Effect of galanin on gastrin and somatostatin release from the rat stomach

Gastric Peptides-Gastrin and Somatostatin

Gastric acid secretion (i) facilitates digestion of protein as well as absorption of micronutrients and certain medications, (ii) kills ingested microorganisms, including Helicobacter pylori, and (iii) prevents bacterial overgrowth and enteric infection. The principal regulators of acid secretion are the gastric peptides gastrin and somatostatin. Gastrin, the major hormonal stimulant for acid secretion, is synthesized in pyloric mucosal G cells as a 101-amino acid precursor (preprogastrin) that is processed to yield biologically active amidated gastrin-17 and gastrin-34. The C-terminal active site of gastrin (Trp-Met-Asp-Phe-NH₂ ) binds to gastrin/CCK₂ receptors on parietal and, more importantly, histamine-containing enterochromaffin-like (ECL) cells, located in oxyntic mucosa, to induce acid secretion. Histamine diffuses to the neighboring parietal cells where it binds to histamine H₂ -receptors coupled to hydrochloric acid secretion. Gastrin is also a trophic hormone that maintains the integrity of gastric mucosa, induces proliferation of parietal and ECL cells, and is thought to play a role in carcinogenesis. Somatostatin, present in D cells of the gastric pyloric and oxyntic mucosa, is the main inhibitor of acid secretion, particularly during the interdigestive period. Somatostatin exerts a tonic paracrine restraint on gastrin secretion from G cells, histamine secretion from ECL cells, and acid secretion from parietal cells. Removal of this restraint, for example by activation of cholinergic neurons during ingestion of food, initiates and maximizes acid secretion. Knowledge regarding the structure and function of gastrin, somatostatin, and their respective receptors is providing novel avenues to better diagnose and manage acid-peptic disorders and certain cancers. Published 2020. Compr Physiol 10:197-228, 2020.

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

Galanin inhibits gastric acid secretion through a somatostatin-independent mechanism in mice

The role of somatostatin in galanin-induced inhibition of gastric acid secretion in urethane-anesthetized mice was investigated by using immunoneutralization of endogenous somatostatin and somatostatin receptor type 2 (SSTR2) knockout mice. Intravenous galanin (10 and 20 microg/kg/h) inhibited pentagastrin-stimulated gastric acid secretion by 47 and 33%, respectively. Somatostatin antibody injected i.v. increased acid secretion by 3.5-fold over basal levels but did not modify the antisecretory effects of galanin. Urethane-anesthetized SSTR2 knockout mice had a basal secretion 14-fold higher than wild-type animals, that was inhibited by galanin (10 and 20 microg/kg/h) by 49 and 31% respectively. In mice galanin inhibits gastric acid secretion through a somatostatin-independent mechanism.

ECL-cell histamine mobilization in conscious rats: effects of locally applied regulatory peptides, candidate neurotransmitters and inflammatory mediators

1. The ECL cells control gastric acid secretion by mobilizing histamine in response to circulating gastrin. In addition, the ECL cells are thought to operate under nervous control and to be influenced by local inflammatory processes. 2. The purpose of the present study was to monitor histamine mobilization from ECL cells in conscious rats in response to locally applied regulatory peptides, candidate neurotransmitters and inflammatory mediators. 3. Microdialysis probes were implanted in the submucosa of the acid-producing part of the rat stomach. Three days later, the agents to be tested were administered via the microdialysis probe and their effects on basal (48 h fast) and stimulated (intravenous infusion of gastrin-17, 3 nmol kg(-1) h(-1)) mobilization of ECL-cell histamine was monitored by continuous measurement of histamine in the perfusate (radioimmunoassay). 4. Locally administered gastrin-17 and sulfated cholecystokinin-8 mobilized histamine as did pituitary adenylate cyclase-activating peptide-27, vasoactive intestinal peptide, peptide YY, met-enkephalin, endothelin and noradrenaline, adrenaline and isoprenaline. 5. While gastrin, sulfated-cholecystokinin-8, met-enkephalin and isoprenaline induced a sustained elevation of the submucosal histamine concentration, endothelin, peptide YY, pituitary adenylate cyclase activating peptide, vasoactive intestinal peptide, noradrenaline and adrenaline induced a transient elevation. 6. Calcitonin gene-related peptide, galanin, somatostatin and the prostanoid misoprostol inhibited gastrin-stimulated histamine mobilization. 7. The gut hormones neurotensin and secretin and the neuropeptides gastrin-releasing peptide, neuropeptide Y and substance P failed to affect ECL-cell histamine mobilization, while motilin and neuromedin U-25 had weak stimulatory effects. Also acetylcholine, carbachol, serotonin and the amino acid neurotransmitters aspartate, gamma-aminobutyric acid, glutamate and glycine were inactive or weakly active as was bradykinin. 8. In summary, a range of circulating hormones, local hormones, catecholamines, neuropeptides and inflammatory mediators participate in controlling the activity of rat stomach ECL cells in situ.

Galanin inhibition of enterochromaffin-like cell function

BACKGROUND & AIMS

Galanin, a 29-amino acid neuropeptide found in the gastric mucosa, inhibits basal and pentagastrin-stimulated acid secretion. Its cellular target is unknown. The aim of this study was to determine whether galanin inhibits Ca2+ signaling and histamine release in enterochromaffin-like (ECL) cells.

METHODS

Isolated rat ECL cells were purified to 85% homogeneity by a combination of elutriation, density gradient centrifugation, and 48-hour culture. Intracellular calcium concentration ([Ca2+]i) was determined using video imaging with Fura-2 in a 37 degreesC superfusion chamber. Histamine was measured by radioimmunoassay.

RESULTS

Reverse-transciption polymerase chain reaction of the ECL cell RNA showed a galanin type I receptor subtype. Galanin inhibited gastrin, Bay K8644, and K+ depolarization-induced calcium mobilization and entry as well as reduced basal calcium levels. Pretreatment with pertussis toxin decreased the effect of galanin. Galanin inhibited basal and gastrin-stimulated histamine release by approximately 60% with a median effective concentration of 1.10(-10) mol/L. The inhibitory actions of galanin on histamine release and Ca2+ influx could be reduced by a galanin antagonist, galantide.

CONCLUSIONS

Galanin's inhibition of acid secretion can be explained in part by inhibition of calcium signaling and histamine release from the ECL cells due to activation of a Gi,o protein-coupled receptor.

Actions of vasodilator nerves on arteriolar smooth muscle and neurotransmitter release from sympathetic nerves in the guinea-pig small intestine

1. Brief constrictions of arterioles of the isolated submucosa of the guinea-pig small intestine were evoked by stimulation of the perivascular sympathetic nerves. Prior stimulation of vasodilator neurones in the submucosal nerve plexus greatly reduced the constrictor response to sympathetic stimulation. 2. Vasodilator nerve stimulation reduced both the amplitude and rate of decay of the excitatory junction potential (EJP) evoked in the arteriolar smooth muscle by sympathetic nerve stimulation. 3. Computer simulation of the effect of membrane resistance changes on the EJP amplitude indicated that the change in amplitude could not be explained by the fall in membrane resistance alone, suggesting that vasodilator nerve activity reduced neurotransmitter release from the sympathetic nerves.

Distribution of galanin-immunoreactive nerves in the canine gastrointestinal tract

The distribution of nerves containing galanin immunoreactivity (GAL-IR) in the canine gastrointestinal tract was determined by immunohistochemistry. GAL-IR was observed in nerve cell bodies and nerve fibers in all layers of the lower esophagus, gastric antrum, pylorus, ileum, and colon, and in the sphincters of lower esophagus and pylorus. There were numerous GAL-IR-containing nerve cell bodies in the myenteric plexus of gastric antrum. GAL-IR nerve somata were also common in the myenteric plexus of lower esophagus, ileum, pylorus, and colon, and in the submucous plexus of stomach, ileum, and colon. GAL-IR-positive nerve fibers were also observed around submucous blood vessels of the stomach. GAL-IR nerves are distributed more widely in the canine enteric nervous system than previously recognized.

Galanin analogues: agonist and antagonist

23 galanin-related peptides were synthesized by solid phase technology or conventional solution method. The purity of the products was carefully assessed by routine analytical criteria. Using these synthetic peptides, we have investigated the effects of galanins and structurally modified galanin peptides on glucose-stimulated insulin release using the isolated perfused rat pancreas, gastrin and somatostatin release using the isolated perfused rat stomach, the neurally-evoked muscle contractions in guinea pig ileum and the C-fiber response in the isolated spinal cord of the new born rat. The results suggest that the galanin amino-terminal 1-15 sequence is crucial for its activity in the above four systems. With the goal of developing a specific antagonist of galanin, synthetic galanin (1-15) analogues [D-Thr6,D-Trp8,9]galanin(1-15)ol, and [D-Trp8,9]galanin(1-15)ol were found to be a potent antagonist for inhibitory effect of galanin on glucose-induced insulin release.

Characterization of porcine gastric galanin

The presence of a peptide capable of producing powerful contractions of rat small intestinal smooth muscle was detected in chromatographic fractions derived from porcine gastric corpus extracts. The pharmacological characteristics of this entity suggested that it might be galanin and on its purification to homogeneity, amino acid composition and sequence analysis demonstrated the identify of the gastric and intestinal forms of galanin. The presence of galanin in the gastric corpus tissue and its ability to affect gastric smooth muscle activity, gastrin release, and gastric acid secretion suggest potential important physiological roles for galanin in the stomach.

Galanin inhibits pancreatic amylase secretion in the pentobarbital-anesthetized rat

The potent inhibitory effect of galanin on basal and pentagastrin-stimulated gastric acid secretion in vivo, and the presence of galanin-containing nerves in gastrointestinal tract and pancreas, suggested that this peptide may regulate the exocrine secretion of the GI system. Male rats were anesthetized with pentobarbital and the dose-dependent inhibitory effects of galanin on basal and stimulated pancreatic protein and amylase secretions were investigated in separate experiments. Galanin was administered intravenously in the following doses: 3, 6, 10, 15 and 20 micrograms/kg/h (0.93, 1.86, 3.1, 4.65 and 6.2 nmol/kg/h), and pancreatic secretions measured. The maximal effective dose of galanin (3.1 nmol/kg/h) on basal pancreatic secretions was found, and was used for evaluating the inhibitory effect of galanin on pancreatic protein and amylase secretions stimulated by bombesin, secretin and cholecystokinin. Galanin potently inhibited basal, bombesin-, secretin- and cholecystokinin-stimulated pancreatic protein and amylase secretion. Inhibitory effect of galanin was dose-dependent and biphasic.

Pickles, peptide hormones and pregnancy: a hypothesis

In western societies a high frequency of heartburn, nausea and vomiting occur during pregnancy. The causes and mechanisms of these clinical symptoms remain poorly understood. Evidence suggests steroid hormones modulate gastrointestinal transit time and plasma peptide hormones, while peptide hormone modulated food intake and preferences. Since diurnal and episodic release of steroid and peptide hormones occur, it is postulated that heartburn and other digestive dysfunction during pregnancy are associated with elevated steroid and peptide (beta-endorphin, NPY) hormone interaction with innate biological rhythms controlling the gastrointestinal tract.

Inhibitory effect of galanin on basal and pentagastrin-stimulated gastric acid secretion in rats

The dose-dependent effects of galanin on basal and pentagastrin-stimulated gastric acid secretion in pentobarbital-anesthetized rats were examined. Intravenous infusion of galanin at doses of 0.31, 0.62, 1.25, 1.87, 3.11, and 6.22 nmol/kg-1/h-1 into pentagastrin-stimulated rats produced a diminution in gastric acid secretion which was maximal (54.7%) at the level of the 1.87 nmol/kg-1/h-1 dose. Furthermore, the effect was biphasic, since both lower and higher doses of peptide were less effective. At the optimum concentration of 1.87 nmol/kg-1/h-1 galanin also inhibited basal gastric acid secretion. We conclude that endogenous galanin might be involved in the physiologic regulation of gastric acid secretion.

Bombesin-like peptides stimulate gastrin release from isolated rat G-cells

Bombesin-like peptides as well as receptor-independent activators were tested for their effect on gastrin release from acutely dispersed rat gastric G-cells. The amphibian peptide bombesin as well as its mammalian analogues neuromedin B and neuromedin C stimulated gastrin release. Maximal responses were achieved with 10(-9) M bombesin (191.0 +/- 16.8% of basal release), 10(-8) M neuromedin C(205.9 +/- 17.6%) and 10(-7) M neuromedin B (162.2 +/- 10.4%), respectively. The phorbol ester 12-O-tetradecanoyl-phorbol 13-acetate (TPA) and the synthetic diacylglycerol analogue 1-oleoyl-2-acetyl-sn-glycerol (OAG) are receptor-independent activators of the protein kinase C. Both TPA (10(-6) M) and OAG (10(-5) M) stimulated gastrin release to 214.0 +/- 29.3% and 198.2 +/- 20.8% of basal, respectively. Calcium ionophore A23187 (10(-5) M) was the most effective stimulant tested (364.7 +/- 39.6%). Its effect was reversed by the calmodulin antagonist W 7 (10(-6)-10(-5) M). Finally, forskolin (10(-5) M), a direct activator of cAMP-formation, as well as the cAMP-analogue dbcAMP (10(-3) M) induced gastrin release. IN conclusion, neuromedin B is less potent and less effective than neuromedin C and bombesin in stimulating rat gastric G-cells. In addition, gastrin release is activated by calcium- and phospholipid-dependent as well as by cAMP-induced cellular signal transduction mechanisms.

Inhibitory action of galanin on gastric acid secretion in pentobarbital-anesthetized rats

The effects of galanin and two galanin fragments, GAL(9-29) and GAL(15-29), were studied for potential effects on pentagastrin- and bethanechol-stimulated gastric acid secretion in a pentobarbital-anesthetized rat experimental model. At a dose of 10 micrograms/kg/h, galanin potently inhibited pentagastrin-stimulated gastric acid secretion whereas inhibition of bethanechol-stimulated gastric acid secretion was not statistically significant. Simultaneous iv infusion of galanin and atropine did not affect the inhibitory action of the former. In similar experiments, a GAL(15-29) fragment was completely inactive whilst GAL(9-29) retained only about 5% potency. These results indicate that galanin probably induces its inhibitory effects by acting directly on the parietal cells rather than through a cholinergic pathway. They also demonstrate that the rat gastric acid inhibitory activity of galanin depends critically on the integrity of the first fourteen N-terminal amino acids.