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

Mucus and bicarbonate secreted from the epithelium are thought to be important for the protection of the duodenal mucosa against acid and pepsin, but so far little is known about the regulation of human duodenal mucosal bicarbonate secretion. After isolating a segment of the proximal human duodenum from gastric and pancreaticobiliary secretion we quantified the secretion of bicarbonate from the human duodenal mucosa. The method was evaluated by measurements of basal and prostaglandin E1 analogue-stimulated bicarbonate secretion. The duodenal mucosal bicarbonate secretion was inhibited 70% after intravenous infusion of morphine in a dose of 73.6 micrograms/kg/h and increased after intravenous administration of naloxone. Thus, the inhibition is most likely mediated by mu-receptors, and the results suggest a role of endogenous opioids in the regulation of the secretion of bicarbonate from the human duodenal mucosa.     

A Role for CagA/VacA in Helicobacter pylori Inhibition of Murine Duodenal Mucosal Bicarbonate Secretion

Duodenal mucosal bicarbonate secretion is diminished in patients with Helicobacter pylori (HP)-associated duodenal ulcer disease. We examined whether HP water extracts inhibit murine duodenal mucosal bicarbonate secretion in vitro, and the mechanisms involved. Murine duodenal mucosae were mounted in Ussing chambers. Short-circuit current and bicarbonate secretion was measured. CagA/VacA-positive HP water extract (HPWE+/+) markedly inhibited PGE2-, carbachol-, or the calcium ionophore A23187-stimulated bicarbonate secretion in a dose-dependent manner. While 3-isobutyl-1-methylxanthine-stimulated bicarbonate secretion was not affected by HPWE+/+, HPWE+/+ did diminish forskolin-stimulated bicarbonate secretion. HPWE+/+ markedly diminished PGE2-induced increases in duodenal mucosal cAMP. CagA/VacA of HP decreases Ca2+-mediated bicarbonate secretion downstream of increases in intracellular Ca2+. Dimunition of PGE2-stimulated bicarbonate secretion occurs, in part, by inhibition of adenylate cyclase, which leads to decreased cAMP levels. The ability of virulent HP strains to inhibit duodenal bicarbonate secretion through multiple intracellular pathways likely contributes to the pathogenesis of HP-associated duodenal ulcer disease.     

Effects of Gamma-Aminobutyric Acid and Peripheral Benzodiazepine Ligands on Duodenal Bicarbonate Secretion

Background: The effects of gamma-aminobutyric (GABA_A)-receptor ligands and peripheral-type benzo\diazepine (BZ) ligands on duodenal mucosal bicarbonate secretion (DBS) were studied in thiobarbiturate-anesthetized rats. Methods: A segment of proximal duodenum was perfused, and bicarbonate secretion was continuously titrated by pH-stat. In some experiments the vagus nerves were dissected free and cut at the neck level. Results: Luminal GABA (10^-8 to 10^-6 M) increased (P < 0.01) DBS dose-dependently, and the GABA_A antagonist (+)-bicuculline (0.1-2.5 mg/kg intravenously) caused a similar increase (P     

Effects of Leukotriene D4, the Antagonist L-649-923, and Arachidonic Acid on Duodenal Bicarbonate Secretion in the Rat in Vivo

Prostaglandins of the E type stimulate bicarbonate secretion by the duodenal mucosa and inhibit gastric acid secretion, effects that have been related to their anti-ulcer activity. Leukotrienes constitute a more recently discovered branch of the arachidonic acid cascade, and C₄ and D₄ have been suggested to be ulcerogenic in the stomach. We have studied the effects of luminal administration of leukotriene D₄ and the leukotriene C₄/D₄ antagonist L-649–923 on duodenal mucosal alkaline secretion in the anaesthetized rat. Leukotriene D₄ (10^?8-10^?6 M) had no significant effects, but the antagonist dose-dependently increased the bicarbonate secretion and also transiently increased the transmucosal electric potential difference. The precursor arachidonic acid (10^?7-10^?6 M) caused a small increase in secretion. The increase in bicarbonate secretion in response to 10^?3 M of the antagonist was of about the same magnitude as that observed with 10^?5 of prostaglandin E₂, and it was abolished by pretreatment with the cyclooxygenase inhibitor indomethacin. The gastroduodenal protective effects of L-649–923 in vivo may reflect an increase in mucosal prostaglandin production rather than leukotriene antagonism.     

Human Proximal Duodenal Alkaline Secretion Is Mediated by Cl-/HCO3- Exchange and HCO3- Conductance

The proximal duodenal epithelium secretesbicarbonate into an adherent mucus layer, therebyprotecting the mucosa from injury by gastric acid andpepsin. While bicarbonate secretion is stimulated andinhibited by a number of agonists and antagonists, theapical anion transport pathways have not been addressedfully. The objective was to assess if apicalCl^-/HCO₃ ^- exchange andCl^-:HCO₃ ^- conductanceare involved in duodenal mucosal bicarbonate secretion(DMBS). In healthy volunteers, the proximal 4 cm ofduodenum was isolated, perfused with either saline or4,4-diisothiocyano-2,2-disulfonic acid(DIDS), and bicarbonate secretion and transepithelial potentialdifference (PD) were stimulated by eitherPGE₂ or the phosphodiesterase inhibitortheophylline to increase cyclic AMP. Luminal DIDSabolished PGE₂-stimulated DMBS, yet had no effect on the increase in PD andfailed to significantly alter theophylline-induced DMBSand PD. Therefore, in human proximal duodenum, itappears that PGE₂ and cAMP activate distinctHCO transport pathways 2 likely involving a DIDS-sensitiveCl^-/HCO₃ ^- exchanger andDIDS-insensitive HCO₃ ^-conductance.     

Indomethacin Decreases Gastroduodenal Mucosal Bicarbonate Secretion in Humans

Background: Cyclooxygenase inhibitors reduce mucosal bicarbonate secretion in the duodenum, but the evidence for their effect on bicarbonate secretion in the stomach remains controversial. We have, therefore, studied how indomethacin influences gastroduodenal bicarbonate secretion and luminal release of prostaglandin (PG) E₂ by means of a method that enables simultaneous measurements in the stomach and the duodenum.

Methods: Gastric and duodenal perfusions were performed twice in random order during control conditions or after pretreatment with indomethacin (100 mg intravenously) in eight healthy volunteers. Bicarbonate and PGE₂ were measured in the gastroduodenal effluents by back-titration and radioimmunoassay, respectively.

Results: Vagal stimulation and duodenal luminal acidification (0.1 M HCI; 20 ml; 5 min) increased gastroduodenal bicarbonate secretion (p < 0.05). Indomethacin markedly inhibited both basal and stimulated gastric and duodenal mucosal bicarbonate secretion, and this reduction was similar to the degree of cyclooxygenase inhibition estimated by the luminal release of PGE₂ (p < 0.05).

Conclusion: These results unequivocally demonstrate that endogenous PG modulates basal and stimulated bicarbonate secretion, both in the stomach and in the duodenum.     

Effect of Stimulation of Mucosal HCO‐ Secretion on Acid-Induced Injury to Porcine Duodenal Mucosa

The effect of stimulation of duodenal mucosal bicarbonate secretion with vasoactive intestinal peptide (VIP) on acid-induced damage to the duodenal mucosa was studied in anaesthetized pigs in which bile and pancreatic juice were diverted from the duodenum. Mucosal damage was quantitatively assessed histologically, and mucosal blood flow was determined by means of radioactively labelled microspheres. Compared with placebo, intravenous infusion of VIP (500 pmol/kg/h) significantly stimulated duodenal mucosal bicarbonate secretion (47 ± 13 versus 249 ± 53 μmol/h) without concomitant changes in mucosal blood flow (51.5 ± 7.8 versus 48.5 ± 9.1 ml/min/100g) or arterial bicarbonate concentration (24.2 ± 1.1 versus 23.4 ± 0.9 mM). The same dose of VIP increased the acid disappearance rate in the duodenum (2.2 ±0.14 versus 3.3 ± 0.09 mmol/h) and reduced the extent of damage to the duodenal surface (16 ± 2% versus 7 ± 2%) during duodenal infusion of 0.03 M HC1 but not 0.1 M HCI. We conclude that the protection offered by VIP against the small dose of acid is most likely secondary to the effect of VIP on mucosal bicarbonate secretion. Thus, this study suggests that duodenal mucosal bicarbonate secretion, independent of mucosal blood flow, is an integral factor in duodenal mucosal defence.     

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.     

Effect of 5-hydroxytryptamine on duodenal mucosal bicarbonate secretion in mice

BACKGROUND & AIMS: 5-hydroxytryptamine (5-HT) is an important neurotransmitter and intercellular messenger that modulates many gastrointestinal functions. Because little is known about the role of 5-HT in the regulation of duodenal bicarbonate secretion, we examined the role of 5-HT on duodenal bicarbonate secretion and define neural pathways involved in the actions of 5-HT. METHODS: Duodenal mucosa from National Institutes of Health Swiss mice was stripped of seromuscular layers and mounted in Ussing chambers. The effect of 5-HT on duodenal bicarbonate secretion was determined by the pH stat technique. Acetylcholine (ACh) release from duodenal mucosa was assessed by preincubating the tissue with [(3)H] choline and measuring 5-HT-evoked release of tritium. RESULTS: 5-HT added to the serosal bath markedly stimulated duodenal bicarbonate secretion and short circuit current (Isc) in a dose-dependent manner (10(-7) mol/L to 10(-3) mol/L; P < 0.0001), whereas mucosally added 5-HT was without effect. 5-HT-stimulated bicarbonate secretion was independent of luminal Cl(-). Pretreatment with tetrodotoxin (TTX) (10(-6) mol/L) or atropine (10(-5) mol/L) markedly reduced 5-HT-stimulated duodenal bicarbonate secretion (by 60% and 65%, respectively; P < 0.001) and Isc (by 45% and 27%, respectively; P < 0.001 and P < 0.05). Pretreatment with N(omega)-nitro-l-arginine methyl ester (l-NAME) (10(-3) mol/L), propranolol (10(-5) mol/L), or phentolamine (10(-5) mol/L) did not significantly alter 5-HT-stimulated duodenal mucosal bicarbonate secretion or Isc. 5-HT concentration-dependently evoked ACh release from duodenal mucosal preparations (P < 0.0001). TTX markedly inhibited 5-HT-evoked ACh release (P < 0.001). CONCLUSIONS: 5-HT is a potent activator of duodenal mucosal bicarbonate secretion in mice. Duodenal bicarbonate secretion induced by 5-HT in vitro occurs principally via a cholinergic neural pathway.     

FFA3 Activation Stimulates Duodenal Bicarbonate Secretion and Prevents NSAID-Induced Enteropathy via the GLP-2 Pathway in Rats

Background

Therapy with nonsteroidal anti-inflammatory drugs (NSAIDs) is associated with enteropathy in humans and experimental animals, a cause of considerable morbidity. Unlike foregut NSAID-associated mucosal lesions, most treatments for this condition are of little efficacy. We propose that the endogenously released intestinotrophic hormone glucagon-like peptide-2 (GLP-2) prevents the development of NSAID-induced enteropathy. Since the short-chain fatty acid receptor FFA3 is expressed on enteroendocrine L cells and on enteric nerves in the gastrointestinal tract, we further hypothesized that activation of FFA3 on L cells protects the mucosa from injury via GLP-2 release with enhanced duodenal HCO₃ ^? secretion. We thus investigated the effects of synthetic selective FFA3 agonists with consequent GLP-2 release on NSAID-induced enteropathy.

Methods

We measured duodenal HCO₃ ^? secretion in isoflurane-anesthetized rats in a duodenal loop perfused with the selective FFA3 agonists MQC or AR420626 (AR) while measuring released GLP-2 in the portal vein (PV). Intestinal injury was produced by indomethacin (IND, 10 mg/kg, sc) with or without MQC (1–10 mg/kg, ig) or AR (0.01–0.1 mg/kg, ig or ip) treatment.

Results

Luminal perfusion with MQC or AR (0.1–10 µM) dose-dependently augmented duodenal HCO₃ ^? secretion accompanied by increased GLP-2 concentrations in the PV. The effect of FFA3 agonists was inhibited by co-perfusion of the selective FFA3 antagonist CF3-MQC (30 µM). AR-induced augmented HCO₃ ^? secretion was reduced by iv injection of the GLP-2 receptor antagonist GLP-2(3-33) (3 nmol/kg), or by pretreatment with the cystic fibrosis transmembrane conductance regulator (CFTR) inhibitor CFTR_inh-172 (1 mg/kg, ip). IND-induced small intestinal ulcers were dose-dependently inhibited by intragastric administration of MQC or AR. GLP-2(3-33) (1 mg/kg, ip) or CF3-MQC (1 mg/kg, ig) reversed AR-associated reduction in IND-induced enteropathy. In contrast, ip injection of AR had no effect on enteropathy.

Conclusion

These results suggest that luminal FFA3 activation enhances mucosal defenses and prevents NSAID-induced enteropathy via the GLP-2 pathway. The selective FFA3 agonist may be a potential therapeutic candidate for NSAID-induced enteropathy.

     

Measurement of Gastric Bicarbonate Secretion in the Human Stomach: Different Methods Produce Discordant Results

Odes HS, Hogan DL, Steinbach JH, Ballesteros MA, Koss MA, Isenberg JI. Measurement of gastric bicarbonate secretion in the human stomach: different methods produce discordant results. Scand J Gastroenterol 1992;27:829-836.

Human gastric bicarbonate secretion has been measured by back-titration, from pH and pressure of carbon dioxide (Pco₂) determinations (using the Henderson-Hasselbalch formula), and from equations based on gastric juice osmolality and [H⁺] (osmolality-[H⁺] method). Since these methods show large quantitative differences in their estimations of gastric bicarbonate secretion, we examined each to define the reasons for these discrepancies and establish guidelines for future work in this area. Bicarbonate recovery from ‘non-parieta' secretions (0 to 80 mM HCO₃) reacting with ‘pure parietal secretion' (160mM HC1) was studied both in vitro and in the pylorus-occluded healthy human stomach during acid suppression, exogenous acidification, and pentagastrin stimulation. The pH/Pco₂ method estimated HC0₃ accurately under anaerobic conditions in vitro, whereas the osmolality-[H⁺] method (with correction factors for osmolality incorporated by us) was accurate under aerobic conditions. In the acid-suppressed stomach back-titration was significantly more accurate than the pH/Pco₂ method. In the exogenously acidified and pentagastrin-stimulated stomachs the pH/Pco₂ method underestimated bicarbonates, and the osmolality-[H⁺] method was spuriously elevated in the low range and diminished at high bicarbonate concentrations. Estimates of'basa' bicarbonate secretion (at zero added bicarbonate) were severalfold higher by the osmolality-[H⁺] method (5.26 ± 0.33 mmol/h) than by the pH/Pco₂ method (1.20 ± 0.23 mmol/h) or back-titration (0.65 ± 0.14 mmol/h). In conclusion, gastric bicarbonate was determined most correctly by back-titration in the acid-suppressed stomach, whereas measurement of bicarbonate in the acid-secreting stomach was not accurate with any method.     

Modulation of Bicarbonate Secretion in Rabbit Duodenum (The Role of Calcium)

Surface epithelial bicarbonate secretionprotects the proximal duodenum from acid peptic injury.Cyclic adenosine monophosphate and calcium serve asintracellular mediators of intestinal transport.Experiments were performed to examine whether calciumparticipates in duodenal bicarbonate transport. Strippedduodenal mucosa from rabbits was studied in Ussingchambers. HCO₃ ^- transport wasstimulated by the calcium ionophore A23187, carbachol, vasoactiveintestinal peptide, prostaglandin E₂,dibutyryl-cyclic adenosine monophosphate, and electricalfield stimulation. A23187 stimulatedHCO₃ ^- secretion andI_SC; tetrodotoxin failed to inhibit this effect. Thecalcium-channel blocker verapamil abolishedHCO₃ ^- secretion stimulated bycarbachol, vasoactive intestinal peptide, and electricalfield stimulation, but failed to alter basal, prostaglandin E₂- ordibutyryl-cyclic adenosine monophosphate-stimulatedHCO₃ ^- secretion. Therefore,calcium is likely required during stimu lation ofduodenal epithelial HCO₃ ^-transport by carbachol, vasoactive intestinal peptide, and electricalfield stimulation. Prostaglandin E₂- anddibutyryl-cyclic adenosine monophosphate appear toactivate duodenal HCO₃ ^- secretionby a calcium-independent pathway(s).     

ACE Inhibition by Enalaprilate Stimulates Duodenal Mucosal Alkaline Secretion via a Bradykinin Pathway in the Rat

The effects of enalaprilate on duodenal mucosalalkaline secretion (in situ titration) and mean arterialblood pressure were investigated inchloralose-anesthetized male rats. A bolus injection ofenalaprilate (0.7 mg/kg intravenously) increased alkalinesecretion by about 60%, and this response was resistantto guanethidine (5 mg/kg intravenously),splanchnicotomy, and vagotomy. Furthermore, angiotensinII infusion (0.25-2.5 g/kg/hr intravenously)following the administration of enalaprilate failed toinfluence this response. Bradykinin (10^-6-10^-4 M) applied topically to theserosal surface of the duodenal segment under study increased dose-dependentlythe duodenal mucosal alkaline secretion, an effect thatcould be blocked by the selective bradykinin receptorsubtype-2 antagonist HOE140 (100 nmol/kg intravenously). HOE140 also antagonized the response toenalaprilate. These data suggest that enalaprilateincreases duodenal mucosal alkaline secretion via alocal bradykinin pathway involving receptors of thebradykinin receptor subtype-2 antagonist, rather than byblockade of endogenous angiotensin II or by centralautonomic neural regulation.     

Human duodenal mucosal bicarbonate secretion. Evidence for basal secretion and stimulation by hydrochloric acid and a synthetic prostaglandin E1 analogue

The factors responsible for prevention of duodenal mucosal injury are not known. This series of experiments was performed to determine whether the human duodenum secretes bicarbonate that could prevent mucosal damage. To isolate a 4-cm segment of proximal (i.e., the duodenal bulb) or distal duodenum free of contamination from either gastric or pancreaticobiliary secretion, or both, methods were developed using occlusive balloons. The test segment was perfused with NaCl (2 ml/min, 37 degrees C) containing [14C]PEG as a nonabsorbable marker, and bicarbonate output was quantitated. Mean (+/- SE) basal proximal duodenal bicarbonate output was 143 +/- 17 mumol/cm X h. A 5-min infusion of 25, 50, and 100 mM HCl directly into the isolated proximal duodenal test segment increased bicarbonate output to 167 +/- 29, 199 +/- 19, and 278 +/- 49 mumol/cm X h, respectively, during the hour after acidification. Distal duodenal acidification (25, 50, and 100 mM) also increased bicarbonate output from the isolated proximal duodenal test segment. A synthetic prostaglandin E1 analogue, misoprostol (1.67-13.3 micrograms/min), infused directly into proximal or distal test segments significantly stimulated bicarbonate outbreak; peak responses were 644 +/- 35 mumol/cm X h and 171 +/- 20 mumol/cm X h (p less than 0.001), respectively. Thus, in humans, the proximal and distal duodenal mucosa secretes bicarbonate at rest; direct acidification of the proximal duodenum stimulates bicarbonate output; acidification of the distal duodenum beyond the isolated test segment also increased proximal duodenal bicarbonate output; and a synthetic prostaglandin E1 analogue stimulated both proximal and distal bicarbonate output; however, distal duodenal bicarbonate output was significantly less, indicating a proximal-to-distal gradient in bicarbonate secretion.     

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 HCI 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.     

A Comparison of Basal and Stimulated Gastric Acid and Duodenal Bicarbonate in Patients with and without Duodenal Ulcer Disease

Basal gastric secretion and the maximum response to pentagastrin and basal secretion into the duodenum and its near-maximum response to secretin were measured in 20 control subjects and ten patients with duodenal ulcer disease. In control subjects the volumes of stimulated gastric and duodenal secretions were significantly correlated. In patients with duodenal ulcer, there were no significant correlations between basal gastric and duodenal secretion but after stimulation both volumes and outputs (but not concentrations) of gastric and duodenal secretions were significantly correlated. The bicarbonate secretory capacity in patients with duodenal ulcer was not only normal but was also comparable to the gastric acid secretory capacity. The basal bicarbonate content of the duodenal aspirate, however, in patients with duodenal ulcer was only half that in control subjects, perhaps another example of inadequate entry of bicarbonate into the duodenum in patients with duodenal ulcer in spite of the normal bicarbonate secretory capacity.     

Lysolecithin and Glyceroglucolipids in Gastric Secretion of Patients with Gastric and Duodenal Ulcer

Basal and pentagastrin-stimulated gastric secretions, collected (at 15-min intervals for 1 h) from six duodenal ulcer and six gastric ulcer patients, were analyzed for their content of lecithin, lysolecithin, and glyceroglucolipids. Whereas the glycero-glucolipid concentrations and the molar ratios of lysolecithin to lecithin (2.5:1) in basal and stimulated secretions from patients with duodenal and gastric ulcer were similar, significant (p < 0.01) differences were noted between these two groups with regard to the lysolecithin contents. The basal secretions of patients with duodenal ulcer contained about 4.5-fold less (204 μmol/l) of lysolecithin than those from patients with gastric ulcer (932 μmol/l). After pentagastrin stimulation, the lysolecithin concentrations in the secretion from duodenal ulcer patients rose slightly (to 212 μmol/l), whereas a twofold decrease (to 440 μmol/l) in lysolecithin was observed in the secretion from patients with gastric ulcer. Statistically significant correlation between concentrations of lysolecithin and glyceroglucolipids was only observed in basal (r = 0.85, p < 0.05) and stimulated (r = 0.93, p < 0.01) secretions from patients with gastric ulcer. It is concluded that high concentrations of lysolecithin in the secretion of gastric ulcer patients results in the weakening of the gastric mucosal barrier by depleting its glyceroglucolipid component.     

Increased Gastric Bicarbonate Secretion in Portal Hypertensive Anesthetized Rats (Role of Prostaglandins and Nitric Oxide)

Gastric bicarbonate secretion might be modifiedin portal hypertension as a consequence of theintramucosal increase in prostaglandins and nitric oxidecontent. Therefore, we studied gastric bicarbonate secretion in control and portal hypertensiverats and investigated the role of prostaglandins andnitric oxide. Basal gastric bicarbonate secretion wasstudied in rats, using a gastric pH back-titration technique, two weeks after partial portal veinligation or a sham operation. The effects of thefollowing drugs were investigated: the prostaglandinsynthase inhibitor indomethacin (5 mg/kg intravenously), prostaglandin (PGE₂) (1 mg/kgintravenously), the nitric oxide synthase inhibitorsN^G-nitro-L-arginine methyl ester (LG NAME, 5mg/kg intravenously) andN^G-monomethyl-L-arginine (L-NMMA, 50 mg/kgintravenously), and the nitric oxide donor nitroprusside (5mmol/liter in the gastric perfusate). Plasma leakage inthe gastric wall was also measured after Evans blue dyeinjection in portal hypertensive and sham-operated rats pretreated by indomethacin (5 mg/kg,intravenously) and L-NAME (5 mg/kg, intravenously).Basal bicarbonate secretion was significantly increasedin portal hypertensive rats as compared to controls. After indomethacin, the bicarbonate secretionwas significantly reduced to a similar level in bothgroups. PGE₂ increased bicarbonate secretionsignificantly more in portal hypertensive rats than insham-operated rats. The NO synthase inhibitor L-NMMAsignificantly increased bicarbonate secretion in portalhypertensive rats only, while the other inhibitor,L-NAME, increased it significantly more in portalhypertensive than in the sham-operated rats. Plasma leakagein portal hypertensive rats, which was increased in thebasal condition as compared to control, was furtherenhanced by indomethacin but not by L-NAME pretreatment. The nitric oxide donor significantly reducedbicarbonate secretion in portal hypertensive rats toreach a similar level as in sham-operated rats. Basalgastric bicarbonate secretion is increased in portal hypertensive rats. This could be due to anenhanced prostaglandin mucosal level. Nitric oxide,which reduces bicarbonate secretion, may contribute tolimiting prostaglandin-induced bicarbonateoverproduction.     

Role of Endogenous Nitric Oxide and Prostaglandin in Duodenal Bicarbonate Response Induced by Mucosal Acidification in Rats

Duodenal HCO₃ ^– secretion increases in response to mucosal acidification by luminal acid. Although this process is known to be mediated by endogenous prostaglandins (PGs), the role of nitric oxide (NO) in this response has been little studied. We examined the effects of indomethacin and N_G-nitro-l-arginine methyl ester (l-NAME) on the acid-induced HCO₃ ^– secretion in the rat duodenum, together with those on PGE₂ generation as well as luminal release of NO metabolites (NO_x). A proximal duodenal loop was perfused with saline, and the HCO₃ ^– secretion was measured at pH 7.0 using a pH-stat method and by adding 10 mM HCl. Mucosal acidification was performed by exposing the loop to 10 or 100 mM HCl for 10 min. Acidification of the duodenal mucosa stimulated the HCO₃ ^– secretion, with concomitant increase of mucosal PGE₂ contents and luminal release of NO_x, the response being much greater in case of 100 mM HCl. Indomethacin significantly inhibited the acid-induced HCO₃ ^– secretion as well as the PGE₂ biosynthetic response, without influence on the NO_x release. Pretreatment of the animals with l-NAME attenuated both the increase of mucosal PGE₂ contents and luminal release of NO_x following the acidification, resulting in a marked inhibition of the acid-induced HCO³> ^– response, and these effects were significantly antagonized by coadministration of l-arginine. Duodenal HCO₃ ^– secretion was also increased by mucosal exposure to NOR-3 (a NO donor), with concomitant increase of PGE₂ generation, but these effects were mitigated in the presence of indomethacin. In addition, the duodenal damage caused by mucosal perfusion with 100 mM HCl for 4 hr was markedly aggravated by pretreatment with l-NAME as well as indomethacin. These results suggest that both endogenous NO and PGs are involved in the mechanism for the acid-induced duodenal HCO₃ ^– secretion, and that NO may increase the HCO₃ ^– secretion by stimulating PG generation.     

Theophylline and prostaglandin E2 on duodenal bicarbonate secretion: role for 5'-cyclic adenosine monophosphate.

Cyclic adenosine monophosphate (cAMP) has been implicated as an intracellular "second" messenger in duodenal mucosal bicarbonate secretion in animals. The purpose of this study was to determine whether cAMP may mediate duodenal mucosal bicarbonate secretion in humans. In healthy volunteers, a 4-cm segment of proximal duodenum was isolated from gastric and pancreaticobiliary secretions. Either the phosphodiesterase inhibitor theophylline, prostaglandin (PG) E2, or a combination thereof was administered topically to the isolated duodenal mucosa. Theophylline (10(-2) mol/L) and PGE2 (10(-5)-10(-4) mol/L) each significantly increased bicarbonate secretion and transmucosal potential difference. Moreover, when theophylline and PGE2 were administered in combination, the duodenal bicarbonate output was additive compared to either agent alone. When theophylline was infused with increasing doses of PGE2, the dose-response curve was shifted to the left. Furthermore, increases in bicarbonate secretion and transmucosal potential difference were correlated significantly. These results suggest that cAMP may act as an intracellular mediator of human duodenal mucosal bicarbonate secretion.