Morphine inhibits secretion of bicarbonate from the human duodenal mucosa. Possible role of endogenous opioids in the regulation of human duodenal mucosal bicarbonate secretion

Opioids and the gut: pharmacology and current clinical experience

This article reviews the pharmacology and physiology of opiate receptors and the current and potential uses of opioid agonists and antagonists in clinical gastroenterology. Mu-receptors are involved in motor and sensory functions, and their modulation is established for treatment of diarrhea. Mu-antagonists have potential to reverse endogenous (e.g., postoperative ileus) or iatrogenic dysmotility (e.g., opioid bowel dysfunction). Modulation of the function of kappa-receptors may be a novel approach to control visceral pain in functional gut disorders. Results of formal testing of novel opioid modulators are keenly awaited.

Inhibition of rabbit duodenal bicarbonate secretion by ulcerogenic agents: histamine-dependent and -independent effects

BACKGROUND & AIMS

The gastroduodenal epithelium is protected from acid-peptic damage, in part, by its ability to secrete bicarbonate. Patients with duodenal ulcer disease have impaired proximal duodenal mucosal bicarbonate secretion. We have shown in vitro that histamine inhibits prostaglandin-stimulated bicarbonate secretion in rabbit duodenal mucosa via histamine H2 receptors and enteric nerves. In this study we examined whether the proulcerogenic compounds aspirin or ethanol regulate duodenal bicarbonate secretion and the involvement of histamine.

METHODS

Bicarbonate secretion by rabbit proximal duodenal mucosa was examined in vitro in Ussing chambers.

RESULTS

Aspirin and ethanol decreased basal and prostaglandin-stimulated bicarbonate secretion; the latter effect was specific for prostaglandin. The inhibitory effects of the two ulcerogenic compounds were at least additive. Ranitidine and tetrodotoxin abolished the inhibitory effects on stimulated, but not basal, secretion. Aspirin and ethanol also induced release of duodenal histamine.

CONCLUSIONS

Aspirin and ethanol act by two distinct pathways to impair duodenal bicarbonate secretion. Both agents inhibit basal secretion via a histamine-independent and neurally independent pathway while they inhibit prostaglandin E2-stimulated secretion via histamine release, likely from mast cells, and actions on enteric nerves. Our findings may be of relevance to the understanding and potential treatment of nonsteroidal anti-inflammatory drug-associated mucosal injury.

Peptic ulcer pathophysiology: acid, bicarbonate, and mucosal function

The previously accepted role of gastric acid hypersecretion in peptic ulcer disease has been modified by studies showing no correlation between acid output and clinical outcome of ulcer disease, or between ulcer recurrence rate after vagotomy and preoperative acid secretion. At the same time, studies have been unable to demonstrate increased acidity in the duodenal bulb in patients with duodenal ulcer, and consequently more emphasis has been given to the mucosal protecting mechanisms. The existence of an active gastric and duodenal mucosal bicarbonate secretion creates a pH gradient from the luminal acid to near neutrality at the surface of the epithelial cells, thereby acting as an important mucosal defence mechanism. The regulation of bicarbonate secretion is a complex process related to motility and neural activity. Stimulation is by acid, PGE2, NO, VIP, cAMP, and mucosal protective agents. Bicarbonate secretion is inhibited by atropine, muscarinic antagonists, alpha-adrenoceptor agonists, indomethacin, bile acids, tobacco smoking, and probably also by infection by Helicobacter pylori. Apart from mucus and bicarbonate, the mucosal defence is supported by a hydrophobic epithelial lining, rapid cell removal and repair regulated by epidermal growth factor. Sufficient mucosal blood flow, including a normal acid/base balance, is important for subepithelial protection. In today's model of ulcer pathogenesis, gastric acid and H. pylori work in concert as aggressive factors, with the open question being: why does only a fraction of the infected population develop an ulcer?

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

BACKGROUND

The purpose of the study was to examine the effect of stimulation and inhibition of duodenal mucosal bicarbonate secretion on pancreatic and hepatic bicarbonate secretion in response to acid.

METHODS

The effect of inhibition (indomethacin) or stimulation (misoprostol) of duodenal mucosal bicarbonate secretion on pancreatic and biliary bicarbonate secretion in response to intraduodenal infusion of HCl or intravenous infusion of secretin was studied in anaesthetized pigs.

RESULTS

The hepatic and pancreatic response to exogenous secretin was not significantly altered by stimulation/inhibition of duodenal bicarbonate secretion. However, pancreatic and biliary bicarbonate secretion in response to duodenal acidification was significantly augmented by inhibition of duodenal mucosal bicarbonate secretion; conversely, it was reduced by stimulation of duodenal bicarbonate secretion. The increase in plasma secretin levels in response to duodenal acidification was reduced by stimulation and augmented by inhibition of duodenal mucosal bicarbonate secretion.

CONCLUSIONS

Duodenal mucosal bicarbonate secretion can serve as a modulator of both pancreatic and biliary bicarbonate secretion in response to luminal acidification, possibly through regulation of the release of secretin.

Review article: gastroduodenal bicarbonate secretion

The gastroduodenal epithelium is covered by an adherent mucus layer into which bicarbonate is secreted by surface epithelial cells. This mucus-bicarbonate barrier is an important first line of defence against damage by gastric acid and pepsin, and has been demonstrated in all species including human. Similar to gastric acid secretion, regulation of gastric and duodenal bicarbonate secretion can be divided into three phases: cephalic, gastric and duodenal. In humans, sham-feeding increases bicarbonate secretion in both the stomach and duodenum which is mediated by cholinergic vagal fibres in the stomach, but seems to be noncholinergic in the duodenum. Gastric distention and luminal acidification increases gastric bicarbonate production. Whereas there are no data relating to the gastric phase of human duodenal bicarbonate secretion, in animals, food and acid in the stomach independently stimulate duodenal bicarbonate output. To date, the duodenal phase of human gastric bicarbonate secretion has not been studied, but data from animals reveal that duodenal acidification augments bicarbonate secretion in the stomach. In all species tested, direct acidification of the duodenum is a potent stimulant of local bicarbonate production. In humans, the pH threshold for bicarbonate secretion is pH 3.0. Mediation of gastroduodenal bicarbonate secretion is provided by a variety of agonists and antagonists, tested mainly in animals, but some have been evaluated in humans. Prostaglandins of the E class and VIP are major factors that control bicarbonate secretion. Bicarbonate secretion, and the mucus-bicarbonate layer in general, is adversely effected by ulcerogenic factors such as aspirin, NSAIDs, bile salts, and cigarette smoking. Furthermore, duodenal ulcer patients have an impairment in bicarbonate production within the duodenal bulb, at rest and in response to stimulation. These findings indicate that the mucus-bicarbonate barrier is an important first line of defence in the pathogenesis of peptic ulcer disease.

Bicarbonate transport by rabbit duodenum in vitro: effect of vasoactive intestinal polypeptide, prostaglandin E2, and cyclic adenosine monophosphate

BACKGROUND

Duodenal surface cells secrete bicarbonate that provides a barrier against injury. The current experiments were performed to identify duodenal bicarbonate regulatory and transport pathways.

METHODS

Rabbit proximal duodenal mucosa were mounted in chambers under short-circuited conditions. Bicarbonate transport, short-circuit current (Isc), and potential difference (PD) were quantitated in response to prostaglandin E2 (PGE2), vasoactive intestinal polypeptide (VIP), and dibutyryl cyclic adenosine monophosphate (db-cAMP). Anoxia (N2), 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) and Cl(-)-free solutions, ouabain, and Na-free solutions were also studied, as was the effect of VIP and PGE2 on duodenocyte cAMP.

RESULTS

PGE2, VIP, db-cAMP, and theophylline significantly increased bicarbonate secretion, Isc, and PD. Ouabain, Na(+)-free bathing solutions, and anoxia (N2) inhibited the responses. DIDS and Cl(-)-free solutions abolished the PGE2-induced response, reduced the response to VIP by about 50%, and had no effect on the response to db-cAMP. After PGE2 and VIP, cAMP concentration increased, yet was likely independent of bicarbonate secretion.

CONCLUSIONS

Mammalian duodenal HCO3- transport requires Na+, Na+/K(+)-adenosine triphosphatase and O2-dependent metabolic pathways and is stimulated by PGE2, VIP, and cAMP, acting by distinct pathways.

Effect of sorbin on duodenal absorption of water and electrolytes in the rat

Sorbin is a newly isolated intestinal peptide that has been purified because of its ability to induce water absorption. The effects that sorbin and some synthetic peptides corresponding to its C-terminal sequence have on duodenal absorption of water, chloride, and sodium were studied in comparison with the effects of vasoactive intestinal peptide (VIP), [D-Ala,Met]-enkephalinamide (DAMA), and angiotensin II. The technique of an in situ ligated duodenal loop in the rat was used for all peptides. Under the experimental conditions used, a low basal secretion of water, chloride, and sodium was obtained; VIP induced an increase of the secretion, whereas DAMA induced an absorption, both in the nanomolar dose range. Angiotensin II and sorbin induced an absorption in the picomolar dose range. The most effective doses of sorbin peptides but not of angiotensin induced the lowest final concentrations of Na+ and Cl- obtainable without inducing secondary water secretion. All synthetic peptides containing the C-terminal heptapeptide of sorbin were active in the picomolar dose range. Contrary to angiotensin, they had no effect on blood pressure.

Relative importance of pancreatic, hepatic, and mucosal bicarbonate in duodenal neutralization of acid in anaesthetized pigs

Pancreatic and hepatic bicarbonate secretion and the disappearance rate of acid during duodenal acidification were measured simultaneously in anaesthetized pigs. Perfusion of the duodenum with HCl resulted in an increase in both hepatic and pancreatic bicarbonate secretion. During all acid loads hepatic bicarbonate secretion was significantly greater than pancreatic secretion. Furthermore, the disappearance rate of acid in the duodenum during diversion of both bile and pancreatic juice was significantly greater than the amount of acid which could be neutralized by the concomitant pancreatic bicarbonate secretion. Diversion of pancreatic juice from the duodenum did not affect the disappearance rate of acid at any acid load, whereas diversion of bile caused a significant decrease. Thus, in the anaesthetized pig the liver and the duodenal mucosa are of greater importance than the pancreas for the neutralization of acid in the duodenum. It is suggested that the importance of the pancreatic contribution to duodenal neutralization should be reevaluated in other species, including man.

Pancreatic, hepatic, and duodenal mucosal bicarbonate secretion during infusion of secretin and cholecystokinin. Evidence of the importance of hepatic bicarbonate in the neutralization of acid in the duodenum of anaesthetized pigs

The effect of infusion of secretin alone or in combination with cholecystokinin (CCK) on pancreatic, hepatic, and duodenal mucosal bicarbonate secretion was studied in anaesthetized pigs. After laparotomy, catheters were inserted into the common bile duct, the pancreatic duct, and both ends of the duodenum. Pancreatic, hepatic, and duodenal mucosal secretions were collected during intraportal infusion of increasing doses of secretin, either alone or in combination with CCK. During infusion of secretin in doses that caused physiologic increases in plasma secretin concentrations the liver produced significantly more bicarbonate than the pancreas. A physiologic dose of CCK augmented the effect of secretin on both hepatic and pancreatic bicarbonate secretion, but the hepatic production of bicarbonate was still larger than the pancreatic production. Neither secretin alone nor secretin combined with CCK caused any changes in duodenal mucosal bicarbonate secretion. These results suggest that the liver plays an important role in the neutralization of acid in the duodenum.