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

GLP-2 Acutely Prevents Endotoxin-Related Increased Intestinal Paracellular Permeability in Rats

BACKGROUND

Circulating endotoxin (lipopolysaccharide, LPS) increases the gut paracellular permeability. We hypothesized that glucagon-like peptide-2 (GLP-2) acutely reduces LPS-related increased intestinal paracellular permeability by a mechanism unrelated to its intestinotrophic effect.

METHODS

We assessed small intestinal paracellular permeability in vivo by measuring the appearance of intraduodenally perfused FITC-dextran 4000 (FD4) into the portal vein (PV) in rats 1-24 h after LPS treatment (5 mg/kg, ip). We also examined the effect of a stable GLP-2 analog teduglutide (TDG) on FD4 permeability.

RESULTS

FD4 movement into the PV was increased 6 h, but not 1 or 3 h after LPS treatment, with increased PV GLP-2 levels and increased mRNA expressions of proinflammatory cytokines and proglucagon in the ileal mucosa. Co-treatment with a GLP-2 receptor antagonist enhanced PV FD4 concentrations. PV FD4 concentrations 24 h after LPS were higher than FD4 concentrations 6 h after LPS, reduced by exogenous GLP-2 treatment given 6 or 12 h after LPS treatment. FD4 uptake measured 6 h after LPS was reduced by TDG 3 or 6 h after LPS treatment. TDG-associated reduced FD4 uptake was reversed by the VPAC1 antagonist PG97-269 or L-NAME, not by EGF or IGF1 receptor inhibitors.

CONCLUSIONS

Systemic LPS releases endogenous GLP-2, reducing LPS-related increased permeability. The therapeutic window of exogenous GLP-2 administration is at minimum within 6-12 h after LPS treatment. Exogenous GLP-2 treatment is of value in the prevention of increased paracellular permeability associated with endotoxemia.

Regulated traffic of anion transporters in mammalian Brunner's glands: a role for water and fluid transport

The Brunner's glands of the proximal duodenum exert barrier functions through secretion of glycoproteins and antimicrobial peptides. However, ion transporter localization, function, and regulation in the glands are less clear. Mapping the subcellular distribution of transporters is an important step toward elucidating trafficking mechanisms of fluid transport in the gland. The present study examined 1) changes in the distribution of intestinal anion transporters and the aquaporin 5 (AQP5) water channel in rat Brunner's glands following second messenger activation and 2) anion transporter distribution in Brunner's glands from healthy and disease-affected human tissues. Cystic fibrosis transmembrane conductance regulator (CFTR), AQP5, sodium-potassium-coupled chloride cotransporter 1 (NKCC1), sodium-bicarbonate cotransporter (NBCe1), and the proton pump vacuolar ATPase (V-ATPase) were localized to distinct membrane domains and in endosomes at steady state. Carbachol and cAMP redistributed CFTR to the apical membrane. cAMP-dependent recruitment of CFTR to the apical membrane was accompanied by recruitment of AQP5 that was reversed by a PKA inhibitor. cAMP also induced apical trafficking of V-ATPase and redistribution of NKCC1 and NBCe1 to the basolateral membranes. The steady-state distribution of AQP5, CFTR, NBCe1, NKCC1, and V-ATPase in human Brunner's glands from healthy controls, cystic fibrosis, and celiac disease resembled that of rat; however, the distribution profiles were markedly attenuated in the disease-affected duodenum. These data support functional transport of chloride, bicarbonate, water, and protons by second messenger-regulated traffic in mammalian Brunner's glands under physiological and pathophysiological conditions.

Differential changes in Substance P, VIP as well as neprilysin levels in patients with gastritis or ulcer

The protective effect of capsaicin-sensitive sensory nerve (CSSN) activation was recently demonstrated in human gastric mucosa. We here examined changes in neuropeptides, specifically Substance P (SP), calcitonin-gene related peptide (CGRP) and vasoactive intestinal peptide (VIP) in patients with chronic gastritis or ulcer. Furthermore changes in neprilysin levels, which hydrolyse these neuropeptides, were determined. Gastric biopsies were obtained from both lesion- and normal-appearing mucosa of 57 patients. The presence of H. pylori infection was verified with rapid urease assay. Neuronal and non-neuronal levels of SP, VIP, CGRP and neprilysin activity were determined in freshly frozen biopsies. Immunohistochemical localization of neprilysin was performed in 30 paraffin embedded specimens. We here found that neuronal SP levels decreased significantly in normally appearing mucosa of patients with gastritis while levels of non-neuronal SP increased in diseased areas of gastritis and ulcer. The presence of H. pylori led to further decreases of SP levels. The content of VIP in both disease-involved and uninvolved mucosa, and expression of neprilysin, markedly decreased in patients with gastritis or ulcer. Since VIP, as well as SP fragments, formed following hydrolysis with neprilysin is recognized to have gastroprotective effects, decreased levels of VIP, SP and neprilysin may predispose to cellular damage.

Umami receptor activation increases duodenal bicarbonate secretion via glucagon-like peptide-2 release in rats

Luminal nutrient chemosensing during meal ingestion is mediated by intestinal endocrine cells, which regulate secretion and motility via the release of gut hormones. We have reported that luminal coperfusion of L-Glu and IMP, common condiments providing the umami or proteinaceous taste, synergistically increases duodenal bicarbonate secretion (DBS) possibly via taste receptor heterodimers, taste receptor type 1, member 1 (T1R1)/R3. We hypothesized that glucose-dependent insulinotropic peptide (GIP) or glucagon-like peptide (GLP) is released by duodenal perfusion with L-Glu/IMP. We measured DBS with pH and CO(2) electrodes through a perfused rat duodenal loop in vivo. GIP, exendin (Ex)-4 (GLP-1 receptor agonist), or GLP-2 was intravenously infused (0.01-1 nmol/kg/h). l-Glu (10 mM) and IMP (0.1 mM) were luminally perfused with or without bolus intravenous injection (3 or 30 nmol/kg) of the receptor antagonists Pro(3)GIP, Ex-3(9-39), or GLP-2(3-33). GIP or GLP-2 infusion dose-dependently increased DBS, whereas Ex-4 infusion gradually decreased DBS. Luminal perfusion of l-Glu/IMP increased DBS, with no effect of Pro(3)GIP or Ex-3(9-39), whereas GLP-2(3-33) inhibited L-Glu/IMP-induced DBS. Vasoactive intestinal peptide (VIP)(6-28) intravenously or N(G)-nitro-L-arginine methyl ester coperfusion inhibited the effect of L-Glu/IMP. Perfusion of L-Glu/IMP increased portal venous concentrations of GLP-2, followed by a delayed increase of GLP-1, with no effect on GIP release. GLP-1/2 and T1R1/R3 were expressed in duodenal endocrine-like cells. These results suggest that luminal L-Glu/IMP-induced DBS is mediated via GLP-2 release and receptor activation followed by VIP and nitric oxide release. Because GLP-1 is insulinotropic and GLP-2 is intestinotrophic, umami receptor activation may have additional benefits in glucose metabolism and duodenal mucosal protection and regeneration.

Phosphatidylinositol 3-kinase is involved in prostaglandin E2-mediated murine duodenal bicarbonate secretion

Prostaglandin E(2) (PGE(2)) plays an important role in the regulation of duodenal bicarbonate (HCO(3)(-)) secretion, but its signaling pathway(s) are not fully understood. In the present study, we investigated the signaling pathways involved in PGE(2)-mediated duodenal HCO(3)(-) secretion. Murine duodenal mucosal HCO(3)(-) secretion was examined in vitro in Ussing chambers by pH-stat titration in the presence of a variety of signal transduction modulators. Phosphatidylinositol 3-kinase (PI3K) activity was measured by immunoprecipitation of PI3K and ELISA, and Akt phosphorylation was measured by Western analysis with anti-phospho-Akt and anti-Akt antibodies. PGE(2)-stimulated duodenal HCO(3)(-) secretion was reduced by the cAMP-dependent signaling pathway inhibitors MDL-12330A and KT-5720 by 23% and 20%, respectively; the Ca(2+)-influx inhibitor verapamil by 26%; and the calmodulin antagonist W-13 by 24%; whereas the PI3K inhibitors wortmannin and LY-294002 reduced PGE(2)-stimulated HCO(3)(-) secretion by 51% and 47%, respectively. Neither the MAPK inhibitor PD-98059 nor the tyrosine kinase inhibitor genistein altered PGE(2)-stimulated HCO(3)(-) secretion. PGE(2) application caused a rapid and concentration-dependent increase in duodenal mucosal PI3K activity and Akt phosphorylation. These results demonstrated that PGE(2) activates PI3K in duodenal mucosa and stimulates duodenal HCO(3)(-) secretion via cAMP-, Ca(2+)-, and PI3K-dependent signaling pathways.

Involvement of the anion exchanger SLC26A6 in prostaglandin E2- but not forskolin-stimulated duodenal HCO3- secretion

BACKGROUND & AIMS

SLC26A6 is a recently identified apical Cl(-)/HCO(3)(-) exchanger with strong expression in murine duodenum. The present study was designed to examine the role of SLC26A6 in prostaglandin E(2) (PGE(2))-, forskolin-, and carbachol-induced duodenal HCO(3)(-) secretion.

METHODS

Murine duodenal mucosal HCO(3)(-) secretion was examined in vitro in Ussing chambers and mucosal SLC26A6 expression levels were analyzed by semiquantitative reverse-transcription polymerase chain reaction.

RESULTS

Basal HCO(3)(-) secretion was diminished by 20%, PGE(2)-stimulated HCO(3)(-) secretory response by 59%, and carbachol-stimulated response was reduced by 35% in SLC26A6-/- compared with +/+ duodenal mucosa, whereas the forskolin-stimulated HCO(3)(-) secretory response was not different. In Cl(-)-free solutions, PGE(2)- and carbachol-stimulated HCO(3)(-) secretion was reduced by 81% and 44%, respectively, whereas forskolin-stimulated HCO(3)(-) secretion was not altered significantly. PGE(2) and carbachol, but not forskolin, were able to elicit a Cl(-)-dependent HCO(3)(-) secretory response in the absence of short-circuit current changes in cystic fibrosis transmembrane conductance regulator knockout mice.

CONCLUSIONS

In murine duodenum, PGE(2)-mediated HCO(3)(-) secretion is strongly SLC26A6 dependent and cystic fibrosis transmembrane conductance regulator independent, whereas forskolin-stimulated HCO(3)(-) secretion is completely SLC26A6 independent and cystic fibrosis transmembrane conductance regulator dependent. Carbachol-induced secretion is less pronounced, but occurs via both transport pathways. This suggests that PGE(2) and forskolin activate distinct HCO(3)(-) transport pathways in the murine duodenum.

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.

Duodenal alkaline secretion: its mechanisms and role in mucosal protection against gastric acid

Duodenal mucosa, especially its proximal portion, is exposed to intermittent pulses of gastric acid (H+). This review summarises the mechanisms of duodenal bicarbonate (HCO3-) secretion and their role in protecting duodenal epithelium against gastric H+. Duodenal epithelium is a leaky barrier against gastric H+, which diffuses into duodenocytes, but fails to damage them due to: (a) an enhanced expression of cyclooxygenase, producing protective prostaglandins and expression of nitric oxide synthase, releasing nitric oxide, both stimulating duodenal HCO3- secretion and (b) the release of several neurotransmitters also stimulating HCO3- secretion such as vasoactive intestinal peptide, pituitary adenylate cyclase-activating polypeptide, acetylcholine and melatonin. At the apical duodenocyte membrane, several HCO3-/Cl- anion exchangers operate in response to luminal H+ to extrude HCO3- into duodenal lumen. In baso-lateral duodenocyte membrane, both non-electrogenic and electrogenic Na+-HCO3- cotransporters are activated after exposure of duodenum to gastric H+, causing inward movement of HCO3- from extracellular fluid to duodenocytes. There are also at least three Na+/H+ exchangers, eliminating H+ which diffused into these cells. The Helicobacter pylori infection with gastric metaplasia in the duodenum and bacterium inoculation results in the inhibition of HCO3- secretion by its endogenous inhibitor dimethyl arginine, resulting in ulcerogenesis.

Low-dose PGE2 mimics the duodenal secretory response to luminal acid in mice

Luminal exposure to concentrated acid, the most accepted physiological stimulus for duodenal bicarbonate secretion (DBS), cannot be used with in vitro preparations due to potential tissue damage. We thus examined whether exposure to PGE(2), a well-characterized physiological duodenal secretagogue, could mimic the effects of acid perfusion. DBS was measured in C57/BL mice by pH-stat/back-titration and measurement of total dissolved CO(2) concentration ([CO(2)](t)). Anion transport inhibitor DIDS, anion channel inhibitor 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB), carbonic anhydrase inhibitor methazolamide, and nonselective cyclooxygenase inhibitor indomethacin were used to inhibit separate components of HCO(3)(-) secretory pathway. Baseline DBS was not altered by exposure to methazolamide (0.1 mM) but was slightly reduced by DIDS (0.5 mM). DBS and [CO(2)](t) increased after acid and PGE(2) exposure. DIDS (0.5 mM) and NPPB (0.2 mM) abolished acid-induced DBS increase. Methazolamide (0.1 mM) and DIDS inhibited acid-induced [CO(2)](t) increase. DIDS, NPPB, or methazolamide had little effect on DBS in response to high concentration PGE(2) (100 microg/ml). Low concentration PGE(2) (1 microg/ml) increased DBS that was inhibited by DIDS, NPPB, and methazolamide. Pretreatment with indomethacin (5 mg/kg) inhibited DBS induced by acid exposure but not by PGE(2). High-dose PGE(2) substantially increases DBS by a mechanism that appears to be different than secretory response to luminal acid perfusion. Secretory response to low-dose PGE(2), at least in terms of inhibitor profile, closely resembles secretion in response to perfusion of physiological acid concentrations and may be a useful stimulus for in vitro study of DBS in isolated mouse duodenum.

Protein kinase C potentiates cAMP-stimulated mouse duodenal mucosal bicarbonate secretion in vitro

PKC has been shown to regulate epithelial Cl(-) secretion in a variety of models. However, the role of PKC in duodenal mucosal bicarbonate secretion is less clear. We aimed to investigate the role of PKC in regulation of duodenal mucosal bicarbonate secretion. Bicarbonate secretion by murine duodenal mucosa was examined in vitro in Ussing chambers using a pH-stat technique. PKC isoform expression and activity were assessed by Western blotting and in vitro kinase assays, respectively. PMA (an activator of PKC) alone had no effect on duodenal bicarbonate secretion or short-circuit current (I(sc)). When PMA and dibutyryl-cAMP (db-cAMP) were added simultaneously, PMA failed to alter db-cAMP-stimulated duodenal bicarbonate secretion or I(sc) (P > 0.05). However, a 1-h preincubation with PMA potentiated db-cAMP-stimulated duodenal bicarbonate secretion and I(sc) in a concentration-dependent manner (from 10(-8) to 10(-5)M) (P < 0.05). PMA preincubation had no effects on carbachol- or heat-stable toxin-stimulated bicarbonate secretion. Western blot analysis revealed that PKCalpha, -gamma, -epsilon, -, -micro, and -iota/lambda were expressed in murine duodenal mucosa. Ro 31-8220 (an inhibitor active against PKCepsilon, -alpha, -beta, and -gamma), but not Gö 6983 (an inhibitor active against PKCalpha, -gamma, -beta, and -delta), reversed the potentiating effect of PMA on db-cAMP-stimulated bicarbonate secretion. PMA also time- and concentration-dependently increased the activity of PKCepsilon, an effect that was prevented by Ro 31-8220 but not Gö 6983. These results demonstrate that activation of PKC potentiates cAMP-stimulated duodenal bicarbonate secretion, whereas it does not modify basal secretion. The effect of PKC on cAMP-stimulated bicarbonate secretion is mediated by the PKCepsilon isoform.

5-HT induces duodenal mucosal bicarbonate secretion via cAMP- and Ca2+-dependent signaling pathways and 5-HT4 receptors in mice

In previous studies, we have found that 5-hydroxytryptamine (5-HT) is a potent stimulant of duodenal mucosal bicarbonate secretion (DMBS) in mice. The aim of the present study was to determine the intracellular signaling pathways and 5-HT receptor subtypes involved in 5-HT-induced DMBS. Bicarbonate secretion by murine duodenal mucosa was examined in vitro in Ussing chambers. 5-HT receptor involvement in DMBS was inferred from pharmacological studies by using selective 5-HT receptor antagonists and agonists. The expression of 5-HT(4) receptor mRNA in duodenal mucosa and epithelial cells was analyzed by RT-PCR. cAMP-dependent signaling pathway inhibitors MDL-12330A, Rp-cAMP, and H-89 and Ca(2+)-dependent signaling pathway inhibitors verapamil and W-13 markedly reduced 5-HT-stimulated duodenal bicarbonate secretion and short-circuit current (I(sc)), whereas cGMP-dependent signaling pathway inhibitors NS-2028 and KT-5823 failed to alter these responses. Both SB-204070 and high-dose ICS-205930 (selective 5-HT(4) receptor antagonists) markedly inhibited 5-HT-stimulated bicarbonate secretion and I(sc), whereas methiothepine (5-HT(1) receptor antagonist), ketanserin (5-HT(2) receptor antagonist), and a low concentration of ICS-205930 (5-HT(3) receptor antagonist) had no effect. RS-67506 (partial 5-HT(4) receptor agonist) concentration-dependently increased bicarbonate secretion and I(sc), whereas 5-carboxamidotryptamine (5-HT(1) receptor agonist), alpha-methyl-5-HT (5-HT(2) receptor agonist), and phenylbiguanide (5-HT(3) receptor agonist) did not significantly increase bicarbonate secretion or I(sc). RT-PCR analysis confirmed the expression of 5-HT(4) receptor mRNA in murine duodenal mucosa and epithelial cells. These results demonstrate that 5-HT regulates DMBS via both cAMP- and Ca(2+)-dependent signaling pathways and 5-HT(4) receptors located in the duodenal mucosa and/or epithelial cells.

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.

Influence of epidermal growth factor on bovine pancreatic duct cell bicarbonate

BACKGROUND AND AIMS

Epidermal growth factor (EGF) is secreted in pancreatic juice and its receptor is expressed on pancreatic duct cells (PDCs), suggesting a physiological role which has yet to be defined. Here we examined the effects of EGF on bicarbonate production and carbonic anhydrase (CA) activity in a PDC explant model.

METHODS

Bovine main PDCs were prepared and maintained in culture as explants. Levels of CA expression, phosphorylation, and enzymatic activity were measured in resting cells and compared to that of cells exposed to 10 nM secretin, 10 nM EGF, or both. Bicarbonate production was measured using the autoburette pH titration technique.

RESULTS

CA protein levels were unchanged with any treatment, but enzyme activity increased by 180% with secretin treatment and was reduced by 54% with EGF. The combination treatment led to a synergistic increase 240% above basal. EGF alone did not affect bicarbonate secretion, but the normal increase observed with secretin stimulation (1.3 +/- 0.4 to 2.9 +/- 0.6 micromol/h/cm(2)) was abolished by acute EGF pretreatment. On the other hand, EGF pretreatment for 24 h significantly increased basal and stimulated secretion (2.2 +/- 0.5 and 3.8 +/- 0.5, respectively) compared to controls.

CONCLUSIONS

EGF exerts a regulatory role on bicarbonate secretion by the pancreatic duct epithelium, independent of its effect on CA activity. Its inhibition of stimulated bicarbonate secretion could play a protective role in the setting of pancreatic inflammation, where increased levels of EGF are associated with reduced pancreatic juice production.

Molecular and functional evidence for electrogenic and electroneutral Na(+)-HCO(3)(-) cotransporters in murine duodenum

Inward Na(+)-HCO(3)(-) cotransport has previously been demonstrated in acidified duodenal epithelial cells, but the identity and localization of the mRNAs and proteins involved have not been determined. The molecular expression and localization of Na(+)-HCO(3)(-) cotransporters (NBCs) were studied by RT-PCR, sequence analysis, and immunohistochemistry. By fluorescence spectroscopy, the intracellular pH (pH(i)) was recorded in suspensions of isolated murine duodenal epithelial cells loaded with 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein. Proximal duodenal epithelial cells expressed mRNA encoding two electrogenic NBC1 isoforms and the electroneutral NBCn1. Both NBC1 and NBCn1 were localized to the basolateral membrane of proximal duodenal villus cells, whereas the crypt cells did not label with the anti-NBC antibodies. DIDS or removal of extracellular Cl(-) increased pH(i), whereas an acidification was observed on removal of Na(+) or both Na(+) and Cl(-). The effects of inhibitors and ionic dependence of acid/base transporters were consistent with both inward and outward Na(+)-HCO(3)(-) cotransport. Hence, we propose that NBCs are involved in both basolateral electroneutral HCO(3)(-) transport as well as basolateral electrogenic HCO(3)(-) transport in proximal duodenal villus cells.

Why should a clinician care about the molecular biology of transport?

This review outlines the progress made over the last few years in three chosen areas of intestinal ion transport. In the field of intestinal secretion, research on the secretion of bicarbonate by pancreatic ducts and duodenal epithelia in cystic fibrosis revealed the crucial role of chloride channel (CFTR) in the control of activity of other transporters involved in bicarbonate secretion. In the area of intestinal absorption, studies on the regulation and physiologic roles of epithelial Na(+)/H(+) exchangers confirmed the suspected involvement of recycling in the acute regulation of NHE3 activity and resulted in formulation of new concepts for the roles of NHE3 and NHE2 in the gastrointestinal tract. Finally, the recent discovery of the first known viral enterotoxin revolutionized our understanding of pathomechanisms of secretory diarrhea during viral infections in humans. All of these findings are discussed in the context of their utility to the practicing gastroenterologist.

Intestinal ion transport in NKCC1-deficient mice

The Na(+)-K(+)-2Cl(-) cotransporter (NKCC1) located on the basolateral membrane of intestinal epithelia has been postulated to be the major basolateral Cl(-) entry pathway. With targeted mutagenesis, mice deficient in the NKCC1 protein were generated. The basal short-circuit current did not differ between normal and NKCC1 -/- jejuna. In the -/- jejuna, the forskolin response (22 microA/cm(2); bumetanide insensitive) was significantly attenuated compared with the bumetanide-sensitive response (52 microA/cm(2)) in normal tissue. Ion-replacement studies demonstrated that the forskolin response in the NKCC1 -/- jejuna was HCO(3)(-) dependent, whereas in the normal jejuna it was independent of the HCO(3)(-) concentration in the buffer. NKCC1 -/- ceca exhibited a forskolin response that did not differ significantly from that of normal ceca, but unlike that of normal ceca, was bumetanide insensitive. Ion-substitution studies suggested that basolateral HCO(3)(-) as well as Cl(-) entry (via non-NKCC1) paths played a role in the NKCC1 -/- secretory response. In contrast to cystic fibrosis mice, which lack both basal and stimulated Cl(-) secretion and exhibit severe intestinal pathology, the absence of intestinal pathology in NKCC1 -/- mice likely reflects the ability of the intestine to secrete HCO(3)(-) and Cl(-) by basolateral entry mechanisms independent of NKCC1.

A novel in vitro model of Brunner's gland secretion in the guinea pig duodenum

A novel in vitro model that combined functional and morphological techniques was employed to directly examine pathways regulating Brunner's gland secretion in isolation from epithelium. In vitro submucosal preparations were dissected from guinea pig duodenum. A videomicroscopy technique was used to measure changes in luminal diameter of glandular acini as an index of activation of secretion. Carbachol elicited concentration-dependent dilations of the lumen (EC(50) = 2 microM) by activating muscarinic receptors on acinar cells. Ultrastructural and histological analyses demonstrated that dilation was accompanied by single and compound exocytosis of mucin-containing granules and the accumulation of mucoid material within the lumen. Inflammatory mediators (histamine, PGE(1), PGE(2)) and intestinal hormones (CCK, gastrin, vasoactive intestinal polypeptide, secretin) also stimulated glandular secretion, whereas activation of submucosal secretomotor neurons by 5-hydroxytryptamine did not. This study directly demonstrates that multiple hormonal, inflammatory, and neurocrine agents activate Brunner's glands, whereas many have dissimilar effects on the epithelium. This suggests that Brunner's glands are regulated by pathways that act both in parallel to and in isolation from those controlling epithelial secretion.

NSP4 elicits age-dependent diarrhea and Ca(2+)mediated I(-) influx into intestinal crypts of CF mice

Homologous disruption of the murine gene encoding the cystic fibrosis (CF) transmembrane conductance regulator (CFTR) leads to the loss of cAMP-mediated ion transport. Mice carrying this gene defect exhibit meconium ileus at birth and gastrointestinal plugging during the neonatal period, both contributing to high rates of mortality. We investigated whether infectious mammalian rotavirus, the recently characterized rotaviral enterotoxin protein NSP4, or its active NSP4(114-135) peptide, can overcome these gastrointestinal complications in CF (CFTR(m3Bay) null mutation) mice. All three agents elicited diarrhea when administered to wild-type (CFTR(+/+)), heterozygous (CFTR(+/-)), or homozygous (CFTR(-/-)) 7- to 14-day-old mouse pups but were ineffective when given to older mice. The diarrheal response was accompanied by non-age-dependent intracellular Ca(2+) mobilization within both small and large intestinal crypt epithelia. Significantly, NSP4 elicited cellular I(-) influx into intestinal epithelial cells from all three genotypes, whereas both carbachol and the cAMP-mobilizing agonist forskolin failed to evoke influx in the CFTR(-/-) background. This unique plasma membrane halide permeability pathway was age dependent, being observed only in mouse pup crypts, and was abolished by either the removal of bath Ca(2+) or the transport inhibitor DIDS. These findings indicate that NSP4 or its active peptide may induce diarrhea in neonatal mice through the activation of an age- and Ca(2+)-dependent plasma membrane anion permeability distinct from CFTR. Furthermore, these results highlight the potential for developing synthetic analogs of NSP4(114-135) to counteract chronic constipation/obstructive bowel syndrome in CF patients.

Regulation of intracellular pH and blood flow in rat duodenal epithelium in vivo

Duodenal mucosal defense was assessed by measuring blood flow and epithelial intracellular pH (pHi) of rat proximal duodenum in vivo. Fluorescence microscopy was used to measure epithelial pHi using the trapped, pHi-indicating dye 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein-AM. Blood flow was measured with laser-Doppler flowmetry. The mucosa was briefly superfused with NH4Cl, pH 2.2 buffer, the potent Na+/H+ exchange inhibitor 5-(N,N-dimethyl)-amiloride (DMA), or the anion exchange and Na+-HCO-3 cotransport inhibitor DIDS. Cryostat sections localized dye fluorescence to the villus tip. Steady-state pHi was 7. 02 +/- 0.01, which remained stable for 60 min. Interventions that load the cells with protons without affecting superfusate pH (NH4Cl prepulse, nigericin with low superfusate K+ concentration, DMA, and DIDS) all decreased pHi, supporting our contention that the dye was faithfully measuring pHi. An acid pulse decreased pHi, followed by a DIDS-inhibitable overshoot over baseline. Intracellular acidification increased duodenal blood flow independent of superfusate pH, which was inhibited by DMA, but not by DIDS. We conclude that we have established a novel in vivo microscopy system enabling simultaneous measurements of pHi and blood flow of duodenal epithelium. Na+/H+ exchange and Na+-HCO-3 cotransport regulate baseline duodenal epithelial pHi. Intracellular acidification enhances duodenal blood flow by a unique, amiloride-inhibitable, superfusate pH-independent mechanism.

Utility of endoscopic biopsy samples to quantitate human duodenal ion transport

Duodenal mucosal bicarbonate secretion (DMBS) prevents acid-peptic damage and facilitates nutrient absorption. DMBS is diminished in patients with duodenal ulcers and is normalized after Helicobacter pylori eradication. The measurement of DMBS in human patients in vivo requires intubation with a multi-lumen balloon tube and permits limited testing with putative agonists and antagonists. Our purpose was to develop a means to investigate transport events in human duodenal biopsy samples in vitro. After validation studies in a modified mini-Ussing chamber were performed, duodenal transport events were examined in proximal endoscopic biopsy samples from normal volunteers (n = 17). Tissues were mounted in modified mini-Ussing chambers (volume 2.5 ml, surface area 3.8 mm2). Short circuit current (Isc), potential difference (PD), and bicarbonate secretion were determined under basal conditions and after stimulation with graded doses of prostaglandin E2 (PGE2)(10(-8) to 10(-4) mol/L) and dibutyryl cAMP (db-cAMP)(10(-4) to 10(-2) mol/L). Duodenal tissues remained viable for at least 2 hours and exhibited stable basal HCO3(-) secretion and electrical parameters. Stimulation with PGE2 and db-cAMP resulted in dose-related increases in both Isc and HCO3(-) secretion (P < .05) that were abolished by ouabain and anoxia. It is concluded (1) that human duodenal bulb biopsy samples maintain their inherent transport function in mini-Ussing chambers and (2) that by using this novel method it will be possible to define the transport events that modulate human duodenal secretion, in particular bicarbonate secretion, in both health and disease.

Dual role of CFTR in cAMP-stimulated HCO3- secretion across murine duodenum

The role of the cystic fibrosis transmembrane conductance regulator (CFTR) in cAMP-stimulated HCO3- secretion across the murine duodenum was investigated. Serosal-to-mucosal flux of HCO3- (Js-->m, in mu eq.cm-2.h-1) and short-circuit current (Isc; in mu eq.cm-2.h-1) were measured by the pH stat method in duodenum from CFTR knockout [CFTR(-)] and normal [CFTR(+)] mice. Under control conditions, forskolin increased Js-->m and Isc (+1.7 and +3.5, respectively) across the CFTR(+) but not CFTR(-) duodenum. Both the forskolin-stimulated delta Js-->m and delta Isc were abolished by the CFTR channel blocker 5-nitro-2-(3-phenylpropylamino)benzoate, whereas inhibition of luminal Cl-/HCO3- exchange by luminal Cl- removal or DIDS reduced the Js-->m by approximately 18% without a consistent effect on the delta Isc. Methazolamide also reduced the Js-->m by 39% but did not affect the delta Isc. When carbonic anhydrase-dependent HCO3- secretion was isolated by using a CO2-gassed, HCO3(-)-free Ringer bath, forskolin stimulated the Js-->m and Isc (+0.7 and +2.0, respectively) across CFTR(+) but not CFTR(-) duodenum. Under these conditions, luminal Cl- substitution or DIDS abolished the Js-->m but not the delta Isc. It was concluded that cAMP-stimulated HCO3- secretion across the duodenum involves 1) electrogenic secretion via a CFTR HCO3- conductance and 2) electroneutral secretion via a CFTR-dependent Cl-/HCO3- exchange process that is closely associated with the carbonic anhydrase activity of the epithelium.

Regulation of intestinal Cl- and HCO3-secretion by uroguanylin

Uroguanylin is an intestinal peptide hormone that may regulate epithelial ion transport by activating a receptor guanylyl cyclase on the luminal surface of the intestine. In this study, we examined the action of uroguanylin on anion transport in different segments of freshly excised mouse intestine, using voltage-clamped Ussing chambers. Uroguanylin induced larger increases in short-circuit current (Isc) in proximal duodenum and cecum compared with jejunum, ileum, and distal colon. The acidification of the lumen of the proximal duodenum (pH 5.0-5.5) enhanced the stimulatory action of uroguanylin. In physiological Ringer solution, a significant fraction of the Isc stimulated by uroguanylin was insensitive to bumetanide and dependent on HCO3- in the bathing medium. Experiments using pH-stat titration revealed that uroguanylin stimulates serosal-to-luminal HCO3- secretion (Js-->lHCO3-) together with a larger increase in Isc. Both Js-->lHCO3- and Isc were significantly augmented when luminal pH was reduced to pH 5.15. Uroguanylin also stimulated the Js-->lHCO3- and Isc across the cecum, but luminal acidity caused a generalized decrease in the bioelectric responsiveness to agonist stimulation. In cystic fibrosis transmembrane conductance regulator (CFTR) knockout mice, the duodenal Isc response to uroguanylin was markedly reduced, but not eliminated, despite having a similar density of functional receptors. It was concluded that uroguanylin is most effective in acidic regions of the small intestine, where it stimulates both HCO3- and Cl-secretion primarily via a CFTR-dependent mechanisms.

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.

A functional CFTR protein is required for mouse intestinal cAMP-, cGMP- and Ca(2+)-dependent HCO3- secretion

1. Most segments of the gastrointestinal tract secrete HCO3-, but the molecular nature of the secretory mechanisms has not been identified. We had previously speculated that the regulator for intestinal electrogenic HCO3- secretion is the cystic fibrosis transmembrane regulator (CFTR) channel. To prove this hypothesis, we have now measured HCO3- secretion by pH-stat titration, and recorded the electrical parameters of in vitro duodenum, jejunum and ileum of mice deficient in the gene for the CFTR protein ('CF-mice') and their normal littermates. 2. Basal HCO3- secretory rates were reduced in all small intestinal segments of CF mice. Forskolin, PGE2, 8-bromo-cAMP and VIP (cAMP-dependent agonists), heat-stable enterotoxin of Escherichia coli (STa), guanylin and 8-bromo-cGMP (cGMP-dependent agonists) and carbachol (Ca2+ dependent) stimulated both the short-circuit current (Isc) and the HCO3- secretory rate (JHCO3-) in all intestinal segments in normal mice, whereas none of these agonists had any effect on JHCO3- in the intestine of CF mice. 3. To investigate whether Cl(-)-HCO3- exchangers, which have been implicated in mediating the response to some of these agonists in the intestine, were similarly active in the small intestine of normal and CF mice, we studied Cl- gradient-driven 36Cl- uptake into brush-border membrane (BBM) vesicles isolated from normal and CF mouse small intestine. Both the time course and the peak value for 4,4'-diisothiocyanostilbene-2',2-disulphonic acid (DIDS)-inhibited 36Cl- uptake was similar in normal and CF mice BBM vesicles. 4. In summary, the results demonstrate that the presence of the CFTR channel is necessary for agonist-induced stimulation of electrogenic HCO3- secretion in all segments of the small intestine, and all three intracellular signal transduction pathways stimulate HCO3- secretion exclusively via activation of the CFTR channel.

Cyclic adenosine-3',5'-monophosphate production is greater in rabbit duodenal crypt than in villus cells

BACKGROUND

Duodenal surface epithelial cells secrete bicarbonate. Agonists of duodenal alkaline secretion (such as vasoactive intestinal polypeptide (VIP), prostaglandin E2 (PGE(2)), and forskolin) increase intracellular cyclic adenosine-3', 5-monophosphate (cAMP), and cAMP stimulates Cl-HCO(3)- exchange in duodenal brush border membrane vesicles. As intestinal villus and crypt cells differ in function, our aims were to contrast cAMP generation in duodenal villus versus crypt cells in response to VIP, PGE(2), and forskolin.

METHODS

Villus and crypt rabbit duodenal enterocytes were isolated by calcium chelation. To prevent the degradation of cAMP in vitro, phosphodiesterase activity was inhibited. cAMP production was quantitated in response to VIP (10(-10)-10(-5)M), PGE(2) (10(-10)-10(-4)M), and forskolin (10(-8)-10(-3)M).

RESULTS

In crypt cells cAMP generation was approximately 10-fold greater (P < 0.001) in response to VIP, PGE(2), and forskolin than to villus cells. The relative orders of potency (that is, D(50), VIP > PGE(2) > forskolin) and efficacy (that is, V max, forskolin > VIP and PGE(2)) were similar in villus and crypt cells.

CONCLUSION

cAMP production is greater in duodenal crypt than in villus enterocytes at rest and in response to forskolin, VIP, and PGE(2), suggesting that alkaline secretion may differ along the villus-to-crypt axis.

Oral delivery of sodium cromolyn: preliminary studies in vivo and in vitro

PURPOSE

Herein we report the discovery of a group of derivatized alpha-amino acids that increase the oral bioavailability of sodium cromolyn.

METHODS

We prepared three N-acylated alpha-amino acids and used these compounds to demonstrate the oral delivery of cromolyn in an in vivo rat model. In vitro experiments, including permeation studies and near infrared spectroscopy, were also performed to initiate an understanding of the mechanism by which these compounds facilitate cromolyn oral delivery.

RESULTS

Following oral administration to rats of solutions containing a combination of cromolyn and the delivery agent, significant systemic plasma concentrations of the drug were detected. In vitro studies suggest that absorption of the drug across the gastrointestinal membrane is a passive process.

CONCLUSION

The absolute oral bioavailability of sodium cromolyn in the rat model is estimated to be approximately 5%. Preliminary mechanistic studies suggest that a complex of the cromolyn/delivery agent facilitates permeation across/through the membrane.

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?

Histamine inhibits prostaglandin E2-stimulated rabbit duodenal bicarbonate secretion via H2 receptors and enteric nerves

BACKGROUND/AIMS

The gastroduodenal epithelium is protected from acid peptic damage by an adherent mucus-bicarbonate layer. Bicarbonate is secreted by the surface epithelial cells into this mucus layer. Patients with duodenal ulcer disease have impaired proximal duodenal bicarbonate secretion. Mast cells, present in large numbers in the duodenal mucosa, release a number of inflammatory mediators, including histamine. Release of such mast cell mediators has been implicated in ulcer disease. In this study, the ability of histamine to regulate bicarbonate secretion was examined.

METHODS

Bicarbonate secretion by rabbit proximal duodenal mucosa was examined in vitro, and the effects of histamine, its agonists, and its antagonists were studied.

RESULTS

Histamine essentially eliminated prostaglandin E2-stimulated duodenal mucosal bicarbonate secretion, an effect reversed both by the neurotoxin, tetrodotoxin, and the histamine H2-receptor antagonist, cimetidine, as well as reproduced by the H2-receptor agonist, dimaprit.

CONCLUSIONS

In addition to the stimulatory action of histamine on gastric acid secretion, histamine expresses an additional antidefensive action by inhibiting prostaglandin E2-stimulated duodenal epithelial bicarbonate secretion. This effect of histamine is likely mediated via H2 receptors located on enteric nerves.

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.

The enteric nervous system modulates mammalian duodenal mucosal bicarbonate secretion

BACKGROUND

Interaction of the enteric nerves in regulating mammalian duodenal mucosal bicarbonate secretion is not well understood. The purpose of the present experiments was to evaluate the role of the enteric nervous system on bicarbonate secretion from rabbit duodenal mucosa in vitro.

METHODS

Proximal duodenum from male New Zealand White rabbits was stripped of seromuscular layers, mounted in Ussing chambers, and studied under short-circuited conditions. Effects of electrical field stimulation, vasoactive intestinal polypeptide (VIP), carbachol, prostaglandin E2 (PGE2), dibutyryl-cyclic adenosine monophosphate (db-cAMP), and the neurotoxin tetrodotoxin (TTX) and muscarinic blockade by atropine were studied.

RESULTS

Electrical field stimulation significantly (P < 0.01) stimulated bicarbonate secretion, short-circuit current (Isc), and electrical potential difference (PD) that was sensitive to both TTX and atropine. VIP-stimulated bicarbonate secretion was significantly inhibited by TTX (-73%), yet Isc and PD remained unchanged. Atropine decreased VIP-induced bicarbonate secretion (-69%) and Isc (-43%). Carbachol-stimulated bicarbonate secretion, Isc, and PD were abolished by atropine, whereas TTX was without affect. Neither TTX nor atropine had a significant effect on PGE2 or db-cAMP-stimulated bicarbonate secretion.

CONCLUSIONS

These results suggest that (1) enteric nerve stimulation activates an acetylcholine receptor that in turn stimulates duodenal epithelial bicarbonate secretion; (2) VIP stimulates bicarbonate secretion, in large part, via the enteric nervous system; and (3) PGE2 and cAMP stimulate bicarbonate secretion independent of the enteric nervous system.