastrin receptor in the absence of high levels of normal regulatory hormones. These studies demonstrate that the trophic effects of gastrin on MC-26 cells are probably mediated by its regulation and maintenance of the binding affinity and capacity of gastrin receptor on the cancer cells, in vivo.

Proglumide (gastrin and cholecystokinin receptor antagonist) inhibits insulin secretion in vitro

CCK-8 and its desulfated analog (des-CCK-8) increase insulin secretion from isolated rat pancreatic islets in the presence of 8.3 mM glucose in a concentration-dependent manner. Proglumide (DL-4-benzamido-N,N-dipropylglutaramic acid), a gastrin and cholecystokinin (CCK) receptor antagonist, inhibits the synergistic effect of CCK on insulin release in the presence of 8.3 mM glucose; its EC50 (half-maximal effective concentration) was 1.2 +/- 0.4 mM. Its effect is specific in that it does not inhibit the glucose- or GIP (glucose dependent insulinotropic peptide) induced insulin secretion to a major degree. CCK-8, des-CCK-8 and proglumide compete for binding of 125I-CCK-33 to rat pancreatic islets; the IC50 of proglumide was 0.8 mM. The affinity of proglumide is in the range of both its EC50 for inhibition of insulin secretion and its IC50 in other in vitro systems tested so far (exocrine pancreas, gall bladder, cortex). Its inhibitory effect presumably is not a gastrin antagonizing effect since gastrin does not stimulate insulin secretion. The data therefore indicate that proglumide should be monitored for diabetic effects in vivo.

Brain CCK receptors: species differences in regional distribution and selectivity

The binding of 125I-CCK-33 to its receptors prepared from cerebral cortex and cerebellum was studied in four species: mouse, rat, hamster, and guinea pig. Only the guinea pig showed significant binding to membranes from cerebellum and this binding was comparable to that observed for cerebral cortex. In all four species, the order of potency of unlabeled analogs to compete for the binding site was CCK-8 greater than CCK-33 greater than desulfated CCK-8 greater than CCK-4. While the affinity for CCK-8 and CCK-33 was similar in the various species, the relative affinity for desulfated CCK-8 and CCK-4 was less for hamster and guinea pig, indicating species differences in receptor specificity, as well as in regional localization.

Synthesis of pseudo-peptide analogues of the C-terminal tetrapeptide of gastrin and evaluation of their biological activity on acid secretion

The syntheses of pseudo-tetrapeptides Boc-Trp-psi (CH2-NH)-Met-Asp-Phe-NH2 21 and Boc-Trp-Met-psi (CH2-NH)-Asp-Phe-NH2 20, representing the C-terminal tetrapeptide sequence of gastrin, in which amide bonds were replaced by CH2-NH bond, are described, as well as the syntheses of pseudo-peptide analogues Boc-Trp-psi (CH2-NH)-Nle-Asp-Phe-NH2 16, Boc-Trp-Nle-psi (CH2-NH)-Asp-Phe-NH2 11, and Boc-Trp-Nle-Asp-psi (CH2-NH)-Phe-NH2 5, in which the methionyl residue was replaced by a norleucyl residue. Pseudo-peptides 16 and 21, in which the amide bond between Trp and Met (or Nle) was substituted by a CH2-NH bond, stimulated gastric acid secretion in the rat in vivo. Pseudo-peptides 11 and 20, where the amide bond between Met (or Nle) and Asp was replaced by a CH2-NH bond, did not exhibit any activity on acid secretion in the rat in vivo but were potent inhibitors of pentagastrin-induced acid secretion. Peptides 11, 16, 20 and 21 all recognize the gastrin receptor on a mucosal cell preparation. Pseudo-peptide 5, in which the amide bond between Asp and Phe was replaced by a CH2-NH bond, was a less potent inhibitor of pentagastin-induced acid secretion and had a weaker affinity than the other pseudo-peptides.

Antagonists of central and peripheral behavioral actions of cholecystokinin octapeptide

Pharmacological studies on the behavioral functions of sulfated cholecystokinin (CCK) in the gut and in the brain require potent, specific antagonists to CCK. Compounds identified as competitive antagonists at the peripheral receptors for CCK were tested for their ability to block the behavioral effects of CCK administered centrally and peripherally. Behavioral effects of CCK (8.8 X 10-10 mmol) administered centrally into the nucleus accumbens, i.e., potentiation of dopamine-induced hyperlocomotion in rats, were effectively blocked by pretreatment with proglumide (6 X 10(-5) mmol of nucleus accumbens), by benzotript (3 X 10(-5) mmol of nucleus accumbens) and by rabbit antiserum raised against CCK (0.2 microliter/nucleus accumbens), but not by CCK26-33 (1.7 X 10(-7) mmol) or unsulfated CCK26-33 (1.9 X 10(-6) mmol). The behavioral effects of peripherally administered CCK, i.e. reduced food consumption and reduced exploratory behaviors in mice, were blocked effectively by pretreatment with proglumide (0.3-0.9 mmol/kg), and by benzotript (0.03 mmol/kg), but not by CCK30-33 (0.003 mmol/kg). None of the compounds administered peripherally significantly affected food consumption or exploratory behaviors when given alone. Furthermore, none of the compounds significantly affected locomotion when administered alone into the nucleus accumbens, or significantly affected dopamine-induced hyperlocomotion when given into the nucleus accumbens before dopamine. Benzotript, proglumide and a CCK antibody appear to act as specific antagonists of the behavioral effects of CCK at both the peripheral gastrointestinal site and at the central nucleus accumbens site. Neither unsulfated CCK26-33 or CCK30-33 were effective as antagonists of peripheral or central behavioral effects of CCK. However, whereas benzotript and proglumide may be useful as pharmacologically specific antagonists, the high doses required suggest that more potent CCK antagonists are required for investigating the behavioral functions of endogenous CCK.

Characterization of [3H]pentagastrin binding in guinea pig gastric glands--an alternative convenient ligand for receptor binding assay

The binding of [3H]pentagastrin to guinea pig gastric glands was specific, saturable and of high affinity (Kd = 5 nM). The relative order of potencies for gastrin and CCK analogs in displacing [3H]pentagastrin binding correlated well with those obtained using [125I]gastrin and their reported biological potencies for stimulating acid secretion. Nonselective CCK/gastrin antagonists including carbobenzoxy-CCK (26-32), proglumide and benzotript, but not the selective peripheral CCK antagonist, asperlicin, inhibited specific [3H]pentagastrin binding. The results indicate that [3H]pentagastrin labels physiologically relevant gastrin receptors in guinea pig gastric glands.

Evidence that cholecystokinin interacts with specific receptors and regulates insulin release in isolated rat islets of Langerhans

To determine the nature of the pancreatic islet cell cholecystokinin (CCK) receptor, we studied CCK receptor binding and biologic activity in isolated rat pancreatic islets. Binding of 70 pM 125I-CCK to collagenase-prepared isolated rat pancreatic islets at 24 degrees C was one-half maximal after 5 min and maximal at 60 min. At 60 min, specific binding was 12% of total radioactivity per 100 micrograms islet protein; nonspecific binding (in the presence of 1 microM CCK 8) was less than 2% of total radioactivity. Unlabeled CCK 33 inhibited labeled hormone binding one-half maximally at 2 nM; Scatchard analysis showed one binding site (Kd, 2.3 +/- 0.4 nM; Bmax, 8.1 pmol/mg protein). The agonist selectivity of this binding site was: CCK 8 = CCK 33 greater than desulfated-CCK 8 greater than CCK 4. Two CCK antagonists were studied; N-carbobenzoxy-L-tryptophan was more potent than dibutyryl-cGMP. When the effect of CCK on insulin release from the islets was studied, the order of potency of CCK agonists and antagonists on insulin secretion was the same as the order of their ability to inhibit 125I-CCK binding. The effect of CCK on insulin secretion was dependent on the glucose concentration in the media. CCK had no effect at 5.6 mM glucose and was fully effective at 11.0 mM glucose. These data, therefore, indicate that: specific binding sites for CCK are present in rat pancreatic beta cells; and CCK acts in concert with glucose to stimulate insulin secretion.

Structure-activity relationship of subtypes of cholecystokinin receptors in the cat lower esophageal sphincter

Cholecystokinin (CCK) causes relaxation of the cat lower esophageal sphincter (LES) by stimulating CCK receptors on the noncholinergic, nonadrenergic inhibitory neurons and causes contraction by stimulating CCK receptors on the sphincter muscle. Studies were performed in anesthetized cats to identify differences between the two CCK receptors by investigating the structure-activity relationships of various fragments of CCK or gastrin molecule. Lower esophageal sphincter pressures were monitored continuously, using continuously perfused catheters, and agents were administered intravenously or close intraarterially. Based on their doses and the presence or absence of tetrodotoxin pretreatment, CCK analogues produced either relaxation or contraction of the sphincter. The relative potencies of CCK analogues on the inhibitory (neural) response were CCK-8 greater than G-17-I greater than or equal to dCCK greater than CCK-4(1:1/14,000: 1/15,000: 1/335,000). Sulfated gastrin was nearly as potent as CCK-8. The relative potency of these agents on the contractile (muscle) response was CCK-8 = G-17-I greater than or equal to dCCK-8 greater than CCK-4 (1:1:1/4.5: 1/2000). Deamidated CCK-8 was inactive. Proglumide shifted the dose-response curves of the inhibitory as well as excitatory effects of CCK analogues to the right. These studies show that there are two distinct species of CCK receptors: (a) The CCK alpha receptors, present on the inhibitory neurons, are very discriminative and are critically dependent on SO4; and (b) the CCK beta receptors, present on the sphincter muscle, are not discriminative and are not critically dependent on SO4. Nonsulfated gastrin may share the CCK beta receptors with CCK.

Two brain cholecystokinin receptors: implications for behavioral actions

The distribution and relative specificity of cholecystokinin (CCK) receptors in the rat brain was mapped by in vitro autoradiography with [125I]CCK-33. We identified two distinct binding patterns, suggesting two CCK receptor types. The first is widespread and relatively non-specific. The second, localized to a few subcortical nuclei, has the specificity demonstrated for pancreatic CCK receptors. Localization of this receptor type to the area postrema provides a possible entry site into brain for circulating CCK that would distinguish between CCK and gastrin and could mediate some of CCK's behavioral effects.

Cholecystokinin antagonists

The three classes of CCK antagonists illustrate the various factors governing affinity of the antagonists for the CCK receptor. The major influence in determining potency of the cyclic nucleotide derivatives, amino acid derivatives and C-terminal fragments of CCK, are hydrophobic forces. In contrast, structural requirements are the major influences in determining potency of the N-terminal fragments of CCK-26-33. The most potent CCK antagonist in each of the three classes is illustrated in Fig. 11. CBZ-CCK-27-32-NH2 is 30 times more potent than N-CBZ-cystine, which is, in turn, slightly more potent than Bt2 cGMP. All these CCK antagonists, however, are relatively weak. For example, CBZ-CCK-27-32-NH2 inhibits binding of 125I-CCK by 50% at a concentration of approximately 5 microM. In contrast, the agonist CCK-26-33 inhibits binding of 125I-CCK by 50% at a concentration of approximately 1 nM. The antagonists remain useful for analyzing those responses that are caused by CCK, though the relatively low potencies of the antagonists may limit their usefulness as antagonists of CCK in vivo systems. This limitation, however, may be only theoretical. For example, proglumide, which requires an in vitro concentration of 0.3 mM to cause half-maximal inhibition of binding of 125I-CCK, can inhibit the actions of CCK and gastrin in animals (Hutchison and Dockray 1980; Stubbs and Stabile 1985) and of gastrin in man (Lamers and Jansen 1983). Nevertheless, the identification of CCK antagonists with greater potency than those hitherto described will facilitate studies of the actions of CCK.

Role of cholecystokinin and opioid peptides in control of food intake

Of the many factors that influence food intake, there is strong evidence that opioid and CCK peptides, which stimulate feeding and elicit satiety, respectively, are important components that may act in concert to regulate energy balance. Cholecystokinin peptides have been isolated in both the brain and gastrointestinal tract, and changes in concentration in the brain and in plasma have been shown to vary with feeding. Peripherally injected CCK has been shown to elicit satiety in many species, including humans, an effect that may be mediated in the CNS via the vagus. In several species, most notably the sheep, direct injection into the CSF potently decreases food intake. Questions remaining regarding the role of CCK peptides in eliciting satiety include the sites and mechanisms of action. It is unknown whether CCK acts directly on receptors, indirectly on some other parameter, or as a neurotransmitter. Although opioid peptides have also been localized in portions of both the periphery and brain, a specific physiological role for their presence has not yet been determined. Opioid peptides from three families--endorphins, enkephalins, and dynorphins--have been shown to stimulate feeding in various species. They have been active at several opioid receptor types in the CNS, but there is limited evidence to suggest they affect food intake when administered peripherally. In contrast, peripheral injection of opiate antagonists has effectively decreased food intake, an observation that led to the original hypothesis that opioids were involved in the hunger component in the control of food intake and that excess concentrations might be involved in the development of obesity. An increasing body of evidence supports the concept that opioid and CCK peptides may interact to control food intake, but the evidence is more suggestive than conclusive.

[3H] Boc [Nle28, 31]CCK27-33, a new highly labelled ligand for CCK receptors: binding on brain and on pancreas

Preliminary results on the binding of [3H]Boc[Nle28, 31]CCK27-33, designated [3H]Boc[diNle]CCK7, on mouse brain and rat pancreas membranes are presented. This new ligand for CCK receptors possesses a high specific activity (144 Ci/mmole), and binds in a saturable manner to mouse brain (Kd = 0.49 nM, Bmax = 49 fmoles/mg protein) and rat pancreas (Kd = 4.4 nM, Bmax = 696 fmoles/mg protein). Unlabelled Boc[diNle]CCK7 displaces [125I]CCK8 from its binding sites on mouse brain membranes with a high affinity, slightly superior to that of CCK8. The order of potencies to displace [3H]Boc[diNle]CCK7 from its binding sites was the same in mouse brain and rat pancreas: [3H]Boc[diNle]CCK7 greater than CCK8, Boc-CCK7 greater than non-sulfated CCK8, the pancreas binding sites being more discriminative than the brain binding sites. Thus, [3H]Boc[diNle]CCK7 is a very promising new probe for the characterization of CCK receptors and their interaction with different CCK fragments.

Gastrin receptors in normal and malignant gastrointestinal mucosa: age-associated changes

Synthetic human gastrin 17-I (MG) and an analogue, [Leu15]gastrin-17-I (LG), were radiolabeled with Na125I by Iodo-Gen, EnzymoBead, and chloramine-T methods, and the characteristics of the radiolabeled peptides were determined. When 125I-MG was iodinated by chloramine-T, its biological activity and its binding activity were almost abolished, whereas the biological activity of 125I-LG, iodinated by either of the methods, and of 125I-MG, iodinated by Iodo-Gen and EnzymoBeads, was not significantly affected. The kinetics, affinity, and specificity of binding of 125I-MG, iodinated by Iodo-Gen, to crude and purified membranes from rat fundic mucosa were examined and found to be similar to that for 125I-LG. Age-associated changes in the number and affinity of gastrin receptors (GR) on the crude membranes of gastrointestinal mucosa of rats was also examined. Significantly fewer GR were observed on the crude membranes of fundic mucosa of aged (24 mo old) compared with young (3- and 6-mo-old) rats. In addition, specific gastrin-binding sites (4.7 +/- 0.9 fmol/mg prot) with low affinity (Kd = 3.7 +/- 1.2 nM) were observed in the antrum of aged rats, the significance of which is not understood. There were, however, no differences in the number and characteristics of GR in other regions of the intestine of old and young rats. The presence of GR was additionally assessed in cell lines of gastrointestinal cancers from humans, mice, and hamsters.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis and biological activities of some pseudo-peptide analogues of tetragastrin: the importance of the peptide backbone

Pseudo-peptide analogues of the C-terminal tetrapeptide of gastrin, in which a peptide bond has been replaced by a CH2-NH bond, i.e. (tert-butyloxycarbonyl)-L-tryptophyl-psi (CH2-NH)-L-leucyl-L-aspartyl-L-phenylalanine amide (8), (tert-butyloxycarbonyl)-L-tryptophyl-L-leucyl-psi (CH2-NH)-L-aspartyl-L-phenylalanine amide (13), (tert-butyloxycarbonyl)-L-tryptophyl-L-leucyl-L-aspartyl-psi (CH2NH)-L-phenylalanine amide (20), were synthesized. The pseudo-peptides 8 and 13 were shown to have the same affinity as (tert-butyloxycarbonyl)-L-tryptophyl-L-leucyl-L-aspartyl-L-phenylalanine amide (21) for the gastrin receptor on isolated mucosal cells. The pseudo-peptide 20 exhibited lower affinity (IC50 congruent to 10(-5) M). The biological activity of these pseudo-peptides was studied on acid secretion in the anesthetized rat. Compound 8 stimulated acid secretion, identically with that of 21. Compound 13 did not exhibit any agonist activity but was able to antagonize the action of gastrin (ED50 = 0.3 mg/kg). Compound 20 did not show any agonist activity but was able to inhibit gastrin-induced acid secretion, with lower potency (ED50 = 15 mg/kg). The importance of the peptide bonds in the mode of action of gastrin is discussed, and a hypothetical approach of the mechanism of action is presented.

Asperlicin, a novel non-peptidal cholecystokinin antagonist from Aspergillus alliaceus. Fermentation, isolation and biological properties

The fermentation and isolation of a new, non-peptide cholecystokinin antagonist, asperlicin, produced by Aspergillus alliaceus is described. The potent and specific interaction of asperlicin with cholecystokinin receptors was shown using in vitro biochemical assays.

A potent nonpeptide cholecystokinin antagonist selective for peripheral tissues isolated from Aspergillus alliaceus

A new, competitive, nonpeptide cholecystokinin (CCK) antagonist, asperlicin, was isolated from the fungus Aspergillus alliaceus. The compound has 300 to 400 times the affinity for pancreatic, ileal, and gallbladder CCK receptors than proglumide, a standard agent of this class. Moreover, asperlicin is highly selective for peripheral CCK receptors relative to brain CCK and gastrin receptors. Since asperlicin also exhibits long-lasting CCK antagonist activity in vivo, it should provide a valuable tool for investigating the physiological and pharmacological actions of CCK.