How does gastrin act to stimulate oxyntic cell secretion?

Gastrin stimulates MMP-1 expression in gastric epithelial cells: putative role in gastric epithelial cell migration

The pyloric antral hormone gastrin plays a role in remodeling of the gastric epithelium, but the specific targets of gastrin that mediate these effects are poorly understood. Glandular epithelial cells of the gastric corpus express matrix metalloproteinase (MMP)-1, which is a potential determinant of tissue remodeling; some of these cells express the CCK-2 receptor at which gastrin acts. We have now examined the hypothesis that gastrin stimulates expression of MMP-1 in the stomach. We determined MMP-1 transcript abundance in gastric mucosal biopsies from Helicobacter pylori negative human subjects with normal gastric mucosal histology, who had a range of serum gastrin concentrations due in part to treatment with proton pump inhibitors (PPI). The effects of gastrin were studied on gastric epithelial AGS-GR cells using Western blot and migration assays. In human subjects with increased serum gastrin due to PPI usage, MMP-1 transcript abundance was increased 2-fold; there was also increased MMP-7 transcript abundance but not MMP-3. In Western blots, gastrin increased proMMP-1 abundance, as well that of a minor band corresponding to active MMP-1, in the media of AGS-GR cells, and the response was mediated by protein kinase C and p42/44 MAP kinase. There was also increased MMP-1 enzyme activity. Gastrin-stimulated AGS-GR cell migration in both scratch wound and Boyden chamber assays was inhibited by MMP-1 immunoneutralization. We conclude that MMP-1 expression is a target of gastrin implicated in mucosal remodeling.

Lack of cholinergic innervation in gastric mucosa does not affect gastrin secretion or basal acid output in neurturin receptor GFRα2 deficient mice

Efferent signals from the vagus nerve are thought to mediate both basal and meal-induced gastric acid secretion, and provide trophic support of the mucosa. However, the underlying mechanisms are incompletely understood. Neurturin, signalling via glial cell line-derived neurotrophic factor (GDNF)-family receptor α2 (GFRα2), is essential for parasympathetic innervation of many target tissues but its role in gastric innervation is unknown. Here we show that most nerve fibres in wild-type mouse gastric mucosa, including all positive for gastrin-releasing peptide, are cholinergic. GFRα2-deficient (KO) mice lacked virtually all cholinergic nerve fibres and associated glial cells in the gastric (oxyntic and pyloric) mucosa but not in the smooth muscle, consistent with the selective expression of neurturin mRNA in the gastric mucosa. 2-Deoxyglucose and hexamethonium failed to affect acid secretion in the GFRα2-KO mice indicating the lack of functional innervation in gastric mucosa. Interestingly, basal and maximal histamine-induced acid secretion did not differ between wild-type and GFRα2-KO mice. Moreover, circulating gastrin levels in both fasted and fed animals, thickness of gastric mucosa, and density of parietal and different endocrine cells were similar. Carbachol-stimulated acid secretion was higher in GFRα2-KO mice, while atropine reduced basal secretion similarly in both genotypes. We conclude that cholinergic innervation of gastric mucosa depends on neurturin-GFRα2 signalling but is dispensable for gastrin secretion and for basal and maximal acid output. Basal acid secretion in the KO mice appears to be, at least partly, facilitated by constitutive activity of muscarinic receptors.

The cholecystokinin CCK2 receptor antagonist, JNJ-26070109, inhibits gastric acid secretion and prevents omeprazole-induced acid rebound in the rat

BACKGROUND AND PURPOSE

JNJ-26070109 [(R)4-bromo-N-[1-(2,4-difluoro-phenyl)-ethyl]-2-(quinoxaline-5-sulfonylamino)-benzamide] is a novel antagonist at cholecystokinin CCK(2) receptors with good pharmacokinetic properties and represents a novel mechanism for the treatment of gastro-oesophageal reflux disease (GORD). The purpose of the present study was to determine whether chronic treatment with JNJ-26070109 could prevent, as well as treat, acid rebound in rats.

EXPERIMENTAL APPROACH

A chronic fistula was surgically inserted into the stomach of rats to enable the measurement of acid secretion under basal, pentagastrin and histamine-stimulated conditions. JNJ-26070109 and omeprazole were administered separately and in combination.

KEY RESULTS

Sustained administration of omeprazole alone and in combination with JNJ-26070109 inhibited gastric acid secretion by >90%. However, 3 days after withdrawing treatment, there was a rebound hypersecretion by ∼1.5-fold in omeprazole-treated animals. No such acid rebound was observed with JNJ-26070109 alone or with co-administration of JNJ-26070109 and omeprazole. The anti-trophic effects of JNJ-26070109 in the gastric mucosal paralleled the effects on acid rebound. Administration of JNJ-26070109 for 3 days after cessation of omeprazole prevented the occurrence of acid rebound. Interestingly, chronic, but not acute, treatment with JNJ-26070109 also inhibited histamine-stimulated acid secretion.

CONCLUSIONS AND IMPLICATIONS

Chronic administration of JNJ-26070109 effectively inhibited gastric acid secretion and suppressed proton pump inhibitor (PPI)-induced acid rebound in the rat. This work advances the field by demonstrating that modest doses of a competitive CCK(2) receptor antagonist have significant and functionally important anti-trophic actions in the gastric mucosa. These properties make JNJ-26070109 a suitable candidate for clinical investigation for the treatment of GORD.

Identification of ezrin as a target of gastrin in immature mouse gastric parietal cells

The gastric acid-secreting parietal cell exhibits profound morphological changes on stimulation. Studies in gastrin null (Gas-KO) mice indicate that maturation of parietal cell function depends on the hormone gastrin acting at the G-protein-coupled cholecystokinin 2 receptor. The relevant cellular mechanisms are unknown. The application of differential mRNA display to samples of the gastric corpus of wild-type (C57BL/6) and Gas-KO mice identified the cytoskeletal linker protein, ezrin, as a previously unsuspected target of gastrin. Gastrin administered in vivo or added to gastric glands in vitro increased ezrin abundance in Gas-KO parietal cells. In parietal cells of cultured gastric glands from wild-type mice treated with gastrin, histamine or carbachol, ezrin was localized to vesicular structures resembling secretory canaliculi. In contrast, in cultured parietal cells from Gas-KO mice, ezrin was typically distributed in the cytosol, and this did not change after incubation with gastrin, histamine or carbachol. However, priming with gastrin for approximately 24 h, either in vivo prior to cell culture or by addition to cultured gastric glands, induced the capacity for secretagogue-stimulated localization of ezrin to large vesicular structures in Gas-KO mice. Similarly, in a functional assay based on measurement of intracellular pH, cultured parietal cells from Gas-KO mice were refractory to gastrin unless primed. The priming effect of gastrin was not attributable to the paracrine mediator histamine, but was prevented by inhibitors of protein kinase C and transactivation of the epidermal growth factor receptor. We conclude that in gastrin null mice there is reduced ezrin expression and a defect in ezrin subcellular distribution in gastric parietal cells, and that both can be reversed by priming with gastrin.

Tachyphylaxis of the ECL-cell response to PACAP: receptor desensitization and/or depletion of secretory products

BACKGROUND AND PURPOSE

Rat stomach ECL cells secrete histamine and pancreastatin in response to gastrin and pituitary adenylate cyclase-activating peptide-27 (PACAP). This study applies microdialysis to explore how ECL cells in situ respond to PACAP and gastrin.

EXPERIMENTAL APPROACH

Both peptides were administered by microinfusion into the gastric submucosa. The microdialysate was analysed for histamine and pancreastatin (ECL-cell markers) and for somatostatin (D-cell marker).

KEY RESULTS

Microinfusion of PACAP (0.01-0.3 nmol microl(-1)) raised microdialysate histamine and pancreastatin dose-dependently. The response was powerful but short-lived. The response to gastrin was sustained at all doses tested. It is unlikely that the transient nature of the histamine response to PACAP reflects inadequate histamine synthesis, since the pancreastatin response to PACAP was short-lived too, and both gastrin and PACAP activated ECL-cell histidine decarboxylase. Unlike gastrin, PACAP mobilized somatostatin. Co-infusion of somatostatin abolished the histamine-mobilizing effect of PACAP. However, pretreatment with the somatostatin receptor type-2 antagonist (PRL-2903) did not prolong the histamine response to PACAP, suggesting that mobilization of somatostatin does not explain the transient nature of the response. Repeated administration of 0.1 nmol microl(-1) of PACAP (1 h infusions, 1 h intervals) failed to induce a second histamine response. Pretreatment with a low dose of PACAP (0.03 nmol microl(-1)) abolished the response to a subsequent near-maximal PACAP challenge (0.3 nmol microl(-1)).

CONCLUSION

The transient nature of the histamine response to PACAP reflects desensitization of the PACAP receptor and/or exhaustion of a specific storage compartment that responds to PACAP but not to gastrin.

Identification of a Gastrin Response Element in the Vesicular Monoamine Transporter Type 2 Promoter and Requirement of 20 S Proteasome Subunits for Transcriptional Activity

Vesicular monoamine transporter type 2 (VMAT2) is crucial for accumulation of monoamine neurotranmitters into neuronal secretory vesicles and histamine into secretory granules of the enterochromaffin-like cell in the acid-secreting gastric mucosa. Gastric VMAT2 expression is regulated by the antral hormone gastrin acting at the CCK(2) receptor. We demonstrate a gastrin response element (-56)ccgccccctc(-47) in the proximal VMAT2 promoter that binds in a gastrin-sensitive manner to nuclear proteins from gastric epithelial cell lines. Mutations within this sequence prevented nuclear protein binding and significantly reduced gastrin-stimulated expression of VMAT2 promoter-reporter constructs in gastric epithelial cells. In a yeast one-hybrid screen of an AR42J cell cDNA library, using the gastrin response element as bait, we identified a beta subunit of the 20 S proteasome, PSMB1, as a potential binding partner. In supershift assays, antibodies to PSMB1 and other proteasome beta subunits disrupted gastrin sensitive nuclear protein binding to the VMAT2 promoter. Moreover, RNA interference of PSMB1 significantly inhibited gastrin-mediated VMAT2 transcription. These data suggest that elements of the 20 S proteasome interact with the VMAT2 promoter to enhance G-protein-coupled receptor-mediated transcription.

Necessity of intracellular cyclic AMP in inducing gastric acid secretion via muscarinic M3 and cholecystokinin2 receptors on parietal cells in isolated mouse stomach

The existence of a direct action of acetylcholine and gastrin on muscarinic M3 and cholecystokinin2 (CCK2) receptors on gastric parietal cells has not yet been convincingly established because these stimulated acid secretions are remarkably inhibited by histamine H2 receptor antagonists. In the present study, we investigated the necessity of intracellular cyclic AMP in inducing gastric acid secretion via muscarinic M3 and CCK2 receptors on parietal cells using an isolated mouse stomach preparation. Bethanechol (10-300 microM) produced a marked increase in acid output and this increase was completely blocked by famotidine (10 microM). In the presence of famotidine, bethanechol (1-30 microM) augmented the acid secretory response to dibutyryl AMP (200 microM) in a concentration-dependent manner. The augmentation was blocked by atropine (1 microM), 4-DAMP (0.1 microM), a muscarinic M3-selective antagonist, and by Ca2+ exclusion from the serosal nutrient solution. Pentagastrin (0.3-3 microM) also concentration-dependently stimulated gastric acid secretion, but the effect was completely inhibited by famotidine. In the presence of famotidine, pentagastrin (0.1-0.3 microM) elicited a definite potentiation of the acid secretory response to dibutyryl cyclic AMP (200 microM). This potentiation was inhibited by YM022 (1 microM), a CCK2 receptor antagonist, and by exclusion of Ca2+ from the serosal nutrient solution. The present results suggest that gastric acid secretion via the activation of muscarinic M3 and CCK2 receptors on the parietal cells is induced by activation of the cyclic AMP-dependent secretory pathway.

Effect of caffeic acid phenethyl ester on gastric acid secretion in vitro

Caffeic acid phenethyl ester (CAPE), one of the major components of propolis (honeybee resin), has demonstrated a wide spectrum of activities including suppression of eicosanoids by inhibition of cyclooxygenase-1 and cyclooxygenase-2 enzyme activities. The aim of this study was to investigate the effect of CAPE on basal and secretagogues-stimulated gastric acid secretion in vitro. In the isolated, lumen-perfused, stomach preparation of mouse, CAPE (10-100 microM) did not affect the basal gastric acid secretion nor the secretion stimulated by histamine, pentagastrin, isobutyl methylxanthine and high levels of K+. By contrast, CAPE increased the gastric acid secretion induced by the muscarinic receptor agonist, 5-methylfurmethide (5-MEF). CAPE also inhibited the acetylcholinesterase activity in an in vitro colorimetric assay. Eserine (10 microM), a well known acetylcholinesterase inhibitor, also increased 5-MEF-stimulated acid secretion. Our results show that CAPE increases gastric acid secretion stimulated by an acetylcholine agonist receptor likely through inhibition of acetylcholinesterase activity.

Stimulation of gastrin-CCKB receptor promotes migration of gastric AGS cells via multiple paracrine pathways

Responses to G protein-coupled receptor stimulation may be mediated by paracrine factors. We have developed a coculture system to study paracrine regulation of migration of gastric epithelial (AGS) cells after stimulation of gastrin-CCK(B) receptors. In cells expressing this receptor, G-17 stimulated migration by activation of protein kinase C. However, G-17 also stimulated the migration of cells expressing green fluorescent protein, but not the receptor, when they were cocultured with receptor-expressing cells consistent with activation of paracrine signals. The use of various pharmacological inhibitors indicated that gastrin stimulated migration via activation of the EGF receptor (EGR-R), the erbB-2 receptor tyrosine kinase, and the MAP kinase pathway. However, gastrin also released fibroblast growth factor (FGF)-1, and migration was inhibited by the FGF receptor tyrosine kinase inhibitor SU-5402. Flow cytometry indicated that in both cell types, gastrin increased MAP kinase via activation of EGF-R but not FGF-R1 or erbB-2. We conclude that gastrin-CCK(B) receptors stimulate epithelial cell migration partly via paracrine mechanisms; transactivation of EGF-R is only one component of the paracrine pathway.

Enhanced calcium signaling and acid secretion in parietal cells isolated from gastrin-deficient mice

Gastrin-deficient mice have impaired basal and agonist-stimulated gastric acid secretion. To analyze whether an intrinsic parietal cell defect contributed to the reduced acid secretion, we analyzed parietal cell calcium responses and acid secretory function in vitro. Parietal cells were purified by light-scatter cell sorting and calcium responses to gastrin, histamine, and carbachol were measured in gastrin-deficient and wild-type mice cell preparations. Surprisingly, basal and histamine-induced calcium concentrations were higher in the mutant cell preparations. [(14)C]aminopyrine uptake analysis in acutely isolated gastric glands revealed that basal acid accumulation was enhanced in gastrin-deficient cell preparations as well as on treatment with carbachol or histamine. These results suggested that an intrinsic parietal cell defect was not responsible for the reduced acid secretion in gastrin-deficient mice. Flow cytometric analysis of dispersed, H(+)-K(+)-ATPase-immunostained gastric mucosal preparations revealed a marked increase in parietal cell number in gastrin-deficient mice, which may have accounted for the enhanced in vitro acid secretion detected in this study. Parietal cells were found to be significantly smaller in the mutant cell preparations, suggesting that gastrin stimulation modulates parietal cell morphology.

Antisecretory effect of somatostatin on gastric acid via inhibition of histamine release in isolated mouse stomach

Somatostatin is known to inhibit gastric acid secretion via both inhibition of histamine release from gastric enterochromaffin-like cells and direct inhibition of parietal cell function. We tried to clarify which of these two mechanisms plays a more important role in the inhibition of gastric acid section by somatostatin using isolated mouse stomach preparations. The gastric acid secretion stimulated by histamine was not inhibited by pretreatment with somatostatin (1 micro M), but somatostatin abolished acid secretion induced by 4-[[[(3-chlorophenyl)amino]carbonyl]oxy]-N,N,N,-trimethyl-2-butynyl-1-aminium chloride (McN-A-343), a muscarinic M(1) receptor agonist. In addition, the histamine-H(2) receptor antagonist famotidine also completely inhibited the secretion stimulated by McN-A-343. Similarly, pretreatment with both somatostatin and famotidine completely abolished pentagastrin-induced acid secretion. Somatostatin partially inhibited the acid secretion stimulated by bethanechol. The late sustained acid secretion induced by bethanechol was reduced more strongly by somatostatin than the initial peak secretion. In addition, somatostatin had no effect on the transient increase in bethanechol-induced acid secretion in famotidine-pretreated preparations. Somatostatin had no effect on basal histamine secretion in isolated mouse stomach preparations, but markedly reduced histamine release induced by McN-A-343 and bethanechol. The present study showed that the acid secretory response via the endogenous histamine-mediated pathway was inhibited by somatostatin, but the response to a direct activation of gastric parietal cells was not. These results suggest that the inhibition of histamine release from enterochromaffin-like cells plays a more important role in the inhibition of gastric acid secretion by somatostatin than the direct inhibition of parietal cells. In addition, somatostatin inhibited the sustained acid secretion more strongly than the initial peak secretion after the cholinergic stimulation.

ECL-cell histamine mobilization in conscious rats: effects of locally applied regulatory peptides, candidate neurotransmitters and inflammatory mediators

1. The ECL cells control gastric acid secretion by mobilizing histamine in response to circulating gastrin. In addition, the ECL cells are thought to operate under nervous control and to be influenced by local inflammatory processes. 2. The purpose of the present study was to monitor histamine mobilization from ECL cells in conscious rats in response to locally applied regulatory peptides, candidate neurotransmitters and inflammatory mediators. 3. Microdialysis probes were implanted in the submucosa of the acid-producing part of the rat stomach. Three days later, the agents to be tested were administered via the microdialysis probe and their effects on basal (48 h fast) and stimulated (intravenous infusion of gastrin-17, 3 nmol kg(-1) h(-1)) mobilization of ECL-cell histamine was monitored by continuous measurement of histamine in the perfusate (radioimmunoassay). 4. Locally administered gastrin-17 and sulfated cholecystokinin-8 mobilized histamine as did pituitary adenylate cyclase-activating peptide-27, vasoactive intestinal peptide, peptide YY, met-enkephalin, endothelin and noradrenaline, adrenaline and isoprenaline. 5. While gastrin, sulfated-cholecystokinin-8, met-enkephalin and isoprenaline induced a sustained elevation of the submucosal histamine concentration, endothelin, peptide YY, pituitary adenylate cyclase activating peptide, vasoactive intestinal peptide, noradrenaline and adrenaline induced a transient elevation. 6. Calcitonin gene-related peptide, galanin, somatostatin and the prostanoid misoprostol inhibited gastrin-stimulated histamine mobilization. 7. The gut hormones neurotensin and secretin and the neuropeptides gastrin-releasing peptide, neuropeptide Y and substance P failed to affect ECL-cell histamine mobilization, while motilin and neuromedin U-25 had weak stimulatory effects. Also acetylcholine, carbachol, serotonin and the amino acid neurotransmitters aspartate, gamma-aminobutyric acid, glutamate and glycine were inactive or weakly active as was bradykinin. 8. In summary, a range of circulating hormones, local hormones, catecholamines, neuropeptides and inflammatory mediators participate in controlling the activity of rat stomach ECL cells in situ.

The gastrins: their production and biological activities

Gastric epithelial organization and function are controlled and maintained by a variety of endocrine and paracrine mediators. Peptides encoded by the gastrin gene are an important part of this system because targeted deletion of the gene, or of the gastrin-CCKB receptor gene, leads to decreased numbers of parietal cells and decreased gastric acid secretion. Recent studies indicate that the gastrin precursor, preprogastrin, gives rise to a variety of products, each with a distinctive spectrum of biological activity. The conversion of progastrin to smaller peptides is regulated by multiple mechanisms including prohormone phosphorylation and secretory vesicle pH. Progastrin itself stimulates colonic epithelial proliferation; biosynthetic intermediates (Gly-gastrins) stimulate colonic epithelial proliferation and gastric epithelial differentiation; and C-terminally amidated gastrins stimulate colonic proliferation, gastric epithelial proliferation and differentiation, and acid secretion. The effects of progastrin-derived peptides on gastric epithelial function are mediated in part by release of paracrine factors that include histamine, epidermal growth factor (EGF)-receptor ligands, and Reg. The importance of the appropriate regulation of this system is shown by the observation that prolonged moderate hypergastrinemia in transgenic mice leads to remodelling of the gastric epithelium, and in the presence of Helicobacter, to gastric cancer.

Isolated rat stomach ECL cells generate prostaglandin E(2) in response to interleukin-1 beta, tumor necrosis factor-alpha and bradykinin

The ECL cells control parietal cells by releasing histamine in their immediate vicinity. Gastrin and pituitary adenylate cyclase-activating peptide (PACAP) stimulate histamine secretion from isolated ECL cells, while somatostatin and galanin inhibit stimulated secretion. Prostaglandin E2 and related prostaglandins likewise suppress ECL-cell histamine secretion. Conceivably, that is how they inhibit acid secretion. In the present study, we examined if prostaglandin E2 can be generated by isolated ECL cells. Rat stomach ECL cells were purified (>90% purity) by counterflow elutriation and gradient centrifugation and cultured for 48 h. ECL cell stimulants (gastrin and PACAP) and inflammatory agents (interleukin-1 beta, tumor necrosis factor-alpha and bradykinin) were tested for their ability to induce prostaglandin E2 accumulation (24-h incubation), measured by radioimmunoassay. Gastrin and PACAP did not affect prostaglandin E2 accumulation but interleukin-1 beta (300 pg/ml), tumor necrosis factor-alpha (10 ng/ml) and bradykinin (1 microM) induced a 2- to 3-fold increase in the amount of prostaglandin E2 accumulated. While the combination of interleukin-1 beta and bradykinin induced a 9-fold increase, the combination interleukin-1 beta+tumor necrosis factor-alpha and bradykinin + tumor necrosis factor-alpha induced additive effects only. The combination of interleukin-1 beta + tumor necrosis factor-alpha + bradykinin did not induce a greater effect than interleukin-1 beta + bradykinin. The effect of interleukin-1 beta + bradykinin was abolished by adding 10 nM hydrocortisone (suppressing phospholipase A2 and cyclooxygenase) or 1 microM indomethacin (inhibiting cyclooxygenase). Incubating ECL cells in the presence of interleukin-1 beta+bradykinin for 24 h reduced their ability to secrete histamine in response to gastrin. The inhibitory effect was reversed by 1 microM indomethacin. Also, increasing the concentrations of hydrocortisone in the medium resulted in an enhanced gastrin-stimulated histamine secretion. Hence, the previously described acid-inhibiting effect of inflammatory agents may be explained by inhibition of ECL-cell histamine mobilization, consequent to enhanced formation of prostaglandin E2 by cells in the oxyntic mucosa, including the ECL cells themselves.

Transcriptional activation of the rat vesicular monoamine transporter 2 promoter in gastric epithelial cells: regulation by gastrin

Vesicular monoamine transporter 2 is important for the accumulation of monoamine neurotransmitters into synaptic vesicles and histamine transport into secretory vesicles of the enterochromaffin-like cell of the gastric corpus. In this study we have investigated the mechanisms regulating the transcriptional activation of the rat vesicular monoamine transporter 2 (VMAT2) promoter in gastric epithelial cells. Maintenance of basal levels of transcription was dependent on the presence of SP1, cAMP-response element (CRE), and overlapping AP2/SP1 consensus sequences within the region of promoter from -86 to +1 base pairs (bp). Gastrin stimulation increased transcriptional activity, and responsiveness was shown to be dependent on the CRE (-33 to -26 bp) and AP2/SP1 (-61 to -48 bp) consensus sites but independent of the SP1 site at -86 to -81 bp. Gastrin-induced transcription was dependent on the cooperative interaction of an uncharacterized nuclear factor of approximately 23.3 kDa that bound to the putative AP2/SP1 site, CRE-binding protein (CREB), and CREB-binding protein/p300. Gastrin stimulation resulted in the increased binding of phosphorylated CREB to the promoter, but it did not result in the increased binding of the AP2/SP1-binding protein. The gastrin responsiveness of the promoter was shown to be dependent on both the protein kinase C and mitogen-activated protein kinase/extracellular signal-regulated kinase kinase-signaling pathways, which may converge on the AP2/SP1-binding protein.

Gastrin and the enterochromaffin-like cell: an acid update

Gastrin is synthesized and secreted mostly in a heptadecapeptide form from neurocrine G cells located in the antrum. The biologically active sequence of the molecule is a C-terminal pentapeptide, which has been conserved across many species. Transcriptional regulation of gastrin mRNA synthesis is positively regulated by transforming growth factor-alpha (TGF-alpha) and inhibited by somatostatin (SST). The inactive precursor form is converted to the active molecule by several post-translation processing steps which include cleavage, C-terminal amidation, glycosylation, phosphorylation and sulfation. Aberrations in processing steps generate incompletely processed forms, particularly glycosylated progastrin, which may act as autocrine growth factors for gastrointestinal neoplasms. Gastrin release is stimulated by luminal aromatic amino acids and inhibited by a decrease in luminal pH. Other gastrin agonists include beta-adrenergic agents, acetylcholine, gastrin-releasing peptide (bombesin), TGF-alpha, and possibly the gastric pathogen, Helicobacter pylori. The principal peptide inhibitor of gastrin release is SST. The major physiological roles of gastrin include stimulation of acid secretion, regulation of mucosal cell lineage and mucosal cell proliferation. The fundic enterochromaffin-like (ECL) cell is the principal cellular transducer of the gastrin-acid signal. Activation of its gastrin/CCKB receptor results in histamine synthesis and release with consequent activation of the fundic parietal cell H2 receptor. An increase in luminal pH caused by acid inhibitory pharmacotherapy agents (particularly proton pump inhibitors) results in hypergastrinemia and ECL cell hyperplasia. Gastric carcinoids however appear occur in patients with multiple endocrine neoplasia type I syndrome, suggesting that an associated genomic defect is necessary. Gastrin is thus both a potent gastrointestinal trophic and histamine secretory agent. As a hormone it is a paradigm in the elucidation of both cellular secretory and growth factor induced cell proliferation.

Gastric submucosal microdialysis: a method to study gastrin- and food-evoked mobilization of ECL-cell histamine in conscious rats

Rat stomach ECL cells are rich in histamine and chromogranin A-derived peptides, such as pancreastatin. Gastrin causes the parietal cells to secrete acid by flooding them with histamine from the ECL cells. In the past, gastric histamine release has been studied using anaesthetized, surgically manipulated animals or isolated gastric mucosa, glands or ECL cells. We monitored gastric histamine mobilization in intact conscious rats by subjecting them to gastric submucosal microdialysis. A microdialysis probe was implanted into the submucosa of the acid-producing part of the stomach (day 1). The rats had access to food and water or were deprived of food (48 h), starting on day 2 after implantation of the probe. On day 4, the rats received food or gastrin (intravenous infusion), and sampling of microdialysate commenced. Samples (flow rate 1.2 microl min(-1)) were collected every 20 or 60 min, and the histamine and pancreastatin concentrations were determined. The serum gastrin concentration was determined in tail vein blood. Exogenous gastrin (4-h infusion) raised microdialysate histamine and pancreastatin dose-dependently. This effect was prevented by gastrin receptor blockade (YM022). Depletion of ECL-cell histamine by alpha-fluoromethylhistidine, an irreversible inhibitor of the histamine-forming enzyme, suppressed the gastrin-evoked release of histamine but not that of pancreastatin. Fasting lowered serum gastrin and microdialysate histamine by 50%, while refeeding raised serum gastrin and microdialysate histamine and pancreastatin 3-fold. We conclude that histamine mobilized by gastrin and food intake derives from ECL cells because: 1) Histamine and pancreastatin were released concomitantly, 2) histamine mobilization following gastrin or food intake was prevented by gastrin receptor blockade, and 3) mobilization of histamine (but not pancreastatin) was abolished by alpha-fluoromethylhistidine. Hence, gastric submucosal microdialysis allows us to monitor the mobilization of ECL-cell histamine in intact conscious rats under various experimental conditions not previously accessible to study. While gastrin receptor blockade lowered post-prandial release of ECL-cell histamine by about 80%, unilateral vagotomy reduced post-prandial mobilization of ECL-cell histamine by about 50%. Hence, both gastrin and vagal excitation contribute to the post-prandial release of ECL-cell histamine.

Stimulatory effect of dibutyryl cyclic GMP on acid secretion in mouse isolated stomach and on histamine release in gastric mucosal cells

We previously reported that endogenous nitric oxide (NO) is involved in the peripheral control of gastric acid secretion induced by some secretagogues, and that endogenous NO is involved in the acid secretion process via histamine release from histamine-containing cells. However, the stimulus-secretion coupling in the cells remains to be clarified. In the present study, we investigated the effect of dibutyryl cyclic GMP on gastric acid secretion in mouse isolated stomach and on histamine release in gastric mucosal cells, in comparison with those of dibutyryl cyclic AMP. Dibutyryl cyclic GMP (300 microM) produced a slight but significant increase of gastric acid secretion, which was completely inhibited by the histamine-H2 receptor antagonist famotidine. In contrast, dibutyryl cyclic GMP (1 mM) markedly inhibited histamine-induced acid secretion. Dibutyryl cyclic AMP (100 microM) produced a sustained increase of gastric acid secretion. The pretreatment with famotidine partially inhibited dibutyryl cyclic AMP-induced gastric acid secretion. Dibutyryl cyclic GMP and dibutyryl cyclic AMP significantly increased the histamine release from gastric mucosal cells. These results suggest that both intracellular cyclic GMP and cyclic AMP act as second messengers for histamine release in the histamine-containing cells, probably ECL cells. On the other hand, in gastric parietal cells, cyclic AMP has a stimulatory effect on gastric acid secretion, whereas cyclic GMP has an inhibitory effect.

Topical review. Gastrin and gastric epithelial physiology

Transepithelial transducing cells, particularly the gastrin (G) cell, co-ordinate gastric acid secretion with the arrival of food in the stomach. Recent work suggests that multiple active products are generated from the gastrin precursor, and that there are multiple control points in gastrin biosynthesis. Biosynthetic precursors and intermediates (progastrin and Gly-gastrins) are putative growth factors; their products, the amidated gastrins, regulate epithelial cell proliferation, the differentiation of acid-producing parietal cells and histamine-secreting enterochromaffin-like (ECL) cells, and the expression of genes associated with histamine synthesis and storage in ECL cells, as well as acutely stimulating acid secretion. Gastrin also stimulates the production of members of the epidermal growth factor (EGF) family, which in turn inhibit parietal cell function but stimulate the growth of surface epithelial cells. Plasma gastrin concentrations are elevated in subjects with Helicobacter pylori, who are known to have increased risk of duodenal ulcer disease and gastric cancer. Studies of the physiology of gastrin may therefore contribute to an understanding of the mechanisms relevant to major upper gastrointestinal tract disease.

CCK2 receptor antagonists: pharmacological tools to study the gastrin-ECL cell-parietal cell axis

Gastrin-recognizing CCK2 receptors are expressed in parietal cells and in so-called ECL cells in the acid-producing part of the stomach. ECL cells are endocrine/paracrine cells that produce and store histamine and chromogranin A (CGA)-derived peptides, such as pancreastatin. The ECL cells are the principal cellular transducer of the gastrin-acid signal. Activation of the CCK2 receptor results in mobilization of histamine (and pancreastatin) from the ECL cells with consequent activation of the parietal cell histamine H2 receptor. Thus, release of ECL-cell histamine is a key event in the process of gastrin-stimulated acid secretion. The oxyntic mucosal histidine decarboxylase (HDC) activity and the serum pancreastatin concentration are useful markers for the activity of the gastrin-ECL cell axis. Powerful and selective CCK2 receptor antagonits have been developed from a series of benzodiazepine compounds. These agents are useful tools to study how gastrin controls the ECL cells. Conversely, the close control of ECL cells by gastrin makes the gastrin-ECL cell axis well suited for evaluating the antagonistic potential of CCK2 receptor antagonists with the ECL-cell HDC activity as a notably sensitive and reliable parameter. The CCK2 receptor antagonists YF476, YM022, RP73870, JB93182 and AG041R were found to cause prompt inhibition of ECL-cell histamine and pancreastatin secretion and synthesis. The circulating pancreastatin concentration is raised, was lowered when the action of gastrin on the ECL cells was blocked by the CCK2 receptor antagonists. These effects were associated with inhibition of gastrin-stimulated acid secretion. In addition, sustained receptor blockade was manifested in permanently decreased oxyntic mucosal HDC activity, histamine concentration and HDC mRNA and CGA mRNA concentrations. CCK2 receptor blockade also induced hypergastrinemia, which probably reflects the impaired gastric acid secretion (no acid feedback inhibition of gastrin release). Upon withdrawal of the CCK2 receptor antagonists, their effects on the ECL cells were readily reversible. In conclusion, gastrin mobilizes histamine from the ECL cells, thereby provoking the parietal cells to secrete acid. While CCK2 receptor blockade prevents gastrin from evoking acid secretion, it is without effect on basal and vagally stimulated acid secretion. We conclude that specific and potent CCK2 receptor antagonists represent powerful tools to explore the functional significance of the ECL cells.

Inhibitory effect of N(omega)-nitro-L-arginine on gastric secretion induced by secretagogues and vagal stimulation in the isolated stomach

The involvement of endogenous nitric oxide (NO) in the control of gastric acid secretion induced by some secretagogues was studied in the mouse isolated whole stomach. The gastric acid secretion induced by McNeil A-343 [4-[[[(3-chlorophenyl)amino]carbonyl]oxy]-N,N,N-trimethyl-2-butyn- 1-aminium chloride], a muscarinic M1 receptor agonist, pentagastrin or electrical vagus nerve stimulation was markedly inhibited by pretreatment with the NO synthase inhibitor N(omega)-nitro-L-arginine (L-NNA). This inhibitory effect of L-NNA was reversed by L-arginine, but not by D-arginine. Histamine-induced gastric acid secretion was not influenced by treatment with L-NNA. Famotidine completely inhibited the gastric acid secretion induced by McNeil A-343, pentagastrin or electrical vagus nerve stimulation, showing that these stimulations induced gastric acid secretion mainly through histamine release from histamine-containing cells in the gastric mucosa. Moreover, the pentagastrin- and bethanechol-induced histamine release from gastric mucosal cells was significantly inhibited by L-NNA. The NO donor, sodium nitroprusside, at a concentration not affecting histamine-induced gastric acid secretion, increased the acid secretory response, and this response was inhibited by famotidine. These results suggest that endogenous NO is involved in the gastric acid secretion via histamine release from histamine-containing cells.

Carbachol stimulation of gastric acid secretion and its effects on the parietal cell

1. The acid secretagogue effect of gastrin is mainly mediated by the release of enterochromaffin-like (ECL) cell histamine, but the mechanism of muscarinic stimulation of acid secretion remains unclear. The results of studying aminopyrine uptake in isolated parietal cells, and histamine release in isolated ECL cells suggest that muscarinic agents may act both directly on the parietal cell and indirectly via histamine release from ECL cells. 2. We examined parietal and ECL cell responses to the muscarinic agent carbamylcholine (carbachol) in conscious rats and in rat isolated vascularly perfused stomachs. 3. Intravenous carbachol stimulated acid secretion in conscious gastric fistula rats and increased H+K+ ATPase mRNA abundance, indicating activation of parietal cells. In these experiments there was no increase in portal venous histamine, or in oxyntic mucosal histidine decarboxylase (HDC) enzyme activity and HDC mRNA abundance. 4. In rat isolated stomachs stimulated with carbachol in the dose range 10 nM(-1) mM only the 1 microM concentration increased venous histamine significantly. 5. We concluded that the muscarinic agent carbachol stimulates acid secretion and H+K+ ATPase mRNA in vivo by a direct effect on the parietal cell, that does not depend on the release of ECL cell histamine.

Sustained cholecystokinin-B/gastrin receptor blockade does not impair basal or histamine-stimulated acid secretion in chronic gastric fistula rats

Gastrin is a physiologically important secretagogue. It is thought to stimulate parietal cells indirectly by mobilizing histamine from enterochromaffin-like (ECL) cells in the oxyntic mucosa. Gastrin stimulates the secretory activity and growth of the ECL cells via an action on cholecystokinin-B/gastrin receptors. Acute cholecystokinin-B/gastrin receptor blockade is known to inhibit gastrin-stimulated acid secretion but whether sustained cholecystokinin-B/gastrin receptor blockade will impair basal, gastrin- and histamine-stimulated acid secretion remains uncertain. The present study was designed to study the effect of long-term (4 weeks) cholecystokinin-B/gastrin receptor blockade on basal and stimulated acid secretion in conscious rats. The selective cholecystokinin-B/gastrin receptor antagonist YM022 (3 mumol.kg-1.hr-1) was given to gastric fistula rats by continuous subcutaneous infusion via osmotic minipumps for various times from 2 hr to 4 weeks. Basal, gastrin- and histamine-stimulated acid secretion were examined during and after cessation of treatment. Basal and histamine-stimulated acid secretion was not affected by YM022 during the 4 week period of administration, whereas gastrin-induced acid secretion was inhibited. YM022 induced hypergastrinaemia in freely fed rats but did not affect the serum gastrin level in fasted rats. The serum gastrin concentration and gastrin-induced acid secretion returned to control levels 3-7 days after termination of YM022 administration.

Autoradiographic identification of a gastrin receptor on the human parietal cell

Gastrin plays an important role in regulating gastric acid secretion and gastrointestinal mucosal growth but its cellular sites of action in man have not been determined. Using cryostat sections of gastric mucosal tissue we have identified (125I-gastrin binding followed by fixation-wet emulsion autoradiography) and characterized (125I-gastrin binding followed by counting) a gastrin receptor binding site in the human stomach. This site displayed binding characteristics similar to those observed in isolated cell systems: specifically, 125I-gastrin binding was rapid (t1/2 approximately 10 min at 37 degrees C), temperature-dependent (3.5 fold more radioligand bound at 22 degrees C than at 4 degrees C) and saturable. The binding of the radioligand was also tissue specific and was five-fold greater in the gastric body than in the gastric antrum and duodenum. In the autoradiographs, silver grains were localized only to parietal cells and not to other epithelial cell types. In the presence of 40 nM gastrin grains were no longer present over parietal cells demonstrating that these sites were both saturable and of high affinity. These data provide the first demonstration of gastrin binding sites (putative receptors) on parietal cells in the human stomach and suggest that gastrin acts directly on these cells to help regulate gastric acid secretion and/or mucosal growth.

Time-course of deactivation of rat stomach ECL cells following cholecystokinin B/gastrin receptor blockade

1 The so-called enterochromaffin-like (ECL) cells constitute 65-75% of the endocrine cells in the acid-producing part of the rat stomach. They produce and secrete histamine and pancreastatin, a chromogranin A (CGA)-derived peptide, in response to gastrin, Cholecystokinin (CCK)B/gastrin receptor blockade is known to suppress their activity. 2 We have examined the time course of the deactivation of the ECL cells following treatment with the selective CCKB receptor antagonists RP73870 and YM022. The drugs were given by continuous subcutaneous infusion for a time span of 1 h to 3 weeks and the serum gastrin concentration and various ECL cell parameters were measured (oxyntic mucosal histidine decarboxylase (HDC) activity, histamine and pancreastatin concentrations, HDC mRNA and CGA mRNA levels, and circulating pancreastatin concentration). 3 The two antagonists caused a prompt and dramatic decline in the oxyntic mucosal HDC activity and HDC mRNA level. The HDC activity started to decline after 1-2 h, was reduced by 60-70% after 6 h and was maximally suppressed (80-90%) after 24-48 h. The HDC mRNA level was reduced after 12 h and was at about 20% of the pretreatment level after 2-4 days of infusion. The ECL cell histamine concentration was lowered by about 50% after 7-10 days. 4 RP73870 and YM022 lowered the serum pancreastatin concentration and the oxyntic mucosal CGA mRNA level. The serum pancreastatin concentration was reduced by 40% after 6 h and the reduction was maximal after 2-3 days. A decline in the oxyntic mucosal CGA mRNA level was noted after 12 h with a maximal reduction after 2-4 days of infusion. The ECL cell pancreastatin concentration was reduced by 30-40% after 3 weeks. 5 The infusion of RP73870 and YM022 induced hypergastrinaemia. The serum gastrin concentration started to rise after 2-4 h, there was a 2 fold increase after 6 h and maximal increase (3-4 fold) after 2-3 days of treatment. 6 In conclusion, CCKB/gastrin receptor blockade promptly deactivates the ECL cells. Deactivation, manifested in a greatly reduced HDC activity, was apparent after 1-2 h of the infusion. The serum pancreastatin concentration and the oxyntic mucosal HDC mRNA and CGA mRNA levels were greatly reduced after 1-2 days. The ECL cell concentrations of histamine and pancreastatin declined quite slowly by comparison.

Effect of cholecystokinin-B/gastrin receptor blockade on gastric acid secretion in conscious rats

Gastrin, histamine and acetylcholine are physiological stimuli of gastric acid secretion. The cholecystokinin-B/gastrin receptor antagonists YM022 and RP73870 were used to study the effect of gastrin receptor blockade on acid secretion. Gastrin, histamine, insulin or bethanechol were administered to conscious gastric fistula rats with or without the concomitant intravenous infusion of YM022 or RP73870. Other rats were subjected to pylorus ligation. YM022 and RP73870 inhibited the gastrin-induced acid secretion in a dose- and time-dependent manner; maximal inhibition was observed at a dose of 0.3 mumol.kg-1.hr-1 for both YM022 and RP73870, the ID50 values being 0.02 mumol.kg-1.hr-1 and 0.05 mumol.kg-1.hr-1 for YM022 and RP7870, respectively. At a dose of 0.3 mumol.kg-1.hr-1 YM022 and RP73870 failed to inhibit basal and histamine-, bethanechol-, and insulin-evoked secretion. They also failed to affect the secretion evoked by infusion of a cocktail of maximally effective doses of gastrin-17, histamine and bethanechol. YM022 and RP73870, finally, were without effect on the acid response to pylorus ligation. We suggest that endogenous gastrin in the conscious rat does not contribute to the basal acid secretion and does not participate in the acid response to histamine or to vagus stimulation.

The effect of ethanol, beer, and wine on histamine release from the dog stomach

The mediator for the action of ethanol on the parietal cell of the stomach is not known. However, because the action of ethanol on gastric acid secretion was proposed to involve the release of histamine, we decided to investigate the effects of ethanol and some alcoholic beverages (red wine and beer) on histamine release from the dog stomach. After performing a splenectomy in anaesthetized beagle dogs, the gastrosplenic vein draining the corpus of the stomach was cannulated for blood withdrawal to evaluate the local release of gastrin and histamine by RIA. Intragastric administration of 200 ml of beer (4.8% ethanol) or red wine (12.5% ethanol) caused a significant enhancement in gastrin and histamine concentrations in venous blood from the stomach. By contrast, intragastric administration of pure ethanol in distilled water at the same concentrations of wine or beer did not significantly modify gastrin and histamine release. Integrated histamine responses for 20 min to beer and wine paralleled gastrin concentrations and were of the same magnitude of those induced by intravenous infusion of pentagastrin at 1 and 6 micrograms/ kg/h, respectively. We conclude that: 1) beer and red wine, but not pure ethanol, are potent releasers of histamine; 2) histamine release seems to be related to the gastrin response and probably occurs at the level of enterochromaffin-like (ECL) cells; 3) the ethanol content of these drinks is not important for their stimulant effect, indicating that some other components of beer and wine are responsible for gastrin and histamine release from the dog stomach.

Comparative evaluation of the role of endogenous gastrin in basal acid secretion in conscious rats provided with chronic fistula and pylorus ligation

We determined the relative contributions of endogenous gastrin, histamine and cholinergic tone to basal acid secretion in chronic fistula rats. Results were compared with those for acid secretion in pylorus-ligated rats. In chronic fistula rats, YM022 ¿(R)-1-[2,3-dihydro-1-(2'-methylphenacyl)-2-oxo-5-phenyl-1 H-1,4-benzodiazepin-3-yl]-3-(3-methylphenyl)urea¿ dose-dependently inhibited pentagastrin-stimulated acid secretion and abolished this secretion at 1 mumol/kg, s.c., but did not affect histamine- and carbachol-induced acid secretion even at 10 mumol/kg. In contrast, famotidine at 1 mumol/kg completely inhibited not only the acid secretion induced by histamine but also those by pentagastrin and carbachol. Furthermore, atropine abolished carbachol- and pentagastrin-stimulated acid secretion and significantly suppressed histamine-stimulated acid secretion at 0.1 mumol/kg. YM022 dose-dependently inhibited basal acid secretion. The YM022 dosage required to inhibit basal acid secretion is consistent with that required to suppress pentagastrin-induced acid secretion. Famotidine (1 mumol/kg) and atropine (0.1 mumol/kg) also abolished basal acid secretion. In pylorus-ligated rats, YM022 inhibited acid secretion in a dose-dependent manner; the inhibition at 1 mumol/kg, i.v. was 65%. No additional effect was observed when rats were dosed at 30 mumol/kg. Famotidine partially inhibited acid secretion in these rats, whereas atropine abolished this secretion. These results indicate that the major part of basal acid secretion in rats is attributable to endogenous gastrin via histamine- and cholinergic tone-dependent pathways. Moreover, pylorus ligation reduces the relative contribution of gastrin to acid secretion due to the activation of cholinergic tone.

Gastric acid secretion after depletion of enterochromaffin-like cell histamine. A study with alpha-fluoromethylhistidine in rats

BACKGROUND

Histamine is thought to play a central role in the regulation of gastric acid secretion. In the rat oxyntic mucosa most of the histamine is synthesized and stored in enterochromaffin-like (ECL) cells, and the rest resides in mast cells. The present study examines the role of ECL-cell histamine in the control of acid secretion in the intact, conscious rat.

METHODS

Rats were treated with alpha-fluoromethylhistidine (alpha-FMH) to inhibit histamine synthesis. alpha-FMH was given by continuous subcutaneous infusion (3 mg/kg/h) for up to 9 days. An additional oral dose of alpha-FMH (50 mg/kg) was given 2 h before each acid secretion test. Acid secretion was studied in pylorus-ligated rats and in chronic gastric fistula rats stimulated with histamine, gastrin-17, or insulin after 2-6 days of alpha-FMH infusion.

RESULTS

Treatment with alpha-FMH lowered oxyntic mucosal histamine synthesis by 80%. From previous observations this is thought to reflect depletion of histamine from the ECL cells. The remaining 20% resides in mucosal and submucosal mast cells, which seem to be resistant to alpha-FMH. Basal acid secretion was inhibited by more than 60% after alpha-FMH treatment and by more than 80% by ranitidine. Histamine-stimulated secretion was unaffected by alpha-FMH and abolished by the histamine H2-receptor antagonist ranitidine. The acid response to gastrin-17 was almost abolished in histamine-depleted rats and abolished by ranitidine. Vagally induced acid secretion (provoked by the injection of insulin or by pylorus ligation) was unaffected by alpha-FMH treatment but abolished by ranitidine and by the muscarinic M1-receptor antagonist pirenzepine.

CONCLUSION

The results suggest that gastrin stimulates acid secretion by releasing histamine from ECL cells. Vagally induced acid secretion is also dependent on a histaminergic pathway but not on ECL-cell histamine.

Comparison of effects of famotidine on vagally and field-electrically stimulated acid secretion in the isolated mouse whole stomach

Effects of famotidine on neuronally evoked acid secretion were investigated by means of vagal (at the lower esophagus level) and field-electrical stimulation (around the stomach) in the isolated mouse whole stomach preparation. Each of the electrical stimulations caused a frequency-dependent (1 to 20 Hz) increase in acid output, and the secretory response was abolished by tetrodotoxin or atropine. In the case of field stimulation, the acid secretion was not completely inhibited by hexamethonium. When 10 Hz frequency was applied with either vagal or field-electrical stimulation, the acid secretion was only partly inhibited by famotidine at doses of up to 30 microM. In contrast, the acid response to 2 Hz stimulation was almost completely inhibited by 1 microM famotidine. In the presence of neostigmine (30 nM), the 2 Hz vagally stimulated acid secretion became partly resistant to the effect of famotidine (10 microM). These results suggest that both vagally and field-electrically stimulated acid secretions have essentially the same characteristics and that the secretory mechanism through histamine release is exclusively dominant with weak stimulation, while the cholinergic mechanism on parietal cells is sufficient for reaching the maximal secretory response with strong stimulation.

Ca(2+)-activated outward-rectifier K+ channels and histamine release by rat gastric enterochromaffin-like cells

Gastric enterochromaffin-like (ECL) cells were isolated from rat gastric fundic mucosa by Percoll density-gradient centrifugation and counter-flow elutriation. About 67% of cells in the purified cell suspension were ECL cells, which were reacted with anti-histidine decarboxylase antibody. A23187, a calcium ionophore, at 0.1-10 microM induced histamine release from ECL cell-rich suspension, indicating that the Ca2+ pathway is involved in the mechanism of histamine release from the ECL cells. A23187 at 5 microM significantly increased outward-rectifier cationic current in 62% of cells in the ECL cell-rich factions. A23187-sensitive cells showed acridine orange uptake. In single-channel recordings, a Ca(2+)-dependent outward-rectifier K+ channel of large conductance (146 +/- 22 picosiemens) was found in the cell that showed acridine orange uptake. The channel opened in a voltage-dependent manner at 0.1 microM of intracellular free Ca2+ concentration. These results may suggest that opening of the Ca(2+)-activated K+ channel is one of the steps involved in the mechanism of histamine release in ECL cells.

The role of gastric histamine release in the acid secretory response to pentagastrin and methacholine in the dog

We have previously demonstrated that both pentagastrin and methacholine can stimulate histamine release from the canine stomach during short term administration of the secretagogues into the gastrosplenic artery. In this study we tested the hypothesis that gastric histamine release determines the acid secretory response to acid secretagogues. Increasing doses of pentagastrin (2, 6, and 20 ng/kg/min) and methacholine (0.1, 0.3, and 1 micrograms/min) were infused into the gastrosplenic artery in dogs, while gastric acid output, histamine and N tau-methyl histamine secretory rates were monitored. Histamine and N tau-methyl histamine concentrations in plasma were measured using GC/NICI-MS. Increasing doses of pentagastrin resulted in increasing gastric output. Total histamine secretory rate expressed as the sum of histamine and N tau-methyl histamine secretory rate showed a significant increase above basal with the two highest doses of pentagastrin. Regression analysis correlating the dose of pentagastrin to gastric acid output gave a correlation coefficient of 0.586 which was very significant. Regression analysis correlating the total histamine secretory rate to acid output gave a correlation coefficient of 0.498 which was also very significant. Increasing doses of methacholine also resulted in a dose-dependent increase in acid output. Histamine secretory rates showed a statistically significant increase above basal only at the 1 microgram/min infusion rate, however, the total histamine secretory rates (histamine + N tau-methyl histamine) were no longer significant at any of the doses of methacholine.(ABSTRACT TRUNCATED AT 250 WORDS)

Application of a model to explore interspecies differences in acetylcholine M-receptor-stimulated gastric acid secretion

1. Concentration-effect curves were obtained, in the absence and presence of histamine H2-receptor blockade, to 5-methylfurmethide (5-MeF) and McN-A 343, high efficacy and low efficacy acetylcholine (ACh) M-receptor agonists, respectively, in isolated stomach preparations from the mouse and immature rat and guinea-pig. 2. In the immature guinea-pig assay, the responses to 5-MeF and McN-A 343 were abolished by histamine H2-receptor blockade suggesting that the responses were totally dependent upon gastric mucosal histamine. However, in the mouse and immature rat assays, although the histamine H2-receptor antagonists produced small but significant rightward shifts and, in some cases, depression of the maximum of the agonist concentration-effect curves, a significant secretory response remained, presumed to be due to direct stimulation of oxyntic cells. 3. Previously, by assuming that the histamine H2-receptor blockade alters the mode of agonist-stimulated acid secretion from mainly an indirect action mediated by histamine release to direct stimulation of the oxyntic cell, we applied an operational model of agonism to similar data obtained in the mouse preparation. In that study we were able to account for the behaviour of 5-MeF and McN-A 343 by assuming that the agonists expressed 6 fold higher efficacy, tau in the operational model of agonism, at ACh M-receptors on the histamine-releasing cells than on the oxyntic cells. In this study it was possible to account for the variation in the behaviour of the agonists both between and within assays by simply varying the efficacy expressed by the agonists at each of the cells in the model. The efficacy variation could be due to receptor concentration variation.4. The data and analysis are discussed in terms of contemporary models for the role of histamine in the regulation of gastric acid secretion.

Stimulatory effect of muscimol on gastric acid secretion stimulated by secretagogues in vagotomized rats under anesthesia

The effect of intravenous administration of muscimol, a GABAA receptor agonist, on gastric acid secretion from perfused stomach was studied in vagotomized rats anesthetized with urethane. Muscimol did not stimulate acid secretion by itself. In contrast, muscimol dose dependently potentiated acid secretion induced by pentagastrin, bethanechol and direct vagal stimulation, but not histamine. Muscimol-potentiated acid secretion induced by pentagastrin and bethanechol was not influenced by pretreatment with atropine or cimetidine, respectively. Muscimol-potentiated acid secretion evoked by direct vagal stimulation was prevented by pretreatment with proglumide, a gastrin receptor antagonist. Muscimol-potentiated acid secretion evoked by bethanechol was dose dependently prevented by bicuculline methiodide, suggesting an involvement of peripheral GABAA receptors. These results suggest that muscimol stimulates acid secretion under certain conditions, and that two mechanisms are involved in this effect. The effects of muscimol on acid secretion may be mediated by increasing the release of histamine by pentagastrin, bethanechol and direct vagal stimulation. In addition, muscimol would also be effective if muscarinic agents were already occupying muscarinic acetylcholine receptors on parietal cells.

Comparative analysis of the vagal stimulation of gastric acid secretion in rodent isolated stomach preparations

1. Electrical field stimulation produced a tetrodotoxin-sensitive, frequency-dependent, release of acid from isolated, lumen-perfused, stomach preparations from mouse, immature rat and guinea-pig. 2. In the guinea-pig and mouse preparations, the frequency-dependent response was abolished by hexamethonium, acetylcholine (ACh) muscarinic (M) and histamine H2-receptor blockade, consistent with the hypothesis that the vagal ACh acts indirectly by stimulating the release of endogenous histamine. 3. In contrast, in the rat preparation the frequency-dependent response was partially refractory to all of these inhibitors. However, a combination of H2- and ACh M-receptor blockade did abolish the effect. 4. We conclude that vagal-stimulated acid secretion in the rat, unlike the other two species, behaves as though there is a direct innervation of the oxyntic cells by either cholinergic or noncholinergic neurones.

The mechanism of action of gastrin releasing peptide (GRP) in stimulating avian gastric acid secretion

The mechanisms of action of gastrin and gastrin releasing peptide (GRP) in stimulating gastric acid secretion were examined in the anaesthetized turkey. Gastrin and GRP produced dose-dependent increases in acid secretion that were inhibited by the gastrin/CCK-B antagonist CI988. The antagonist did not affect the acid secretory responses to histamine or carbachol. A GRP antagonist (M216140) inhibited the acid response to GRP, but not gastrin. The results suggest that in birds, GRP stimulates acid secretion by release of gastrin, which acts in turn on classical gastrin/CCK-B receptors in the proventriculus.

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.

Helicobacter pylori potentiates histamine release from serosal rat mast cells in vitro

Helicobacter pylori seems to be involved in the etiology of peptic ulcer and chronic gastritis. Histamine is fundamental in gastric secretion modulation, and some features of H. pylori-associated gastritis (edema, vasodilatation, inflammatory cell infiltration) are typical of the histamine-mediated response. This in vitro study has been undertaken as a preliminary step, in order to find a possible link between H. pylori and histamine release. H. pylori isolated from gastric biopsies has been tested as whole washed bacterium, whole formalin-killed bacterium, and crude cell wall preparation with serosal mast cells obtained by density gradient centrifugation or elutriation from peritoneal and pleural washings of male Wistar albino rats. Histamine release was assayed fluorimetrically. No significant histamine release was obtained by testing the various bacterial preparations alone with mast cells. On the contrary, whole washed cells, whole formalin-killed cells and crude cell walls from H. pylori have been found to potentiate compound 48/80 or calcium ionophore A23187-induced histamine release. Crude cell walls showed the highest activity, whereas filtered supernatants from broth cultures constantly appeared inactive. The enhancement in histamine release differed between the different strains. The present in vitro study, which shows the potentiating effect of H. pylori on histamine release, may prove interesting; however, at present, clinical implications cannot be inferred and further studies as well as in vivo demonstrations are needed.

Comparative effects of cimetidine and famotidine on the vagally stimulated acid secretion in the isolated mouse whole stomach

We investigated the effects of cimetidine and famotidine on the acid secretory response to electrical vagal stimulation, bethanechol and histamine in the isolated mouse whole stomach preparation. The acid secretion elicited by electrical vagal stimulation at the position of the esophagus (10 Hz, 0.3 msec, 10 V for 5 min) was reproducible by repeated stimulation in each preparation, and it was abolished by tetrodotoxin, atropine and hexamethonium. This vagally stimulated acid secretion was abolished by cimetidine (3 mM), while it was only partly inhibited by famotidine (10-100 microM). Histamine (100 microM)-induced acid secretion was inhibited by cimetidine and famotidine, and the doses of these drugs required for complete inhibition were 3 mM and 10 microM, respectively. In contrast, bethanechol (10 microM)-induced acid secretion was slightly reduced by famotidine (1-100 microM), but markedly reduced by cimetidine (3 mM). In the guinea pig ileum, millimolar concentrations of cimetidine and famotidine shifted the dose-response curve of the contractile response to acetylcholine rightward. These findings suggest that the inhibitory effect of cimetidine on the vagally stimulated or bethanechol-induced acid secretion is elicited at least partly through mechanisms different from H2-antagonism.

Hyperplasia of histamine-depleted enterochromaffinlike cells in rat stomach using omeprazole and alpha-fluoromethylhistidine

In the rat, gastric histamine is stored mainly in the enterochromaffinlike cells. Gastrin releases histamine from these cells, and long-term hypergastrinemia results in hyperplasia. The effect of sustained hypergastrinemia on histamine-depleted enterochromaffinlike cells was studied by measuring histidine decarboxylase activity and histamine concentrations and by using quantitative histology. Hypergastrinemia maintained for 6 weeks was induced by inhibition of gastric acid secretion with omeprazole (400 mumol.kg-1.day-1) given orally, and histamine synthesis was inhibited for the same length of time with alpha-fluoromethylhistidine (3 mg.kg-1.h-1) given via osmotic minipumps. In rats given omeprazole alone, the effects of the resulting hypergastrinemia on the enterochromaffinlike cells was reflected in increased histidine decarboxylase activity, increased histamine concentration, and increased number of enterochromaffinlike cells. The general trophic effects on the stomach were seen as increased stomach and oxyntic mucosal weight and increased mucosal thickness. Treatment with alpha-fluoromethylhistidine plus omeprazole markedly reduced the histidine decarboxylase activity and histamine concentration, but the weight of the stomach and oxyntic mucosa, the enterochromaffinlike cell density, and intensity of histidine decarboxylase immunostaining were increased to at least the same extent as after omeprazole alone. These observations indicate that enterochromaffinlike cell histamine is not important for a full expression of gastrin-evoked trophic effects in the stomach.

Effects of gastrin on cytosolic free Ca2+ in individual, acid-secreting rat parietal cells

The effects of gastrin on cytosolic free Ca2+ ([Ca2+]i) in single, isolated rat gastric parietal cells were investigated using the fluorescent probe Fura-2 and digital image analysis. [Ca2+]i was increased by gastrin (100 nM) in approximately 30% of the parietal cells, which were identified by using either the fluorescent probe acridine orange or a parietal cell-specific monoclonal antibody. In the dominant pattern observed, [Ca2+]i was elevated 50-150% and returned within 1-2 min to a value 30-60% over the basal, which was sustained until withdrawal of the stimulant or addition of the gastrin inhibitor L-365,260 (1 microM). The second, but not the first phase, was abolished in the absence of extracellular Ca2+. The results indicate the existence of functional gastrin receptors in a subpopulation of rat parietal cells.