Rat gastric histamine release: a sensitive gastrin bioassay

Gastric cancer and gastrin: on the interaction of Helicobacter pylori gastritis and acid inhibitory induced hypergastrinemia

Gastric cancer, a disease with a reduced frequency for decades, now appears to be on the rise again in young Americans. The epidemiology of gastric cancer differs between tumors in the cardia and those of the more distal parts of the stomach. The tumors are divided into the intestinal type showing glandular growth pattern and the diffuse type with a different pattern. The latter often expresses neuroendocrine and more specifically ECL-cell markers suggesting that they originate from the ECL cell, the target cell for the antral hormone, gastrin. Helicobacter pylori gastritis is accepted as the major cause of gastric cancer, but only after having induced oxyntic atrophy which reduces gastric acid secretion and thus induces hypoacidity leading to hypergastrinemia. Long-term hypergastrinemia is known to induce malignant neoplasia in the stomach of animals as well as man. Recently treatment with proton pump inhibitor after Helicobacter pylori eradication in patients with gastroesophageal reflux disease, has been reported to predispose to gastric cancer. Since profound acid inhibition is a well-known cause of gastric neoplasia, it is to be expected that Helicobacter pylori infection and profound acid inhibition has an additive or possibly potentiating effect on the development of gastric cancer.

The Enterochromaffin-like [ECL] Cell-Central in Gastric Physiology and Pathology

Background: Studies on the regulation of gastric and pancreatic secretion began more than 100 years ago. Secretin was the first hormone postulated to exist, initiating the field of endocrinology. Gastrin produced in the antral mucosa was the second postulated hormone, and together with histamine and acetylcholine, represent the three major gastric acid secretagogues known since 1920. For a long time, the mast cell was the only recognized histamine-producing cell in the oxyntic mucosa and, in the mid-1980s, the ECL cell was recognized as the cell producing histamine, taking part in the regulation of gastric acid secretion. Methods: This review is based upon literature research and personal knowledge. Results: The ECL cell carries the gastrin receptor, and gastrin regulates its function (histamine release) as well as proliferation. Long-term hypergastrinemia results in gastric neoplasia of variable malignancies, implying that gastric hypoacidity resulting in increased gastrin release will induce gastric neoplasia, including gastric cancer. Conclusions: The trophic effect of gastrin on the ECL cell has implications to the treatment with inhibitors of acid secretion.

Not only stem cells, but also mature cells, particularly neuroendocrine cells, may develop into tumours: time for a paradigm shift

Stem cells are considered the origin of neoplasms in general, and malignant tumours in particular, and the stage at which the stem cells stop their differentiation determines the degree of malignancy. However, there is increasing evidence supporting an alternative paradigm. Tumours may develop by dedifferentiation from mature cells able to proliferate. Studies of gastric carcinogenesis demonstrate that mature neuroendocrine (NE) cells upon long-term overstimulation may develop through stages of hyperplasia, dysplasia, and rather benign tumours, into highly malignant carcinomas. Dedifferentiation of cells may change the histological appearance and impede the identification of the cellular origin, as seen with gastric carcinomas, which in many cases are dedifferentiated neuroendocrine tumours. Finding the cell of origin is important to identify risk factors for cancer, prevent tumour development, and tailor treatment. In the present review, we focus not only on gastric tumours, but also evaluate the role of neuroendocrine cells in tumourigenesis in two other foregut-derived organs, the lungs and the pancreas, as well as in the midgut-derived small intestine.

Helicobacter pylori and gastric acid: an intimate and reciprocal relationship

Helicobacter pylori (Hp) is the main cause of gastritis, peptic ulcer disease and gastric cancer. There are still unanswered questions related to the interaction between Hp and man, like what determines the susceptibility for the initial infection and the mechanisms for the carcinogenic effect. The initial infection seems to require a temporal gastric hypoacidity. For Hp to survive in the gastric mucous layer, some acidity is necessary. Hp itself is probably not directly carcinogenic. Only when inducing oxyntic mucosal inflammation and atrophy with hypoacidity, Hp predisposes for gastric cancer. Gastrin most likely plays a central role in the Hp pathogenesis of duodenal ulcer and gastric cancer.

Upper gastrointestinal physiology and diseases

Nordic research on physiology and pathophysiology of the upper gastrointestinal tract has flourished during the last 50 years. Swedish surgeons and physiologists were in the frontline of research on the regulation of gastric acid secretion. This research finally led to the development of omeprazole, the first proton pump inhibitor. When Swedish physiologists developed methods allowing the assessment of acid secretion in isolated oxyntic glands and isolated parietal cells, the understanding of mechanisms by which gastric acid secretion is regulated took a great step forward. Similarly, in Trondheim, Norway, the acid producing isolated rat stomach model combined with a sensitive and specific method for determination of histamine made it possible to evaluate this regulation qualitatively as well as quantitatively. In Lund, Sweden, the identification of the enterochromaffin-like cell as the cell taking part in the regulation of acid secretion by producing and releasing histamine was of fundamental importance both physiologically and clinically. Jorpes and Mutt established a center at Karolinska Institutet in Stockholm for the purification of gastrointestinal hormones in the 1960s, and Danes followed up this work by excelling in the field of determination and assessment of biological role of gastrointestinal hormones. A Finnish group was for a long period in the forefront of research on gastritis, and the authors' own studies on the classification of gastric cancer and the role of gastrin in the development of gastric neoplasia are of importance. It can, accordingly, be concluded that Nordic researchers have been central in the research on area of the upper gastrointestinal physiology and diseases.

Gastrin May Mediate the Carcinogenic Effect of Helicobacter pylori Infection of the Stomach

Gastric cancer occurs almost exclusively in patients with gastritis. Since Helicobacter pylori (Hp) was proved to cause gastritis, Hp was also expected to play a role in gastric carcinogenesis. Despite extensive studies, the mechanisms by which Hp cause gastric cancer are still poorly understood. However, there is evidence that the anatomical site of Hp infection is of major importance. Infection confined to the antral mucosa protects against gastric cancer but predisposes to duodenal ulcer, whereas Hp infection of the oxyntic mucosa increases the risk of gastric cancer. Hp infection does not predispose to cancers in the gastric cardia. In patients with atrophic gastritis of the oxyntic mucosa, the intragastric pH is elevated and the concentration of microorganisms in the stomach is increased. This does not lead to increased risk of gastric cancer at all anatomical sites. The site specificity of Hp infection in relation to cancer risk indicates that neither Hp nor the changes in gastric microflora due to gastric hypoacidity are carcinogenic per se. However, reduced gastric acidity also leads to hypergastrinemia, which stimulates the function and proliferation of enterochromaffin-like (ECL) cells located in the oxyntic mucosa. The ECL cell may be more important in human gastric carcinogenesis than previously realized, as every condition causing long-term hypergastrinemia in animals results in the development of neoplasia in the oxyntic mucosa. Patients with hypergastrinemia will far more often develop carcinomas in the gastric corpus. In conclusion, hypergastrinemia may explain the carcinogenic effect of Hp.

The regulation of gastric acid secretion - clinical perspectives

The purpose of this review, based upon 40 years of research, is to clear old controversies. The gastric juice is a strong acid with active enzymes (pepsin and lipase); ideal for killing swallowed microorganisms. Totally isolated rat stomach and histamine determination. Human gastric carcinomas were examined for ECL cell differentiation because tumours found in rodents after dosing with inhibitors of acid secretion were reclassified to be of ECL cell origin. The gastrin receptor is localized to the ECL cell only, where gastrin stimulates the function and growth. Drug-induced hypo-acidity induces hypergastrinaemia and ECL cell hyperplasia responsible for rebound acid hypersecretion. Every condition with long-term hypergastrinaemia disposes to ECL cell neoplasia. In man, both atrophic gastritis and gastrinoma lead to ECL cell carcinoids. Proton pump inhibitors induce hypergastrinaemia with ECL cell hyperplasia and ECL cell carcinoids that disappear when stopping treatment. The gastrin antagonist netazepide induces regression of ECL cell carcinoids due to atrophic gastritis. Human gastric carcinomas of diffuse type, particularly the signet-ring subtype, show ECL cell differentiation, suggesting involvement of gastrin in the carcinogenesis. Helicobacter pylori (Hp) causes gastritis and peptic ulcer, and when infecting the antrum only gives a slight hypergastrinaemia with acid hypersecretion predisposing to duodenal ulcer, but protecting from gastric cancer. When Hp infection spreads to oxyntic mucosa, it induces atrophy, reduced acid secretion and marked hypergastrinaemia and cancer.It is remarkable that the interaction between Hp and gastrin may explain the pathogenesis of most diseases in the upper gastrointestinal tract.

Rebound acid hypersecretion from a physiological, pathophysiological and clinical viewpoint

OBJECTIVE

The recent description of dyspepsia in healthy individuals after stopping treatment with proton-pump inhibitors (PPIs) indicates that reflux disease may worsen due to this treatment. The aim of this paper is to review current knowledge of the regulation of gastric acid secretion, including maximal acid secretion, and to improve understanding of the pathogenesis of acid-related conditions.

MATERIAL AND METHODS

We reviewed our findings from three decades of studies on gastric acid secretion in the isolated rat stomach and in humans as well as studies by the group of Robert Jensen involving gastrinoma patients.

RESULTS

The parietal cell has receptors for histamine and acetylcholine, whereas the gastrin receptor is localized to the enterochromaffin-like (ECL) cell. Gastrin-stimulated histamine release depends on the ECL cell mass, which is regulated by gastrin. The parietal cell mass is also influenced by gastrin. All conditions with hypergastrinemia concomitant with a normal oxyntic mucosa result in an increase in acid secretion. Helicobacter pylori infection in the antral mucosa may induce duodenal ulcers by its effect on acid secretion, as in patients with gastrinoma. Whereas PPIs induce clinically important rebound acid hypersecretion, histamine-2 blockers do not, since they also induce tolerance.

CONCLUSION

From a biological and physiological point of view, patients should be given treatment that disturbs the normal physiology as little as possible.

Rebound hypersecretion after inhibition of gastric acid secretion

Drugs inhibiting gastric acid secretion are widely used because of the high prevalence of acid-related disorders. However, from clinical experience it seems that symptom relapse is common after withdrawal of these drugs. Experimental as well as clinical studies have demonstrated an increased acid secretion after a period of treatment with either histamine 2 receptor antagonists or proton pump inhibitors. Rebound hypersecretion is likely to reflect the following sequence of events: Long-term inhibition of acid output is accompanied by elevated serum gastrin levels, leading to enterochromaffin-like cell activation and proliferation, resulting in increased amounts of histamine being mobilized from these cells to stimulate the parietal cells. The clinical consequences of rebound hypersecretion have not been settled.

Glycine-extended gastrin-17 stimulates acid secretion only via CCK-2 receptor-induced histamine release in the totally isolated vascularly perfused rat stomach

The effects of gastrin precursors have been discussed during recent years. However, the mechanism for their action, whether through a novel receptor on the parietal cell or a cholecystokinin-2 (CCK-2) receptor on the enterochromaffin like (ECL) cells, is still not settled. This study examines the effect of glycine-extended gastrin-17 (Gly-G-17), the main non-amidated gastrin precursor, on gastric acid secretion and histamine release in the totally isolated vascularly perfused rat stomach. Glycine-extended gastrin-17 at the concentrations from 0.52 to 520 nmol L(-1) was administered to the totally isolated vascularly perfused rat stomach. Glycine-extended gastrin-17 at 52 or 520 nmol L(-1), and gastrin-17 at 0.52 nmol L(-1)were co-administered to examine whether glycine-extended gastrin augmented maximal gastrin stimulated acid secretion and histamine release. Both Gly-G-17 at 52 nmol L(-1) and gastrin-17 (G-17) at 0.52 nmol L(-1) were administered together with the histamine-2 receptor antagonist ranitidine at 10 micromol L(-1). Gastric acid and venous histamine output were measured. Glycine-extended gastrin-17 at lower concentrations from 0.52 to 5.2 nmol L(-1) did not stimulate gastric acid output or histamine release, whereas higher concentrations from 52 to 520 nmol L(-1) elicited a concentration-dependent increase in acid secretion and histamine release. The outputs of acid and histamine at 520 nmol L(-1) Gly-G-17 were at the same level as those found for G-17 at its maximally effective concentration of 0.52 nmol L(-1). Glycine-extended gastrin-17 at maximally effective concentration of 520 nmol L(-1) did not augment maximal gastrin stimulated acid secretion or histamine release. Ranitidine inhibited G-17 and Gly-G-17 stimulated acid secretion to a similar degree. This study confirms that the stimulatory effect of Gly-G-17 on gastric acid secretion is via a CCK-2 receptor on the ECL cell.

Enhancement of peptone-induced gastric acid secretion in streptozotocin-induced diabetic rats

We compared the acid secretory response to peptone in normal and streptozotocin-induced diabetic rats. Animals were injected with streptozotocin and used after 5 weeks of diabetes with blood glucose levels of >350 mg/dl. Under urethane anesthesia, 2 ml peptone solution (2-8%) was instilled in the stomach through an acute fistula every 30 min. Peptone increased acid secretion in a concentration-dependent manner in normal rats, the maximal response being obtained at 8%. Likewise, the increased acid response was observed in diabetic rats, yet the maximal response observed at 4% was significantly greater than that in normal rats. In both cases, this response was inhibited potently by famotidine as well as YM-022 (a CCKB antagonist) and partially inhibited by atropine. Peptone increased luminal histamine and plasma gastrin levels in both normal and diabetic rats, and the former response was significantly greater in diabetic animals. The altered acid secretion and histamine output in diabetic rats were reverted by insulin treatment. Pentagastrin- but not histamine-induced acid secretion was also increased in diabetic rats. We conclude that peptone-induced acid secretion is increased in diabetic conditions. This phenomenon is insulin-dependent and associated with an enhanced release of histamine but not with an increased sensitivity of the parietal cells.

Vagal stimulation augments maximal (penta) gastrin-stimulated acid secretion in humans

Since the late sixties, the pentagastrin test has been the standard method to examine maximal gastric acid secretion in humans. However, studies on rats and dogs have shown that maximal pentagastrin-stimulated acid secretion can be augmented by concomitant cholinergic stimulation. The aim of this study was to examine whether the combined stimulation of the vagal nerves and pentagastrin infusion could increase maximal gastric acid secretion compared with pentagastrin alone. Eight healthy medical students (seven males) were included in the study. Gastric acid secretion was determined thrice in each subject. On day one, pentagastrin (6 microg kg(-1) h(-1)) was infused. On day two, insulin-induced hypoglycaemia (plasma glucose approximately 2.3 mM during 30 min) was obtained by applying the glucose clamp technique. On day three, pentagastrin infusion and insulin induced-hypoglycaemia were combined. The combination of insulin-induced hypoglycaemia and pentagastrin infusion increased peak acid output about 20% (P = 0.018) compared with pentagastrin alone. The hypoglycaemia did not cause significant release of gastrin. It is concluded that vagal stimulation of gastric acid secretion may be safely obtained by insulin-induced hypoglycaemia when applying the glucose clamp technique. In addition, maximal pentagastrin-stimulated acid secretion does not represent the maximal acid secretory capacity in humans.

Biological activity of carboxy-terminal gastrin analogs

Amidated forms of gastrin are derived by post-translational processing of a large precursor peptide and stimulate gastric acid secretion via the gastrin/CCK(B) receptor. Non-amidated biosynthetic intermediates may exert biological effects through other mechanisms, but their effect on gastric acid secretion is unclear. Amidated gastrins stimulate acid secretion mainly by releasing histamine from mucosal enterochromaffin-like cells. This study examines the effects on histamine release from the vascularly perfused rat stomach of amidated gastrin-17, COOH-terminal glycine-extended gastrin-17, gastrin-17 extended at the COOH-terminal including the remaining progastrin sequence, and carboxy-terminal progastrin fragments (SAEDEN and GRRSAEDEN). Carboxy-terminal extended gastrins induced histamine release which was inhibited by the gastrin/CCK(B) antagonist L-740,093, but had to be given in concentrations 100-fold higher than amidated gastrin-17 to produce comparable effects. These progastrin-derived peptides are found in high concentrations in some patients with the Zollinger-Ellison syndrome and may contribute to acid hypersecretion and other gastrin/CCK(B) receptor mediated responses.

Treatment with proton pump inhibitors induces tolerance to histamine-2 receptor antagonists in Helicobacter pylori-negative patients

BACKGROUND

Treatment with H2 receptor antagonists (H2RAs) and proton pump inhibitors (PPIs) induces hypergastrinemia and causes rebound hypersecretion of gastric acid after treatment, and during treatment with H2RAs tolerance develops. In the present study we investigated whether a treatment period with a PPI induced tolerance to an H2RA.

METHODS

Thirteen patients with esophagitis were given omeprazole for 90 days. Twenty-four-hour pH monitorings without and with ranitidine were performed before and after treatment with omeprazole. Blood samples and biopsy specimens from the oxyntic mucosa were analyzed for gastrin, histamine, and chromogranin A.

RESULTS

An increase in mucosal histamine and a reduction in the effect of ranitidine on gastric pH was found 14 days after discontinuing omeprazole compared with before treatment in Helicobacter pylori-negative but not in H. pylori-positive patients.

CONCLUSIONS

Treatment with omeprazole reduces the effect of ranitidine in H. pylori-negative patients. This is caused by an increase in histamine released by the enterochromaffin-like cell secondarily to hypergastrinemia, corresponding to the tolerance towards H2RAs seen in patients with Zollinger-Ellison syndrome.

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.

Evaluation of biologic gastrin activity of compound CI-988 in the isolated, vascularly perfused rat stomach

BACKGROUND

The peptoid CI-988 has previously been shown to have high affinity for the cholecystokinin (CCK)-B/gastrin receptor and has been reported to be a powerful CCK antagonist in many systems, although it has agonist activity on histidine decarboxylase in the rat.

METHODS

In the present study the effect of CI-988 on acid secretion and histamine release in the totally isolated, vascularly perfused rat stomach was assessed.

RESULTS

CI-988 was found to be a gastrin agonist with regard to the stimulation of both histamine release and acid secretion.

CONCLUSION

Thus, in this stomach model CI-988 behaved as a CCKB/gastrin agonist. The present study underlines the importance of testing the biologic activity of ligands in models with sufficient sensitivity.

Bioassay of gastrin using the isolated vascularly perfused rat stomach. A new, simplified and sensitive method

Radioimmunoassays are sensitive and specific methods for measurement of the concentrations of regulatory peptides. However, aspects of physiological, pathophysiological and pharmacological research require knowledge about the biological activity which does not necessarily vary concomitantly with immunological activity. The present work describes a simplified bioassay for gastrin based on the gastric histamine releasing properties of this peptide, using an isolated vascularly perfused rat stomach preparation with a crystalline perfusate and a specific radioimmunoassay for histamine. The establishment of a dose-response curve is described, as well as the utilization of the bioassay on sera from patients with hypergastrinaemia. The method is sensitive for gastrin in the low (4 pmol L-1) picomolar range.

Enterochromaffin-like (ECL) cells and their growths: relationships to gastrin, reduced acid secretion and gastritis

ECL cells are argyrophilic endocrine cells of the stomach. Their distribution is species specific, however they are consistently located in the oxyntic mucosa and, in particular, in very close contact with the adenomeres of acidopeptic glands. ECL cells store histamine and are considered a key element in the mechanisms of gastric acid secretion as controlled by gastrin stimulus. Their peculiar anatomical disposition and secretory properties strongly suggest that ECL cells exert their function by a paracrine mechanism, i.e. by releasing histamine in the extracellular spaces surrounding acid-producing parietal cells. ECL cell activity is strongly stimulated by gastrin, which, once applied as a long-standing stimulus, also exerts a potent proliferating effect. Long-lasting hypergastrinaemia has been demonstrated to elicit ECL cell proliferation in laboratory animals, inducing ECL cell hyperplasia, dysplasia and ECL cell tumours, i.e. argyrophilic gastric carcinoids. However, in experimental rodents it is believed that hypergastrinaemia is not per se a stimulus capable of inducing ECL cell transformation, a predisposing genetic background being required for tumour development in endocrine organs. In man, long-standing hypergastrinaemia exerts the same proliferative pressure on ECL cells and is associated with hyperplasia with or without dysplastic changes and carcinoid development. Clinical evidence suggests that other factors, both genetic and environmental, are required to induce ECL cell transformation and carcinoid development. For this reason human gastric argyrophilic ECL carcinoids are subdivided into three main groups depending on their clinical background: (1) gastric carcinoids in patients with chronic atrophic gastritis; (2) gastric carcinoids in patients with Zollinger-Ellison and multiple endocrine neoplasia type 1 syndrome (MEN-ZES); and (3) solitary, sporadic gastric carcinoids. The clinical assessment of carcinoid-bearing patients is strongly recommended for better diagnosis and management of patients.

Biologic and immunologic gastrin activity in serum of patients with gastrinoma. Bioassay of gastrin activity in serum

The biologic gastrin activity in serum from 14 patients with the Zollinger-Ellison syndrome was assessed by the stimulation of histamine release and acid secretion from the isolated vascularly perfused rat stomach and compared with the immunologic activity as determined by radioimmunoassay using an antibody directed towards the active site of the gastrin molecule. Biologic gastrin activity assessed by the stimulation of histamine release was more closely correlated to immunologic gastrin activity than biologic activity assessed by the stimulation of gastrin acid secretion. This study does not contradict the concept that gastrin stimulates acid secretion at least partly by releasing histamine and also shows that the immunologic gastrin activity determined with the help of an antibody directed towards the active site reflects biologic activity.

Trophic effect of gastrin on the enterochromaffin like cells of the rat stomach: establishment of a dose response relationship

Gastrin was given to rats by continuous subcutaneous infusion through implanted osmotic minipumps in doses covering a wide range of the dose response relationship for gastrin with regard to the trophic effect on the enterochromaffin like cells of the oxyntic mucosa. Thirty five rats were divided into five groups (each of seven rats), one group receiving a control solution of 1% albumin, the others receiving gastrin in 1% albumin at doses of 2.5, 5, 10, and 15 micrograms/kg/h, respectively. The plasma gastrin concentrations in the various groups increased in the same order of magnitude as expected from the gastrin doses given. Gastrin induced a dose dependent increase in enterochromaffin like cell density, oxyntic mucosal histamine concentration and histidine decarboxylase activity up to the dose of 5 micrograms/kg/h, where the increase levelled off. Hence, the dose response relationship for the trophic effect of gastrin on the enterochromaffin like cells seems to follow a polynomial rather than a linear function. These findings may also contribute to the understanding of the trophic effect of gastrin on enterochromaffin like cells in man with conditions associated with hypergastrinaemia.

Radioimmunoassay of histamine

Histamine is formed by decarboxylation of the amino acid histidine and is found both in plants and in animals, including man. In man it has important biologic functions. To assess the physiologic role of histamine, however, it is necessary to have a reliable and convenient method to determine its concentration in biologic fluids and tissue. Histamine has been determined by bioassay, chemically by different modification of a fluorometric method, by radioenzymatic methods, and, recently, by immunoassays. Immunoassay of histamine has, however, been difficult to establish, mainly as a result of problems with the production of an antibody with histamine specificity. This is due to the general occurrence of histamine in all animal species. By binding histamine to different ligands, several researchers have succeeded in producing antibodies against antigens in which histamine is integrated. Treating samples and histamine standard with the same coupling agent, reliable and specific radioimmunoassays of histamine have been established. We have for some years utilized a commercial radioimmunoassay of histamine and confirmed its convenience, specificity, and sensitivity. In some patients taking a histamine-2 blocker (cimetidine or ranitidine) we have detected an increase in plasma histamine which also tended to be increased after proximal gastric vagotomy and in patients with gastric ulcer compared with patients with duodenal ulcer. In rats treated with high doses of omeprazole for 90 days we found an increase in the enterochromaffin-like cell mass and in histamine concentration in the oxyntic mucosa which was reflected by an increase in plasma histamine.(ABSTRACT TRUNCATED AT 250 WORDS)

Aspects of the regulation of gastric histamine release

Histamine is found in large amounts in the gastric mucosa and plays an essential role in the regulation of acid secretion. It is thought to stimulate acid secretion directly after being released by the other two major secretagogues (gastrin and acetylcholine) (the mediator hypothesis) or to potentiate the action of the other two secretagogues (the interaction hypothesis). Recent studies with isolated, vascularly perfused rat stomach have shown that gastrin in physiologic concentrations elicits a release of histamine sufficient to explain its acid-stimulatory effect. Vagal nerve stimulation, on the other hand, only gives a faint histamine release, indicating that the vagal acid stimulation is mainly mediated by a direct stimulation of the parietal cell. Furthermore, the gastrin-stimulated histamine release seems to be mediated by a calcium-dependent mechanism. Somatostatin inhibits gastrin-stimulated histamine release via a paracrine mechanism, and a prostaglandin E1 analogue (misoprostol) has been shown to be a potent inhibitor of base-line and gastrin-stimulated histamine release. These studies show that the modulation of histamine release may be a central regulatory mechanism of gastric acid secretion. Although these studies have been done in rats, there are indications that these results are of a general nature nd valid for other species as well.

The gastrin receptor assay

Gastrin is a major physiologic regulator of gastric acid secretion and growth of the oxyntic mucosa. Biologically active radiolabelled hormones may be used to characterize and localize receptors for peptide hormones. The cellular localization of the gastrin receptor in the fundic mucosa, however, is still a matter of great debate owing to difficulties in developing a gastrin receptor binding assay. Despite considerable work in several laboratories, the criteria for true receptor binding have not yet been fulfilled. The preparation of a suitable tissue receptor material (plasma membranes or isolated cells) from the heterocellular fundic mucosa seems to be the major problem. This problem may be related to the fact that the receptors are only present on the enterochromaffin-like cells (ECL), which constitute but a minor fraction of the cells in the oxyntic mucosa. Furthermore, the second messenger of gastrin is still not known, and the poor functional responsiveness of isolated cells and the oxyntic glands to gastrin further complicates the evaluation of the gastrin receptor. In this review the different steps in the gastrin receptor assay (the labelling of gastrin, preparation of the receptor, and the incubation and correlation of the binding and biologic effect of gastrin) are discussed.