Duodenal mucosal bicarbonate secretion in pigs is accompanied by compensatory changes in pancreatic and biliary HCO3- secretion

Multiple actions of secretin in the human body

The discovery of secretin initiated the field of endocrinology. Over the past century, multiple gastrointestinal functions of secretin have been extensively studied, and it was discovered that the principal function of this peptide in the gastrointestinal system is to facilitate digestion and to provide protection. In view of the late identification of secretin and the secretin receptor in various tissues, including the central nervous system, the pleiotropic functions of secretin have more recently been an area of intense focus. Secretin is a classical hormone, and recent studies clearly showed secretin's involvement in neural and neuroendocrine pathways, although the neuroactivity and neural regulation of its release are yet to be elucidated. This chapter reviews our current understanding of the pleiotropic actions of secretin with a special focus on the hormonal and neural interdependent pathways that mediate these actions.

Duodenal secretion in humans mediated by the EP4 receptor subtype

AIM

Assessment of functional EP receptor subtypes involved in PGE2-induced secretion in human duodenum. The spectrum of activities by PGE2 in mammals, including cytoprotective bicarbonate secretion in duodenum, is mediated through four G protein-coupled receptor subtypes (EP1-EP4).

METHODS

Biopsies from the second part of duodenum from patients undergoing endoscopy were mounted in modified Ussing chambers. Basal and stimulated short circuit current (SCC) and slope conductance (SG) were measured. Dose-response relations for PGE2 and subtype receptors EP1/EP3 (sulprostone), EP2 (butaprost), and EP4 (1-OH PGE1) were assessed by cumulated doses of single agonists.

RESULTS

PGE2 caused a dose-dependent increase in SCC, maximum 101 +/- 20 microA cm(-2) with an EC50 of 35.6 +/- 5.8 nm (n = 3). Likewise 1-OH PGE1 caused a dose-dependent SCC increase, maximum 63.3 +/- 28.6 microA cm(-2) with an EC50 of 56.7 +/- 7.2 nm (n = 3). 1-OH PGE1 at 500 nm increased SCC by 18.0 +/- 3.0 microA cm(-2) (n = 10) and SG by 2.9 +/- 0.4 mS cm(-2) (n = 6). Sulprostone (n = 6) and butaprost (n = 6) had no effects on SCC or SG. SCC was inhibited 31.4 +/- 13.2% (n = 10) by bumetanide (25 microM serosa) and 18.6 +/- 5.8% (n = 10) by acetazolamide (250 microM lumen). Diphenylamine-2-carboxylate (DPC) (250 microM mucosa) and SITS (10 microM mucosa) had almost no effect.

CONCLUSIONS

Effects of PGE2 on secretion in the second part of human duodenum is mediated through the EP4 receptor and not through EP1, EP2, or EP3.

Gastroduodenal mucus bicarbonate barrier: protection against acid and pepsin

Secretion of bicarbonate into the adherent layer of mucus gel creates a pH gradient with a near-neutral pH at the epithelial surfaces in stomach and duodenum, providing the first line of mucosal protection against luminal acid. The continuous adherent mucus layer is also a barrier to luminal pepsin, thereby protecting the underlying mucosa from proteolytic digestion. In this article we review the present state of the gastroduodenal mucus bicarbonate barrier two decades after the first supporting experimental evidence appeared. The primary function of the adherent mucus gel layer is a structural one to create a stable, unstirred layer to support surface neutralization of acid and act as a protective physical barrier against luminal pepsin. Therefore, the emphasis on mucus in this review is on the form and role of the adherent mucus gel layer. The primary function of the mucosal bicarbonate secretion is to neutralize acid diffusing into the mucus gel layer and to be quantitatively sufficient to maintain a near-neutral pH at the mucus-mucosal surface interface. The emphasis on mucosal bicarbonate in this review is on the mechanisms and control of its secretion and the establishment of a surface pH gradient. Evidence suggests that under normal physiological conditions, the mucus bicarbonate barrier is sufficient for protection of the gastric mucosa against acid and pepsin and is even more so for the duodenum.

Quantitative assessment of motility-associated changes in gastric and duodenal luminal pH in humans

BACKGROUND

Interdigestive pain relieved by food is a common feature of ulcer disease. We tested the hypothesis that the duodenal bulb is intermittently acidified in association with phase III of the interdigestive motility cycle, and tried to quantify the balance between acid and duodenal bicarbonate secretion during this particular period.

METHODS

The experiments were performed in Helicobacter-negative healthy volunteers. Gastric and duodenal luminal pH was measured with a triple antimon electrode before, during, and after phase III of the migrating motor complex. Gastric acid secretion rate was measured in real time with a perfusion system and duodenal bicarbonate secretion was estimated from a continuous recording of the transmucosal potential difference (PD) in the duodenal bulb.

RESULTS

No significant changes in bulb pH occurred before, during, or after phase III. During the studied time window, the stomach produced 2.24 +/- 0.55 mmol of acid at a peak pH of 1.74 +/- 0.10. Basal HCO3- secretion calculated from bulb PD was 0.82 +/- 0.12 mmol x 30 min(-1) to which was added 0.47 +/- 0.07 mmol of HCO3- during duodenal phase III. The contribution of retroperistalsis-driven HCO3- reflux was small (0.08 +/- 0.02 mmol).

CONCLUSIONS

Both the pH recording and the quantitative assessment of secretion rates show that in healthy subjects, fasting gastric acid production and duodenal bicarbonate secretion are of similar magnitude and dynamically coordinated. The mechanism behind the linkage may be reflex activation by motor activity, or a luminal PCO2 rise during phase III activity.

Nitric oxide prevents rat duodenal contractions induced by potentially noxious agents

After abdominal surgery, luminal HCl fails to induce duodenal contractions in anaesthetized rats. Elevated tissue levels of nitric oxide (NO) and prostaglandins possibly contribute to this observation. The aim of this study was to compare the effects of luminal capsaicin (1.2 mg mL-1), ethanol (15%) and high partial pressure of CO2 (>250 mmHg) with those of HCl (10 mM) in anaesthetized rats. Motility (intraluminal pressure), mucosal permeability [blood-to-lumen clearance of 51Cr-EDTA (51Chromium-labelled ethylenediaminetetraacetate)] and duodenal mucosal bicarbonate secretion (DMBS) were recorded. Three groups of animals were studied: (1) controls, (2) pretreatment with the NO synthase inhibitor N-nitro-L-arginine-methyl-ester (L-NAME) and (3) pretreatment with the cyclo-oxygenase inhibitor indomethacin. Neither capsaicin, ethanol, CO2 nor HCl induced duodenal contractions or affected DMBS in control rats. However, L-NAME induced duodenal contractions that were augmented by capsaicin, ethanol and HCl, but not by CO2. Indomethacin also induced contractions that were reversibly diminished by capsaicin and HCl, but not by ethanol or CO2. Significant increases in mucosal permeability occurred during ethanol perfusion in indomethacin- and L-NAME pretreated rats. In conclusion, NO probably plays a key role in preventing duodenal contractions in response to luminally HCl, capsaicin and ethanol. The HCl-induced effect on motility appears to be independent of CO2 and is not caused by alteration in mucosal integrity.