Stimulation of colonic anion secretion by monochloramine: action sites

Molecular mechanisms of caffeine-mediated intestinal epithelial ion transports

BACKGROUND AND PURPOSE

As little is known about the effect of caffeine, one of the most widely consumed substances worldwide, on intestinal function, we aimed to study its action on intestinal anion secretion and the underlying molecular mechanisms.

EXPERIMENTAL APPROACH

Anion secretion and channel expression were examined in mouse duodenal epithelium by Ussing chambers and immunocytochemistry. Ca²⁺ imaging was also performed in intestinal epithelial cells (IECs).

KEY RESULTS

Caffeine (10 mM) markedly increased mouse duodenal short-circuit current (I_sc ), which was attenuated by a removal of either Cl^- or HCO₃ ^- , Ca²⁺ -free serosal solutions and selective blockers of store-operated Ca²⁺ channels (SOC/Ca²⁺ release-activated Ca²⁺ channels), and knockdown of Orai1 channels on the serosal side of duodenal tissues. Caffeine induced SOC entry in IEC, which was inhibited by ruthenium red and selective blockers of SOC. Caffeine-stimulated duodenal I_sc was inhibited by the endoplasmic reticulum Ca²⁺ chelator (N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine), selective blockers (ruthenium red and dantrolene) of ryanodine receptors (RyR), and of Ca²⁺ -activated Cl^- channels (niflumic acid and T16A). There was synergism between cAMP and Ca²⁺ signalling, in which cAMP/PKA promoted caffeine/Ca²⁺ -mediated anion secretion. Expression of STIM1 and Orai1 was detected in mouse duodenal mucosa and human IECs. The Orai1 proteins were primarily co-located with the basolateral marker Na⁺ , K⁺ -ATPase.

CONCLUSIONS AND IMPLICATIONS

Caffeine stimulated intestinal anion secretion mainly through the RyR/Orai1/Ca²⁺ signalling pathway. There is synergism between cAMP/PKA and caffeine/Ca²⁺ -mediated anion secretion. Our findings suggest that a caffeine-mediated RyR/Orai1/Ca²⁺ pathway could provide novel potential drug targets to control intestinal anion secretion.

Anoctamin 6 Contributes to Cl- Secretion in Accessory Cholera Enterotoxin (Ace)-stimulated Diarrhea: AN ESSENTIAL ROLE FOR PHOSPHATIDYLINOSITOL 4,5-BISPHOSPHATE (PIP2) SIGNALING IN CHOLERA

Accessory cholera enterotoxin (Ace) of Vibrio cholerae has been shown to contribute to diarrhea. However, the signaling mechanism and specific type of Cl- channel activated by Ace are still unknown. We have shown here that the recombinant Ace protein induced ICl of apical plasma membrane, which was inhibited by classical CaCC blockers. Surprisingly, an Ace-elicited rise of current was neither affected by ANO1 (TMEM16A)-specific inhibitor T16A(inh)-AO1(TAO1) nor by the cystic fibrosis transmembrane conductance regulator (CFTR) blocker, CFTR inh-172. Ace stimulated whole-cell current in Caco-2 cells. However, the apical ICl was attenuated by knockdown of ANO6 (TMEM16F). This impaired phenotype was restored by re-expression of ANO6 in Caco-2 cells. Whole-cell patch clamp recordings of ANO currents in HEK293 cells transiently expressing mouse ANO1-mCherry or ANO6-GFP confirmed that Ace induced Cl- secretion. Application of Ace produced ANO6 but not the ANO1 currents. Ace was not able to induce a [Ca2+]i rise in Caco-2 cells, but cellular abundance of phosphatidylinositol 4,5-bisphosphate (PIP2) increased. Identification of the PIP2-binding motif at the N-terminal sequence among human and mouse ANO6 variants along with binding of PIP2 directly to ANO6 in HEK293 cells indicate likely PIP2 regulation of ANO6. The biophysical and pharmacological properties of Ace stimulated Cl- current along with intestinal fluid accumulation, and binding of PIP2 to the proximal KR motif of channel proteins, whose mutagenesis correlates with altered binding of PIP2, is comparable with ANO6 stimulation. We conclude that ANO6 is predominantly expressed in intestinal epithelia, where it contributes secretory diarrhea by Ace stimulation in a calcium-independent mechanism of RhoA-ROCK-PIP2 signaling.

Role of epithelial ion transports in inflammatory bowel disease

Inflammatory bowel disease (IBD) is a chronic inflammatory disorder with a complex pathogenesis. Diarrhea is a highly prevalent and often debilitating symptom of IBD patients that results, at least in part, from an intestinal hydroelectrolytic imbalance. Evidence suggests that reduced electrolyte absorption is more relevant than increased secretion to this disequilibrium. This systematic review analyses and integrates the current evidence on the roles of epithelial Na(+)-K(+)-ATPase (NKA), Na(+)/H(+) exchangers (NHEs), epithelial Na(+) channels (ENaC), and K(+) channels (KC) in IBD-associated diarrhea. NKA is the key driving force of the transepithelial ionic transport and its activity is decreased in IBD. In addition, the downregulation of apical NHE and ENaC and the upregulation of apical large-conductance KC all contribute to the IBD-associated diarrhea by lowering sodium absorption and/or increasing potassium secretion.

Traditional Chinese formula, lubricating gut pill, stimulates cAMP-dependent CI(−) secretion across rat distal colonic mucosa

AIM OF THE STUDY

Lubricating gut pill (LGP), a traditional Chinese formula, had been conformed to improve the loperamide-induced rat constipation by stimulation of Cl(-) secretion, but its mechanism has not been fully explored. Thus, the purpose of this study was to identify the action sites of LGP-stimulated Cl(-) secretion across rat distal colonic mucosa.

MATERIALS AND METHODS

Rat distal colonic mucosa was mounted in Ussing chambers and short circuit current (I(SC)), apical Cl(-) current and basolateral K(+) current were recorded. Intracellular cyclic adenosine monophosphate (cAMP) content and protein kinase A (PKA) activity were determined with ELISA kit and the non-radioactive PepTag test, respectively.

RESULTS

LGP at 800μg/ml elicited a sustained increase in Cl(-) secretory response, which was inhibited by CFTR(inh)172, a cystic fibrosis transmembrane conductance regulator (CFTR) inhibitor. Permeabilizing apical membrane with nystatin revealed that LGP-stimulated basolateral K(+) current was significantly inhibited by KCNQ1 K(+) channel inhibitor chromanol 293B. LGP-stimulated I(SC) was markedly reduced by pretreatment with cis-N-[2-phenylcyclopentyl]-azacyclotridec-1-en-2amine (MDL-12,330A) and N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide (H-89), but not with inhibitors of Ca(2+)-dependent signaling pathway. Treatment of tissue with LGP resulted in an increase in intracellular cAMP level and the activation in protein kinase A. The E-prostanoid(4) (EP)(4) receptor antagonist L-161,982 completely eliminated LGP-induced response.

CONCLUSIONS

The results showed that LGP enhances Cl(-) and fluid secretion via prostanoid receptor signaling and also cAMP and protein kinase A pathway, subsequently triggering the activation of apical Cl(-) channels mostly CFTR and basolateral cAMP-dependent K(+) channel.

Molecular bases of impaired water and ion movements in inflammatory bowel diseases

The intestine is dedicated to the absorption of water and nutrients. Fine tuning of this process is necessary to maintain an adequate balance and inflammation disrupts the equilibrium. This review summarizes the current evidence in this field. Classical mechanisms proposed include alteration of epithelial integrity, augmented secretion, and reduced absorption. In addition, intestinal inflammation is associated with defects in epithelial barrier function. However, our understanding of the phenomenon has been complicated by the fact that ionic secretion is in fact diminished in vivo, even after inflammation has subsided. Inhibited ionic secretion can be reversed partially or totally in vitro by maneuvers such as blockade of inducible nitric oxide synthase or removal of the submucosal layer. Disturbances in ionic absorption are less well characterized but clearly involve both electroneutral and electrogenic Na(+) absorption. Altered ionic transport is associated with changes in the expression and function of the transporters, including the Na(+)/K(+) ATPase, the sodium/potassium/chloride cotransporter 1 (NKCC1), the sodium/hydrogen exchanger 3 (NHE3), and the epithelial sodium channel (ENaC), as well as to the modulation of intracellular signaling. Further investigation is needed in this area in order to provide an integrated paradigm of ionic transport in the inflamed intestine. In particular, we do not know exactly how diarrhea ensues in inflammation and, consequently, we do not have specific pharmacological tools to combat this condition effectively and without side effects. Moreover, whether transport disturbances are reversible independently of inflammatory control is unknown.

Characterization of ryanodine receptors in rat colonic epithelium

AIM

Functional evidence suggests the presence of two types of intracellular Ca(2+) channels responsible for the release of Ca(2+) from Ca(2+)-stores, i.e. inositol-1,4,5-trisphosphate (IP(3)R) and ryanodine receptors (RyR), in rat colonic epithelium. Generally, three ryanodine receptor isoforms (RyR1-RyR3) are known; however, the type of RyR at this epithelium is unknown and was the focus of the present study.

METHODS

RyRs were characterized by molecular biological and immunohistochemical methods in the rat colon.

RESULTS

A transcript of RyR1 was found in mRNA from colonic crypts. In contrast, RyR2 and RyR3 were found in their corresponding reference tissues, but not in the cDNA from colonic crypts suggesting a predominant expression of the RyR1 isoform in this epithelium. In order to characterize the subcellular localization of RyR1, immunohistochemical experiments were performed. They showed that RyR1 is present in the lamina epithelialis mucosae and smooth muscle cells and is distributed equally along the whole crypt axis with no difference between surface and crypt cells. A double staining with IP(3)R3, the dominant cytoplasmic isoform of IP3Rs in this epithelium, revealed that there is only little colocalization of the two receptor subtypes within the epithelial cells. Furthermore, the epithelium is equipped with the enzyme CD38 responsible for the production of cyclic adenosine diphosphate ribose, the physiological agonist of RyR. RyRs are known to be activated by changes in the redox state. The oxidant, monochloramine evoked a ruthenium red-sensitive Ca(2+) release all over the crypt axis. This release was unaffected by prior stimulation of IP(3) receptors with ATP (and vice versa).

CONCLUSION

The present data suggest a functional separation of IP(3)- and ryanodine receptor-carrying Ca(2+) stores in the colonic epithelium.

Sites of action of hydrogen peroxide on ion transport across rat distal colon

BACKGROUND AND PURPOSE

The aim of this study was the identification of the mechanism of oxidant-induced intestinal secretion.

EXPERIMENTAL APPROACH

The action of H2O2 on ion transport across rat distal colon was evaluated in Ussing chambers. Changes in cytosolic Ca2+ concentration were measured using fura-2.

KEY RESULTS

H2O2 concentration-dependently induced an increase in short-circuit current (Isc), which was due to a stimulation of Cl(-) secretion. The effect of H2O2 was dependent on the presence of serosal Ca2+. It was inhibited after emptying of intracellular Ca2+ stores by cyclopiazonic acid or blockade of ryanodine receptors by ruthenium red, whereas a blocker of inositol 1,4,5-trisphosphate receptors was less effective. Fura 2-experiments confirmed an increase in the cytosolic Ca2+ concentration in the presence of H2O2. Measurements of Cl- currents across the apical membrane at basolaterally depolarized epithelia revealed the activation of a glibenclamide-sensitive, SITS-resistant Cl- conductance by the oxidant. The activation of this conductance was inhibited after blockade of protein kinases with staurosporine. When the apical membrane was permeabilized with nystatin, two sites of action of H2O2 were identified at the basolateral membrane. The oxidant stimulated a basolateral tetrapentylammonium-sensitive K+ conductance and increased the current generated by the Na+-K+ pump. Pretreatment of the tissues with H2O2 reduced the action of subsequently administered Ca2+-, cAMP- and cGMP-dependent secretagogues demonstrating a long-term downregulation after the initial secretory response evoked by the oxidant.

CONCLUSIONS AND IMPLICATIONS

H2O2 affects colonic anion secretion by action sites at both the apical, as well as the basolateral membrane.

Palmatine, a protoberberine alkaloid, inhibits both Ca(2+)- and cAMP-activated Cl(-) secretion in isolated rat distal colon

BACKGROUND AND PURPOSE

The protoberberine alkaloid berberine has been reported to inhibit colonic Cl(-) secretion. However, it is not known if other protoberberine alkaloids share these effects. We have therefore selected another protoberberine alkaloid, palmatine, to assess its effects on active ion transport across rat colonic epithelium.

EXPERIMENTAL APPROACH

Rat colonic mucosa was mounted in Ussing chambers and short circuit current (I (SC)), apical Cl(-) current and basolateral K(+) current were recorded. Intracellular cAMP content was determined by an enzyme immunoassay. Intracellular Ca(2+) concentration was measured with Fura-2 AM.

KEY RESULTS

Palmatine inhibited carbachol-induced Ca(2+)-activated Cl(-) secretion and the carbachol-induced increase of intracellular Ca(2+) concentration. Palmatine also inhibited cAMP-activated Cl(-) secretion induced by prostaglandin E(2) (PGE(2)) or forskolin. Palmatine prevented the elevation of intracellular cAMP by forskolin. Determination of apical Cl(-) currents showed that palmatine suppressed the forskolin-stimulated, apical cAMP-activated Cl(-) current but not the carbachol-stimulated apical Ca(2+)-activated Cl(-) current. Following permeabilization of apical membranes with nystatin, we found that palmatine inhibited a carbachol-stimulated basolateral K(+) current that was sensitive to charybdotoxin and resistant to chromanol 293B. However, the forskolin-stimulated basolateral K(+) current inhibited by palmatine was specifically blocked by chromanol 293B and not by charybdotoxin.

CONCLUSIONS AND IMPLICATIONS

Palmatine attenuated Ca(2+)-activated Cl(-) secretion through inhibiting basolateral charybdotoxin-sensitive, SK4 K(+) channels, whereas it inhibited cAMP-activated Cl(-) secretion by inhibiting apical CFTR Cl(-) channels and basolateral chromanol 293B-sensitive, KvLQT1 K(+) channels.

Rutaecarpine induces chloride secretion across rat isolated distal colon

The present study evaluated the effect of rutaecarpine (Rut) on Cl(-) secretion across rat distal colonic mucosa. Basolateral application of Rut elicited an increase in short-circuit current (I(SC)) response in a concentration-dependent manner. Evidence that Rut-stimulated I(SC) was due to Cl(-) secretion is based on 1) inhibition of current by bumetanide; 2) Cl(-) channel blockers diphenylamine-2-carboxylate, 5-nitro-2-(3-phenylpropylamino)-benzoic acid, and glibenclamide; and 3) removal of Cl(-) ions in bath solution. Determination of neurogenic blockers on Rut-induced I(SC) indicated that pretreatment of tissues with tetrodotoxin or indomethacin, but not atropine or hexamethonium, inhibited Rut-induced response. Treatment with Rut led to release and synthesis of prostaglandin E(2) in rat colonic mucosa. Rut-stimulated I(SC) was markedly reduced by pretreatment with MDL-12,330A [cis-N-[2-phenylcyclopentyl]-azacyclotridec-1-en-2-amine] and N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide (H-89), but not with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester, bisindolylmaleimide, and thapsigargin. Elimination of the extracellular Ca(2+) also did not alter Rut response. Rut treatment resulted in the increase in intracellular cAMP levels and the activation of protein kinase A. Depolarizing the basolateral membrane with high K(+) showed that Rut-stimulated apical Cl(-) current was largely prevented by cystic fibrosis transmembrane conductance regulator (CFTR) inhibitors. Permeabilizing apical membrane with nystatin revealed that Rut-stimulated basolateral K(+) current was specifically inhibited by Ba(2+) ions and chromanol 293B. The evidence derived from present study suggests that Rut-stimulated Cl(-) secretion is mediated by generation of endogenous prostaglandin E(2) and that it also involves the stimulation of cAMP and protein kinase A pathways, which subsequently lead to the activation of apical Cl(-) channels, mostly the CFTR and basolateral cAMP-dependent K(+) channels.

Modelling dysregulated Na+ absorption in airway epithelial cells with mucosal nystatin treatment

In cystic fibrosis (CF), the absence of functional CFTR leads to dysregulated Na(+) absorption across airway epithelia. We established an in vitro model of dysregulated Na(+) absorption by treating polarized normal human bronchial epithelial cells (HBEs) with nystatin (Nys), a polyene antibiotic that enables monovalent cations to permeate biological membranes. Acute mucosal Nys produced a rapid increase in short circuit current (I(sc)) that reflected increased transepithelial Na(+) absorption and required Na(+)/K(+)ATPase activity. The acute increase in I(sc) was associated with increased mucosal liquid absorption. Prolonged mucosal Nys treatment resulted in sustained Na(+) hyperabsorption, associated with increased mucosal liquid absorption in comparison with naïve (nontreated, kept under air-liquid interface conditions) or vehicle-treated cultures. Nys treatment was not toxic. Increased lactate accumulation in Nys-treated culture media suggested a higher metabolic rate associated with the higher energy demand for Na(+) transport. After chronic Nys treatment, the increased I(sc) was rapidly lost when the cultures were mounted in Ussing chambers, indicating that Nys could be rapidly removed from the apical membrane. Importantly, chronic Nys treatment promoted sustained mucosal liquid depletion and caused mucus dehydration, compaction, and adhesion to the apical surface of Nys-treated cultures. We conclude that mucosal Nys treatment of HBEs provides a simple in vitro model to recapitulate the Na(+) and volume hyperabsorptive features of CF airway epithelia.

Involvement of ryanodine-operated channels in tert-butylhydroperoxide-evoked Ca2+ mobilisation in pancreatic acinar cells

Reactive oxygen species and related oxidative damage have been implicated in the initiation of acute pancreatitis, a disease characterised in its earliest stages by disruption of intracellular Ca2+ homeostasis. The present study was carried out in order to establish the effect of the organic pro-oxidant, tert-butylhydroperoxide (tBHP), on the mobilisation of intracellular Ca2+ stores in isolated rat pancreatic acinar cells and the mechanisms underlying this effect. Cytosolic free Ca2+ concentrations ([Ca2+]c) were monitored using a digital microspectrofluorimetric system in fura-2 loaded cells. In the presence of normal extracellular Ca2+ concentrations ([Ca2+]o), perfusion of pancreatic acinar cells with 1 mmol l-1 tBHP caused a slow sustained increase in [Ca2+]c. This increase was also observed in a nominally Ca2+-free medium, indicating a release of Ca2+ from intracellular stores. Pretreatment of cells with tBHP abolished the typical Ca2+ response of both the physiological agonist CCK-8 (1 nmol l-1) and thapsigargin (TPS, 1 micromol l-1), an inhibitor of the SERCA pump, in the absence of extracellular Ca2+. Similar results were observed with carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP, 0.5 micromol l-1), a mitochondrial uncoupler. In addition, depletion of either agonist-sensitive Ca2+ pools by CCK-8 or TPS or mitochondrial Ca2+ pools by FCCP were unable to prevent the tBHP-induced Ca2+ release. By contrast, simultaneous administration of TPS and FCCP clearly abolished the tBHP-induced Ca2+ release. These results show that tBHP releases Ca2+ from agonist-sensitive intracellular stores and from mitochondria. On the other hand, simultaneous application of FCCP and of 2-aminoethoxydiphenylborane (2-APB), a blocker of IP3-mediated Ca2+ release, was unable to suppress the increase in [Ca2+]c induced by tBHP, while the application of 50 micromol l-1 of ryanodine (which is able to block the ryanodine channels) inhibits tBHP-evoked Ca2+ mobilisation. These findings indicate that tBHP releases Ca2+ from non-mitochondrial Ca2+ pools through ryanodine channels.