The effect of gastrin-releasing peptide on acid secretion and the release of gastrin, somatostatin, and histamine in the totally isolated, vascularly perfused rat stomach

Scandinavian Journal of Gastroenterology - the editors-in-chief

The SJG has enjoyed a sequence of active and dynamic editors-in-chief following the initial period with Professor Myren in charge. This paper gives a short overview of the subsequent editors including their clinical and scientific merits.

Somatostatin-receptor 2 (sst2)-mediated effects of endogenous somatostatin on exocrine and endocrine secretion of the rat stomach

Somatostatin is a potent inhibitor of gastric acid secretion. Its effects are mediated through five specific receptor subtypes (sst(1-5)), of which sst(2) is dominant on the enterochromaffin-like (ECL) cell and the parietal cell. To study the paracrine mechanisms of somatostatin, the sst(2)-specific antagonist PRL-2903 was used. Effects of PRL-2903 on acid secretion and release of histamine were studied in the totally isolated, vascularly perfused rat stomach. Further, the release of histamine and gastrin after bombesin, alone and in combination with PRL-2903, were studied. Results are presented as mean+/-standard error of the mean (s.e.m.). PRL-2903 concentration-dependently increased the venous histamine concentration from basal 55.6+/-7.5 to 367+/-114 nM at 50 microM PRL-2903. With 10 microM PRL-2903, venous histamine output increased from baseline 6.2+/-0.5 to 20.9+/-4.9 nmol h(-1); P=0.008. The combination of 520 pM gastrin and 10 microM PRL-2903 increased venous histamine output from 41.7+/-7.3 nmol h(-1) with gastrin alone to 95.2+/-9.8 nmol h(-1); P=0.016. Further, 10 microM PRL-2903 increased acid output from baseline 8.5+/-1.8 to 37.4+/-11 micromol h(-1); P=0.017. When combined with 10 microM ranitidine, PRL-2903 did not significantly stimulate acid secretion. Bombesin/PRL-2903 increased venous histamine concentration from 50.4+/-14.8 to 292+/-64.2 nM; P=0.008, and gastrin concentration from 38.6+/-13.1 to 95.8+/-20.3 pM; P=0.037. Endogenous somatostatin exerts a continuous restraint on histamine and gastrin release from the gastric mucosa and significantly reduces baseline acid secretion.

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.

Gastric atrial natriuretic peptide regulates endocrine secretion in antrum and fundus of human and rat stomach

Atrial natriuretic peptide (ANP) is present in gastric mucosa and preferentially binds to two subtypes of natriuretic peptide receptors (NPR), NPR-A and NPR-C. The present study examines the role of endogenous ANP in regulating endocrine secretion in rat and human stomachs. NPR-A protein expression and transcripts were identified in rat antral and fundic mucosa by Western blot and RT-PCR. In superfused rat and human antral and fundic segments, ANP (0.1 pM to 0.1 microM) caused a concentration-dependent increase in somatostatin secretion. In antrum, this was accompanied by a decrease in gastrin, and in fundus, this was accompanied by a decrease in histamine secretion. Changes in gastrin and histamine secretion reflected changes in somatostatin secretion and were abolished by somatostatin antibody. The NPR-A receptor antagonist anantin 1) inhibited basal somatostatin secretion and 2) abolished the somatostatin, gastrin, and histamine responses to ANP. We conclude that endogenous ANP, acting via the NPR-A receptor, stimulates somatostatin secretion from both antrum and fundus of rat and human stomach. Stimulation of somatostatin secretion is coupled to inhibition of gastrin secretion in the antrum and inhibition of histamine secretion in the fundus.

Adrenomedullin stimulates somatostatin and thus inhibits histamine and acid secretion in the fundus of the stomach

Adrenomedullin has recently been localized to enterochromaffin-like (ECL) and chief cells in the gastric fundus. It has been proposed that adrenomedullin may play a role in gastric mucosal defense and repair. In the present study, we have used the isolated, luminally perfused mouse stomach and superfused rat fundic segments to examine the effect of adrenomedullin on exocrine and endocrine secretion in this region of the stomach. Addition of adrenomedullin (1 pM to 1 microM) to the isolated mouse stomach caused a concentration-dependent decrease in acid secretion. The EC(50) value was 1.4 x 10(-9) and maximal inhibition of acid secretion was obtained at a concentration of 1 microM (31+/-4% below basal level, P<0.001). In rat fundic segments, superfusion with adrenomedullin (0.1 pM to 0.1 microM) caused a concentration-dependent increase in somatostatin secretion (EC(50), 1 x 10(-10)) that was accompanied by a reciprocal decrease in histamine secretion (EC(50), 1.2 x 10(-11)). Maximal stimulation of somatostatin secretion (60+/-5% above basal level, P<0.001) and inhibition of histamine secretion (50+/-5% below basal level, P<0.01) was obtained at a concentration of 0.1 microM. Changes in acid and histamine secretion induced by adrenomedullin reflected changes in somatostatin secretion and could be abolished by addition of somatostatin antibody. The axonal blocker, tetrodotoxin, also abolished the somatostatin and, consequently, the acid and histamine responses to adrenomedullin, implying that the effect of adrenomedullin on somatostatin secretion was mediated via activation of intramural neurons. We conclude that adrenomedullin, acting via intramural fundic neurons, stimulates somatostatin and thus inhibits histamine and acid secretion. This represents one mechanism by which adrenomedullin might enhance mucosal defense and repair.

Reciprocal paracrine pathways link atrial natriuretic peptide and somatostatin secretion in the antrum of the stomach

Atrial natriuretic peptide (ANP) as well as its receptor, NPR-A, have been identified in gastric antral mucosa, suggesting that ANP may act in a paracrine fashion to regulate gastric secretion. In the present study, we have superfused antral mucosal segments obtained from rat stomach to examine the paracrine pathways linking ANP and somatostatin secretion in this region.ANP (0.1 pM to 0.1 microM) caused a concentration-dependent increase in somatostatin secretion (EC(50), 0.3 nM). The somatostatin response to ANP was unaffected by the axonal blocker tetrodotoxin but abolished by addition of the selective NPR-A antagonist, anantin. Anantin alone inhibited somatostatin secretion by 18+/-3% (P<0.005), implying that endogenous ANP, acting via the NPR-A receptor, stimulates somatostatin secretion. Somatostatin (1 pM to 1 microM) caused a concentration-dependent decrease in ANP secretion (EC(50), 0.7 nM) that was abolished by addition of the somatostatin subtype 2 receptor (sst2) antagonist, PRL2903. Neutralization of ambient somatostatin with somatostatin antibody (final dilution 1:200) increased basal ANP secretion by 70+/-8% (P<001), implying that endogenous somatostatin inhibits ANP secretion. We conclude that antral ANP and somatostatin secretion are linked by paracrine feedback pathways: endogenous ANP, acting via the NPR-A receptor, stimulates somatostatin secretion, and endogenous somatostatin, acting via the sst2 receptor, inhibits ANP secretion.

Amylin, released from the gastric fundus, stimulates somatostatin and thus inhibits histamine and acid secretion in mice

BACKGROUND & AIMS

Amylin, a peptide that displays 50% homology with calcitonin gene-related peptide (CGRP), is colocalized with somatostatin in endocrine cells of the gastric fundus. The present study was designed to determine the mechanism of action of amylin on gastric exocrine and endocrine secretion.

METHODS

Acid secretion was measured in the isolated mouse stomach by titration. Somatostatin and histamine secretion were measured in rat fundic segments by radioimmunoassay.

RESULTS

In isolated mouse stomach, amylin caused a concentration-dependent decrease in acid secretion. In rat fundic segments, amylin and CGRP each caused a concentration-dependent increase in somatostatin and a decrease in histamine secretion. Changes in histamine secretion induced by amylin reflected changes in somatostatin secretion and could be abolished by addition of somatostatin antibody. Both the somatostatin and the histamine responses to amylin were abolished by the selective amylin antagonist AC187 but were unaffected by the CGRP antagonist CGRP8-37. In contrast, the responses to CGRP were abolished by CGRP8-37 but were unaffected by AC187. AC187 alone decreased somatostatin and increased histamine in fundic segments and increased acid secretion in isolated stomach, indicating that endogenous amylin participates in the regulation of gastric endocrine (somatostatin and histamine) and exocrine (acid) secretion.

CONCLUSIONS

In gastric fundus, release of amylin from somatostatin cells interacts with distinct amylin receptors to enhance somatostatin secretion via an autocrine pathway that leads to inhibition of histamine and acid secretion.

Neurotransmitters regulating acid secretion in the proventriculus of the Houbara bustard (Chlamydotis undulata): a morphological viewpoint

Endocrine cells containing somatostatin (Som), gastrin-releasing peptide (GRP), and neuronal nitric oxide synthase (nNOS) and nerve fibers containing choline acetyl transferase (ChAT), tyrosine hydroxylase (TH), galanin (Gal), substance P (SP), and vasoactive intestinal polypeptide (VIP) were immunolocalized in the proventriculus of the Houbara bustard, Chlamydotis undulata. While GRP-immunoreactive (GRP-IR) cells occur in the inner zone, somatostatin (Som-IR) and polyclonal nNOS (nNOS-IR) immunoreactive cells were localized mainly in the peripheral zone of submucosal glands. GRP-IR, Som-IR, and nNOS-IR cells were occasionally observed in the walls of the gastric glands. Endocrine cells are of the closed variety and usually possess apical processes extending along the basal surfaces of adjacent nonreactive cells. Ultrastructural features of these cells are typical. ChAT, Gal, SP, VIP, and TH were immunolocalized in nerve fibers and terminals in the walls of arterioles and capillaries at the periphery of submucosal glands. Immunoreactivity to monoclonal nNOS occurred mainly in neuronal cell bodies in ganglia located around the submucosal glands. ChAT and TH immunoreactive cell bodies were also occasionally seen around the submucosal glands in the peripheral region. Immunoreactivity to Gal, SP, and VIP, but not ChAT or TH, was discernible around the walls of gastric glands. It was concluded that the distribution of neurotransmitters in neuronal structures is similar, but that of the endocrine cells varies from that of some avian species. The roles of these neurotransmitters in the regulation of acid secretion are discussed.

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.

Review article: gastrin releasing peptide and its value in assessing gastric secretory function

Gastrin releasing peptide (GRP) has proved to be a particularly valuable tool in detecting disturbances of gastric secretory function associated with duodenal ulcer disease and Helicobacter pylori infection, and it has furthered understanding of the pathophysiology of these conditions. Its attractiveness lies in the fact that it simultaneously activates many physiological control processes, both stimulatory and inhibitory. This facilitates the detection of a defect in any of the many controls involved in regulating biological function. Other gastrointestinal functions such as gall-bladder contraction, pancreatic secretion and gastrooesophageal motility are also subject to complex regulatory controls, and GRP may also be of value in investigating disturbances of these processes.

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.

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.

Vagal control of the release of somatostatin, vasoactive intestinal polypeptide, gastrin-releasing peptide, and HCl from porcine non-antral stomach

We studied the secretion of somatostatin and HCl and the release of vasoactive intestinal polypeptide (VIP) and gastrin-releasing peptide (GRP) from isolated, vascularly perfused, porcine non-antral stomach. Electric vagus stimulation increased acid secretion and the release of VIP and GRP and inhibited somatostatin secretion as determined in the venous effluent. Atropine abolished the HCl response and reversed the somatostatin inhibition to a three-fold increase, whereas GRP and VIP responses were unchanged. Both intra-arterial carbachol (10(-6) M) and GRP (10(-8) M) increased acid secretion and inhibited somatostatin secretion. VIP (10(-8) M) increased somatostatin secretion and had no effect on acid secretion. By immunohistochemistry, somatostatin was localized to both open-type and closed-type cells equally spread in the various parts of the gastric glands without particular relation to the parietal cells. Numerous GRP- and VIP-immunoreactive nerve fibers were seen between the glands. It is concluded that the fundic and antral secretion of somatostatin, investigated in a previous study, are differently regulated. The relation of fundic somatostatin release to acid secretion seems to be complex.

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.

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.

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.

Gastrin-releasing peptide: neuronal distribution and spatial relation to endocrine cells in the human upper gut

By using immunocytochemical techniques, we have studied the distribution of gastrin releasing peptide (GRP)-containing neurons as well as the spatial relationship between these neurons and the endocrine cells in the human stomach and duodenum. Moderate numbers of immunoreactive fibers were distributed in the smooth muscle and submucosa of the stomach; they were more rare in the duodenal wall. Numerous GRP-containing nerve fibers were found in the oxyntic mucosa, the antral mucosa harboured only few GRP immunoreactive nerve fibers. The mucosa of the proximal duodenum was found to be virtually devoid of such fibers. Only occasionally did we observe signs of a direct contact between GRP-containing nerve fibers and gastrin and somatostatin cells in the antral mucosa. In the oxyntic mucosa GRP-containing nerve fibers sometimes seemed to contact endocrine cells, including somatostatin cells as well as individual parietal cells. In conclusion, although GRP-containing nerve fibers were quite numerous in the wall of the human upper gastro-intestinal (GI)-tract, we observed a lack of intimate spatial relationship between these fibers and endocrine cells in the antral mucosa, suggesting additive mechanisms to a direct innervation of gastrin cells and somatostatin cells by GRP nerve fibers explaining the physiological effects on hormonal release.

Effect of antibodies to the neuropeptide GRP on distention-induced gastric acid secretion in the rat

The present study examined and compared the effects of muscarinic blockade, beta-adrenergic blockade and immunoneutralization of the neuropeptide gastrin-releasing peptide (GRP) on distention-induced gastric acid secretion and gastrin release. In response to distention of rat stomachs with 0.9% NaCl, acid output rose from 3.5 +/- 0.5 mumol H+/30 min to 15.4 +/- 2.5 mumol H+/30 min (P less than 0.01). Intravenous administration of 4 mg/kg propranolol did not affect the acid secretory response to distention, however both 2 mg/kg atropine and 6 mg/kg pirenzepine significantly decreased gastric acid secretion by 44.8 +/- 7.8% and 40.9 +/- 5.7% (P less than 0.05), respectively. When specific antibodies to GRP were infused intravenously, the acid secretory response to distention was nearly abolished, decreasing to 5.1 +/- 0.8 mumol H+/30 min (P less than 0.01). In contrast to the effects on acid secretion, GRP antiserum did not significantly alter the gastrin release observed following distention. Results of these studies indicate that, under the conditions of these experiments, the acid secretory response to gastric distention may be independent of its effect on gastrin release. Although distention-induced gastric acid secretion may be partially governed by muscarinic pathways, the acid secretory response to distention in the rat appears to involve GRP-containing neurons.