Gastric somatostatin: a paracrine regulator of acid secretion

Pancreatic endocrine and exocrine signaling and crosstalk in physiological and pathological status

The pancreas, an organ with dual functions, regulates blood glucose levels through the endocrine system by secreting hormones such as insulin and glucagon. It also aids digestion through the exocrine system by secreting digestive enzymes. Complex interactions and signaling mechanisms between the endocrine and exocrine functions of the pancreas play a crucial role in maintaining metabolic homeostasis and overall health. Compelling evidence indicates direct and indirect crosstalk between the endocrine and exocrine parts, influencing the development of diseases affecting both. From a developmental perspective, the exocrine and endocrine parts share the same origin-the "tip-trunk" domain. In certain circumstances, pancreatic exocrine cells may transdifferentiate into endocrine-like cells, such as insulin-secreting cells. Additionally, several pancreatic diseases, including pancreatic cancer, pancreatitis, and diabetes, exhibit potential relevance to both endocrine and exocrine functions. Endocrine cells may communicate with exocrine cells directly through cytokines or indirectly by regulating the immune microenvironment. This crosstalk affects the onset and progression of these diseases. This review summarizes the history and milestones of findings related to the exocrine and endocrine pancreas, their embryonic development, phenotypic transformations, signaling roles in health and disease, the endocrine-exocrine crosstalk from the perspective of diseases, and potential therapeutic targets. Elucidating the regulatory mechanisms of pancreatic endocrine and exocrine signaling and provide novel insights for the understanding and treatment of diseases.

The cckOMA syndrome and its relation to the Zollinger-Ellison syndrome: a diagnostic challenge

OBJECTIVE

Among patients with enteropancreatic neuroendocrine tumor syndromes only one case with a cholecystokinin (CCK) secreting tumor has been reported. She had significant hyperCCKemia leading to a specific syndrome of severe diarrheas, weight loss, repeated duodenal ulcers and a permanently contracted gallbladder with gallstones. There are, however, reasons to believe that further CCKomas exist, for instance among Zollinger-Ellison patients with normal plasma gastrin concentrations. The present review is a call to gastroenterologists for awareness of such CCKoma patients.

METHOD

After a short case report, the normal endocrine and oncological biology of CCK is described. Subsequently, the CCKoma symptoms are discussed with particular reference to the partly overlapping symptoms of the Zollinger-Ellison syndrome. In this context, the diagnostic use of truly specific CCK and gastrin assays are emphasized. The discussion also entails the problem of access to accurate CCK measurements.

CONCLUSION

Obviously, the clinical awareness about the CCKoma syndrome is limited. Moreover, it is also likely that the knowledge about the necessary specificity demands of diagnostic gastrin and CCK assays have obscured proper diagnosis of the CCKoma syndromes in man.

Reassessment of SST4 Somatostatin Receptor Expression Using SST4-eGFP Knockin Mice and the Novel Rabbit Monoclonal Anti-Human SST4 Antibody 7H49L61

Among the five somatostatin receptors (SST1–SST5), SST4 is the least characterized, which is in part due to the lack of specific monoclonal antibodies. We generated a knockin mouse model that expresses a carboxyl-terminal SST4-eGFP fusion protein. In addition, we extensively characterized the novel rabbit monoclonal anti-human SST4 antibody 7H49L61 using transfected cells and receptor-expressing tissues. 7H49L61 was then subjected to immunohistochemical staining of a series of formalin-fixed, paraffin-embedded normal and neoplastic human tissues. Characterization of SST4-eGFP mice revealed prominent SST4 expression in cortical pyramidal cells and trigeminal ganglion cells. In the human cortex, 7H49L61 disclosed a virtually identical staining pattern. Specificity of 7H49L61 was demonstrated by detection of a broad band migrating at 50–60 kDa in immunoblots. Tissue immunostaining was abolished by preadsorption of 7H49L61 with its immunizing peptide. In the subsequent immunohistochemical study, 7H49L61 yielded a predominant plasma membrane staining in adrenal cortex, exocrine pancreas, and placenta. SST4 was also found in glioblastomas, parathyroid adenomas, gastric and pancreatic adenocarcinomas, pheochromocytomas, and lymphomas. Altogether, we provide the first unequivocal localization of SST4 in normal and neoplastic human tissues. The monoclonal antibody 7H49L61 may also prove of great value for identifying SST4-expressing tumors during routine histopathological examinations.

Effects of pirenzepine on vonoprazan-induced gastric acid inhibition and hypergastrinemia

BACKGROUND

Compared to proton pump inhibitors, vonoprazan exerts a greater inhibitory effect on gastric acid secretion and is useful for treating acid-related diseases, such as gastro-esophageal reflux disease. However, there is a problem that vonoprazan causes hypergastrinemia, which confers a risk of carcinoid tumor. A previous report demonstrated that pirenzepine, an M1 muscarinic receptor antagonist, enhances the acid inhibitory effects while suppressing hypergastrinemia induced by omeprazole. Here, we examined whether pirenzepine enhances the gastric acid inhibitory effects of vonoprazan without further increasing serum gastrin levels.

METHODS

Eleven healthy volunteers were subjected to 24-h intragastric pH monitoring and serum gastrin measurements on day 7 of three different regimens: pirenzepine 75 mg alone, vonoprazan 10 mg alone, and vonoprazan 10 mg plus pirenzepine 75 mg administered in a randomized crossover fashion.

RESULTS

Median pH 4 holding time ratios (range) achieved with pirenzepine 75 mg, vonoprazan 10 mg, and vonoprazan 10 mg plus pirenzepine 75 mg were 6.9% (2.4-32.8%), 88.4% (54.6-100%), and 84.2% (40.3-100%), respectively. Respective serum gastrin levels were 79 (75-210) pg/ml, 310 (110-870) pg/ml, and 170 (140-930) pg/ml. In cases with hypergastrinemia (gastrin ≥ 200 pg/ml) induced by vonoprazan 10 mg alone, concomitant treatment with pirenzepine significantly reduced serum gastrin levels from 370 to 180 pg/ml (P = 0.028).

CONCLUSION

Although pirenzepine does not enhance acid inhibition, it does improve hypergastrinemia induced by vonoprazan to some extent.

Gastric Peptides-Gastrin and Somatostatin

Gastric acid secretion (i) facilitates digestion of protein as well as absorption of micronutrients and certain medications, (ii) kills ingested microorganisms, including Helicobacter pylori, and (iii) prevents bacterial overgrowth and enteric infection. The principal regulators of acid secretion are the gastric peptides gastrin and somatostatin. Gastrin, the major hormonal stimulant for acid secretion, is synthesized in pyloric mucosal G cells as a 101-amino acid precursor (preprogastrin) that is processed to yield biologically active amidated gastrin-17 and gastrin-34. The C-terminal active site of gastrin (Trp-Met-Asp-Phe-NH₂ ) binds to gastrin/CCK₂ receptors on parietal and, more importantly, histamine-containing enterochromaffin-like (ECL) cells, located in oxyntic mucosa, to induce acid secretion. Histamine diffuses to the neighboring parietal cells where it binds to histamine H₂ -receptors coupled to hydrochloric acid secretion. Gastrin is also a trophic hormone that maintains the integrity of gastric mucosa, induces proliferation of parietal and ECL cells, and is thought to play a role in carcinogenesis. Somatostatin, present in D cells of the gastric pyloric and oxyntic mucosa, is the main inhibitor of acid secretion, particularly during the interdigestive period. Somatostatin exerts a tonic paracrine restraint on gastrin secretion from G cells, histamine secretion from ECL cells, and acid secretion from parietal cells. Removal of this restraint, for example by activation of cholinergic neurons during ingestion of food, initiates and maximizes acid secretion. Knowledge regarding the structure and function of gastrin, somatostatin, and their respective receptors is providing novel avenues to better diagnose and manage acid-peptic disorders and certain cancers. Published 2020. Compr Physiol 10:197-228, 2020.

Neuropods

Enteroendocrine cells (EECs) are sensory cells of the gastrointestinal tract. Most EECs reside in the mucosal lining of the stomach or intestine and sense food in the gut lumen. Food signals stimulate the release of hormones into the paracellular space where they either act locally or are taken up into the blood and circulate to distant organs. It recently was recognized that many EECs possess basal processes known as neuropods that not only contain hormones but also connect to nerves. This review describes how neuropods contribute to EEC function beyond typical hormonal actions. For example, gastrointestinal hormones not only act on distant organs, but, through neuropods, some act locally to stimulate other mucosal cells such as intestinal stem cells, enterocytes, or other EECs. With the recent discovery that EECs communicate directly with enteric nerves, EECs not only have the ability to sense food and bacteria in the gastrointestinal tract, but can communicate these signals directly to the nervous system.

Transcriptional and Functional Characterization of the G Protein-Coupled Receptor Repertoire of Gastric Somatostatin Cells

In the stomach, somatostatin (SST) acts as a general paracrine negative regulator of exocrine secretion of gastric acid and pepsinogen and endocrine secretion of gastrin, ghrelin, and histamine. Using reporter mice expressing red fluorescent protein (RFP) under control of the SST promotor, we have characterized the G protein-coupled receptors expressed in gastric Sst-RFP-positive cells and probed their effects on SST secretion in primary cell cultures. Surprisingly, besides SST, amylin and PYY were also highly enriched in the SST cells. Several receptors found to regulate SST secretion were highly expressed and/or enriched. 1) The metabolite receptors calcium-sensing receptor and free fatty acid receptor 4 (GPR120) functioned as positive and negative regulators, respectively. 2) Among the neurotransmitter receptors, adrenergic receptors α1a, α2a, α2b, and β1 were all highly expressed, with norepinephrine and isoproterenol acting as positive regulators. The muscarinic receptor M3 acted as a positive regulator, whereas M4 was conceivably a negative regulator. 3) Of the hormone receptors, the GLP-1 and GIP receptors, CCKb (stimulated by both CCK and gastrin) and surprisingly the melanocortin MC1 receptor were all positive regulators. 4) The neuropeptide receptors for calcitonin gene-related peptide, adrenomedullin, and vasoactive intestinal peptide acted as positive regulators, no effect was observed using galanin and nociceptin although transcripts for the corresponding receptors appeared highly expressed. 5) The SST receptors 1 and 2 functioned in an autocrine negative feedback loop. Thus, the article provides a comprehensive map of receptors through which SST secretion is regulated by hormones, neurotransmitters, neuropeptides and metabolites that act directly on the SST cells in the gastric mucosa.

Gastric acid secretion: changes during a century

The advances in knowledge of gastric physiology within the past century have been the most exciting and important in this area of interest for many decades. The aim of this presentation consists of a comprehensive review of the extensive recent literature on this topic in order to highlight milestones in the field of gastric physiology, in particular in gastric acid secretion, gastric pathophysiology, acid-related diseases and use of acid regulatory drugs. Moreover, in the 21st century there have been many epidemiologic changes as well as a decrease of Helicobacter pylori infection and gastric cancer together with an increase of gastroesophageal reflux disease and the related increase of pomp proton inhibitor wide use.

PREDICTING EFFICACY OF PROTON PUMP INHIBITORS IN REGULATING GASTRIC ACID SECRETION

Developing drugs to treat gastric acid related illnesses such as ulcers and acid reflux disease is the leading focus of pharmaceutical companies. In fact, expenditure for treating these disorders is highest among all illnesses in the US. Over the last few decades, a class of drugs known as a proton pump inhibitors (PPIs) appeared on the market and are highly effective at abating gastric illnesses by raising stomach pH (reducing gastric acid levels). While much is known about the action of PPIs , there are still open questions regarding their efficacy, dosing and long-term effects. Here we extend a previous gastric acid secretion model developed by our group to incorporate a pharmacodynamic/pharmacokinetic model to study proton pump inhibitor (PPI) action. Model-relevant parameters for specific drugs such as omeprazole (OPZ) , lansoprazole (LPZ) and pantoprazole (PPZ) were used from published data, and we conducted simulations to study various aspects of PPI treatment. Clinical data suggests that duration of acid suppression is dependent on proton pump turnover rates and this is supported by our model. We found the order of efficacy of the different PPIs to be OPZ>PPZ>LPZ for clinically recommended dose values, and OPZ>PPZ=LPZ for equal doses. Our results indicate that a breakfast dose for once-daily dosing regimens and a breakfast-lunch dose for twice-daily dosing regimens is recommended. Simulation of other gastric disorders using our model provides atypical applications for the study of drug treatment on homeostatic systems and identification of potential side-effects.

Control of gastric acid secretion in health and disease

Recent milestones in the understanding of gastric acid secretion and treatment of acid-peptic disorders include the (1) discovery of histamine H(2)-receptors and development of histamine H(2)-receptor antagonists, (2) identification of H(+)K(+)-ATPase as the parietal cell proton pump and development of proton pump inhibitors, and (3) identification of Helicobacter pylori as the major cause of duodenal ulcer and development of effective eradication regimens. This review emphasizes the importance and relevance of gastric acid secretion and its regulation in health and disease. We review the physiology and pathophysiology of acid secretion as well as evidence regarding its inhibition in the management of acid-related clinical conditions.

Long slender cytoplasmic extensions: a common feature of neuroendocrine cells?

We have recently developed a new method for visualisation of gut mucosal cells and demonstrated that enterochromaffin (EC) and enterochromaffin-like (ECL) cells possess cytoplasmic extensions. The aim of the present study was to characterise the morphology of D- and G-cells. The D-cells in the stomach differed morphologically from intestinal D-cells, suggesting two distinct subpopulations of D-cells. Some D-cells appeared to be interconnected. No cell-to-cell contact between parietal and D-cells was found. Both D- and G-cells possessed long cytoplasmic extensions corresponding with our previous descriptions of EC and ECL cells. We propose that all neuroendocrine cells have the ability to develop cytoplasmic extensions, enabling them to signal to their target cells in a neurocrine manner.

Somatostatin-receptor 2 (sst2)-mediated effects of endogenous somatostatin on exocrine and endocrine secretion of the rat stomach

Somatostatin is a potent inhibitor of gastric acid secretion. Its effects are mediated through five specific receptor subtypes (sst(1-5)), of which sst(2) is dominant on the enterochromaffin-like (ECL) cell and the parietal cell. To study the paracrine mechanisms of somatostatin, the sst(2)-specific antagonist PRL-2903 was used. Effects of PRL-2903 on acid secretion and release of histamine were studied in the totally isolated, vascularly perfused rat stomach. Further, the release of histamine and gastrin after bombesin, alone and in combination with PRL-2903, were studied. Results are presented as mean+/-standard error of the mean (s.e.m.). PRL-2903 concentration-dependently increased the venous histamine concentration from basal 55.6+/-7.5 to 367+/-114 nM at 50 microM PRL-2903. With 10 microM PRL-2903, venous histamine output increased from baseline 6.2+/-0.5 to 20.9+/-4.9 nmol h(-1); P=0.008. The combination of 520 pM gastrin and 10 microM PRL-2903 increased venous histamine output from 41.7+/-7.3 nmol h(-1) with gastrin alone to 95.2+/-9.8 nmol h(-1); P=0.016. Further, 10 microM PRL-2903 increased acid output from baseline 8.5+/-1.8 to 37.4+/-11 micromol h(-1); P=0.017. When combined with 10 microM ranitidine, PRL-2903 did not significantly stimulate acid secretion. Bombesin/PRL-2903 increased venous histamine concentration from 50.4+/-14.8 to 292+/-64.2 nM; P=0.008, and gastrin concentration from 38.6+/-13.1 to 95.8+/-20.3 pM; P=0.037. Endogenous somatostatin exerts a continuous restraint on histamine and gastrin release from the gastric mucosa and significantly reduces baseline acid secretion.

Preventive activity of pyrrolizidine alkaloids from Seneciobrasiliensis (Asteraceae) on gastric and duodenal induced ulcer on mice and rats

The alkaloid extract of Senecio brasiliensis inflorescences contain a mixture of the pyrrolizidine alkaloids (PA) senecionine, integerrimine, retrorsine, usaramine and seneciphylline. We evaluated this PA mixture on preventive antiulcerogenic effects on standard rodent models of induced gastric and duodenal ulcers. In the HCl/ethanol, indomethacin-bethanechol and hypothermic-restraint-induced gastric ulcer, the lesion was significantly inhibited by PA (p.o.) (p < 0.001). In the pylorus-ligature, PA (i.d.), significantly increased the gastric juice content and the pH values and decreased the acid output. In the cysteamine induced duodenal ulcers, PA (p.o.) showed significant inhibition (p < 0.001) of the duodenal lesions when compared to the respective control. The levels of the somatostatin hormone in the blood samples of animals pre-treated with the PA (12.5 mg/kg) and the free mucus and prostaglandin synthesis also increased (p < 0.001) after administration of PA extract (p.o.). The results suggested that the PA extract from Senecio brasiliensis inflorescences presents a significant anti-ulcer effect in the selected ulcer models. The mechanism involved with the action of the PA extract is the cytoprotection. Additional studies are in progress to determine other possible mechanisms involved with effect of the PA as anti-ulcer agents.

A model for the study of Helicobacter pylori interaction with human gastric acid secretion

We present a comprehensive mathematical model describing Helicobacter pylori interaction with the human gastric acid secretion system. We use the model to explore host and bacterial conditions that allow persistent infection to develop and be maintained. Our results show that upon colonization, there is a transient period (day 1-20 post-infection) prior to the establishment of persistence. During this period, changes to host gastric physiology occur including elevations in positive effectors of acid secretion (such as gastrin and histamine). This is promoted by reduced somatostatin levels, an inhibitor of acid release. We suggest that these changes comprise compensatory mechanisms aimed at restoring acid to pre-infection levels. We also show that ammonia produced by bacteria sufficiently buffers acid promoting bacteria survival and growth.

Ghrelin stimulates gastric emptying but is without effect on acid secretion and gastric endocrine cells

Ghrelin, a recently discovered peptide hormone, is produced by endocrine cells in the stomach, the so-called A-like cells. Ghrelin binds to the growth hormone (GH) secretagogue receptor and releases GH. It is claimed to be orexigenic and to control gastric acid secretion and gastric motility. In this study, we examined the effects of ghrelin, des-Gln14-ghrelin, des-octanoyl ghrelin, ghrelin-18, -10 and -5 (and motilin) on gastric emptying in mice and on gastric acid secretion in chronic fistula rats and pylorus-ligated rats. We also examined whether ghrelin affected the activity of the predominant gastric endocrine cell populations, G cells, ECL cells and D cells. Ghrelin and des-Gln14-ghrelin stimulated gastric emptying in a dose-dependent manner while des-octanoyl ghrelin and motilin were without effect. The C-terminally truncated ghrelin fragments were effective but much less potent than ghrelin itself. Ghrelin, des-Gln14-ghrelin and des-octanoyl ghrelin neither stimulated nor inhibited gastric acid secretion, and ghrelin, finally, did not affect secretion from either G cells, ECL cells or D cells.

Cholecystokinin inhibits gastrin secretion independently of paracrine somatostatin secretion in the pig

BACKGROUND

Cholecystokinin inhibits the secretion of gastrin from antral G cells, an effect that is speculated to be mediated by D cells secreting somatostatin. The aim of the study was to test directly whether cholecystokinin inhibition of antral gastrin secretion is mediated by somatostatin.

METHODS

The effects of CCK on gastrin and somatostatin secretion were studied in isolated vascularly perfused preparations of pig antrum before and after immunoneutralization brought about by infusion of large amounts of a high affinity monoclonal antibody against somatostatin.

RESULTS

CCK infusion at 10(-9) M and 10(-8) M decreased gastrin output to 70.5% +/- 7.6% (n = 8) and 76.3% +/- 3.6% (n = 7) of basal output, respectively. CCK at 10(-10) M had no effect (n = 6). Somatostatin secretion was dose-dependently increased by CCK infusion and increased to 268 +/- 38.2% (n = 7) of basal secretion during infusion of CCK at 10(-8) M. Immunoneutralization of somatostatin caused a doubling of the basal secretion of gastrin, but did not affect the CCK-induced decrease in gastrin secretion.

CONCLUSION

CCK inhibits gastrin secretion independently of paracrine somatostatin secretion.

Altered control of gastric acid secretion in gastrin-cholecystokinin double mutant mice

BACKGROUND & AIMS

Three pathways control gastric acid secretion: the gastrin-enterochromaffin-like (ECL) cell axis, the vagus-parietal cell axis, and the cholecystokinin (CCK)-D cell axis. Mice lacking gastrin or both gastrin and CCK were examined to determine the role of the hormones.

METHODS

Acid was measured after pylorus ligation, and biopsies from gastrin knockout (KO), gastrin-CCK double-KO, and wild-type (WT) mice were collected for biochemical, immunocytochemical, and electron-microscopic examination.

RESULTS

The ECL cells were inactive in both groups of mutant mice but the cell number was unaffected. Both parietal cell number and level of H(+)/K(+)-ATPase messenger RNA (mRNA) were reduced in the mutant strains, but gastrin-CCK double-KO mice displayed more active parietal cells and larger acid output than the gastrin KO mice. The acid response to histamine in double-KO mice was unchanged whereas that to gastrin was diminished, but it could be restored by infusion of gastrin. Oxyntic D-cell density was the same in both mutant strains, but the D cells were more active in the gastrin KO than in the double-KO mice. CCK infusion in gastrin-CCK double-KO mice raised the somatostatin mRNA level and inhibited acid secretion to the level seen in gastrin KO mice. Vagotomy and atropine abolished acid secretion in all 3 groups of mice.

CONCLUSIONS

Lack of gastrin impairs the gastrin-ECL axis, whereas lack of gastrin and CCK impairs both hormonal pathways. In the gastrin-CCK double-KO mice, acid secretion is only controlled by cholinergic vagal stimulation, which normalizes the acid output.

A model for integrative study of human gastric acid secretion

We have developed a unique virtual human model of gastric acid secretion and its regulation in which food provides a driving force. Food stimulus triggers neural activity in central and enteric nervous systems and G cells to release gastrin, a critical stimulatory hormone. Gastrin stimulates enterochromaffin-like cells to release histamine, which, together with acetylcholine, stimulates acid secretion from parietal cells. Secretion of somatostatin from antral and corpus D cells comprises a negative-feedback loop. We demonstrate that although acid levels are most sensitive to food and nervous system inputs, somatostatin-associated interactions are also important in governing acidity. The importance of gastrin in acid secretion is greatest at the level of transport between the antral and corpus regions. Our model can be applied to study conditions that are not yet experimentally reproducible. For example, we are able to preferentially deplete antral or corpus somatostatin. Depletion of antral somatostatin exhibits a more significant elevation of acid release than depletion of corpus somatostatin. This increase in acid release is likely due to elevated gastrin levels. Prolonged hypergastrinemia has significant effects in the long term (5 days) by promoting enterochromaffin-like cell overgrowth. Our results may be useful in the design of therapeutic strategies for acid secretory dysfunctions such as hyper- and hypochlorhydria.

Pharmacological control of gastric acid secretion for the treatment of acid-related peptic disease: past, present, and future

Pharmacological agents, such as histamine H(2) receptor antagonists and acid pump inhibitors, are now the most frequently used treatment for such acid-related diseases as gastroduodenal ulcers and reflux esophagitis. Based on increased understanding of the precise mechanisms of gastric acid secretion at the level of receptors, enzymes, and cytoplasmic signal transduction systems, further possibilities exist for the development of effective antisecretory pharmacotherapy. Gastrin CCK(2) receptor antagonists and locally active agents appear to represent promising therapies for the future. Development of gene targeting techniques has allowed production of genetically engineered transgenic and knockout mice. Such genetic technology has increased the investigative power for pharmacotherapy for not only antisecretory agents, but also treatment of mucosal diseases, such as atrophy, hyperplasia, and cancer. Elucidation of the origin of gastric parietal cells also represents an interesting investigative target that should allow a better understanding of not only acid-related diseases, but also the evolution of the stomach as an acid-secreting organ.

Morphological support for paracrine inhibition of gastric acid secretion by nitric oxide in humans

BACKGROUND

Functional studies have shown that nitric oxide (NO) inhibits gastric acid secretion in a variety of species, including man. We have performed a morphological study with the intention of localizing the endothelial NO synthase (eNOS) in the human gastric mucosa.

METHODS

Fifteen healthy subjects voluntarily participated in the study, and mucosal biopsies were obtained from the cardia, corpus and antrum. The presence and localization of eNOS were studied using immunohistochemical techniques.

RESULTS

eNOS-immunoreactivity (eNOS-IR) is found in surface mucous cells of cardia, corpus and antrum. Unique to the oxyntic mucosa is the presence of eNOS-IR in 'endocrine-like' cells, found in close contact with parietal cells.

CONCLUSIONS

eNOS-IR cells in close apposition to parietal cells provide morphological support for paracrine inhibition of gastric acid secretion by NO.

Neurotransmitters regulating acid secretion in the proventriculus of the Houbara bustard (Chlamydotis undulata): a morphological viewpoint

Endocrine cells containing somatostatin (Som), gastrin-releasing peptide (GRP), and neuronal nitric oxide synthase (nNOS) and nerve fibers containing choline acetyl transferase (ChAT), tyrosine hydroxylase (TH), galanin (Gal), substance P (SP), and vasoactive intestinal polypeptide (VIP) were immunolocalized in the proventriculus of the Houbara bustard, Chlamydotis undulata. While GRP-immunoreactive (GRP-IR) cells occur in the inner zone, somatostatin (Som-IR) and polyclonal nNOS (nNOS-IR) immunoreactive cells were localized mainly in the peripheral zone of submucosal glands. GRP-IR, Som-IR, and nNOS-IR cells were occasionally observed in the walls of the gastric glands. Endocrine cells are of the closed variety and usually possess apical processes extending along the basal surfaces of adjacent nonreactive cells. Ultrastructural features of these cells are typical. ChAT, Gal, SP, VIP, and TH were immunolocalized in nerve fibers and terminals in the walls of arterioles and capillaries at the periphery of submucosal glands. Immunoreactivity to monoclonal nNOS occurred mainly in neuronal cell bodies in ganglia located around the submucosal glands. ChAT and TH immunoreactive cell bodies were also occasionally seen around the submucosal glands in the peripheral region. Immunoreactivity to Gal, SP, and VIP, but not ChAT or TH, was discernible around the walls of gastric glands. It was concluded that the distribution of neurotransmitters in neuronal structures is similar, but that of the endocrine cells varies from that of some avian species. The roles of these neurotransmitters in the regulation of acid secretion are discussed.

Urethane-induced somatostatin mediated inhibition of gastric acid: reversal by the somatostatin 2 receptor antagonist, PRL-2903

Urethane increases the release of somatostatin (SRIF) which inhibits gastric acid secretion. The SRIF monoclonal antibody, CURE.S6 and the novel sst2 antagonist, PRL-2903 injected intravenously at maximal effective doses increased gastric acid secretion by 2 and 10 fold respectively from basal values within 30 min in urethane-anesthetized rats. Plasma gastrin levels were elevated 2.5 fold within 15 min by PRL-2903 (1.3 micromol/kg, i.v.). These data indicate that the low gastrin and acid secretion levels induced by urethane result from endogenous SRIF acting on sst2 and that PRL-2903 is a valuable SRIF antagonist to assess sst2 mediated events.

High basal gastric acid secretion in somatostatin receptor subtype 2 knockout mice

BACKGROUND & AIMS

Somatostatin receptor subtype 2 (sst2) agonists inhibit gastric secretion. The role of sst2 in the regulation of acid secretion was assessed using sst2 knockout mice and urethane to induce somatostatin release.

METHODS

Acid secretion was monitored every 10 minutes by gastric perfusion and backtitration of perfusates in fasted, urethane-anesthetized C57/129 sst2 (-/-) mice and wild-type (+/+) mice. The ileal vein was cannulated for drug injection. Intragastric pH and serum gastrin were monitored 1 hour after anesthesia without perfusion.

RESULTS

Gastric pH values were lower in sst2 (-/-) mice (3.8 +/- 0.3) than in wild-type mice (7.1 +/- 0.1, P < 0.05), and there was no difference in gastrin levels. Basal acid output per 2 hours was 10-fold higher in sst2 knockout mice compared with wild-type mice. The gastrin antibody abolished the high basal acid secretion in sst2 (-/-) mice and had no effect in wild-type mice. The somatostatin antibody increased basal secretion by 4-fold in wild-type and had no effect in knockout mice. Somatostatin 14 or the sst2 agonist DC 32-87 inhibited pentagastrin-stimulated acid secretion in wild-type mice, but did not alter basal secretion in knockout mice.

CONCLUSIONS

These results indicate that sst2 is the main subtype whereby endogenous somatostatin suppresses gastric acid secretion through inhibition of gastrin action.

Effects of lanreotide, a somatostatin analogue, on postprandial gastric functions and biliopancreatic secretions in humans

AIMS

Lanreotide is a novel synthetic somatostatin analogue. A long-acting formulation of lanreotide has been shown to be effective for the treatment of gastroentero-pancreatic hormone-producing tumours but effects on postprandial digestive and absorptive functions remain obscure. The aim of the present study was to evaluate the effects of intravenous lanreotide on gastric and biliopancreatic secretions in man as well as the absorption of nutrients and the duodeno-caecal transit time after ingestion of an homogenized meal (500 kcal, 55% carbohydrates, 15% proteins, 30% lipids).

METHODS

Eight healthy male volunteers were studied on two occasions within a 2 weeks interval, using a perfusion method. They received in single-blind and random order continuous i.v. infusion of either placebo or lanreotide (100 micrograms h-t after a bolus of 100 micrograms 15 min before the beginning of the study).

RESULTS

Lanreotide significantly decreased gastric acid secretion (90%) for the initial 3 h period. Gastric emptying was not significantly modified by lanreotide infusion. Compared with placebo, lanreotide almost completely abolished both bile salts and lipase responses to the meal. It largely increased the duodeno-caecal transit time and decreased significantly the duodenal absorption of carbohydrates and triglycerides.

CONCLUSIONS

Since lanreotide has powerful effects on gastrointestinal functions, it could be useful in the prevention or in the treatment of pancreatic and bowel fistulas as well as short bowel syndrome.

Passive immunization against somatostatin increases resistance to Eimeria vermiformis infection in susceptible mice

The effect of in vivo immunoneutralization of somatostatin (SRIF) on Eimeria vermiformis intestinal infection was studied in resistant (BALB/c), and susceptible (C57BL/6) mouse strains. An anti-SRIF monoclonal antibody (MAb-SRIF) was used to passively immunize the mice by intraperitoneal injection. The animals were subsequently orally infected with oocysts of E. vermiformis. Individual fecal samples were collected daily for 21 days to monitor the kinetics of oocyst shedding. The fecal oocyst shedding was significantly higher in the C57BL/6 strain than in the BALB/c strain (P < 0.01). Passive immunization with MAb-SRIF in the C57BL/6 mice significantly reduced the number of oocysts in feces (P < 0.05), when compared to the infected non-immunized mice of the same strain. Infected BALB/c mice showed no difference in oocyst shedding in response to the passive immunoneutralization with MAb-SRIF. In conclusion, passive immunization with MAb-SRIF increased resistance to E. vermiformis-infection in the susceptible C57BL/6 mice, but not in the resistant BALB/c mice. This suggests that SRIF modulates gut immune function in parasitic infection.

Effect of cold-restraint stress on immunoreactive thyrotropin-releasing hormone and immunoreactive somatostatin in the rat stomach

The effects of cold-restraint stress on immunoreactive thyrotropin-releasing hormone (ir-TRH) and immunoreactive somatostatin (ir-SOM) concentrations in the rat stomach were investigated. Rats immobilized with a spring-loaded metallic plate were placed in a room maintained at 4 degrees C for 1-3 h and then decapitated serially for investigation. Gastric ir-TRH and ir-SOM concentrations were measured by individual radioimmunoassays. Cold-restraint stress induced gastric mucosal lesions as well as a decrease of the ir-TRH concentration in the glandular stomach, an increase of the ir-TRH concentration in the gastric juice, and a decrease in gastric pH. In contrast, this stress caused an increase of ir-SOM in the glandular stomach and a decrease of ir-SOM in the gastric juice. However, cold or restraint stress alone did not induce gastric mucosal lesions or changes in gastric ir-TRH and ir-SOM concentrations or the gastric pH. To clarify the endocrine influence of peripheral TRH, pretreatment with thyroid hormone was performed to inhibit elevation of the serum TRH level during cold-restraint stress. Despite this pretreatment, cold-restraint stress still induced ulcer formation, along with changes in gastric ir-TRH and ir-SOM concentrations and gastric pH. These findings suggest that changes in gastric ir-TRH and ir-SOM concentrations may be closely related to ulcer formation due to cold-restraint, and that TRH may act in a paracrine manner in the stomach.

Comparative effects of GLP-1-(7-36) amide, oxyntomodulin and glucagon on rabbit gastric parietal cell function

We have investigated in vitro, the effects of glucagon-like peptide-1-(7-36) amide (GLP-1-(7-36) amide), oxyntomodulin and glucagon on two rabbit parietal cell-enriched fractions (F3, F3n), with parietal cell contents of 60% and 88%, respectively. Histamine (10(-5) M) stimulated [14C]aminopyrine accumulation to an amount of 850% in excess of the basal level, whereas GLP-1-(7-36) amide (10(-7) M) and oxyntomodulin (10(-6) M) induced increases of 50% and 30%, respectively. With a histamine concentration of 10(-6) M, [14C]aminopyrine accumulation was stimulated to 498% in excess of the basal level; GLP-1-(7-36) amide (10(-7) M) and oxyntomodulin (10(-7) M) induced increases of 18% and 15%, respectively. With these parameters, oxyntomodulin[19-37] and glucagon were without effect. Specific binding of [125I]GLP-1-(7-36) amide to parietal cell plasma membranes was inhibited dose-dependently by GLP-1-(7-36) amide, oxyntomodulin and glucagon with inhibitory concentrations of 0.25 nM, 65 nM and 800 nM, respectively. No specific binding of [125I]oxyntomodulin or [125I]glucagon was detectable. GLP-1-(7-36) amide receptor mRNA was only detected in parietal cell-enriched fractions. GLP-1-(7-36) amide, oxyntomodulin and glucagon stimulated parietal cell cAMP production to similar maximal levels with median values close to 0.28 nM, 10.5 nM and 331.7 nM, whereas oxyntomodulin[19-37] had no effect. The maximal cAMP production induced by GLP-1-(7-36) amide, oxyntomodulin or glucagon was additive to that induced by histamine.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of hypochlorhydria and hypergastrinemia on structure and function of gastrointestinal cells. A review and analysis

Since hypochlorhydria can induce hypergastrinemia, and gastrin has a trophic effect on some gastrointestinal cells, states that cause elevated plasma gastrin levels are of interest in terms of effects on cell growth and function. This article reviews the relationship between gastric mucosal cells during periods of acid stimulation and inhibition and analyses the effects of hypochlorhydria and hypergastrinemia on gastric and colonic cells and tumors. Hypochlorhydria releases the inhibitory effect of antral gastrin cells, inducing them to release gastrin in the presence of peptides or amino acids in the gastric lumen or in response to antral distension. Gastrin stimulates the oxyntic mucosa, which may lead to hyperplasia of enterochromaffin-like cells, resulting in enterochromaffin-like carcinoid tumors in aged rats and, rarely, in patients with chronic atrophic gastritis or gastrinomas. In addition to hypergastrinemia, other factors appear to be required for the progression of enterochromaffin-like hyperplasia to carcinoids; genetic factors may be involved. Gastrin elevations due to antisecretory drug therapy are indirectly proportional to the degree of acid inhibition and are reversible upon cessation of therapy. The gastrin levels during omeprazole therapy are similar to those caused by gastric vagotomy. Available evidence does not support a relationship between hypergastrinemia and the occurrence or growth of gastric carcinoma or colonic tumors.

Cholecystokinin is a negative regulator of gastric acid secretion and postprandial release of gastrin in humans

BACKGROUND/AIMS

The role of cholecystokinin (CCK) in the regulation of gastric acid secretion is still controversial. This study examined the effect of the CCK-A receptor antagonist loxiglumide (lox) on gastrin- or CCK-induced gastric acid secretion and meal-stimulated plasma gastrin levels in a placebo-controlled study.

METHODS

Acid output was studied in eight subjects who received intravenously gastrin-17 (15, 30, and 60 pmol.kg-1.h-1); gastrin-17 plus lox; cholecystokinin octapeptide (CCK-8) (15, 30, and 60 pmol.kg-1.h-1); CCK-8 plus lox; or gastrin plus CCK-8. Sham feeding-induced acid output and meal-stimulated gastrin secretion were studied during lox infusion.

RESULTS

Gastrin-17 dose-dependently stimulated acid output to near-maximal levels. CCK-8 (15 pmol.kg-1.h-1) increased acid secretion 2.5-fold over basal; higher infusion rates had less or no effect. When combined with lox, CCK-8 produced a near-maximal acid response (6-fold over basal). CCK-8 together with gastrin-17 inhibited gastrin-induced acid output by 67%. Meal-stimulated plasma gastrin concentrations were elevated 3.2-fold, whereas sham feeding-induced acid secretion was not modified by lox.

CONCLUSIONS

Blockade of CCK-A receptors converts CCK-8 into a potent acid secretagogue and augments postprandial gastrin secretion. A CCK-mediated stimulation of paracrine somatostatin secretion from antral and fundic D cells represents a candidate mechanism for the inhibition of the parietal and gastrin cell in humans.

Expression of somatostatin receptors in normal, inflamed, and neoplastic human gastrointestinal tissues

The multiple actions of somatostatin are mediated by specific membrane-bound receptors present in all somatostatin target tissues, such as brain, pituitary, pancreas, gastrointestinal tract, and kidney. For instance, in the human gastrointestinal tract, three different types of tissue compartments express somatostatin receptors: the gastrointestinal mucosa, the peripheral nervous system, and the gut-associated lymphoid tissue, where the receptors are preferentially located in germinal centers. In all these cases, somatostatin binding is of high affinity and specific for bioactive somatostatin analogues. Somatostatin receptors are also expressed in pathological states, such as cancers. A particular abundance is found in neuroendocrine tumors of the gastrointestinal tract. Ninety percent of the carcinoids and a majority of islet cell carcinomas, including their metastases, usually have a high density of somatostatin receptors. Several different somatostatin-receptor subtypes can be expressed by these tumors, the SSTR2 subtype being the most frequently and abundantly expressed. The somatostatin receptors in tumors are identified with in vitro-binding methods, molecular biology techniques, or in vivo-imaging techniques; the latter allow the precise localization of the tumors and their metastases in the patients. Because somatostatin receptors in human gastroenteropancreatic tumors are functional, their identification can be used to predict the therapeutical efficacy of octreotide to inhibit excessive hormone release. Of differential diagnostic importance is the fact that other pathological processes in the gastrointestinal tract may be associated with a high density of somatostatin receptors. Ninety percent of lymphomas, including those with intestinal involvement express somatostatin receptors. Furthermore, a moderate number of colorectal carcinomas contain somatostatin receptors, whereas exocrine pancreatic carcinomas do not. Finally, an increased expression of SS receptors in nonneoplastic conditions, such as in intestinal veins in inflammatory bowel disease, has been recently observed. These observations demonstrate the ability of the human body to regulate SS receptors in a wide number of tissues and conditions.

Effect of CGRP antagonist, alpha-CGRP 8-37, on acid secretion in the dog

The recently synthesized calcitonin gene-related peptide (CGRP) antagonist, human alpha-CGRP 8-37, was used to study its effects on gastric acid secretion. Four dogs with gastric fistula were used to measure the antagonist's physiologic effects in the stomach. All dogs received a bactopeptone dextrose meal (intragastric titration to pH 5.5) with either continuous CGRP 8-37 (1000 pmol/kg/hr) or saline (control). Additionally, intravenous bombesin (75-600 ng/kg/hr) and bethanechol (12.5-100 micrograms/kg/hr) was tested in the presence of the antagonist. Plasma gastrin levels also were measured via radioimmunoassay (RIA) in control and CGRP 8-37-stimulated animals. Gastric acid secretion increased by 100% with infusion of 1000 pmol/kg/hr CGRP 8-37 when compared to the control. Acid output increased 98% with both intravenous antagonist and 600 ng/kg/hr bombesin when compared to bombesin alone. However, no augmentation of acid secretion by CGRP 8-37 was shown with 25 micrograms/kg/hr bethanechol. RIA of plasma gastrin demonstrated no effect with the antagonist when given alone and did not increase bombesin-stimulated gastrin release. We conclude that CGRP 8-37 blocks native CGRP inhibitory effects on gastric acid secretion. Our findings of potentiation of acid secretion by bombesin as well as no change in gastrin levels in the presence of the antagonist is likely due to a blockage in a noncholinergic neuron to the somatostatin cell. Furthermore, CGRP 8-37 did not increase bethanechol-stimulated acid secretion, most likely due to bethanechol's (acetylcholine) nearly ubiquitous positive effects on acid secretion.

Somatostatin effectively prevents ethanol- and NSAID-induced gastric mucosal damage in rats

The interrelationship between somatostatin and its synthetic analog, sandostatin, with neuropeptides and inflammatory mediators, as well as their protection of gastric mucosal damage, were tested in rats. Rats were treated intragastrically with 1.0 ml of 96% ethanol with or without intravenous or intraperitoneal coadministration of somatostatin (1.0 microM/kg). Mucosal damage was also induced by the administration of either indomethacin (30 mg/kg subcutaneously) with or without intravenous sandostatin (10 micrograms/rat), given 30 min prior to damage induction. Somatostatin levels in ethanol-damaged gastric mucosa were significantly lower than in control rats. Substance P and vasoactive intestinal peptide (VIP) levels were significantly higher in the damaged mucosa in rats treated with ethanol, as was the mucosal generation of leukotriene B4 (LTB4) and cysteinyl-containing leukotrienes. The coadministration of somatostatin with ethanol significantly reduced gastric mucosal injury induced by ethanol alone. The protection of the mucosa was accompanied by reduction of mucosal substance P and VIP levels, as well as the generation of leukotrienes, an effect that was reversed by intraperitoneal or intravenous coadministration of somatostatin antagonist, cyclo-(7-aminoheptanoyl-PH-E-D-Trp-Lys-THR), 1.0 microM/100 g, with somatostatin (1.0 microM/kg) and ethanol. When given by itself somatostatin significantly reduced mucosal leukotriene generation compared with their generation in saline-treated rats. Sandostatin completely abolished gastric mucosal damage induced by indomethacin administration. In rats treated with somatostatin and indomethacin, this effect was accompanied by reduction of mucosal leukotriene generation. Administration of sandostatin to pylorus-ligated rats significantly reduced gastric acid output.(ABSTRACT TRUNCATED AT 250 WORDS)

Somatostatin: physiology and clinical applications

Somatostatin (SOM) was originally isolated as the hypothalamic inhibitor of growth hormone release but was subsequently shown to have a widespread distribution including the gastrointestinal tract. In fact the gastrointestinal tract contains about 70% of the total body SOM. SOM has inhibitory actions on gastrointestinal exocrine and endocrine secretions, motility and blood flow. Within the gut it functions as an endocrine, paracrine, autocrine and neurocrine factor. SOM is released by a meal, and a number of neurotransmitters and regulatory peptides also influence SOM release. SOM is a key component of the gastrin-acid feedback loop as luminal acid releases SOM, which in turn has inhibitory effects on both gastrin and gastric acid. Consistent with the diverse functions of SOM, a number of different although related SOM receptors with distinct distribution patterns and intracellular mediators have been cloned and sequenced. SOM is the first of the gut regulatory peptides to have a significant therapeutic use. By inhibiting both the target cell (e.g. parietal cell) and the release of the active agent (e.g. gastrin) the therapeutic potential of SOM is magnified. To date most of the clinical experience has been with the one analogue, octreotide. This analogue has a longer half-life than SOM (hours versus minutes) but has only minimal oral activity, therefore requiring subcutaneous injections several times a day. The definite gastrointestinal applications include treatment of gastroenteropancreatic tumours. It is also becoming a favoured treatment for gastrointestinal fistulae, variceal bleeding and diarrhoea. However, octreotide has no consistent effect on tumour growth. The high density of SOM receptors on tumours has allowed localization of tumours using in vivo scintography with labelled octreotide. The sequencing of a variety of SOM receptors with different distributions and differing cellular effector systems raises the likelihood of developing SOM analogues for specific clinical applications.

Somatostatin is released in response to cholecystokinin by activation of type A CCK receptors

Cholecystokinin is a principal mediator of intestinal fat-induced inhibition of gastric acid secretion, indicating that it is an important physiological enterogastrone. Cholecystokinin has been shown to inhibit acid secretion by activation of type A CCK receptors and through a mechanism involving somatostatin. In the present study, we investigated the possibility that these two mechanisms are directly related such that activation of type A CCK receptors by CCK causes the release of somatostatin. We tested this hypothesis in vivo in a study of CCK-stimulated release of somatostatin in dogs and in vitro in a study of CCK-stimulated release of somatostatin from an enriched culture of canine fundic D cells. In dogs, IV infusion of CCK (50 pmol/kg/h, IV) significantly increased circulating somatostatin concentrations above basal. Further, systemic administration of somatostatin MAb F(ab)1 fragments of a somatostatin monoclonal antibody prevented most of CCK-induced inhibition of meal-stimulated acid secretion. In canine fundic D cells in culture, CCK-stimulated somatostatin release was blocked in a dose-dependent fashion by application of a type A CCK receptor antagonist. This study indicates that CCK activates type A CCK receptors to release somatostatin from canine fundic mucosal D cells, and accounts for somatostatin-dependent CCK-induced inhibition of acid secretion.

Comparative study of the control of basal acid output from rodent isolated stomachs

1. Isolated, lumen-perfused, whole stomach preparations from mouse and immature rat produced a stable basal acid output which, although not blocked by histamine H2-, acetylcholine M- or CCKB/gastrin receptor antagonists, was almostly completely blocked by the H+/K(+)-ATPase inhibitor, omeprazole, and the metabolic inhibitor, sodium thiocyanate (NaSCN). 2. Fully-defined concentration-effect curves could be obtained on both assays with the phosphodiesterase inhibitor, isobutyl methylxanthine (IBMX) and with dibutyryl cyclic AMP. 3. On the rat stomach assay, histamine H2-receptor blockade had no effect on the IBMX curve. In contrast, the IBMX response in the mouse was abolished by histamine H2-receptor blockade. On both assays responses to dibutyryl cyclic AMP were resistant to H2-receptor blockade. 4. In the absence of suprathreshold endogenous histamine, it is argued that H+/K(+)-ATPase mediated basal acid secretion from the mouse stomach assay is regulated by something other than cyclic AMP.

Histamine secretion from rat enterochromaffinlike cells

BACKGROUND

In vivo studies have suggested an important role for gastric enterochromaffinlike (ECL) cells in mediating acid secretion. Direct evidence for this function is lacking and requires a preparation of highly purified ECL cells. This work investigates the possible role and mechanism of histamine release from the ECL cell in the peripheral regulation of acid secretion, using purified ECL cells from rat fundic mucosa.

METHODS

A combination of elutriation and density-gradient centrifugation was used to purify rat fundic ECL cells. Enrichment was determined by the presence of acidic vacuoles containing a V type adenosine triphosphatase, electron microscopy, immunostaining, and histamine content and release.

RESULTS

ECL cells were enriched at least 65-fold with respect to the fundic epithelium. Gastrin (EC50 0.2 nmol/L) and cholecystokinin octapeptide (nonsulfated, EC50 0.04 nmol/L) stimulated histamine release in a time- and dose-dependent manner, suggesting a CCK-B receptor subtype, confirmed by the inhibition of gastrin/CCK stimulation with the CCK-B antagonist L365,260. Somatostatin also inhibited gastrin-mediated histamine release. Single cell imaging showed that gastrin elevated intracellular cytosolic calcium concentration biphasically. Carbachol and the C kinase activator 120-tetradecanoylphorbol-13-acetate also stimulated histamine release. Epinephrine (blocked by propranolol), forskolin, and dibutyryl-5'-cyclic adenosine monophosphate were also effective, implicating a beta-adrenergic pathway. The H3 agonist R-alpha-methyl-histamine inhibited, whereas the H3-antagonist thioperamide potentiated gastrin/CCK stimulated histamine release.

CONCLUSIONS

These in vitro results support a central role for the ECL cell in the peripheral regulation of gastric acid secretion.

Intestinal fat does not inhibit gastric function through a hormonal somatostatin mechanism in dogs

In awake dogs with chronic gastric, duodenal, and jejunal fistulas, F(ab)1 fragments of somatostatin monoclonal antibody (mAb S607) were administered intravenously (IV) to test the hypothesis that intraintestinal lipid inhibits peptone-stimulated gastric acid secretion and emptying by a hormonal somatostatin mechanism. Plasma somatostatin was increased significantly by duodenal and jejunal perfusion with 20% lipid. Somatostatin administered IV caused dose-dependent inhibition of meal-stimulated gastric acid secretion and gastric emptying similar to that seen after intestinal perfusion with lipid. Administration of mAb S607 F(ab)1 fragments significantly reversed somatostatin (400 pmol.kg-1.h-1, IV)-induced inhibition of peptone-stimulated acid output and gastric emptying. Acid output inhibited by intraduodenal lipid was reversed partially after F(ab)1 administration, but the inhibitory effect of intrajejunal lipid was not altered. Inhibition of acid secretion by IV somatostatin and by intraintestinal fat was not caused by a decrease in circulating gastrin concentrations. Gastric emptying delayed by intraintestinal lipid was unaffected by antibody administration. Somatostatin does not appear to be a major hormonal mediator of intestinal fat-induced inhibition of gastric acid secretion or delayed gastric emptying in dogs.

Effect of loxiglumide on basal and gastrin- and bombesin-stimulated gastric acid and serum gastrin levels

The effect of the specific cholecystokinin-receptor antagonist loxiglumide on basal and bombesin-, and gastrin 17-I-stimulated gastric acid secretion and serum gastrin levels was studied in 12 healthy subjects. Loxiglumide (10 mg.kg-1.h-1) significantly augmented basal gastric acid output from 1.8 +/- 0.3 to 3.9 +/- 0.6 mmol H+/h (P less than 0.005) but did not significantly influence integrated basal serum gastrin concentrations (2 +/- 21 vs. 32 +/- 21 pmol L-1.h-1). Both gastric acid secretion and integrated serum gastrin concentrations stimulated by bombesin infusion (92.6 pmol.kg-1.h-1) were significantly augmented by loxiglumide [from 4.0 +/- 0.3 to 10.0 +/- 1.3 mmol H+/h (P less than 0.005) and from 1251 +/- 93 to 2558 +/- 206 pmol.L-1.h-1 (P less than 0.005), respectively]. Gastric acid output and serum gastrin concentrations during infusion of 5 pmol.kg-1.h-1 of synthetic human gastrin 17-I (9.6 +/- 2.9 mmol H+/h and 1045 +/- 177 pmol.L-1.h-1) and during infusion of 15 pmol.kg-1.h-1 of gastrin 17-I (14.5 +/- 3.1 mmol H+/h and 2412 +/- 312 pmol.L-1.h-1) were not significantly influenced by loxiglumide (10.3 +/- 2.3 mmol H+/h and 1291 +/- 257 pmol.L-1.h-1 for the 5-pmol.kg-1.h-1 gastrin 17-I infusion dose with loxiglumide and 13.6 +/- 3.4 mmol H+/h and 2611 +/- 305 pmol.L-1.h-1 for the 15-pmol.kg-1.h-1 gastrin 17-I infusion dose with loxiglumide). These data indicate that endogenous cholecystokinin inhibits gastric acid secretion under basal conditions and gastrin release and gastric acid secretion during infusion of bombesin in humans and suggest that the augmented effect of loxiglumide on bombesin-stimulated gastric acid secretion may be explained largely by enhanced gastrin release.

Validation of the antral mucosal/submucosal sleeve preparation: studies of gastrin and acetylcholine release in response to luminal stimulation

In the present study we developed an experimental model for direct assessment of antral endocrine cell and cholinergic neural responses to luminal stimulation. A sleeve of antral mucosal/submucosal tissue was prepared from rat antrum, mounted in perfusion chamber, and perfused in both luminal and submucosal compartments. Morphological and functional integrity of the antral sleeve were confirmed by histological examination and measurement of protein synthesis. Antral gastrin release was assessed in response to luminal stimulation with acid, peptone and distension. Luminal acid (pH3) inhibited basal gastrin release by -70.4% and luminal peptone stimulated gastrin release to 210% above control (p < 0.02). Distention of the antral sleeve by hydrostatic pressure (3-25cm H2O) caused stepwise and significant increase in gastrin release that was reversible. 3H-acetylcholine was stimulated significantly by KCl (56mM) to values twice control. In summary, these results establish the integrity and responsiveness of the antral sleeve to pharmacological and luminal stimulation. The antral sleeve may be a useful model in assessing antral function in response to luminal stimulation.