Regulation of gastroduodenal bicarbonate secretion by capsaicin-sensitive sensory neurons in rats

FFA2 activation combined with ulcerogenic COX inhibition induces duodenal mucosal injury via the 5-HT pathway in rats

Serotonin (5-HT), predominantly synthesized and released by enterochromaffin cells, is implicated in gastrointestinal symptoms such as emesis, abdominal pain, and diarrhea. Because luminal short-chain fatty acids (SCFAs) release 5-HT from enterochromaffin cells, which express the SCFA receptor free fatty acid receptor 2 (FFA2) in rat duodenum, we examined the effects of the selective FFA2 agonist phenylacetamide-1 (PA1) on duodenal 5-HT release with consequent bicarbonate secretion [duodenal bicarbonate secretion (DBS)] and on indomethacin (IND)-induced enteropathy. Intestinal injury was induced by IND (10 mg/kg sc) with or without PA1. We measured DBS in vivo in a duodenal loop perfused with PA1 while measuring 5-HT released in the portal vein. Duodenal blood flow was measured by laser-Doppler flowmetry. IND induced small intestinal ulcers with duodenal sparing. PA1 given with IND (IND + PA1) dose dependently induced duodenal erosions. IND + PA1-induced duodenal lesions were inhibited by the FFA2 antagonist GLPG-0974, ondansetron, or omeprazole but not by RS-23597 or atropine. Luminal perfusion of PA1 augmented DBS accompanied by increased portal blood 5-HT concentrations with approximately eight times more release at 0.1 mM than at 1 µM, with the effects inhibited by coperfusion of GLPG-0974. Luminal PA1 at 1 µM increased, but at 0.1 mM diminished, duodenal blood flow. Cosuperfusion of PA1 (0.1 mM) decreased acid-induced hyperemia, further reduced by IND pretreatment but restored by ondansetron. These results suggest that, although FFA2 activation enhances duodenal mucosal defenses, FFA2 overactivation during ulcerogenic cyclooxygenase inhibition may increase the vulnerability of the duodenal mucosa to gastric acid via excessive 5-HT release and 5-HT₃ receptor activation, implicated in foregut-related symptoms such as emesis and epigastralgia.NEW & NOTEWORTHY Luminal free fatty acid receptor 2 agonists stimulate enterochromaffin cells and release serotonin, which enhances mucosal defenses in rat duodenum. However, overdriving serotonin release with high luminal concentrations of free fatty acid 2 ligands such as short-chain fatty acids injures the mucosa by decreasing mucosal blood flow. These results are likely implicated in serotonin-related dyspeptic symptom generation because of small intestinal bacterial overgrowth, which is hypothesized to generate excess SCFAs in the foregut, overdriving serotonin release from enterochromaffin cells.

The Gastrointestinal Circulation: Physiology and Pathophysiology

The gastrointestinal (GI) circulation receives a large fraction of cardiac output and this increases following ingestion of a meal. While blood flow regulation is not the intense phenomenon noted in other vascular beds, the combined responses of blood flow, and capillary oxygen exchange help ensure a level of tissue oxygenation that is commensurate with organ metabolism and function. This is evidenced in the vascular responses of the stomach to increased acid production and in intestine during periods of enhanced nutrient absorption. Complimenting the metabolic vasoregulation is a strong myogenic response that contributes to basal vascular tone and to the responses elicited by changes in intravascular pressure. The GI circulation also contributes to a mucosal defense mechanism that protects against excessive damage to the epithelial lining following ingestion of toxins and/or noxious agents. Profound reductions in GI blood flow are evidenced in certain physiological (strenuous exercise) and pathological (hemorrhage) conditions, while some disease states (e.g., chronic portal hypertension) are associated with a hyperdynamic circulation. The sacrificial nature of GI blood flow is essential for ensuring adequate perfusion of vital organs during periods of whole body stress. The restoration of blood flow (reperfusion) to GI organs following ischemia elicits an exaggerated tissue injury response that reflects the potential of this organ system to generate reactive oxygen species and to mount an inflammatory response. Human and animal studies of inflammatory bowel disease have also revealed a contribution of the vasculature to the initiation and perpetuation of the tissue inflammation and associated injury response.

Estrogen potentiates prostaglandin E₂-stimulated duodenal mucosal HCO₃⁻ secretion in mice

The cause of lower prevalence of duodenal ulcer in young women compared with men is largely unknown. We recently found that sex difference in duodenal mucosal HCO₃⁻ secretion existed in humans and mice, but the mechanisms are not clear. Prostaglandin E₂ (PGE₂) is an important endogenous mediator that plays an important role in the regulation of duodenal HCO₃⁻ secretion. Therefore, in the present study, we investigated the effect of estrogen on PGE₂-stimulated duodenal HCO₃⁻ secretion and the underlying mechanisms. The results showed that 17β-estradiol at the physiological concentration (1 nM) had no significant effects on duodenal mucosal HCO₃⁻ secretion or short-circuit current (I(sc)) in mice. However, the pretreatment of 17β-estradiol (1 nM) markedly potentiated PGE₂-stimulated duodenal HCO₃⁻ secretion and I(sc) (P < 0.01 and P < 0.05). Global estrogen receptor (ER) antagonist ICI-182,780 and ERα-specific antagonist MPP, but not the ERβ-specific antagonist PHTPP, abolished estrogen-potentiated PGE₂-stimulated duodenal HCO₃⁻ secretion and I(sc). 17β-Estradiol and PGE₂ additively increased phosphatidylinositol 3-kinase (PI3K) activity and Akt phosphorylation. Wortmannin, a specific PI3K inhibitor, inhibited estrogen-potentiated PGE₂-stimulated duodenal HCO₃⁻ secretion and I(sc). In conclusion, estrogen at the physiological concentration potentiates PGE₂-stimulated duodenal mucosal HCO₃⁻ secretion through the activation of ERα and the PI3K-dependent mechanism, which may contribute to the sex difference in duodenal mucosal HCO₃⁻ secretion and the lower prevalence of duodenal ulcer in young women.

The effects of capsaicin on gastrin secretion in isolated human antral glands: before and after ingestion of red chilli

BACKGROUND

Capsaicin is known to have regulatory effects on gastrointestinal functions via the vanilloid receptor (VR1). We reported previously that endocrine-like cells in the human antrum express VR1.

AIM

To identify VR1-expressing endocrine-like cells in human antral glands and to examine whether stimulation with capsaicin causes release of gastrin, somatostatin, and serotonin. Further, to investigate the effects of a chilli-rich diet.

METHODS

Gastroscopic biopsies were received from 11 volunteers. Seven of the 11 subjects agreed to donor gastric biopsies a second time after a 3-week chilli-rich diet containing 1.4-4.2 mg capsaicin/day. VR1-immunoreactive cells were identified by double-staining immunohistochemistry against gastrin, somatostatin, and serotonin. For the stimulation studies, we used an in vitro method where antral glands in suspension were stimulated with 0.01 mM capsaicin and physiological buffer was added to the control vials. The concentrations of secreted hormones were detected and calculated with radioimmunoassay (RIA). Results The light microscopic examination revealed that VR1 was localized in gastrin cells. The secretory studies showed an increase in release of gastrin and somatostatin compared to the control vials (P = 0.003; P = 0.013). Capsaicin-stimulation caused a consistent raise of the gastrin concentrations in the gland preparations from all subjects. A chilli-rich diet had an inhibitory effect on gastrin release upon stimulation compared to the results that were obtained before the start of the diet.

CONCLUSION

This study shows that capsaicin stimulates gastrin secretion from isolated human antral glands, and that a chilli-rich diet decreases this secretion.

Expression of vanilloid receptor-1 in epithelial cells of human antral gastric mucosa

OBJECTIVE

Capsaicin, which acts by binding to the vanilloid receptor-1 (VR1), has been shown to give protection against gastric mucosal injury and to enhance healing of gastric ulcers. Although VR1 has recently been reported to be present in non-neural tissues, it is primarily considered to be expressed in nociceptor sensory neurons of small diameter. The aim of the present study was to evaluate the distribution of VR1 immunoreactivity in the normal human gastric mucosa.

MATERIALS AND METHODS

Ten volunteers underwent gastroscopy and biopsies were obtained from the corpus and the antrum. The specimens were labelled immunohistochemically using polyclonal goat anti-VR1 and evaluated at the light- and electronmicroscopic level. Moreover, post-embedding immunogold labelling was performed and subsequently analysed at the electronmicroscopic level.

RESULTS

In the antrum, VR1 immunoreactivity was located in epithelial cells that fulfilled the criteria of endocrine cells of the "open type". These cells were located primarily in the neck region of the antral glands and the labelling was concentrated on the microvilli of these cells. At the ultrastructural level, round granulae with differences in electron density were identified in the basal compartment of the labelled cells. VR1 immunoreactivity was also identified in axon-like structures that were located in the lamina propria, often in close vicinity of vessels, in the corpus as well as in the antrum.

CONCLUSIONS

VR1-immunoreactivity was evident in antral epithelial cells exhibiting characteristics of endocrine-like cells. This may indicate that the gastroprotective effects of capsaicin, which hitherto have been attributed to primary afferent neurons, at least partly may be explained by an action on specific epithelial cells in the antrum.

Neural pathways regulating Brunner's gland secretion in guinea pig duodenum in vitro

This study examined the neural pathways innervating Brunner's glands using a novel in vitro model of acinar secretion from Brunner's glands in submucosal preparations from the guinea pig duodenum. Neural pathways were activated by focal electrical stimulation and excitatory agonists, and videomicroscopy was used to monitor dilation of acinar lumen. Electrical stimulation of perivascular nerves evoked large dilations that were blocked by TTX (1 microM) or the muscarinic receptor antagonist 4-diphenylacetoxy-N-(2-chloroethyl)-piperidine hydrochloride (1 microM). The nicotinic agonist 1,1-dimethyl-4-phenylpiperazinium iodide (100 microM) had no effect, and the nerve-evoked responses were not inhibited by hexamethonium (200 microM). Dilations were abolished in preparations from chronically vagotomized animals. Activation of submucosal ganglia significantly dilated submucosal arterioles but not Brunner's glands. Effects of electrical stimulation of perivascular and submucosal nerves were not altered by guanethidine. Capsaicin and substance P also dilated arterioles but had no effect on Brunner's glands. Cholinergic (choline acetyltransferase-immunoreactive) nerve fibers were found in Brunner's glands. These findings demonstrate that Brunner's glands are innervated by cholinergic vagal fibers but not by capsaicin-sensitive or intrinsic enteric nerves.

The mechanism of action of amtolmetin guacyl, a new gastroprotective nonsteroidal anti-inflammatory drug

Amtolmetin guacyl (2-methoxyphenyl-1-methyl-5-p-methylbenzoyl-pyrrol-2-acetamido acetate) (MED15) is a new nonsteroidal anti-inflammatory drug (NSAID) with anti-inflammatory, analgesic and antipyretic properties similar to the traditional drugs, but with unexpected gastroprotective effects. In an in vivo rat model, amtolmetin guacyl administered orally demonstrates inhibition of gastric acid secretion following stimulation by various agonists, and up-regulation of gastric bicarbonate production. Pretreatment with MED15 also shows a significant reduction of indomethacin-induced gastric damage in the rat. The reason behind this behaviour appears to be bound to the presence in the MED15 molecule of a vanillic moiety known to stimulate capsaicin receptors. In fact, the antisecretive effect of MED15 is blocked by capsazepine (a specific capsaicin receptor antagonist). This effect is confirmed by the interference found with anti-histamine H(1) drugs. Owing to the connection between capsaicin and calcitonin gene-related peptide (CGRP), a possible effect of MED15 on CGRP receptors was hypothesized, considering the leading role played on gastric mucosa by the predominant sensory neuropeptide of the stomach wall, CGRP. In fact, the anti-secretive and gastroprotective effect of MED15 is abolished by CGRP-(8-37) (the specific CGRP receptor antagonist). The unmodified MED15 molecule is found throughout the gastroenteric tract for long periods of time following oral administration, as further confirmation of the mechanism of action being based on the presence of the vanillic moiety at receptor level.