Ghrelin stimulates gastric emptying but is without effect on acid secretion and gastric endocrine cells

Ghrelin, a recently discovered peptide hormone, is produced by endocrine cells in the stomach, the so-called A-like cells. Ghrelin binds to the growth hormone (GH) secretagogue receptor and releases GH. It is claimed to be orexigenic and to control gastric acid secretion and gastric motility. In this study, we examined the effects of ghrelin, des-Gln¹⁴-ghrelin, des-octanoyl ghrelin, ghrelin-18, -10 and -5 (and motilin) on gastric emptying in mice and on gastric acid secretion in chronic fistula rats and pylorus-ligated rats. We also examined whether ghrelin affected the activity of the predominant gastric endocrine cell populations, G cells, ECL cells and D cells. Ghrelin and des-Gln¹⁴-ghrelin stimulated gastric emptying in a dose-dependent manner while des-octanoyl ghrelin and motilin were without effect. The C-terminally truncated ghrelin fragments were effective but much less potent than ghrelin itself. Ghrelin, des-Gln¹⁴-ghrelin and des-octanoyl ghrelin neither stimulated nor inhibited gastric acid secretion, and ghrelin, finally, did not affect secretion from either G cells, ECL cells or D cells.     

Ghrelin stimulates motility in the small intestine of rats through intrinsic cholinergic neurons

BACKGROUND AND PURPOSE: Ghrelin is a peptide discovered in endocrine cells of the stomach. Since ghrelin mRNA expression and plasma levels are elevated in the fasting state, we investigated the effects of ghrelin on the interdigestive migrating myoelectric complex (MMC) in the small intestine in vivo and compared with motor effects of ghrelin in vitro. Methods: Sprague-Dawley rats were supplied with a venous catheter and bipolar electrodes in the duodenum and jejunum for electromyography of small intestine in awake rats. In organ baths, isometric contractions of segments of rat jejunum were studied. RESULTS: Ghrelin dose-dependently shortened the MMC cycle length at all three recording points. At the duodenal site, the interval shortened from 17.2+/-2.0 to 9.9+/-0.8 min during infusion of ghrelin (1000 pmol kg(-1) min(-1)) and at the jejunal site from 17.5+/-2.2 to 10.5+/-0.8 min. Ghrelin contracted the muscle strips with a pD2 of 7.97+/-0.47. Atropine (10(-6) M) in vitro and (1 mg kg(-1)) in vivo blocked the effect of ghrelin. CONCLUSION: Ghrelin stimulates interdigestive motility through cholinergic neurons. Ghrelin also stimulates motility, in vitro, suggesting that ghrelin receptors are present in the intestinal neuromuscular tissue and mediate its effects via cholinergic mechanisms.     

Intracisternal injection of TRH precursor, TRH-glycine, stimulates gastric acid secretion in rats

Intracisternal injection of thyrotropin-releasing hormone (TRH)-Gly (pGlu-His-Pro-Gly) produced a dose-dependent (1-100 micrograms) stimulation of gastric acid secretion in urethane-anesthetized rats implanted acutely with a gastric fistula. The peak response occurred 20-30 min after intracisternal injection and lasted for more than 2 h. Intravenous injection of TRH-Gly (100 micrograms) did not modify gastric acid secretion. Following intracisternal injection of TRH-Gly, a peak elevation of both TRH-Gly and TRH levels is observed in the cerebrospinal fluid (CSF) within 15 min. Thereafter, TRH values are returned to basal levels at 75 min after the injection, whereas TRH-Gly concentrations remain significantly elevated throughout the 2-h period of measurement. Compartmental analysis revealed that CSF conversion of TRH-Gly to TRH was only 0.0072%/min. Medullary coronal sections containing the dorsal vagal complex and the raphé nucleus revealed increased content of TRH-Gly, but not TRH, 40 min after administration of TRH-Gly at an intracisternal dose effective in stimulating gastric acid secretion (100 micrograms). In addition, TRH but not TRH-Gly (10(-7)-10(-5) M) displaced [3H]MeTRH binding from rat medullary blocks containing the dorsal vagal complex. These data suggest that the intracisternal TRH-Gly-induced stimulation of gastric acid secretion is not related to its conversion to TRH in the CSF, or direct activation of TRH receptors in the medulla. The acid secretory response of TRH-Gly may be due to the formation of TRH at the active brain sites, or alternatively to activation of its own specific receptors.     

Ghrelin directly stimulates glucagon secretion from pancreatic alpha-cells

Previous work has demonstrated that the peptide hormone ghrelin raises blood glucose. Such has been attributed to ghrelin's ability to enhance GH secretion, restrict insulin release, and/or reduce insulin sensitivity. Ghrelin's reported effects on glucagon have been inconsistent. Here, both animal- and cell-based systems were used to determine the role of glucagon in mediating ghrelin's effects on blood glucose. The tissue and cell distribution of ghrelin receptors (GHSR) was evaluated by quantitative PCR and histochemistry. Plasma glucagon levels were determined following acute acyl-ghrelin injections and in pharmacological and/or transgenic mouse models of ghrelin overexpression and GHSR deletion. Isolated mouse islets and the α-cell lines αTC1 and InR1G9 were used to evaluate ghrelin's effects on glucagon secretion and the role of calcium and ERK in this activity. GHSR mRNA was abundantly expressed in mouse islets and colocalized with glucagon in α-cells. Elevation of acyl-ghrelin acutely (after sc administration, such that physiologically relevant plasma ghrelin levels were achieved) and chronically (by slow-releasing osmotic pumps and as observed in transgenic mice harboring ghrelinomas) led to higher plasma glucagon and increased blood glucose. Conversely, genetic GHSR deletion was associated with lower plasma glucagon and reduced fasting blood glucose. Acyl-ghrelin increased glucagon secretion in a dose-dependent manner from mouse islets and α-cell lines, in a manner requiring elevation of intracellular calcium and phosphorylation of ERK. Our study shows that ghrelin's regulation of blood glucose involves direct stimulation of glucagon secretion from α-cells and introduces the ghrelin-glucagon axis as an important mechanism controlling glycemia under fasting conditions.     

Ghrelin acts in the central nervous system to stimulate gastric acid secretion

Ghrelin is a novel acylated peptide that functions in the regulation of growth hormone release and energy metabolism. It was isolated from rat stomach as an endogenous ligand for growth hormone secretagogue receptor. Ghrelin is also localized in the arcuate nucleus of rat hypothalamus. Intracerebroventricular (ICV) administration increases food intake and body weight. We examined the effect of ghrelin on gastric acid secretion in urethane-anesthetized rats and found that ICV administration of ghrelin increased gastric acid output in a dose-dependent manner. Vagotomy and administration of atropine abolished the gastric acid secretion induced by ghrelin. ICV administration of ghrelin also induced c-fos expression in the neurons of the nucleus of the solitary tract and the dorsomotor nucleus of the vagus, which are key sites in the central nervous system for regulation of gastric acid secretion. Our results suggest that ghrelin participates in the central regulation of gastric acid secretion by activating the vagus system.     

Peptide histidine valine-42 stimulates gastric acid secretion in man

The effect of peptide histidine valine-42 (PHV-42) on gastric acid secretion was studied in man. PHV-42 was infused into 5 healthy volunteers at a dose of 10 pmol/kg/min. This dose caused a significant stimulation of basal gastric acid and potassium output. there were no significant changes in circulating gastrin throughout the infusion. In 2 subjects with a background of submaximal pentagastrin stimulation, PHV-42 infusion at the same dose did not alter acid secretion in either subject. The previous observation that PHV-42 is found particularly in the stomach and the new finding that it stimulates basal gastric secretion suggest the possibility that PHV-42 could have a role in local control of acid secretion.     

PACAP stimulates gastric acid secretion in the rat by inducing histamine release

Previous studies have shown that pituitary adenylate cyclase-activating peptide (PACAP) stimulates enterochromaffin-like (ECL) cell histamine release, but its role in the regulation of gastric acid secretion is disputed. This work examines the effect of PACAP-38 on aminopyrine uptake in enriched rat parietal cells and on histamine release and acid secretion in the isolated vascularly perfused rat stomach and the role of PACAP in vagally (2-deoxyglucose) stimulated acid secretion in the awake rat. PACAP has no direct effect on the isolated parietal cell as assessed by aminopyrine uptake. PACAP induces a concentration-dependent histamine release and acid secretion in the isolated stomach, and its effect on histamine release is additive to gastrin. The histamine H2 antagonist ranitidine potently inhibits PACAP-stimulated acid secretion without affecting histamine release. Vagally stimulated acid secretion is partially inhibited by a PACAP antagonist. The results from the present study strongly suggest that PACAP plays an important role in the neurohumoral regulation of gastric acid secretion. Its effect seems to be mediated by the release of ECL cell histamine.     

Sauvagine: inhibition of gastric acid secretion in rats

Sauvagine (SV) powerfully inhibits gastric acid secretion by both the central and peripheral mechanisms. We examined whether adrenergic mechanisms or prostaglandin pathways might mediate the inhibitory action of SV on acid production in pylorus-ligated rats. Adrenalectomy altered the extent of the SV suppressive effect, suggesting that adrenal-derived substances participate in the action of the peptide. Blockade of adrenergic receptors by propranolol did not modify the antisecretory effect of SV, while the alpha-adrenergic antagonist, phentolamine, and the dopaminergic antagonist, haloperidol, potentiated the gastric response to the peptide. The action of SV appeared to be independent of prostaglandin pathways. We conclude that the antiacid effect of SV may be mediated by the adrenal but probably not by adrenergic or prostaglandin mechanisms.     

Modulation of gastric acid secretion by cannabinoids in rats

The current study aimed to evaluate the role of cannabinoid receptors in the regulation of gastric acid secretion and oxidative stress in gastric mucosa. To fulfill this aim, gastric acid secretion stimulated with histamine (5 mg/kg, subcutaneous [SC]), 2‐deoxy‐ d ‐glucose (D‐G) (200 mg/kg, intravenous) or ‐carbachol (4 μg/kg, SC) in the 4‐hour pylorus‐ligated rats. The CB1R agonist ( N‐arachidonoyl dopamine, 1 mg/kg, SC) inhibited gastric acid secretion stimulated by D‐G and carbachol but not in histamine, reduced pepsin content, and increased mucin secretion. Furthermore, it decreased malondialdehyde (MDA) and nitric oxide (NO) contents with an increase in glutathione (GSH) and paraoxonase 1 (PON‐1). Meanwhile, CB2R antagonist (AM630, 1 mg/kg, SC) inhibited gastric acid secretion stimulated by D‐G and reduced MDA and NO contents with an increase in GSH and PON‐1. Meanwhile, CB1R antagonist rimonabant or CB2R agonist GW 405833 had no effect on stimulated gastric acid secretion. Therefore, both CB1R agonist and CB2R antagonist may exert antisecretory and antioxidant potential in the stomach.     

Sauvagine: effects on gastric acid secretion in rats

Intracerebroventricular (ICV) and subcutaneous (SC) injections of sauvagine powerfully inhibited gastric acid secretion stimulated by gastric distension and by 2-deoxy-D-glucose, but not by histamine in pylorus-ligated rats. Naloxone failed to antagonize the antisecretory effects of SC and ICV sauvagine. Intravenous infusion of sauvagine completely suppressed bethanechol-stimulated gastric secretion, significantly decreased pentagastrin-stimulated gastric secretion and did not modify histamine-stimulated gastric secretion in gastric-perfused rats. The inhibitory effect of sauvagine on gastric secretory response is not mediated through opioid or histamine receptors. It appears to be dependent on a vagal mechanism as well as other mechanisms that await further elucidation.     

Nociceptin stimulates thyrotropin secretion in rats

Effects of nociceptin on thyrotropin (TSH) and thyrotropin-releasing hormone (TRH) secretion in rats were studied. Nociceptin (150 microgram/kg) was injected intravenously and rats were serially decapitated after the injection. The effects of nociceptin on TRH release from the hypothalamus and TSH release from the anterior pituitary in vitro were also investigated. TRH and thyroid hormones were measured by individual radioimmunoassays. TSH was determined by enzyme immunoassay. TRH contents in the hypothalamus decreased significantly after nociceptin injection, whereas plasma TRH concentrations showed no changes. Plasma TSH concentrations increased significantly in a dose-related manner. The TRH release from the hypothalamus was enhanced significantly in a dose-related manner with the addition of nociceptin. The TSH release from the anterior pituitary in vitro was not affected by the addition of nociceptin. The plasma thyroxine and 3,3',5-triiodothyronine levels did not change significantly after nociceptin administration. The inactivation of TRH by plasma or hypothalamus in vitro after nociceptin injection did not differ from that of controls. The findings suggest that nociceptin acts on the hypothalamus to stimulate TRH and TSH secretion.     

Capsaicin effects on stress pathology and gastric acid secretion in rats

The effects of capsaicin on stress ulcer formation, plasma corticosterone levels and gastric acid secretion were examined in rats. Capsaicin desensitization did not affect restraint stress ulcer formation but was associated with markedly elevated corticosterone levels, even in non-stressed rats. Acute, orally administered capsaicin augmented ulcer formation only if its administration was followed immediately by restraint stress. Delays of 1, 2, or 3 h between drug administration and stress produced ulceration comparable to control values. Capsaicin did not affect basal (non-stimulated) gastric acid secretion but substantially decreased pentagastrin-stimulated acid output.     

Modulation of gastric acid secretion by adenosine in conscious rats

Basal (nonstimulated) gastric acid output was determined in conscious rats fitted with indwelling gastric cannulae. The adenosine deaminase resistant analog of adenosine, R-phenylisopropyladenosine, elevated intraluminal pH beyond 7.0 and decreased gastric acid secretion when given at doses of 0.10 or 1.0 mg/kg, while S-phenylisopropyladenosine at similar doses did not affect either gastric acid output or pH. The potent adenosine receptor antagonist, 8-phenyltheophylline, given at doses of 0.1, 1.0, and 2.5 mg/kg augmented gastric acid output and, at doses of 0.01, 0.1, 1.0, and 2.5 mg/kg, blocked the acid-reducing effect of R-phenylisopropyladenosine (0.1 mg/kg). These data suggest that adenosine systems may be important regulators of gastric function.     

Effect of atrial peptide on gastric acid secretion in rats

The effect of synthetic rat atriopeptin (AP) II was examined on basal, vagally and carbachol-induced gastric acid secretion in anesthetized rats. AP II infusion, at stepwise increasing doses of 2, 20 and 100 ng/kg/min, had no effect on basal acid secretion. At doses of 2 and 20 ng/kg/min, AP II augmented vagally induced acid secretion significantly. The secretory response to vagal stimulation + AP II 20 ng/kg/min was completely abolished by atropine. In contrast a higher dose of AP II (50 ng/kg/h) reduced vagally induced acid secretion significantly. This dose of AP II also reduced acid secretion during direct cholinergic stimulation by carbachol, while the lower dose of 20 ng/kg/min had no effect on carbachol-induced acid secretion. The present data demonstrate for the first time an effect of atrial peptide on gastric acid secretion. At lower doses AP II augments the vagal influence on parietal cell function perhaps by augmenting vagally induced acetylcholine release. At higher doses AP II exerts an inhibitory effect on parietal cell function during vagally and carbachol-induced acid secretion, suggesting different and as yet unknown mechanisms of action. These results raise the possibility that the heart can exert a hormonally mediated influence on the regulation of gastric acid secretion.     

Pancreatic somatostatin-like immunoreactivity suppresses gastric acid secretion in rats.

Somatostatin-like immunoreactivity (SLI) was extracted from the canine pancreas and purified by ion exchange, affinity chromatography and gel filtration. The 1600 dalton fraction, which is physicochemically similar to synthetic somatostatin was infused into the peripheral circulation of anesthetized rats and its effect upon gastric acid secretion was compared with that of synthetic somatostatin. Both synthetic somatostatin and pancreatic SLI in a dose of 7–8 μg/kg/h suppressed pentagastrin-stimulated gastric acid secretion. It is concluded that the highly purified 1600 dalton fraction of canine pancreatic SLI, like synthetic somatostatin, can exert biological activity upon the stomach of rats.     

Histamine dependence of pentagastrin-stimulated gastric acid secretion in rats.

Does gastrin stimulate gastric acid secretion by direct action on oxyntic cells, by releasing histamine, or by being potentiated by histamine? Previous studies in the mouse pointed to gastrin-regulated histamine release. Guinea pig and rat are well known to vary in their sensitivity to histamine. Therefore, the effects of histamine and pentagastrin were compared quantitatively on isolated, lumen-perfused, stomach preparations from these species in the absence and presence of histamine H2-receptor blockade. The loss of potency of histamine in the rat was mirrored by a loss of potency of pentagastrin consistent with the idea that pentagastrin acts by releasing histamine. In the rat, a well-defined pentagastrin curve was obtained in the presence of histamine H2-receptor block as though pentagastrin acts both directly on the oxyntic cell and indirectly by releasing histamine. It was not necessary to invoke a potentiating interaction between histamine and pentagastrin at the oxyntic cell; the two effects appeared simply to add. Potentiation was observed, however, between other combinations of stimuli, for example, between vagal nerve and pentagastrin stimulation. The physiological consequences of these results are discussed.     

Neurotensin stimulates pancreatic exocrine secretion in rats

Neurotensin (NT) stimulates pancreatic exocrine secretion in dogs and humans. The purpose of this study was to examine the effect of exogenous neurotensin on pancreatic exocrine secretion in rats. Five Sprague-Dawley male rats were prepared with pancreatic, gastric and duodenal fistulas. Bile was shunted into the duodenum in order to collect pure pancreatic juice. 24 h later, neurotensin (0.05, 0.1, 0.2, 0.3, 1.0 nmol/kg) was infused intravenously in a random fashion. Pancreatic juice was collected every 10 min, and the volume was recorded and protein and bicarbonate were measured. Neurotensin stimulated, in a dose-related manner, the pancreatic secretion of water, protein and bicarbonate. Neurotensin may be involved in the physiologic control of pancreatic secretion in rats.