Divergent effects of bombesin and bethanechol on stimulated gastric secretion in duodenal ulcer and in normal men

Somatostatin receptor type 2 mediates bombesin-induced inhibition of gastric acid secretion in mice

Studies in isolated mouse stomach showed that bombesin releases somatostatin. We characterized the effects of exogenous bombesin on gastric acid secretion in mice and determined the involvement of somatostatin and somatostatin receptor type 2 (SSTR2) by using somatostatin immunoneutralization, the SSTR2 antagonist, PRL-2903, and SSTR2 knockout mice. Gastric acid secretion was monitored under basal and pentagastrin-, histamine- or bethanechol-stimulated conditions in urethane-anaesthetized mice. Bombesin (10-40 micro g kg-1 h-1) and somatostatin-14 (20 micro g kg-1 h-1) were infused I.V. 10 and 30 min after PRL-2903 or somatostatin antibody pretreatment, respectively. Urethane-anaesthetized wild-type mice had low basal acid secretion (0.12 +/- 0.01 micro mol (10 min)-1) compared with SSTR2 knockout mice (1.43 +/- 0.10 micro mol (10 min)-1). Somatostatin antibody and PRL-2903 increased basal secretion in wild-type mice but not in SSTR2 knockout animals. In wild-type mice, bombesin inhibited secretagogue-stimulated acid secretion in a dose-dependent manner, and somatostatin-14 inhibited pentagastrin-stimulated secretion. In wild-type mice pretreated with somatostatin antibody or PRL-2903 and in SSTR2 knockout mice, bombesin and somatostatin-14 I.V. infusion did not alter the increased gastric acid secretion. These results indicate that, in mice, bombesin inhibits gastric acid secretion through the release of somatostatin and the activation of SSTR2. These observations strengthen the important role of SSTR2 in mediating somatostatin inhibitory actions on gastric acid secretion.

Helicobacter pylori infection and abnormalities of acid secretion in patients with duodenal ulcer disease

BACKGROUND & AIMS

The mechanism by which Helicobacter pylori predisposes to duodenal ulcers (DUs) remains unclear. The aim of this study was to investigate the effect of the infection on acid secretion.

METHODS

Acid output was examined basally and in response to gastrin-releasing peptide (GRP) and gastrin in healthy volunteers with and without H. pylori infection and in patients with DUs before and after eradication of the infection.

RESULTS

Compared with H. pylori-negative healthy volunteers, patients with DUs with H. pylori had the following abnormalities of acid secretion: (1) threefold increase in basal acid output, (2) sixfold increase in acid response to GRP, (3) increased maximal acid response to exogenous gastrin, (4) increased ratio of basal acid output to maximal gastrin-stimulated output, and (5) increased ratio of maximal GRP-stimulated acid output to maximal gastrin-stimulated output. All of these abnormalities resolved fully after H. pylori eradication except for increased maximal acid output to gastrin, which was unchanged. Infected healthy volunteers showed a threefold increase in acid response to GRP that resolved after eradication of H. pylori infection.

CONCLUSIONS

These disturbances in acid secretion caused by H. pylori infection are consistent with impaired inhibitory control and are likely to be relevant to the mechanism by which the infection predisposes to DU.

Gastrointestinal motor and secretory responses to cholinergic stimulation in humans. Differential modulation by muscarinic and cholecystokinin receptor blockade

The present study investigated how a cholinergic agonist modifies interdigestive motility and secretion of the upper gastrointestinal tract and how muscarinic and cholecystokinin receptor blockade interfere with this direct cholinergic stimulation. In eight healthy volunteers, gastrointestinal motor and secretory responses to bethanechol (12.5, 25, and 50 micrograms kg-1 h-1) with and without a background of atropine (5 micrograms kg-1 h-1) or loxiglumide (10 mg kg-1 h-1) were studied. Stepdoses of bethanechol caused a parallel stimulation of antroduodenal motility and gastropancreatic secretion (P < 0.01) without inducing a fed pattern. However, duration of phase I was shortened (P < 0.05). Only high doses of bethanechol enhanced gastrin (P < 0.05), cholecystokinin (P < 0.05), and pancreatic polypeptide (P < 0.01) release. Atropine completely antagonized motor and secretory responses to cholinergic stimulation. Loxiglumide left cholinergically stimulated motility and pancreatic enzyme secretion unaltered. With co-infusion of bethanechol and loxiglumide, PP release dropped by 63% (P < 0.01); gastric acid output, gastrin and CCK release increased by 56%, 16%, and 25%, respectively (P < 0.05). We conclude that stimulation by a cholinergic agonist preserves the interdigestive pattern. Low dose muscarinic receptor blockade abolishes cholinergic stimulation over the full dose range. Inhibition of somatostatin release would explain stimulation of gastrin release and gastric acid secretion with co-infusion of bethanechol and loxiglumide. Endogenous CCK appears to interact with direct cholinergic stimulation at the pancreatic PP cell and the gastric D-cell but not at pancreatic acinar and antroduodenal smooth muscle cells.

Bombesin does not stimulate pepsinogen release in isolated gastric chief cells

Bombesin (BN)-related peptides, such as gastrin-releasing peptide (GRP), have been shown in vivo to stimulate release of pepsinogen. However, whether this is due to a direct interaction with chief cells is not clear. To clarify this we prepared isolated chief cells (> 90% pure) from guinea pig stomach. BN, GRP, or neuromedin B (NMB), at concentrations up to 1 microM, did not stimulate pepsinogen release or affect the stimulation caused by vasoactive intestinal peptide (VIP) (100 nM) or CCK-8 (10 nM), respectively. In addition, BN, GRP, or NMB at a concentration of 1 microM did not increase cAMP nor did they alter the increase in cAMP caused by VIP or secretin. BN (1 microM) did not alter basal cytosolic calcium [Ca2+]i or affect the increase in [Ca2+]i caused by CCK-8 (1 microM). Furthermore, BN, GRP, or NMB at a concentration of 1 microM did not increase the generation of inositol phosphates (IP) or alter the increase in [3H]IP1, [3H]IP2, or [3H]IP3, caused by CCK-8 (1 microM) or carbachol (1 mM). Binding studies demonstrated no saturable binding of either [125I][Tyr4]BN or [125I][D-Tyr0]NMB using experimental conditions where binding with other peptide ligands to other receptors on chief cells is seen. We conclude that BN-related peptides do not interact directly with specific receptors on chief cells to stimulate or alter stimulated pepsinogen secretion, increase the breakdown of inositol phosphates, or alter [Ca2+]i or cAMP.

Somatostatin antibody does not influence bombesin-induced inhibition of gastric acid secretion in rats

The influence of peripheral somatostatin immunoneutralization on intravenous (i.v.) and intracerebroventricular (ICV) bombesin-induced inhibition of gastric acid secretion (GAS) was investigated with the somatostatin monoclonal antibody, CURE.S6, in rats. The somatostatin antibody, injected i.v. in conscious rats with a chronic gastric fistula and i.v. catheter, did not modify basal GAS, whereas in urethane-anesthetized rats the basal GAS was increased by 150%. In conscious rats, somatostatin (15 micrograms/kg/h, i.v.) inhibited basal GAS by 50% after injection of control antibody but not after pretreatment with the somatostatin antibody. With pretreatment with a control antibody, bombesin (10 micrograms/kg/h, i.v.) inhibited basal GAS by 60% in conscious rats and by 50% the acid response to pentagastrin infusion in urethane-anesthetized rats. Bombesin injected ICV (3 and 10 ng/10 microliters/rat) inhibited basal GAS by 50% and 70%, respectively, in conscious rats pretreated with a control antibody. The somatostatin antibody injected i.v. before i.v. or ICV injection of bombesin did not influence bombesin-induced inhibition of GAS in conscious or anesthetized rats. These results show that peripheral somatostatin does not play a major role in the inhibition of gastric acid secretion induced either by ICV or i.v. administration of bombesin or basal acid secretion in conscious rats.

Effects of cholinergic, histaminergic, and peptidergic stimulation on pepsinogen secretion by isolated human peptic cells

BACKGROUND

Whereas pepsin secretion by the human stomach has been extensively investigated, the characteristics of the peptide cell cannot be fully understood from in vivo studies. We therefore studied isolated human peptic cells to test directly cellular responses to different agents.

METHODS

Eight endoscopic biopsy specimens yielded 10(6) cells, > 90% pure and > 95% viable. Secreted pepsinogen was measured with a sensitive hemoglobin digestion method at pH 2.

RESULTS

Pepsinogen secretion was concentration-dependently stimulated by acetylcholine (ACh) (EC50 = 0.3 microM), cholecystokinin (CCK)-8 (2 nM), histamine (2 microM), and gastrin-I (30 nM) but not by bombesin or pentagastrin. ACh stimulation was inhibited 40 times more potently by atropine (IC50 = 12 nM) than by pirenzepine (IC50 = 0.5 microM). Histamine was inhibited by 10(-4) cimetidine. CCK-8 stimulation was inhibited 80% by the CCK-A-selective antagonist L364,718 (IC50 = 12 nM) but not by the CCK-B-selective antagonist L365,260.

CONCLUSION

Isolated human peptic cells from endoscopic biopsy specimens secrete pepsinogen in response to ACh > CCK-8 > histamine > gastrin-I. The human peptic cell muscarinic-cholinergic receptor is not of the M1 subtype, and the CCK-8 response is predominantly mediated by a CCK-A receptor subtype.

Helicobacter pylori-associated exaggerated gastrin release in duodenal ulcer patients. The effect of bombesin infusion and urea ingestion

Recent studies have shown that the exaggerated meal-stimulated gastrin release in patients with duodenal ulcer abates after eradication of Helicobacter pylori infection. Bombesin-stimulated gastrin release was compared in 11 H. pylori-infected patients with chronic duodenal ulcer and 8 uninfected healthy volunteers both before and after therapy to eradicate H. pylori. Bombesin infusion significantly increased the gastrin release both in control subjects and in patients with duodenal ulcer. Antimicrobial therapy (bismuth, tetracycline, and metronidazole) to eradicate the H. pylori infection was associated with a significant reduction in bombesin-stimulated gastrin release in patients with duodenal ulcer (from 116.9 +/- 19 pg/mL to 69.5 +/- 7 pg/mL following 50 pmol.kg-1.h-1 bombesin; and from 158 +/- 29 to 83.4 +/- 10 following 200 pmol.kg-1.h-1 bombesin: P = 0.01 for each). Antimicrobial therapy had no effect on gastrin release in uninfected volunteers, thus excluding a nonspecific effect of antimicrobial therapy on antral G-cell function. Serum gastrin was also not increased by feeding 500 mg of urea to 5 H. pylori-infected volunteers. This suggests that access of hydrogen ion to the pH-sensitive sites governing gastrin release by mucosal ammonia produced by H. pylori urease is not a critical factor. These data suggest that exaggerated gastrin release present in patients with duodenal ulcer disease is secondary to H. pylori infection.