Inhibition Of Gastrin Release Research Articles

A network map of the gastrin signaling pathway

Gastrin is the primary hormone that induces gastric acid secretion (Edkins 1906). In humans, the gene encoding for gastrin is located on chromosome 17q21 (Lund et al. 1986). This hormone is produced by the G-cells of the antrum of stomach as preprogastrin, which comprises of 101 amino acids and is cleaved between Ala-21 and Ser-22 to yield progastrin (Reeve et al. 1984). Progastrin is then sequentially cleaved by prohormone convertase and carboxypeptidase E to yield glycine-extended gastrins- a 35-amino acid gastrin-34-Gly (G34-Gly) or an 18-amino acid gastrin-17-Gly (G17-Gly) in endocrine cells (Varro et al. 1995; Lacourse et al. 1997). G34-Gly and G17-Gly are amidated at their carboxyl terminal groups by the enzyme peptidyl alpha-amidating mono-oxygenase to form amidated gastrin. Gastrin interacts with the membrane-bound G-protein coupled cholecystokinin receptor group consisting of Cholecystokinin A receptor (CCKAR) and Cholecystokinin B receptor (CCKBR). CCKAR possesses high affinity for cholecystokinin and has a negligible affinity for gastrin (Ferrand and Wang 2006; Dufresne et al. 2006). However, CCKBR has a high affinity for gastrin and their carboxyl amidated analogues. CCKBR is expressed in the brain, smooth muscle cells, and parietal cells (Kopin et al. 1992; Berna et al. 2007). In addition to this, gastrin has also been reported to interact with annexin 2, and thereby exerts proliferation effects in gastrointestinal cancers (Singh et al. 2007; Singh 2007; Sarkar et al. 2012). The release of gastrin is induced by gastrin-releasing peptide, a neurotransmitter which acts on its basolateral receptor in the G-cells. Binding of gastrin to CCKBR present in parietal and enterochromaffin-like (ECL) cells (Schmitz et al. 2001; Kulaksiz et al. 2000)induces gastric acid secretion by parietal cells as well as histamine release by ECL cells (Dockray et al. 2005). The released histamine reaches parietal cells by paracrine diffusion where it binds H2 receptors and induces gastric acid secretion. The secretion of gastric acid inhibits the release of gastrin hormone rendering a negative feedback control, further preventing excess acid secretion. In addition, low pH value in the stomach inhibits gastrin release through stimulating somatostatin secretion by antral D-cells (Bloom et al. 1974). Gastrin is a well-known growth factor for the gastrointestinal tract. Gastrin stimulates proliferation of gastric mucosal cells (Hansen et al. 1976), maturation of parietal cells and enterochromaffin-like cells (Jain and Samuelson 2006), and promotes islet differentiation in the pancreas (Wang et al. 1993). It has also been shown to stimulate proliferation of gastric (Ishizuka et al. 1992) and colon cancer cells (Watson et al. 1989). Gastrin modulates invasion (Wroblewski et al. 2002), apoptosis (Przemeck et al. 2008; Todisco et al. 2001) and migration (Noble et al. 2003) in epithelial cells. Both glycine-extended gastrin and amidated gastrin have been reported to induce angiogenesis (Clarke et al. 2006; Lefranc et al. 2004). Moreover, the hypergastrinemic state has been associated with diverse physiological disorders in humans and other mammals. These include atrophic gastritis (Lehy et al. 2000), pernicious anemia (Orlando et al. 2007), excess acid secretion leading to duodenal ulcer disease in Helicobacter pylori infection (Berna et al. 2006; Jensen 2002; Scarpignato et al. 1996) and gastrinoma (Kloppel and Anlauf 2007). Recently, a number of research groups have explored the feasibility of using gastrin to treat various diseases. Gastrin has been used in combination therapy with epidermal growth factor to increase beta-cell mass which reversed hyperglycemia in diabetic mice (Suarez-Pinzon et al. 2005). Gastrin-stimulated beta cell neogenesis when used in combination with glucagon-like peptide 1 in human pancreatic duct cell transplanted in immunodeficient diabetic mice (Suarez-Pinzon et al. 2008). Gastrins and their receptors have been suggested as potential targets to treat gastrointestinal and pancreatic cancers (Rengifo-Cam and Singh 2004). Gastrin has also been shown to have therapeutic promise as it inhibits the growth of cholangiocarcinoma cells by promoting apoptosis (Kanno et al. 2001). On account of the functional significance of different natural forms of gastrin, we have assembled signaling pathway reactions induced by them upon binding to gastrin receptor(s) in different human cell types. These manually curated signaling events are made available through NetPath (http://www.netpath.org/) (Kandasamy et al. 2010) in different formats for analysis by the scientific community.

G cells and gastrin in chronic alcohol-treated rats

Numerous reports have described gastric mucosal injury in rats treated with high ethanol concentrations. However, to the best of our knowledge, ultrastructural characteristics of G cells and antral gastrin levels have not been previously reported, either in rats that chronically consumed alcohol or in human alcoholics. The goal of this study was to examine the effect of ethanol consumption (8.5 g/kg) over a 4-month period, under controlled nutritional conditions, on antral and plasma levels of gastrin, ultrastructure of G cells, morphometric characteristics of G cells by stereological methods, and analysis of endocrine cells in the gastric mucosa by immunohistochemistry. The chronic alcohol consumption resulted in a nonsignificant decrease in gastrin plasma levels and unchanged antral gastrin concentrations. A slightly damaged glandular portion of the gastric mucosa and dilatation of small blood vessels detected by histological analysis, suggests that ethanol has a toxic effect on the mucosal surface. Chronic alcohol treatment significantly decreased the number of antral G cells per unit area, and increased their cellular, nuclear, and cytoplasmatic profile areas. In addition, the volume density and diameter of G-cell granules, predominantly the pale and lucent types, were increased, indicating inhibition of gastrin release. Ethanol treatment also decreased the number of gastric somatostatin-, serotonin-, and histamine-immunoreactive cells, except the somatostatin cells in the pyloric mucosa, as well as both G: D: enterochromaffin cells (EC) cell ratios in the antrum and D: ECL cell ratios in the fundus. These results indicate that the change of morphometric parameters in G cells may be related to cellular dysfunction. Our findings also suggest that regulation of G-cell secretion was not mediated by locally produced somatostatin in ethanol-consuming rats, but may involve gastric luminal content and/or neurotransmitters of gastric nerve fibers.

Gastrin-producing endocrine cells: a novel source of histamine in the rat stomach.

Gastrin and histamine both potently stimulate secretion of acid into the gastric lumen. How these agents interact and how their release is controlled is poorly understood. Therefore, we decided to look for histamine in the antral portion of the rat stomach where the gastrin-producing G cells are located. We used immunocytochemical methods to visualize histamine, histidine decarboxylase (HDC, the enzyme that converts histidine to histamine), and the type 1 vesicular monoamine transporter (VMAT1, the protein responsible for moving histamine into vesicles for storage and release). We were surprised to find that histamine, HDC, and VMAT1 were all present in G cells. Our results suggest that G cells synthesize and secrete gastrin and histamine. Whether histamine acts in concert with gastrin to stimulate acid secretion, or functions as an autocrine inhibitor of gastrin release remains to be seen.

Somatostatin inhibits gastrin release and acid secretion by activating sst2 in dogs.

Somatostatin is a potent inhibitor of gastrin-stimulated acid secretion by activation of somatostatin receptor type 2 (sst2) in vivo, probably in part by blocking gastrin-stimulated histamine release from enterochromaffin-like cells expressing sst2. We propose that activation of sst2 may also regulate meal-stimulated acid secretion by blocking gastrin release from antral G cells. Using peptide analogs relatively selective for sst2 (NC-8-12), sst3 (BIM-23058), and sst5 (BIM-23052), we tested this hypothesis in two ways: first, in vivo by measuring plasma gastrin release during meal-stimulated acid secretion in dogs, and second, in vitro by measuring bombesin-stimulated gastrin release from an enriched culture of canine antral G cells. In vivo, a low dose (0.05 nmol.kg-1.h-1) of NC-8-12 inhibited acid secretion 56 +/- 16% without blocking gastrin release. A higher dose (1 nmol.kg-1.h-1) of NC-8-12 abolished acid secretion and inhibited gastrin release by 61 +/- 4%, whereas the highest dose (5 nmol.kg-1.h-1) inhibited gastrin release by 84 +/- 3%. Only the highest doses (5 nmol.kg-1.h-1) of BIM-23058 and BIM-23052 significantly inhibited gastrin release and acid secretion. In vitro, NC-8-12 (10(-9) M) reduced bombesin-stimulated gastrin release from antral G cells by 49 +/- 5%, whereas BIM-23058 and BIM-23052 were at least 100-fold less effective. These results indicate that somatostatin activation of sst2, but not sst3 or sst5, is the major pathway for somatostatin-induced inhibition of meal-stimulated gastrin release and acid secretion.

Role of vagal fibers and bombesin/gastrin-releasing peptide-neurons in distention-induced gastrin release in rats

In the rat the exact role of vagal fibers and the interaction between the extrinsic and intrinsic neural system in distention-induced gastrin release are still a matter of debate. Accordingly, the aim of the present study was to examine the contribution of afferent and efferent vagal fibers as well as intrinsic neurons on gastrin response to gastric distention. In anesthetized rats graded gastric distention by 5, 10 and 15 ml saline for 20 min caused a significant volume-dependent increase of plasma gastrin levels by 12±6 pg/ml (5 ml saline, n=8, P=0.05), 26±7 pg/ml (10 ml saline, n=10, P<0.05) and 37±7 pg/ml (15 ml saline, n=8, P<0.01), respectively. To examine the role of the extrinsic vagal innervation, gastrin response to distention was studied in anesthetized rats after bilateral truncal vagotomy ( n=9) or selective afferent vagotomy following pretreatment with capsaicin ( n=6). Stimulation of gastrin release by 10 ml distention in sham-operated control rats was reversed to an inhibition after truncal vagotomy (26±7 vs. −11±4 pg/ml; P<0.05) and capsaicin-treatment (37±18 vs. −34±11 pg/ml; P<0.05). A contribution of cholinergic mechanisms to this vagovagal-mediated stimulation of distention-induced gastrin release was excluded, since atropine (100 μg/kg/h; n=8) further augmented distention-stimulated gastrin release. Since bombesin/gastrin-releasing peptide (GRP)-neurons contribute to vagally stimulated gastrin secretion, we have examined gastrin response to distention in the presence of the specific bombesin-receptor antagonist d-Phe 6-BN(6-13)OMe (400 μg/kg/h; n=10). This bombesin-antagonist completely reduced distention-stimulated gastrin release in vivo. In contrast, distention of the isolated, extrinsically denervated stomach significantly decreased gastrin release by 13±5 pg/min (5 ml saline, n=8, P<0.05), 28±8 pg/min (10 ml saline, n=11, P<0.05) and 35±10 pg/min (15 ml saline, n=8, P<0.01), respectively, without changing the activity of bombesin/GRP-neurons. Distention-induced decrease of gastrin release was attenuated to 50 percent by atropine (10 −7 M; n=10) or tetrodotoxin (TTX) (10 −6 M; n=10), respectively. These data demonstrate, that in anesthetized rats distention-stimulated gastrin secretion depends on the activation of a vagovagal reflex and intrinsic bombesin/ GRP-neurons. In contrast distention of the isolated rat stomach inhibits gastrin release in part via intrinsic cholinergic pathways and other as yet unknown mechanisms.

Effect of cholinergic agonists on gastrin release from primary cultures of human antral G cells

BACKGROUND & AIMS: There is conflicting evidence concerning whether muscarinic regulation of gastrin release in humans is a direct or indirect effect on antral G cells. The present experiments were designed to resolve this question using an isolated human G-cell preparation.METHODS: The ability of muscarinic agonists to stimulate or inhibit gastrin release was assessed with or without an immunoneutralizing somatostatin antibody or an m3 receptor antagonist. The effect of secretogogues on G and D cells was monitored by intracellular calcium imaging.RESULTS: Muscarinic agonists failed to stimulate gastrin release, even after the removal of somatostatin- induced inhibition. Our group has previously shown that muscarinic agonists stimulated somatostatin release from antral D cells. Methacholine (100 mumol/L) increased intracellular calcium levels in cocultured D cells; this increase was inhibited by the m3 receptor antagonist 4-diphenylacetoxy-n-methylpiperidine methiodide. Gastrin cells in the same field of view lacked a response to methacholine but showed a clear response to 10 nmol/L bombesin.CONCLUSIONS: The experiments indicate that vagal control of gastrin release in humans is indirect; stimulation would be achieved by the activation of intrinsic gastrin-releasing peptide neurons, and inhibition would be via the paracrine action of somatostatin released from adjacent D cells.(Gastroenterology 1997 Feb;112(2):357-63)

Evidence that endogenous GRP in rat stomach mediates omeprazole-induced hypergastrinemia.

To investigate the physiological role of endogenous gastrin-releasing peptide (GRP) in regulating the release of gastrin, we evaluated the response of intragastric pH, gastrin, and GRP after omeprazole treatment in rats. A significant elevation of the plasma level of GRP (P < 0.01) and a significant reduction of the antral content of GRP (P < 0.05) were observed after the administration of 100 mg/kg omeprazole. The antral content of GRP was significantly decreased 12 h after omeprazole administration and thereafter gradually returned to control levels. Peak values for intragastric pH and plasma GRP were observed 3 h after omeprazole administration and before the peak of serum gastrin. The bombesin antagonist [D-Phe6]-bombesin-(6,13)-methyl ester significantly inhibited gastrin release after omeprazole treatment (P < 0.05). These observations indicate that omeprazole-induced inhibition of acid secretion stimulates the release of GRP and suggest that the secretion of GRP induced by omeprazole may stimulate the secretion of gastrin, at least in the early phase.

Infection of Sheep with Adult and Larval Ostertagia circumcincta: Effects on Abomasal pH and Serum Gastrin and Pepsinogen

Lawton D. E. B., Reynolds G. W., Hodgkinson S. M., Pomroy W. E. & Simpson H. V. 1996. Infection of sheep with adult and larval Ostertagia circumcincta: effects on abomasal pH and serum gastrin and pepsinogen. International Journal for Parasitology 26: 1063–1074. Infection of sheep with adult or larval O. circumcincta increased serum pepsinogen and gastrin and abomasal pH. The upper limits of the normal range, calculated from over 1000 samples collected from parasite-naive sheep, were set at 2 standard deviations above the mean; these were for serum pepsinogen, 454 mU tyrosine 1−1; serum gastrin, 64 pM and abomasal pH, 3.26. Five infection regimes were used: sheep previously exposed to field parasitism were infected with 30,000 larvae intraruminally (Group A), while parasite-naive sheep were administered either 50,000 larvae intraruminally (Group B), 150,000 larvae intraruminally followed by a trickle infection of 10,000 larvae thrice weekly from days 21 to 45 (Group C), 150,000 exsheathed larvae via an abomasal cannula (Group D) or 15,000 adult worms via an abomasal cannula (Group E). Whereas the presence of adult worms rapidly increased serum pepsinogen (after 8 h) and abomasal pH and serum gastrin (after about 19 h), the early infective larval stages, regardless of the infection regime, had minimal effects until the abrupt rise in all parameters 5–6 days after infection, Abomasal pH returned to near normal levels when the infections became patent and was not re-elevated by a subsequent trickle infection, whereas serum gastrin and pepsinogen remained high. The initial hypergastrinaemia was coincident with the increased abomasal pH, but was preceded by the increase in serum pepsinogen. In several sheep, serum pepsinogen increased very little during the parasitism, although there were typical effects on abomasal pH and serum gastrin. Serum gastrin was depressed when the abomasal pH exceeded about 5.5. It is suggested that an inhibitor of gastrin release is generated by proliferating abomasal microbes under these conditions and that this is a limitation to the use of elevated serum gastrin in the diagnosis of parasitism in individual sheep.

Infection of sheep with adult and larval Ostertagia circumcincta: Effects on abomasal pH and serum gastrin and pepsinogen

Lawton D. E. B., Reynolds G. W., Hodgkinson S. M., Pomroy W. E. & Simpson H. V. 1996. Infection of sheep with adult and larval Ostertagia circumcincta: effects on abomasal pH and serum gastrin and pepsinogen. International Journal for Parasitology 26: 1063–1074. Infection of sheep with adult or larval O. circumcincta increased serum pepsinogen and gastrin and abomasal pH. The upper limits of the normal range, calculated from over 1000 samples collected from parasite-naive sheep, were set at 2 standard deviations above the mean; these were for serum pepsinogen, 454 mU tyrosine 1 −1; serum gastrin, 64 pM and abomasal pH, 3.26. Five infection regimes were used: sheep previously exposed to field parasitism were infected with 30,000 larvae intraruminally (Group A), while parasite-naive sheep were administered either 50,000 larvae intraruminally (Group B), 150,000 larvae intraruminally followed by a trickle infection of 10,000 larvae thrice weekly from days 21 to 45 (Group C), 150,000 exsheathed larvae via an abomasal cannula (Group D) or 15,000 adult worms via an abomasal cannula (Group E). Whereas the presence of adult worms rapidly increased serum pepsinogen (after 8 h) and abomasal pH and serum gastrin (after about 19 h), the early infective larval stages, regardless of the infection regime, had minimal effects until the abrupt rise in all parameters 5–6 days after infection, Abomasal pH returned to near normal levels when the infections became patent and was not re-elevated by a subsequent trickle infection, whereas serum gastrin and pepsinogen remained high. The initial hypergastrinaemia was coincident with the increased abomasal pH, but was preceded by the increase in serum pepsinogen. In several sheep, serum pepsinogen increased very little during the parasitism, although there were typical effects on abomasal pH and serum gastrin. Serum gastrin was depressed when the abomasal pH exceeded about 5.5. It is suggested that an inhibitor of gastrin release is generated by proliferating abomasal microbes under these conditions and that this is a limitation to the use of elevated serum gastrin in the diagnosis of parasitism in individual sheep.

Pyloric antral inhibition of gastrin release.

Inconsistencies and omissions in current explanations for the well known depressions of gastric acid secretion and blood gastrin levels following acidification by antral stimulants are discussed. Evidence is presented which favors a reciprocal sensitivity relationship between the fundic mucosa and the antral G cells, such that blood gastrin levels rise when the secreting fundic mucosa is compromised and acid secretion in response to exogenous gastrin is increased when G cells are depressed or reduced. The functional connections between the two phenomena are considered to be nervous.

Effect of lactic acid on postprandial plasma-glucose and -insulin responses in rats administered glucose solution

The effect of lactic acid or sodium lactate on the glycemic response was studied in rats. The lightly anesthetized, overnight-fasted rats were administered a glucose solution (200 g/L) with or without lactic acid or sodium lactate by a stomach tube to provide 250 mg of glucose per 100 g body weight. The addition of lactic acid (300 mmol/L) to a glucose solution reduced the initial rise in plasma glucose and insulin concentrations, slowed the rate of gastric emptying and brought a drop of pH in gastric contents. However, such effects were not recognized in the addition of sodium lactate (300 mmol/L as the lactate ion). Lactic acid stimulated cholecystokinin (CCK) and secretin release, but inhibited gastrin release. The study suggests that lactic acid is effective in modulating postprandial plasma glucose concentration, which is due to delayed gastric emptying accompanied by the increase of secretin and CCK release following the fall in intraduodenal pH.

Lack of Effect of H2-Receptor Antagonists and Antacids on the Gastric and Duodenal Gastrin-, Somatostatin- and Serotonin-Producing Cells in Patients with Acid Peptic Disorders

Standard therapeutic approaches to acid peptic disorders have dealt with neutralizing or inhibiting aggressive factors and/or bolstering defensive factors. Gastric and duodenal mucosal biopsies were examined from 90 patients with various acid peptic disorders, as follows: reflux esophagitis (n=24), gastric ulcer (n=13), duodenal ulcer (n=47) and nonulcer dyspepsia (n=6). Seven patients with minimal dyspeptic symptoms and an endoscopically and histologically normal stomach and duodenum served as controls. Immunoperoxidase staining for gastrin-producing G cells, somatostatin-producing D cells and serotonin-producing EC cells was carried out on fundic, antral and duodenal biopsies, and quantitated using a Zeiss MOP videoplan. No significant effects secondary to treatment with antacid, ranitidine or cimetidine were observed on endocrine cell densities and ratios. Biopsies obtained on different occasions over time indicated that in patients on enprostil (a synthetic E2prostaglandin), there was a trend towards increasing cell counts, suggesting that the serum gastrin-lowering effect of this drug may result from inhibition of gastrin release. Thus, H2-receptor antagonists and antacids do not alter gastric or duodenal mucosal G, D or EC cells in patients with acid peptic disorders.

Somatostatin-14 mediates gastric and inhibition of gastrin release through muscarinic pathways: [formula omitted], S. Pokol-Daniel, G.R. Greenberg, Departments of Medicine and Physiology, University of Toronto, Toronto, Canada

Somatostatin-14 mediates gastric and inhibition of gastrin release through muscarinic pathways: [formula omitted], S. Pokol-Daniel, G.R. Greenberg, Departments of Medicine and Physiology, University of Toronto, Toronto, Canada

Effect of plaunotol on hypergastrinemia induced by long-term omeprazole administration in humans

Omeprazole markedly inhibits basal and stimulated gastric acid secretion and has the ability to produce hypergastrinemia and hyperplasia of enterochromaffin-like cells in humans. On the other hand, plaunotol, an acyclic diterpene alcohol, has been reported to inhibit gastrin release by stimulating endogenous secretion release. We investigated the effect of plaunotol on serum gastrin levels after six to eight weeks of omeprazole (20 mg/day) administration in 22 patients (16 males, 6 females; mean age 52.3, range 36-70 years) with peptic ulcer disease. The patients were randomized to the following two groups: 11 subjects with omerprazole alone (single group) and 11 with omeprazole plus plaunotol (240 mg/day) (combination group) treatment. There were no significant differences between the two groups concerning age, sex, ulcer stage, ulcer history, environmental factors, and Helicobacter pylori (HP) prevalence. After complete drug(s) administration, serum immunoreactive (ir) -gastrin levels increased significantly in the single group (P < 0.001) in contrast to the combination group, and plaunotol significantly inhibited hypergastrinemia induced by omeprazole administration (P < 0.001). Significant increases in serum ir-calcitonin gene-related peptide concentrations were observed in the combination group compared to the single group (P < 0.05). However, there were no significant changes in sereum ir-secretin, somatostatin, and vasoactive intestinal polypeptide levels as well as ulcer healing and HP prevalence between the two groups. These findings suggest that plaunotol may suppress hypergastrinemia induced by long-term omeprazole administration, at least partly, via a certain brain-gut hormone affecting gastrin release.

Functional characterization of a muscarinic receptor stimulating gastrin release from rabbit antral G-cells in primary culture.

In previous studies carbachol-induced stimulation of gastrin release from antral G-cells in primary culture suggested the presence of muscarinic acetylcholine receptors on this cell type. Therefore, we attempted to pharmacologically characterize the muscarinic acetylcholine receptor subtype involved. Enzymatically isolated rabbit antral mucosal cells (0.8% G-cells) were separated by counterflow elutriation yielding a fraction (1.7% G-cells) that was placed in culture on collagen-coated well plates. After 24-36 h of culture 13.0 +/- 2.4% of total adherent cells were immunoreactive for gastrin as shown by immunocytochemical staining using the avidin-biotin complex method. In this preparation basal gastrin release ranged from 3.3 +/- 0.3 to 4.1 +/- 0.3% of total cellular content. Maximal gastrin release in response to the acetylcholine receptor agonist carbachol (10(-4) M) or the selective muscarinic receptor agonist arecaidine propargyl ester (10(-4) M) was 8.5 +/- 0.4% and 7.6 +/- 0.4% of total cellular content, respectively. The EC50 values were 3.7 +/- 0.5 x 10(-6) M carbachol and 1.8 +/- 0.4 x 10(-6) M arecaidine propargyl ester. At a concentration of 10(-6) M the non-selective muscarinic receptor antagonist atropine and the muscarinic M3 receptor preferring antagonist hexahydro-sila-difenidol (HHSiD; M3 > or = M1 > M2) completely inhibited gastrin release in response to carbachol (Ki values: 52 x 10(-9) M atropine and 29 x 10(-9) M HHSiD) and arecaidine propargyl ester (Ki values: 11 x 10(-9) M atropine and 13 x 10(-9) M HHSiD).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of systemic gastric acid stimulation and intragastric pH changes on synchronous antral gastrin and somatostatin release in anesthetized, nonatropinized duodenal ulcer patients and controls

The synchronous changes in antral gastrin and somatostatin release in anesthetized, nonatropinized duodenal ulcer patients and control subjects were investigated by serial intraoperative blood sampling from the right gastroepiploic vein. The mean basal antral plasma gastrin and somatostatin concentrations of the two groups did not differ significantly. The significantly greater gastric acid secretory response to systemic gastric acid stimulation (pentagastrin stimulation) in duodenal ulcer patients compared with that of control subjects was not linked to any difference in antral somatostatin release pattern. The decrease in antral plasma gastrin release was significantly lower after acid instillation and the increase was significantly higher after alkali instillation in duodenal ulcer patients compared with those of controls, indicating an abnormal gastrin response to intragastric pH changes in duodenal ulcer patients, which was again not found to be coupled to any significant difference in antral somatostatin release. The results suggest that an abnormal somatostatin-mediated inhibition of gastrin release and/or gastric acid secretion does not exist in duodenal ulcer patients.

Relative potencies of the somatostatin analogs octreotide, BIM-23014, and RC-160 on the inhibition of hormone release by cultured human endocrine tumor cells and normal rat anterior pituitary cells.

In the present study we investigated the effects of the somatostatin (SS) analogs octreotide, RC-160, and BIM-23014 on GH release by cultured cells of human GH-secreting pituitary tumors, in normal rat anterior pituitary cells, and on gastrin release by cultured cells from a human gastrinoma. In all GH-secreting adenomas and in rat anterior pituitary cells, RC-160 was the most potent compound. RC-160 significantly inhibited GH-, PRL, and/or alpha-subunit release by human GH-secreting pituitary adenoma cells in concentrations as low as 10(-12)-10(-14) M, whereas at the same concentrations, octreotide and BIM-23014 did not inhibit or were significantly less effective in inhibiting GH release (P < 0.01, RC-160 vs. octreotide and BIM-23014). In rat anterior pituitary cell cultures, the IC50 values for inhibition of GH release were, in rank order of potency, 0.1, 5.3, 47, 48, and 99 pM for RC-160, SS-14, BIM-23014, octreotide, and SS-28, respectively. Maximal inhibitory effects by the three analogs were the same in the human GH adenoma cell cultures and the rat anterior pituitary cell cultures (-60%). On the basis of these data, RC-160 appears to be about 500 times more potent than octreotide and BIM-23014 in inhibiting GH release by rat anterior pituitary cells in vitro. Forskolin (100 microM) as well as pretreatment of the cells with pertussis toxin significantly diminished the inhibitory effects of the three SS analogs and those of SS-14 and SS-28 to the same extent. The latter data suggest that octreotide, RC-160, and BIM-23014 act mainly via a pertussis toxin-sensitive G-protein and an adenylyl cyclase-dependent mechanism. In the human gastrinoma culture, RC-160 inhibited gastrin release significantly more than octreotide at 10(-12)- and 10(-14)-M concentrations (P < 0.01). In conclusion, the SS analogs octreotide, RC-160, and BIM-23014 may have significant different potencies of inhibition of hormone release in vitro, with RC-160 being the most potent SS analog and octreotide and BIM-23014 having similar potencies. Depending on the pharmacokinetic properties of these three octapeptide SS analogs, these observations may have consequences for the medical therapy of patients with SS receptor-positive endocrine tumors.

The stomach is the etiologic organ for immobilization-induced hypocalcemia in rats.

Our previous study revealed that the gastric vagus nerve plays an etiologic role in immobilization (IMB) stress-induced hypocalcemia. The purpose of the present study is to identify exactly what parts of the stomach are involved in the development of IBM-induced hypocalcemia and to determine whether or not gastric acid secretion is involved. A total gastrectomy, but not a resection of the upper intestine, eliminated the hypocalcemic effect of IMB. In addition, either an antrectomy (removal of the source of gastrin) or a fundectomy (depriving the origin of gastric histamine and gastric acid) was sufficient for eliminating IMB-induced hypocalcemia, while a partial (50%) fundectomy failed to suppress it. An intraperitoneal injection of galanin (an inhibitor of gastrin release) or ranitidine (a blocker of histamine H2-receptor) also suppressed the calcium-lowering effect of IBM, whereas omeprazole (an inhibitor of the proton pump) had no effect. These findings suggest that the antrum and the fundus of the stomach play essential roles in IMB-induced hypocalcemia through the vagus-induced release of gastrin and histamine but not through the secretion of gastric acid per se.

Gastric vagus mediates immobilization-induced hypocalcemia in rats.

The involvement of the parasympathetic nervous system in the etiology of stress-induced hypocalcemia was investigated in the rat. Atropine methyl bromide (0.1 and 0.6 mg/kg ip) given 20 min before immobilization (IMB) was observed to suppress the induction of hypocalcemia in a dose-dependent manner. A vagotomy of the bilateral cervical trunks also abolished the IMB-induced hypocalcemia. A vagotomy on either the thyroid/parathyroid branches or the celiac branches had no effect on the IMB-induced hypocalcemia, but a vagotomy on the gastric branches completely abolished it. Pretreatment with either secretin (2 and 6 micrograms/kg ip), an inhibitor of gastrin release, or cimetidine (5 and 10 mg/kg ip), a histamine H2-receptor antagonist, diminished the IMB-induced hypocalcemia. The concentration of serum gastrin increased significantly during IMB. It is thus concluded that the decreased levels of plasma calcium caused by IMB are due to the activation of the vagus innervating the stomach. Gastrin and histamine are also involved as a consequence of the activation of the vagus.

High potency of a new bombesin antagonist (RC-3095) in inhibiting serum gastrin levels; comparison of different routes of administration

This study was performed to evaluate the efficacy and duration of action of a new bombesin antagonist d-Tpi 6,Leu 13 ψ (CH 2NH)Leu 14-bombesin (6–14) (RC-3095), given by different routes of administration, in suppressing gastrin releasing-peptide (GRP(14–27))-stimulated gastrin release in rats. First, we showed that GRP(14–27) itself was highly active when administered by different routes. GRP(14–27), given to rats at a dose of 25 μg/100 g b.w. significantly increased serum gastrin levels 3 and 6 min after intravenous and for more than 30 min after subcutaneous administration or pulmonary inhalation. RC-3095 was then injected subcutaneously, intravenously and also delivered by pulmonary inhalation at a dose of 10 μg/100 g b.w. in each case to seven male rats 2, 30, 60 or 120 min prior to i.v. administration of 5 μg GRP(14–27). RC-3095 administered 2 min prior to GRP(14–27) decreased the gastrin response to GRP(14–27), measured as area under the curve, by 81% in the intravenously injected group and 64% in the pulmonary inhalation group in the first 6 min. When GRP(14–27), was given 30 min after administration of RC-3095, the gastrin response was decreased by 52% in the subcutaneous group, 49% in the pulmonary inhalation group and 11% in the intravenous group during the first 6 min. RC-3095 delivered subcutaneously or by pulmonary inhalation 1 h before GRP(14–27) was also able to significantly inhibit gastrin release. Analysis of the data revealed that the biovailability of RC-3095 given by the pulmonary inhalation route was about 69% of the s.c. route. Our results indicate high activity of bombesin antagonist RC-3095, administered by different in vivo routes, in suppressing serum gastrin in rats. The administration by pulmonary inhalation evaluated here in rats, may prove to be practical and useful in the clinical setting.