The interaction of histamine with gastrin and carbamylcholine on oxygen uptake by isolated mammalian parietal cells

Gastrin receptor pharmacology

C-terminally amidated gastrins act at cholecystokinin-2 receptors (CCK2R), which are normally expressed by gastric parietal and enterochromaffin-like (ECL) cells and smooth muscle; there is also extensive expression in the CNS where the main endogenous ligand is cholecystokinin. A variety of neoplasms express CCK2R, or splice variants, including neuroendocrine, pancreatic, medullary thyroid and lung cancers. Other products of the gastrin gene (progastrin, the Gly-gastrins) may stimulate cell proliferation but are not CCK2R ligands. Depending on the cell type, stimulation of CCK2R evokes secretion, increases proliferation and cell migration, inhibits apoptosis, and controls the expression of various genes. These effects are mediated by increased intracellular calcium and activation of protein kinase C, MAPkinase and other protein kinase cascades. There has been recent progress in developing CCK2R ligands that can be used for imaging tumours expressing the receptor. New antagonists have also been developed, and there is scope for using these for suppression of gastric acid and for treatment of neuroendocrine and other CCK2R-expressing tumours.

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

Role of taurine on acid secretion in the rat stomach

Background

Taurine has chemical structure similar to an inhibitory neurotransmitter, γ-aminobutyric acid (GABA). Previous studies on GABA in the stomach suggest GABAergic neuron is involved in acid secretion, but the effects of taurine are poor understood.

Methods

The effects of taurine on acid secretion, signal transduction, and localization of taurinergic neurons were determined in the rat stomach using everted whole stomach, RIA kit and immunohistochemical methods.

Results

We used antibodies against taurine-synthesizing enzyme, cysteine sulfuric acid decarboxylase (CSAD), and taurine. CSAD- and taurine-positive cells were found in the muscle and mucosal layers. Distributions of CSAD- and taurine-positive cells in both mucosal and muscle layers were heterogeneous in the stomach. Taurine at 10^-9~10^-4 M induced acid secretion, and the maximum secretion was at 10^-5 M, 1.6-fold higher than the spontaneous secretion. Taurine-induced acid secretion was completely inhibited by bicuculline and atropine but not by cimetidine, proglumide, or strychnine. Atropine and tetrodotoxin (TTX) completely inhibited the acid secretion induced by low concentrations of taurine and partially inhibited induced by high concentrations. Verapamil, a calcium blocker agent, inhibited acid output elicited by taurine. We assumed all Ca²⁺ channels involved in the response to these secretagogues were equally affected by verapamil. Intracellular cAMP (adenosine 3', 5'-monophosphat) in the stomach significantly increased with taurine treatment in a dose-dependent manner. High correlation (r=0.859, p < 0.001) of taurine concentrations with cAMP was observed.

Conclusions

Our results demonstrated for the first time in taurine-induced acid secretion due to increase intracellular calcium may act through the A type of GABA receptors, which are mainly located on cholinergic neurons though cAMP pathway and partially on nonneuronal cells in the rat stomach.

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.

Reduced Shh expression in TFF2-overexpressing lesions of the gastric fundus under hypochlorhydric conditions

Expression of sonic hedgehog (Shh), a morphogen for the gastric fundic glands, is reduced in the atrophic mucosa that develops in association with Helicobacter pylori infection, resulting in impaired differentiation of the fundic gland cells, increased expression of trefoil factor family 2 (TFF2) and the formation of spasmolytic polypeptide (SP)-expressing metaplasia (SPEM), a preneoplastic lesion. However, it is still unresolved whether H. pylori-induced inflammation and the resultant reduction in parietal cell number or reduced parietal cell function per se reduces Shh expression. The present study was designed to clarify the expression of Shh and TFF2 in the context of parietal cell dysfunction in the absence of inflammation, using histamine H(2) receptor-knockout (H(2)R-null) mice and an acid exposure model. Age-matched H(2)R-null mice and wild-type (WT) mice were used. The expression of Shh and TFF2 mRNA was quantified by quantitative RT-PCR. Immunohistochemistry was also performed to detect the expression of Shh, TFF2 and cell markers. To study the effects of acid exposure, HCl solution was administered to the animals. The H(2)R-null mice exhibited higher gastric pH, increased TFF2 expression and reduced Shh expression. Impaired mucous neck-to-zymogenic cell differentiation was observed in the H(2)R-null mice. Furthermore, Shh expression increased in the presence of gastric acid and showed a significant correlation with gastric surface pH. In conclusion, our results suggest that persistent parietal cell dysfunction alone (suppressed gastric acid secretion), in the absence of inflammation or parietal cell loss caused by H. pylori infection, may be sufficient to down-regulate Shh expression in TFF2-overexpressing preneoplastic lesions of the gastric fundus. Since exposure to acid restored fundic Shh expression, appropriate gastric acid secretion may play an important role in the morphogen dynamics involved in the maintenance of gastric fundic gland homeostasis.

Structural and functional characterization of gastric mucosa and central nervous system in histamine H2 receptor-null mice

To examine the physiological role of the histamine H(2) receptor, histamine H(2) receptor-null mice were generated by homologous recombination. Histamine H(2) receptor-null mice, which developed normally and were fertile and healthy into adulthood, exhibited markedly enlarged stomachs and marked hypergastrinemia. The former was due to hyperplasia of gastric gland cells (small-sized parietal cells, enterochromaffin-like cells and mucous neck cells which were rich in mucin), but not of gastric surface mucous cells, which were not increased in number as compared with those in wild-type mice despite the marked hypergastrinemia. Basal gastric pH was slightly but significantly higher in histamine H(2) receptor-null mice. Although carbachol but not gastrin induced in vivo gastric acid production in histamine H(2) receptor-null mice, gastric pH was elevated by both muscarinic M(3) and gastrin antagonists. Thus, both gastrin and muscarinic receptors appear to be directly involved in maintaining gastric pH in histamine H(2) receptor-null mice. Interestingly, gastric glands from wild-type mice treated with an extremely high dose of subcutaneous lansoprazole (10 mg/kg body weight) for 3 months were very similar to those from histamine H(2) receptor-null mice. Except for hyperplasia of gastric surface mucous cells, the findings for gastric glands from lansoprazole-treated wild-type mice were almost identical to those from gastric glands from histamine H(2) receptor-null mice. Therefore, it is possible that the abnormal gastric glands in histamine H(2) receptor-null mice are secondary to the severe impairment of gastric acid production, induced by the histamine H(2) receptor disruption causing marked hypergastrinemia. Analyses of the central nervous system (CNS) of histamine H(2) receptor-null mice revealed these mice to be different from wild-type mice in terms of spontaneous locomotor activity and higher thresholds for electrically induced convulsions. Taken together, these results suggest that (1) gastrin receptors are functional in parietal cells in histamine H(2) receptor-null mice, (2) abnormal gastric glands in histamine H(2) receptor-null mice may be secondary to severe impairment of gastric acid production and secretion and (3) histamine H(2) receptors are functional in the central nervous system.

Control of gastric acid secretion:the gastrin-ECL cell-parietal cell axis

Gastric acid secretion is under nervous and hormonal control. Gastrin, the major circulating stimulus of acid secretion, probably does not stimulate the parietal cells directly but acts to mobilize histamine from the ECL cells in the oxyntic mucosa. Histamine stimulates the parietal cells to secrete HCl. The gastrin-ECL cell pathway has been investigated extensively in situ (gastric submucosal microdialysis), in vitro (isolated ECL cells) and in vivo (intact animals). Gastrin acts on CCK2 receptors to control the synthesis of ECL-cell histamine, accelerating the expression of the histamine-forming enzyme histidine decarboxylase (HDC) at both the transcription and the translation/posttranslation levels. Depletion of histamine by alpha-fluoromethylhistidine (an irreversible inhibitor of HDC) prevents gastrin-induced but not histamine-induced gastric acid secretion. Acute CCK2 receptor blockade inhibits gastrin-evoked but not histamine-induced acid secretion. Studies both in vivo/in situ and in vitro have suggested that while acetylcholine seems capable of activating parietal cells, it does not affect histamine secretion from ECL cells. Unlike acetylcholine, the neuropeptides pituitary adenylate cyclase-activating peptide and vasoactive intestinal peptide mobilize ECL-cell histamine. Whether vagally stimulated acid secretion reflects an effect of the enteric nervous system on the ECL cells (neuropeptides) and/or a direct one on the parietal cells needs to be further investigated.

Abnormal functional and morphological regulation of the gastric mucosa in histamine H2 receptor-deficient mice

To clarify the physiological roles of histamine H2 receptor (H2R), we have generated histamine H2R-deficient mice by gene targeting. Homozygous mutant mice were viable and fertile without apparent abnormalities and, unexpectedly, showed normal basal gastric pH. However, the H2R-deficient mice exhibited a marked hypertrophy with enlarged folds in gastric mucosa and an elevated serum gastrin level. Immunohistochemical analysis revealed increased numbers of parietal and enterochromaffin-like (ECL) cells. Despite this hypertrophy, parietal cells in mutant mice were significantly smaller than in wild-type mice and contained enlarged secretory canaliculi with a lower density of microvilli and few typical tubulovesicles in the narrow cytoplasm. Induction of gastric acid secretion by histamine or gastrin was completely abolished in the mutant mice, but carbachol still induced acid secretion. The present study clearly demonstrates that H2R-mediated signal(s) are required for cellular homeostasis of the gastric mucosa and normally formed secretory membranes in parietal cells. Moreover, impaired acid secretion due to the absence of H2R could be overcome by the signals from cholinergic receptors.

Inhibition of gastric acid secretion in rat stomach by PACAP is mediated by secretin, somatostatin, and PGE(2)

Pituitary adenylate cyclase-activating polypeptide (PACAP), existing in two variants, PACAP-27 and PACAP-38, is found in the enteric nervous system and regulates function of the digestive system. However, the regulatory mechanism of PACAP on gastric acid secretion has not been well elucidated. We investigated the inhibitory action of PACAP-27 on acid secretion and its mechanism in isolated vascularly perfused rat stomach. PACAP-27 in four graded doses (5, 10, 20, and 50 microg/h) was vascularly infused to determine its effect on basal and pentagastrin (50 ng/h)-stimulated acid secretion. To study the inhibitory mechanism of PACAP-27 on acid secretion, a rabbit antisecretin serum, antisomatostatin serum, or indomethacin was administered. Concentrations of secretin, somatostatin, PGE(2), and histamine in portal venous effluent were measured by RIA. PACAP-27 dose-dependently inhibited both basal and pentagastrin-stimulated acid secretion. PACAP-27 at 10 microg/h significantly increased concentrations of secretin, somatostatin, and PGE(2) in basal or pentagastrin-stimulated state. The inhibitory effect of PACAP-27 on pentagastrin-stimulated acid secretion was reversed 33% by an antisecretin serum, 80.0% by an antisomatostatin serum, and 46.1% by indomethacin. The antisecretin serum partially reduced PACAP-27-induced local release of somatostatin and PGE(2). PACAP-27 at 10 microg/h elevated histamine level in portal venous effluent, which was further increased by antisomatostatin serum. However, antisomatostatin serum did not significantly increase acid secretion. It is concluded that PACAP-27 inhibits both basal and pentagastrin-stimulated gastric acid secretion. The effect of PACAP-27 is mediated by local release of secretin, somatostatin, and PGE(2) in isolated perfused rat stomach. The increase in somatostatin and PGE(2) levels in portal venous effluent is, in part, attributable to local action of the endogenous secretin.

Up-regulation of H2 receptor and adenylate cyclase in rabbit parietal cells during prolonged treatment with H2-receptor antagonists

Intragastric hyperacidity occurs after abrupt withdrawal of histamine H2-receptor antagonists, and the prolonged administration of these agents induces tachyphylaxis of the inhibitory effects on gastric acid secretion. We examined the effect of the prolonged administration of H2-receptor antagonists on the H2-receptor signaling system in parietal cells isolated from rabbits that had received H2-receptor antagonists for 14 days. [125I]aminopotentidine (APT) binding sites to H2 receptors in parietal cell membranes were increased without any significant change in the affinity for [125I]APT. The expression of Gs(alpha), guanosine triphosphate (GTP)-binding protein coupled to H2 receptor, was slightly increased. Basal as well as GTP- or histamine-stimulated cAMP production was increased, but no significant change was observed in the presence of an H2-receptor antagonist. The up-regulation of the H2 receptor and adenylate cyclase appeared to cause hypersecretion of acid after withdrawal of H2-receptor blockade.

Enterochromaffin-like cells, a cellular source of uroguanylin in rat stomach

Uroguanylin is an endogenous peptide ligand for guanylyl cyclase-C, an apical membrane receptor predominantly located in the gastrointestinal epithelium. It regulates intestinal and renal fluid and electrolyte transport through the second messenger, cyclic GMP. Uroguanylin messenger RNA and the peptide are present in rat stomach, but the cellular source has not been identified. We separated gastric mucosal cells by size into seven fractions (F1-F7) and enriched endocrine cells into F1-F3 using counterflow elutriation. Uroguanylin messenger RNA and peptide were found in F1-F3 by Northern blot analysis and an RIA specific for rat uroguanylin. Uroguanylin-producing cells were identified as endocrine cells by immunocytochemical methods using antisera for uroguanylin, prouroguanylin, and chromogranin A, as well as by in situ hybridization cytochemistry. Double-staining showed that uroguanylin and histamine are colocalized in enterochromaffin-like (ECL) cells that release histamine, leading to the stimulation of gastric acid secretion from parietal cells. Uroguanylin is synthesized in ECL cells. These findings should contribute to elucidating the physiological functions of ECL cells and the cyclic GMP-mediated gastric ion transport mechanism.

L-365,260 inhibits in vitro acid secretion by interacting with a PKA pathway

The aim of this study was to analyse the antisecretory mechanism of L-365,260 in vitro in isolated rabbit gastric glands. We showed that compound L-365,260, described as a non-peptide specific competitive CCK-B receptor antagonist, was able to dose-dependently inhibit [14C]-aminopyrine accumulation induced by histamine (10(-4) M), carbachol (5x10(-5) M), 3-isobutyl-1-methyl-xanthine (IBMX) (5x10(-6) M) and forskolin (5x10(-7) M) with similar IC50 values respectively of 1.1+/-0.6x10(-7) M, 1.9+/-1.2x10(-7) M, 4.2+/-2.0x10(-7) M and 4.0+/-2.8x10(-7) M. We showed that L-365,260 acted beyond receptor activation and production of intracellular second messengers and that it had no action on the H+/K+ -ATPase. We found that L-365,260 inhibited cyclic AMP-induced [14C]-aminopyrine accumulation in digitonin-permeabilized rabbit gastric glands, suggesting that this compound acted, at least in part, as an inhibitor of the cyclic AMP-dependent protein kinase (PKA) pathway.

Effect of excitatory amino acid neurotransmitters on acid secretion in the rat stomach

Excitatory amino acids (EAAs), in particular, L-aspartate (L-Asp) neurons and their processes, were localized in the rat stomach using a immunohistochemical method with specific antibodies against either L-Asp or its synthesizing enzyme, aspartate aminotransferase (AAT). Myenteric ganglia and nerve bundles in the circular muscle and in the longitudinal muscle were found to be AAT- or L-Asp-positive. In addition, AAT- or L-Asp-positive cells were also found in the muscle layer and the deep mucosal layer. The distribution of AAT- or L-Asp-positive cells in both the mucosal and muscle layers was heterogeneous in the stomach. In addition, L-Asp at 10(-6) M negligibly influenced acid secretion in an everted preparation of isolated rat stomach. However, according to our results, L-Asp markedly inhibited the histamine-stimulated acid secretion, but not the oxotremorine- or the pentagastrin-stimulated acid secretion. Furthermore, L-Asp also inhibited histamine-induced elevation of cAMP. L- Asp itself did not affect the cAMP level although it elevated the cGMP level in the stomach. Moreover, either (+)2-amino-5-phosphonovaleric acid or (+/-)3-(2-carboxypiperazin-4-yl)prophyl-1-phosphonic acid, i.e. two specific antagonists for N-methyl-D-aspartic acid (NMDA) receptors, blocked the inhibitory effect of L-Asp on histamine-stimulated acid secretion or histamine-induced elevation of cAMP. Since cAMP has been strongly implicated as the second messenger involved in histamine-induced acid secretion, we believe that L-Asp regulates acid secretion in the stomach by inhibiting histamine release through the NMDA receptors, subsequently lowering the level of cAMP and ultimately reducing acid secretion.

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.

Characterization of somatostatin receptor subtypes mediating inhibition of nutrient-stimulated gastric acid and gastrin in dogs

Five somatostatin receptor subtypes (SSTR) have been cloned and characterized in various tissues, including the gastrointestinal tract. This study examined which receptor subtypes mediate the inhibitory actions of somatostatin on gastric acid secretion and gastrin release in conscious dogs. Peptide agonists with relatively high specificity for SSTR1-5 (somatostatin-14), SSTR2 (MK-678), SSTR3 (L-362823), and SSTR5 (L-362855) were infused i.v. after nutrient-stimulated gastric acid secretion and gastrin release with intraduodenal perfusions of 8% peptone and after secretagogue-stimulated acid secretion with gastrin (75 pmol kg-1 h-1) or histamine (20 micrograms kg-1 h-1). At 1000 pmol kg-1 h-1, the SSTR2 agonist inhibited peptone-stimulated acid output to baseline (P < 0.001), whereas the SSTR3 agonist decreased acid output by 58 +/- 6% (P < 0.01): the SSTR5 agonist was without effect. The SSTR2 agonist at 100 pmol kg-1 h-1 also abolished the rise of plasma gastrin. At 50 pmol kg-1 h-1 i.v. infusions of S-14, to simulate circulating S-14 rises after nutrients, decreased peptone-stimulated acid secretion by 58 +/- 8% (P < 0.01), whereas the SSTR2 agonist inhibited gastric acid by 96 +/- 2% (P < 0.001); the SSTR3 agonist was without effect. S-14 or the agonists at 50 pmol kg-1 h-1 did not alter elevations of plasma gastrin. S-14 and the SSTR2 agonist at 50 pmol kg-1 h-1 decreased gastrin-stimulated acid secretion by 42 +/- 8% (P < 0.01) and 78 +/- 4% (P < 0.001), respectively but the SSTR3 and SSTR5 agonists were without effect. In contrast, histamine-stimulated acid secretion was not altered by 1000 pmol kg-1 h-1 S-14 or the agonists. These results in conscious dogs suggest that the inhibitory actions of circulating S-14 on nutrient and gastrin-stimulated acid secretion include activation of the SSTR-2 subtype. Regulation of gastrin release by S-14 may also occur via SSTR-2, but not through an endocrine mechanism. Factors in addition to gastrin and histamine modulate intestinal protein-stimulated acid secretion yet include peripheral S-14 inhibition via SSTR2 activation.

Physiology of isolated gastric endocrine cells

The regulation of gastric acid secretion is achieved in the periphery by interplay between three major gastric endocrine cells: the enterochromaffin-like (ECL) cell, the gastrin or G cell, and the somatostatin or D cell. Regulation of these cells is via stimulatory or inhibitory paracrine, endocrine, and neural pathways. Upregulation of ECL function is determined by activation of CCK-B receptors, by gastrin, and by activation of beta-adrenergic receptors, as well as by acetylcholine in some (10-29%) of the cells. Gastrin and acetylcholine produce typical biphasic calcium signals. Inhibition of ECL cell histamine release and calcium signaling is produced by somatostatin acting at a type 2 receptor, histamine acting at a histamine-3 receptor, and by peptide PYY. Stimulation of ECL cells results in activation of chloride channels, and there is evidence that voltage-dependent calcium channels, along with the receptor-operated calcium channels, also are responsible for elevation of [Ca]i. Depolarization-activated K+ channels presumably restore the potential after depolarization by activation of the chloride channel. The D cell is activated by either gastrin or CCK and appears to be inhibited by acetylcholine and somatostatin. The G cell is activated by acetylcholine and gastrin-releasing peptide (GRP) and is inhibited by somatostatin. The functional integration of these three cell types is the primary determinant of the degree of stimulation of the parietal cell.

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.

Are C-terminal octapeptide of cholecystokinin and [Leu11]gastrin-(5-17) different in stimulating acid secretion in isolated rabbit gastric glands?

In the present study we compared various CCK(B) receptor antagonists and tried to detect a difference in biological activity between the C-terminal octapeptides of cholecystokinin (CCK-8) and [Leu11]gastrin-(5-17) in isolated rabbit gastric glands. Binding experiments showed that different CCK(B)/gastrin receptor agonists bound with high affinity and that antagonists inhibited this binding in accordance with a CCK(B)/gastrin pharmacological profile. [Leu11]gastrin-(5-17), CCK-8 and cionin were found to induce [14C]aminopyrine accumulation to 25% above the basal level. Under the same experimental conditions, histamine induced a response twice as great as the response obtained with [Leu11]gastrin-(5-17) or CCK-8. [Leu11]gastrin-(5-17) (10(-7) M), CCK-8 (10(-8) M) and cionin (10(-8) M) appeared to be full agonists. CCK(B)/gastrin receptor antagonists including L-365,260 (3R-(+)-N-(2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzodiazepin++ +-3-yl)-N-(3-methylphenyl) urea), L-364,718 (3S-(-)-N-(2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzodiazepin++ +-3-yl)-1H-indole-2-carboximide) (a selective CCK(A) receptor antagonist), PD-135,158 (4([2-[[3-(1H-indol-3-yl)-2-methyl-1-oxo-2-[[[1.7.7-trimethyl-bicyclo[2. 2.1]hept-2-yl)oxy]carbonyl]amino]propyl]amino]-1-phenylethyl] amino-4-oxo-[1S-1alpha.2beta[S*(S*)]4alpha]]-butano nate N-methyl-D-glucamine) (bicyclo system 1S-endo), YM-022 ((R)-1-[2,3-dihydro-1-(2'-methylphenacyl)-2-oxo-5-phenyl-1H-1,4-++ +benzodiazepin-3-yl]-3-(3-methylphenyl)urea) and JMV-180 (Boc-Tyr(SO3H)-Nle-Gly-Trp-Nle-Asp-O-CH2-CH2-C6H5) exhibited the same profile for inhibition of [Leu11]gastrin-(5-17) or CCK-8-induced [14C]aminopyrine accumulation in rabbit gastric glands. These results suggested that [Leu11]gastrin-(5-17) and CCK-8 induced [14C]aminopyrine accumulation by the same mechanism. [Leu11]gastrin-(5-17)- or CCK-8-induced [14C]aminopyrine accumulation was inhibited by about 40% by the histamine H2 receptor blocker cimetidine. These results are consistent with there being cooperativity between [Leu11]gastrin-(5-17) (or CCK-8) and histamine in the acid secretory pathway. Similarly, the CCK(B)/gastrin receptor antagonists were tested against histamine-induced [14C]aminopyrine accumulation and surprisingly, only compound L-365,260 appeared active and even more potent than cimetidine.

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.

Inhibitory effect of leminoprazole on acid secretion in parietal cells isolated from guinea pig gastric mucosa

Isolated guinea pig parietal cells, the function of which is similar to that of human parietal cells, were used in this study. The accumulation of 14C-aminopyrine (14C-AP) was measured to study the inhibitory mechanism of leminoprazole in cells. Stimulation by 10 microM histamine, 0.1 mM carbachol, 1 microM gastrin or 1 mM db-cAMP brought about satisfactory incorporation of 14C-AP, and leminoprazole concentration-dependently inhibited acid secretion induced by these stimulants. At 10(-5) M, almost 100% inhibition was observed. The IC50 values of leminoprazole obtained from its inhibitory action on histamine, carbachol, gastrin and db-cAMP-stimulated acid secretion were 4.0 x 10(-7) M, 3.5 x 10(-7) M, 2.5 x 10(-7) M and 5.6 x 10(-7) M, respectively. Thus the extent of inhibition was the same for the responses to all the secretagogues. These results indicate that the site of action of leminoprazole is intracellular and distal from cAMP (intracellular second messenger), but not at the receptor sites. The results also strongly suggest that the inhibitory action of leminoprazole on H+,K(+)-ATPase may contribute to the inhibitory effect of this drug on gastric acid secretion.

Stimulatory effect of dopamine on acid secretion from the isolated rat stomach

The effect of dopamine (DA) on acid secretion was studied using the everted preparation of isolated rat stomachs. DA concentrations, measured by HPLC-ECD in the rumen, corpus and antrum were 1.06 nmol/mg protein, 0.49 nmol/mg protein and 2.92 nmol/mg protein, respectively. DA stimulated acid secretion at a concentration of 10 nM and elicited the maximum response at 10 microM, which was at a level approximately 1.56-fold that of the spontaneous secretion but only about half that of secretion induced by histamine at a concentration of 0.3 mM. The concentration-dependent stimulation by DA was antagonized by octopamine and SCH 23390. Failure of proglumide and cimetidine to affect this stimulation ruled out the participation of histamine and/or gastrin. Scopolamine and tetrodotoxin completely inhibited the acid secretion induced by low concentrations of DA but inhibited only partially the response induced by high concentrations of DA. The results obtained indicate that DA induces acid secretion via activation of the dopamine D1 receptor, located on the cholinergic neurons and on some nonneuronal cells, in the rat stomach.

Synthesis and characterization of a new labeled gastrin ligand, 125-I-BH-[Leu15]-gastrin-(5-17), on binding to canine fundic mucosal cells and Jurkat cells

In the course of our study concerning gastrin and cholecystokinin (CCK) receptors, we have synthesized and characterized a new labeled gastrin ligand, 125I-BH-[Leu15]-gastrin-(5-17) [(3-[125I]iodo-4-hydroxyphenyl)-propionyl-[Leu15]-gastrin-(5-17)]. Binding of 125I-BH-[Leu15]-gastrin-(5-17) to isolated canine fundic mucosal cells was specific, saturable and of high affinity. 125I-BH-[Leu15]-gastrin- (5-17) and 125I-BH-CCK-8[(3-[125I]iodo-4-hydroxyphenyl)-propionyl-CCK-8] interact with isolated canine fundic mucosal cells with small differences in maximal binding capacities and affinities, 3800 +/- 900 binding sites/cell (Kd = 0.52 +/- 0.23 nM) and 6200 +/- 1100 binding sites/cell (Kd = 0.31 +/- 0.18 nM), respectively. The relative order of potencies for gastrin and CCK analogs in displacing 125I-BH-[Leu15]-gastrin-(5-17) binding correlated well with those obtained using 125I-BH-CCK-8. Selective CCK/gastrin antagonists L-364,718 (MK-329) and L-365,260 also inhibited 125I-BH-[Leu15]-gastrin-(5-17) binding. These results indicate that 125I-BH-[Leu15]-gastrin-(5-17) binds to gastrin receptors in isolated canine fundic mucosal cells. We have also characterized 125I-BH-[Leu15]-gastrin-(5-17) binding to the human Jurkat lymphoblastic cell line (Jurkat cells) known to express the CCK-B/gastrin receptor. Saturation experiments have shown that both 125I-BH-[Leu15]-gastrin-(5-17) and 125I-BH-CCK-8 interact with a single class of high-affinity binding sites in the Jurkat cell line.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of L-glutamic acid on acid secretion and immunohistochemical localization of glutamatergic neurons in the rat stomach

Glutamatergic neurons in the rat stomach were localized immunohistochemically using antibodies against L-glutamate (L-Glu) as well as glutamate synthesizing enzyme, glutaminase (GLNase). Myenteric ganglia and nerve bundles in the circular muscle and the longitudinal muscle were found to contain GLU- and GLNase-positive nerve fibers, while submucosa and mucosa were devoid of glutamatergic innervation. The distribution of glutamatergic neurons and their processes in both myenteric ganglia and circular muscle is heterogeneous within the stomach. The effect of L-Glu on gastric acid secretion was investigated on an everted preparation of isolated rat stomach. L-Glu at 10(-7) and 10(-8) M alone had no effect on acid secretion. It was found that the oxotremorine-, histamine-, or gastrin-stimulated acid secretion was markedly reduced by L-Glu at 10(-8) M, whereas L-Glu had little effect on the acid secretion stimulated by dimethyl-phenylpiperazinium (DMPP) at this concentration. However, at higher concentration, e.g., 10(-7) M, L-Glu also markedly reduced DMPP-induced acid secretion. Among L-Glu receptor agonists tested, quisqualic acid (QA) is most potent, followed by kainic acid (KA) and N-methyl-D-aspartic acid (NMDA) in inhibiting oxotremorine-stimulated acid secretion. Furthermore, this inhibitory effect of L-Glu on oxotremorine-stimulated acid secretion is blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a specific non-NMDA receptor antagonist. All these results suggest that glutamatergic neurons are involved in the modulation of gastric acid secretion via ionotropic QA/KA receptors, probably through openings of Ca2+ channels.

Inhibitory effect of DS-4574, a mast cell stabilizer with peptidoleukotriene receptor antagonism, on gastric acid secretion in rats

We examined the inhibitory effect of DS-4574 (6-(2-cyclohexylethyl)[1,3,4]thiadiazolo[3,2-alpha]-1,2,3- triazolo[4,5-d] pyrimidin-9(3H)-one), a mast cell stabilizer with peptidoleukotriene receptor antagonism, on gastric acid secretion stimulated by several secretagogues in rats. In anesthetized rats with acute gastric fistulas, DS-4574 (50 mg/kg, intraduodenal) significantly inhibited gastric acid secretion induced by both carbachol (50 micrograms/kg, s.c.) and pentagastrin (75 micrograms/kg, s.c.) but not by histamine (2.5 mg/kg, s.c.). In unanesthetized pylorus-ligated rats, DS-4574 (10 and 25 mg/kg, intraduodenal) markedly suppressed increases in gastric acid output and histamine leakage into the gastric juice produced by carbachol (0.1 mg/kg, s.c.) or pentagastrin (1 mg/kg, s.c.). When the relationship between acid output and histamine leakage elicited by carbachol and pentagastrin was assessed, there was a close correlation (r = 0.84) that was highly significant (P < 0.01). In the in vitro study with rat gastric tissues, DS-4574 (10(-7)-10(-5) M) had no effect on the K(+)-dependent ATPase activity or on aminopyrine uptake into mucosal preparations containing parietal cells stimulated by carbachol (10(-5) M), histamine (10(-4) M), or dibutyryl-cyclic AMP (10(-3) M). These results suggest that the effect of DS-4574 may be mediated by inhibition of endogenous histamine from histamine-storing cells in the stomach.

Gut hormones in gastric function

This chapter has focused on many of the gut hormones that regulate gastric function. Gastrin remains the principal, and only, gastric hormone controlling gastric acid secretion during the cephalic, gastric and intestinal phases of secretion. Several other hormones, including cholecystokinin, peptide YY and secretin, released from intestinal endocrine cells in response to food substrates, have significant inhibitory effects on gastric acid secretion. Many of these hormones, including enteroglucagon and glucagon-like peptide, may act through paracrine release of somatostatin, which in turn acts as the final mediator of acid inhibition. In addition, several peptides contained in nerves, including gastrin releasing peptide and vasoactive intestinal peptide, have been shown to regulate gastric acid secretion and motor function. With the creation of specific monoclonal antibodies for use in in vivo immunoneutralization studies, and the development of selective chemical antagonists for use in receptor blockade experiments, the specific contributions of the different gut hormones in the regulation of gastric function, can be assessed.

Gastrin action on aminopyrine accumulation in isolated pig parietal cells requires cAMP

The mechanism of action of gastrin on pig parietal cells was investigated. The aminopyrine accumulation technique was used to estimate acid production in gastric mucosal cells, containing 10-20% parietal cells, and in enriched parietal cells, containing 65-95% parietal cells. The gastrin analogue pentagastrin stimulated aminopyrine accumulation in a dose-dependent fashion irrespective of the proportion of non-parietal cells present. The apparent EC50 for pentagastrin was 5 nM and the maximally effective concentration was 100 nM. The histamine H2-receptor antagonist ranitidine did not affect the action of pentagastrin. The stimulatory effects of various doses of histamine on aminopyrine accumulation in highly enriched parietal cells were potentiated by the inclusion of 100 nM pentagastrin in the incubation medium. In another series of experiments using mucosal cells, the action of effective doses of pentagastrin were potentiated by the phosphodiesterase inhibitor isobutylmethyl xanthine (IBMX), which alone elicited an aminopyrine accumulation equal to 50% of that obtained by 100 microM histamine. When ranitidine (100 microM) was included, the action of IBMX was almost completely abolished. However, the dose-response curve for pentagastrin in the presence of ranitidine plus IBMX was similar to that obtained in the absence of IBMX. Dibutyryl-cAMP (DBcAMP, 1 mM) in the presence of ranitidine (100 microM) also potentiated the action of all effective doses of pentagastrin on mucosal cells. The protein kinase A inhibitor Rp-cAMPS, present at 500 microM in the incubation medium, significantly reduced the action of each effective concentration of pentagastrin on aminopyrine accumulation in enriched parietal cells. These results in pig parietal cells were interpreted as indicative of: (i) an action of gastrin exerted directly on the parietal cells; (ii) elevation of intracellular cAMP having a permissive role in the action of gastrin on aminopyrine accumulation.

The enterochromaffin-like (ECL) cell. Physiological and pathophysiological role

Histamine has a central role in the regulation of gastric acid secretion. This histamine is produced by and released from the enterochromaffin-like (ECL) cell which accordingly has a key-regulatory role in the oxyntic mucosa. Gastrin and the vagal nerves stimulate the formation and release of histamine from the ECL cell. Moreover, gastrin and the vagal nerves also stimulate the proliferation of the ECL cell. An increased ECL cell density may partly explain the increased acid secretion in patients with duodenal ulcer, particularly in patients with Zollinger-Ellison syndrome. The reduced potency of histamine-2 blockers in patients with Zollinger-Ellison syndrome is probably due to increased histamine release by an elevated ECL cell mass. Prolonged and profound hypergastrinemia may lead to ECLomas. Moreover, a proportion of diffuse gastric carcinomas may originate from ECL cells.

Verapamil decreases ethanol-induced gastric acid secretion in rats

The present study examined the effect of verapamil, a calcium channel blocker, on gastric acid secretion and circulating gastrin levels in rats after ethanol challenge. Normal saline or verapamil were given intraperitoneally to different groups of rats at 1 min, 1 h or 2 h before the administration of ethanol. One hour later, gastrin and gastric acid concentrations were determined. Regression analysis revealed the relationship between gastric acid output and serum gastrin levels in the group receiving saline intraperitoneally and ethanol orogastrically and the group receiving saline both intraperitoneally and orogastrically is similar. The slope of the regression line of the ethanol-challenged group treated with verapamil, however, was significantly lower than the slopes of the other two groups (P less than 0.001). Furthermore, verapamil decreased gastrin levels and acid output significantly in the ethanol-challenged group (P less than 0.01). When given 10 min prior to ethanol challenge, verapamil had a greater acid suppression effect than when given 60 or 120 min before the challenge. Verapamil at 20 mg/kg was more effective in acid secretion than at 10 mg/kg bodyweight, when administered 60 min before ethanol challenge.

Acid secretion and suppression

Hydrochloric acid is involved in the causation of peptic ulcer, but the exact role has not been defined. Suppression of acid secretion is associated with ulcer healing. The acid secreting cell is the parietal cell, which possesses a proton pump in the secretory membrane; morphologic changes accompany and facilitate the active secretion of hydrochloric acid. Stimulation of acid secretion occurs by three major pathways, which utilize acetylcholine, histamine, and gastrin. The predominant effects of histamine are mediated by adenylate cyclase, whereas those of gastrin and acetylcholine involve cytosolic calcium. There is a complicated arrangement of receptors and pathways that culminate in the activation of the proton pump. The parietal cell is influenced by neurocrine, hormonal, and paracrine mechanisms. Peptides join the more familiar neurotransmitters in affecting the parietal cell. Somatostatin is present in the gut and acts to decrease acid secretion. The hormone gastrin is released, in a feedback fashion, when the antrum is alkalinized. Most stimuli of acid secretion are blocked by H2-antagonists. Inhibitory hormones are released when acid arrives in the intestine. Inhibition of acid secretion can be achieved by influencing the parietal cell at the level of histamine, gastrin, and muscarinic receptors. The proton pump itself can be blocked by drugs that inhibit the final phase of acid secretion.

Muscarinic responses of gastric parietal cells

Isolated rabbit gastric glands were used to study the nature of the muscarinic cholinergic responses of parietal cells. Carbachol (CCh, 100 microM) stimulation of acid secretion, as measured by the accumulation of aminopyrine, was inhibited by the M1 antagonist, pirenzepine, with an IC50 of 13 microM; by the M2 antagonist, 11,2-(diethylamino)methyl-1 piperidinyl acetyl-5,11-dihydro-6H-pyrido 2,3-b 1,4 benzodiazepin-6-one (AF-DX 116), with an IC50 of 110 microM; and by the M1/M3 antagonist, diphenyl-acetoxy-4-methylpiperidinemethiodide (4-DAMP), with an IC50 of 35 nM. The three antagonists displayed equivalent IC50 values for the inhibition of carbachol-stimulated production of 14CO2 from radiolabeled glucose, which is a measure of the turnover of the H,K-ATPase, the final step of acid secretion. Intracellular calcium levels were measured in gastric glands loaded with FURA 2. Carbachol was shown to both release calcium from an intracellular pool and to promote calcium entry across the plasma membrane. The calcium entry was inhibitable by 20 microM La3+. The relative potency of the three muscarinic antagonists for inhibition of calcium entry was essentially the same as for inhibition of acid secretion or pump related glucose oxidation. Image analysis of the glands showed the effects of carbachol, and of the antagonists, on intracellular calcium were occurring largely in the parietal cell. The rise in cell calcium due to release of calcium from intracellular stores was inhibited by 4-DAMP with an IC50 of 1.7 nM, suggesting that the release pathway was regulated by a low affinity M3 muscarinic receptor or state; Ca entry and acid secretion are regulated by a high affinity M3 muscarinic receptor or state, inhibited by higher 4-DAMP concentrations (greater than 30 nM), suggesting that it is the steady-state elevation of Ca that is related to parietal cell function rather than the [Ca]i transient. Displacement of 3H N-methyl scopolamine (NMS) binding to purified parietal cells by CCh showed the presence of two affinities for CCh, but only a single affinity for 4-DAMP and lower affinity for pirenzepine and AFDX 116, providing further evidence for the parietal cell location of the [Ca]i response. Elevation of steady-state [Ca]i levels with either ionomycin or arachidonic acid did not replicate M3 stimulation of acid secretion or glucose oxidation, hence elevation of [Ca]i is necessary but not sufficient for acid secretion.

Second messengers in the gastric gland: a focus on calcium

The rabbit gastric gland model was used to study the nature of the muscarinic cholinergic and gastrin responses of parietal cells. Carbachol (100 microM) stimulation of acid secretion, as measured by the accumulation of aminopyrine, was inhibited by the M1 antagonist pirenzepine with an IC50 of 13 microM; by the M2 antagonist 11,2-(diethylamino)methyl-1-piperidinyl acetyl-5,11-dihydro-6H-pyrido 2,3-b 1,4-benzodiazepin-6-one (AF-DX 116) with an IC50 of 110 microM; and by the M3 antagonist diphenylacetoxy-4-methylpiperidinemethiodide (4-DAMP) with an IC50 of 35nM. The three antagonists displayed similar IC50 values for the inhibition of carbachol-stimulated production of 14CO2 from radiolabeled glucose, which is a measure of the turnover of the H(+)-H(+)-ATPase. Intracellular calcium levels wer measured in gastric glands loaded with FURA2. Carbachol was shown both to release calcium from an intracellular pool and to promote calcium entry across the plasma membrane. The calcium entry was inhibitable by 20 microM La3+. The relative potency of the three muscarinic antagonists for inhibition of calcium entry was essentially the same as for inhibition of acid secretion or metabolism. However, the rise in cell calcium due to release of calcium from intracellular stores was inhibited by 4-DAMP with an IC50 of 1.7 nM. Image analysis confirmed that the effect of carbachol and of the antagonists on intracellular calcium was occurring in the partial cell. In particular, the high-affinity inhibition of calcium release by 4-DAMP occurs in the parietal cell. Accordingly, it appears that the secretory receptor of the parietal cell is of the M3 type, and acid secretion depends on the entry of calcium rather than on calcium release from intracellular stores. In parallel experiments gastrin (G-17-sulfated) produced a dose-dependent increase in intracellular calcium (EC50, 0.14 +/- 0.013 microM). No stimulation of acid secretion was observed, but pepsinogen secretion was stimulated dose-dependently (EC50 = 1.17 +/- 0.21 microM).

Effects of some gastrointestinal peptides on isolated human and rabbit gastric glands

The isolated gastric gland preparation, with aminopyrine accumulation as an index of the parietal cell response, has been used to study the effects of somatostatin (S-14), gastrin-releasing peptide (GRP), cholecystokinin (CCK-8), vasoactive intestinal peptide (VIP), and peptide YY (PYY) on the in vitro acid secretion in human and rabbit oxyntic mucosa. Somatostatin was able to inhibit the parietal cell response to histamine in both human and rabbit isolated gastric glands (maximal inhibition, 22% and 34%, respectively) but failed to inhibit the parietal cell response to db-cAMP. However, other peptides capable of inhibiting gastric acid secretion in vivo, such as CCK, VIP, and PYY, were unable to induce any inhibition of the parietal cell response to db-cAMP or histamine in the isolated gastric gland preparation irrespective of the species studied. GRP was not able to induce a parietal cell response, a finding that is in accord with the assumption that the stimulatory effect of GRP on gastric acid secretion in vivo is by releasing gastrin from antral G-cells.

Gastrin-histamine interactions: direct and paracrine elements

The receptors mediating the physiologic actions of gastrin on acid secretion and growth have thus far not been localized to specific cells or fully characterized. Studies in canine fundic mucosa indicate that gastrin receptors are present on several cell types, including parietal cells and somatostatin cells. There is also increasing evidence for a gastrin-inducible pool of histamine in the fundic mucosa which is presumably stored in histamine-enterochromaffin-like cells. From the vantage point of studies in the canine fundic mucosa, the issue is no longer which cell type has the gastrin receptor but to sort out the mechanisms by which the effects of the gastrin receptors on endocrine/paracrine (histamine and somatostatin) cells and exocrine (parietal) cells are integrated to regulate secretory function and mucosal growth and differentiation.

Neuropeptide Y (NPY) inhibits dimethylphenylpiperazinium (DMPP)-induced gastric acid secretion in isolated rat stomach

The effect of neuropeptide Y (NPY) on gastric acid secretion was investigated on an everted preparation of isolated rat stomach. Perfusion with synthetic NPY did not modify the basal secretion of gastric acid. However, NPY reduced dimethylphenylpiperazinium (DMPP)-stimulated acid secretion at concentrations insufficient to affect acid secretion stimulated by muscarine, histamine or gastrin. The decrease in acetylcholine (ACh) release from postganglionic cholinergic neurons by NPY is therefore considered to be responsible. Determination of NPY content by radioimmunoassay, in mucosal and muscular layers of the stomach, indicates that NPY possibly produces cholinergic inhibition under physiological levels. The present study suggests, therefore, that NPY has the ability to inhibit the release of ACh from postganglionic cholinergic neurons, thus producing a decrease in gastric acid secretion.

Gastrin is a major mediator of the gastric phase of acid secretion in dogs: proof by monoclonal antibody neutralization

We developed a monoclonal antibody, 28.2, that binds specifically to the amidated carboxyl terminal region common to gastrin and cholecystokinin. This immunoglobulin G1 antibody has high affinity (ID50 = 30-70 pM for gastrin and cholecystokinin peptides), binds labeled gastrin similarly at 37 degrees C and 4 degrees C, and shows minimal inhibition of binding in the presence of 40% canine serum. Antibody 28.2 was used to carry out in vivo immunoneutralization studies in 8 dogs previously prepared with chronic gastric fistulas. Preliminary studies revealed that a single intravenous dose of 0.75 mg of partially purified immunoglobulin G of monoclonal antibody 28.2 completely inhibited the acid stimulatory effect of exogenous gastrin-17 given intravenously at 200 pmol/kg.h, a physiologic dose, and inhibited by 70% the acid response to a supraphysiologic dose, 800 pmol/kg.h. The same dose of antibody decreased the acid secretory response obtained during distention of the stomach with 300 ml of 5.8% glucose solution by 98% and decreased the response to distention with 300 ml of 8% peptone solution by 68%. A 10-fold higher dose of antibody decreased the acid response to peptone by 96%. The gastrin antibody had no effect on the acid response to exogenous histamine. A control antibody, specific for the biologically inactive glycine-extended gastrin/cholecystokinin peptapeptide region, had no significant effect on gastric acid secretion stimulated by gastrin or by gastric distention with nutrients. These studies indicate that circulating gastrin is of major importance in the gastric phase of gastric acid stimulation caused by distention of the stomach with nutrients.

Effect of pirenzepine on aminopyrine uptake by isolated guinea pig parietal cells

The inhibitory action of pirenzepine on acid secretion of isolated guinea pig parietal cells was investigated by the aminopyrine method. Pirenzepine markedly inhibited acid secretion of isolated parietal cells induced by carbachol in a dose-dependent manner but showed no inhibition on acid secretion stimulated by histamine. These results may suggest a direct action of pirenzepine on muscarinic receptors in parietal cells.