Pentagastrin-stimulated gastric acid secretion by the isolated perfused rat stomach

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.

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.

Bravo capsule system optimizes intragastric pH monitoring over prolonged time: Effects of ghrelin on gastric acid and hormone secretion in the rat

AIM: To evaluate measurements of intragastric pH with the Bravo capsule system over a prolonged time.

METHODS: A Bravo capsule was placed inside the rat gastric body and pH was studied for periods up to five consecutive days. For comparison, a gastric fistula model was used. Effects of ghrelin and esomeprazole, with or without pentagastrin, on gastric pH were studied. In addition, effects of esomeprazole on plasma ghrelin, gastrin and somatostatin were analyzed.

RESULTS: All rats recovered after surgery. The average 24-h pH during free feeding was 2.3 ± 0.1 (n = 20) with a variation of 18% ± 6% over 5 d. Ghrelin, 2400 pmol/kg, t.i.d. increased pH from 1.7 ± 0.1 to 3.1 ± 0.3 (P < 0.01) as recorded with the Bravo system. After esomeprazole (1 mg/kg, 3 mg/kg and 5 mg/kg) there was a dose-dependent pH increase of maximally 3.4 ± 0.1, with day-to-day variation over the entire period of 8% ± 3%. The fistula and pH studies generated similar results. Acid inhibition with esomeprazole increased plasma ghrelin from 10 ± 2 pmol/L to 65 ± 26 pmol/L (P < 0.001), and somatostatin from 10 ± 2 pmol/L to 67 ± 18 pmol/L (P < 0.001).

CONCLUSION: pH measurements with the Bravo capsule are reliable, and comparable to those of the gastric fistula model. The Bravo system optimizes accurate intragastric pH monitoring over prolonged periods and allows both short- and long-term evaluation of effects of drugs and hormones.

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.

Bombesin inhibits histamine release from the rat oxyntic mucosa by a somatostatin-dependent mechanism

BACKGROUND AND METHODS

This study examines the effect of bombesin on endogenous somatostatin and the histamine-synthesizing enterochromaffin-like cells. Somatostatin and histamine were measured in the venous effluent of isolated/antrectomized vascularly perfused rat stomachs after administration of bombesin and gastrin alone or combined. Histidine decarboxylase (HDC) enzyme activity and mRNA abundance were measured in the gastric corpus after intravenous administration of bombesin to conscious rats.

RESULTS

Bombesin released somatostatin from the isolated stomachs and reduced basal and gastrin-stimulated venous histamine. Somatostatin antiserum partially reversed the effect of bombesin on basal and gastrin-stimulated histamine release. In conscious fed rats, intravenous bombesin doubled serum gastrin concentrations and increased HDC activity.

CONCLUSION

We conclude that endogenous (paracrine) somatostatin inhibits basal and gastrin-stimulated histamine release from the ECL cell. In intact animals this effect is surmountable by simultaneously released gastrin, suggesting that a balance between the effects of gastrin and somatostatin determines the activation of the ECL cell.

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.

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.

Pentagastrin-stimulated histamine release and acid secretion from the totally isolated vascularly perfused rat stomach

This study was performed to examine the effect of cAMP levels on histamine release in the isolated rat stomach. Pentagastrin-induced histamine release was unaffected by phosphodiesterase inhibition, and pentagastrin itself had no phosphodiesterase-like effect. The results support previous observations showing that histamine is likely to be the mediator of the acid secretagogue effect of (penta) gastrin.

Rat gastric histamine release: a sensitive gastrin bioassay

Acid secretion and histamine release from the totally isolated vascularly perfused rat stomach are sensitive parameters for the biological effects of gastrin. By optimalizing the Ca2+ concentration of the vascular perfusate the sensitivity for stimulation of acid secretion was improved from 65 to 16 pmol/l gastrin 1-17 (G-17). Measurement of venous histamine before and after gastrin in each stomach preparation excluded inter-stomach variability and improved the sensitivity to 2 pmol/l G-17. This is the most sensitive gastrin bioassay reported, with a detection limit comparable to that of radioimmunoassay.

Gastric acid secretion in the totally isolated, vascularly perfused rat stomach. A selective muscarinic-1 agent does, whereas gastrin does not, augment maximal histamine-stimulated acid secretion

The gastric acid secretion in response to graded doses of gastrin, histamine, impromidine (a selective H2-receptor agonist), and the muscarinic-1 agonist McN-A-343 was studied in the totally isolated, vascularly perfused rat stomach. Combinations of stimulants at doses giving maximal acid secretion for each secretatogue were thereafter tested. All stimulants increased the gastric acid output significantly compared with the base-line output (7.2 +/- 2.0 mu eq/h). Gastrin induced significant increases in acid outputs at very low and physiologically relevant concentrations with a threshold concentration of 65 pM. Nevertheless, maximal gastrin-stimulated acid secretion represented only 55% of the maximal histamine-stimulated acid output of 154.8 +/- 10.0 mu eq/h. Impromidine and McN-A-343 induced a maximum 59% and 34% of maximal histamine-stimulated acid output, respectively. Adding gastrin to the maximal histamine of impromidine-stimulated stomachs did not increase the acid secretion further. Histamine and McN-A-343 in combination, however, induced a more than additive increase in the gastric acid output (232.0 +/- 14.7 mu eq/h) than did histamine and McN-A-343 separately (154.8 +/- 10.0 and 53.0 +/- 6.7 mu eq/h, respectively). The results indicate that in the rat, gastrin stimulates the parietal cell indirectly via histamine release, whereas muscarinic agents (cholinergic stimulation) act directly via a separate receptor.

Effect of pentagastrin on gastric acid secretion in the totally isolated vascularly perfused rat stomach stimulated with the phosphodiesterase inhibitor isobutyl methylxanthine

The development of a very sensitive stomach biomodel, using the totally isolated rat stomach, is reported. Pentagastrin-stimulated acid secretion in anesthetized and conscious rats was also studied, and the capacity to produce acid and the sensitivity towards pentagastrin in the different rat stomach models were compared. The secretory capacity and the sensitivity were evaluated by means of the maximal acid output (MAO) and threshold dose (TD), respectively. Conscious rats provided with gastric fistulas had a MAO of 48 +/- 6.3 mu eq/10 min and a TD of 6 micrograms/kg-h. In anesthetized rats with luminally perfused stomach MAO was reduced to 11.6 +/- 1.2 mu eq/10 min, whereas the sensitivity was increased, as indicated by a TD of 0.5 micrograms/kg-h. Totally isolated vascularly perfused rat stomachs without isobutyl methylxanthine (IMX) produced even less acid (MAO, 2.2 +/- 0.4 mu eq/10 min) without any change in demand for stimulant (TD, 0.25 micrograms/stomach-h). Addition of IMX to the vascular perfusate in the isolated stomach preparation increased the MAO to 8.0 +/- 1.0 mu eq/10 min and induced a remarkable reduction in demand for stimulant, with a TD obtained already at a pentagastrin dose of 0.008 micrograms/stomach-h, corresponding to a concentration of pentagastrin in the vascular perfusate medium of 84.5 pM. Accordingly, the totally isolated vascularly perfused rat stomach stimulated with IMX is the most sensitive bioassay for (penta)gastrin so far reported.

The effect of pentagastrin on the gastric secretion by the totally isolated vascularly perfused rat stomach

Gastric acid and pepsin secretion by the totally isolated vascularly perfused rat stomach was studied. Rat stomachs were vascularly perfused with Krebs-Ringer buffer gassed with 96% O2-4% CO2 and containing 5 mM glucose, 5 mM pyruvate, 4% bovine serum albumin, 10% ovine erythrocytes, and various concentrations of pentagastrin. The gastric lumen was perfused with distilled water gassed with 100% O2. To remove preformed juice and to enable preformed pepsin to be washed out, the initial 20-min gastric juice was discarded. The gastric effluent was then collected continuously for six 10-min periods, and the H+ concentration was determined by titration and the pepsin concentration estimated by a hemoglobin digestion method. Inclusion of pentagastrin in the perfusion buffer increased the acid output dose-dependently from 2.3 +/- 0.4 mueq/h during the control perfusion to a maximum of 12.7 +/- 2.1 mueq/h (p less than 0.01). With the higher doses of pentagastrin and concomitant high acid secretion, gastric acid secretion faded during the collection period. In contrast, gastric pepsin secretion was virtually unaffected by pentagastrin. The present model should be useful for studying the regulation of gastric acid and pepsin secretion.

Effect of antibodies to somatostatin on acid secretion and gastrin release by the isolated perfused rat stomach

The present studies were directed toward examining the effect of somatostatin on gastrin release and acid secretion by the isolated vascularly perfused rat stomach. Rat stomachs were perfused in situ with Krebs-Ringer bicarbonate buffer containing 10% ovine erythrocytes and gassed with 95% O2-5% CO2. Inclusion of pentagastrin in the perfusion buffer increased acid output from 2.2 +/- 0.4 microEq H+/h during control perfusion to 18.8 +/- 1.8 microEq H+/h (p less than 0.01). In order to determine the effect of somatostatin on acid secretion and gastrin release, specific antibodies to somatostatin were included in the perfusate. Inclusion of antibodies to somatostatin in the buffer without pentagastrin did not significantly enhance acid output; however, gastrin concentration in the portal venous effluent increased from 15.1 +/- 2.0 to 25.2 +/- 5.2 pg/ml at 45 min (p less than 0.05). When antibodies to somatostatin were perfused in the presence of pentagastrin, acid output increased by 32% to 24.9 +/- 1.2 microEq H+/h (p less than 0.05); however, no change in gastrin concentration over basal was detected in the portal effluent. Results of these studies indicate that the capacity of the isolated rat stomach to secrete acid permits direct assessment of factors involved in the regulation of acid secretion. Under the conditions of these experiments, gastric somatostatin inhibits basal gastrin release and directly inhibits pentagastrin-stimulated acid secretion without affecting gastrin release.