HCO3- transport in a mathematical model of the pancreatic ductal epithelium

A mathematical model of ENaC and Slc26a6 regulation by CFTR in salivary gland ducts

Cystic fibrosis (CF) is a genetic disease caused by the mutations of cystic fibrosis transmembrane conductance regulator (CFTR), the cystic fibrosis transmembrane conductance regulator gene. Cftr is a critical ion channel expressed in the apical membrane of mouse salivary gland striated duct cells. Although Cftr is primarily a Cl- channel, its knockout leads to higher salivary Cl- and Na+ concentrations and lower pH. Mouse experiments show that the activation of Cftr upregulates epithelial Na+ channel (ENaC) protein expression level and Slc26a6 (a 1Cl-:2[Formula: see text] exchanger of the solute carrier family) activity. Experimentally, it is difficult to predict how much the coregulation effects of CFTR contribute to the abnormal Na+, Cl-, and [Formula: see text] concentrations and pH in CF saliva. To address this question, we construct a wild-type mouse salivary gland model and simulate CFTR knockout by altering the expression levels of CFTR, ENaC, and Slc26a6. By reproducing the in vivo and ex vivo final saliva measurements from wild-type and CFTR knockout animals, we obtain computational evidence that ENaC and Slc26a6 activities are downregulated in CFTR knockout in salivary glands.NEW & NOTEWORTHY This paper describes a salivary gland mathematical model simulating the ion exchange between saliva and the salivary gland duct epithelium. The novelty lies in the implementation of CFTR regulating ENaC and Slc26a6 in a CFTR knockout gland. By reproducing the experimental saliva measurements in wild-type and CFTR knockout glands, the model shows that CFTR regulates ENaC and Slc26a6 anion exchanger in salivary glands. The method could be used to understand the various cystic fibrosis phenotypes.

Bicarbonate Transport in Cystic Fibrosis and Pancreatitis

CFTR, the cystic fibrosis (CF) gene-encoded epithelial anion channel, has a prominent role in driving chloride, bicarbonate and fluid secretion in the ductal cells of the exocrine pancreas. Whereas severe mutations in CFTR cause fibrosis of the pancreas in utero, CFTR mutants with residual function, or CFTR variants with a normal chloride but defective bicarbonate permeability (CFTR^BD), are associated with an enhanced risk of pancreatitis. Recent studies indicate that CFTR function is not only compromised in genetic but also in selected patients with an acquired form of pancreatitis induced by alcohol, bile salts or smoking. In this review, we summarize recent insights into the mechanism and regulation of CFTR-mediated and modulated bicarbonate secretion in the pancreatic duct, including the role of the osmotic stress/chloride sensor WNK1 and the scaffolding protein IRBIT, and current knowledge about the role of CFTR in genetic and acquired forms of pancreatitis. Furthermore, we discuss the perspectives for CFTR modulator therapy in the treatment of exocrine pancreatic insufficiency and pancreatitis and introduce pancreatic organoids as a promising model system to study CFTR function in the human pancreas, its role in the pathology of pancreatitis and its sensitivity to CFTR modulators on a personalized basis.

Bicarbonate-rich fluid secretion predicted by a computational model of guinea-pig pancreatic duct epithelium

KEY POINTS

The ductal system of the pancreas secretes large volumes of alkaline fluid containing HCO₃^- concentrations as high as 140 mm during hormonal stimulation. A computational model has been constructed to explore the underlying ion transport mechanisms. Parameters were estimated by fitting the model to experimental data from guinea-pig pancreatic ducts. The model was readily able to secrete 140 mm HCO₃^- . Its capacity to do so was not dependent upon special properties of the cystic fibrosis transmembrane conductance regulator (CFTR) anion channels and solute carrier family 26 member A6 (SLC26A6) anion exchangers. We conclude that the main requirement for secreting high HCO₃^- concentrations is to minimize the secretion of Cl^- ions. These findings help to clarify the mechanism responsible for pancreatic HCO₃^- secretion, a vital process that prevents the formation of protein plugs and viscous mucus in the ducts, which could otherwise lead to pancreatic disease.

ABSTRACT

A computational model of guinea-pig pancreatic duct epithelium was developed to determine the transport mechanism by which HCO₃^- ions are secreted at concentrations in excess of 140 mm. Parameters defining the contributions of the individual ion channels and transporters were estimated by least-squares fitting of the model predictions to experimental data obtained from isolated ducts and intact pancreas under a range of experimental conditions. The effects of cAMP-stimulated secretion were well replicated by increasing the activities of the basolateral Na⁺ -HCO₃^- cotransporter (NBC1) and apical Cl^- /HCO₃^- exchanger (solute carrier family 26 member A6; SLC26A6), increasing the basolateral K⁺ permeability and apical Cl^- and HCO₃^- permeabilities (CFTR), and reducing the activity of the basolateral Cl^- /HCO₃^- exchanger (anion exchanger 2; AE2). Under these conditions, the model secreted ∼140 mm HCO₃^- at a rate of ∼3 nl min^-1  mm^-2 , which is consistent with experimental observations. Alternative 1:2 and 1:1 stoichiometries for Cl^- /HCO₃^- exchange via SLC26A6 at the apical membrane were able to support a HCO₃^- -rich secretion. Raising the HCO₃^- /Cl^- permeability ratio of CFTR from 0.4 to 1.0 had little impact upon either the secreted HCO₃^- concentration or the volume flow. However, modelling showed that a reduction in basolateral AE2 activity by ∼80% was essential in minimizing the intracellular Cl^- concentration following cAMP stimulation and thereby maximizing the secreted HCO₃^- concentration. The addition of a basolateral Na⁺ -K⁺ -2Cl^- cotransporter (NKCC1), assumed to be present in rat and mouse ducts, raised intracellular Cl^- and resulted in a lower secreted HCO₃^- concentration, as is characteristic of those species. We conclude therefore that minimizing the driving force for Cl^- secretion is the main requirement for secreting 140 mm HCO₃^- .

Role of CFTR in epithelial physiology

Salt and fluid absorption and secretion are two processes that are fundamental to epithelial function and whole body fluid homeostasis, and as such are tightly regulated in epithelial tissues. The CFTR anion channel plays a major role in regulating both secretion and absorption in a diverse range of epithelial tissues, including the airways, the GI and reproductive tracts, sweat and salivary glands. It is not surprising then that defects in CFTR function are linked to disease, including life-threatening secretory diarrhoeas, such as cholera, as well as the inherited disease, cystic fibrosis (CF), one of the most common life-limiting genetic diseases in Caucasian populations. More recently, CFTR dysfunction has also been implicated in the pathogenesis of acute pancreatitis, chronic obstructive pulmonary disease (COPD), and the hyper-responsiveness in asthma, underscoring its fundamental role in whole body health and disease. CFTR regulates many mechanisms in epithelial physiology, such as maintaining epithelial surface hydration and regulating luminal pH. Indeed, recent studies have identified luminal pH as an important arbiter of epithelial barrier function and innate defence, particularly in the airways and GI tract. In this chapter, we will illustrate the different operational roles of CFTR in epithelial function by describing its characteristics in three different tissues: the airways, the pancreas, and the sweat gland.

A model of ionic transport and osmotic volume control in cochlear outer hair cells

A computational model of the outer hair cell (OHC) of the mammalian cochlea is presented. It addresses the way in which movement of ions controls the cell shape and regulates pH. The model takes into account the possible chloride-bicarbonate exchange function of prestin, a protein highly expressed in the plasma membrane of OHCs. A model of intracellular pH transients following the imposition of a rapid acid load upon the cell has been adapted from the squid axon literature and further extended in order to investigate the effects of ion transport upon the osmotic flux of water into the cell. The model predicts the slow length changes of OHCs reported in the literature a feature which may control the contribution of OHCs to cochlear amplification.

Multiscale modelling of saliva secretion

We review a multiscale model of saliva secretion, describing in brief how the model is constructed and what we have so far learned from it. The model begins at the level of inositol trisphosphate receptors (IPR), and proceeds through the cellular level (with a model of acinar cell calcium dynamics) to the multicellular level (with a model of the acinus), finally to a model of a saliva production unit that includes an acinus and associated duct. The model at the level of the entire salivary gland is not yet completed. Particular results from the model so far include (i) the importance of modal behaviour of IPR, (ii) the relative unimportance of Ca(2+) oscillation frequency as a controller of saliva secretion, (iii) the need for the periodic Ca(2+) waves to be as fast as possible in order to maximise water transport, (iv) the presence of functional K(+) channels in the apical membrane increases saliva secretion, (v) the relative unimportance of acinar spatial structure for isotonic water transport, (vi) the prediction that duct cells are highly depolarised, (vii) the prediction that the secondary saliva takes at least 1mm (from the acinus) to reach ionic equilibrium. We end with a brief discussion of future directions for the model, both in construction and in the study of scientific questions.

Dynamic modulation of ANO1/TMEM16A HCO3(-) permeability by Ca2+/calmodulin

Anoctamin 1 (ANO1)/transmembrane protein 16A (TMEM16A) is a calcium-activated anion channel that may play a role in HCO(3)(-) secretion in epithelial cells. Here, we report that the anion selectivity of ANO1 is dynamically regulated by the Ca(2+)/calmodulin complex. Whole-cell current measurements in HEK 293T cells indicated that ANO1 becomes highly permeable to HCO(3)(-) at high [Ca(2+)](i). Interestingly, this result was not observed in excised patches, indicating the involvement of cytosolic factors in this process. Further studies revealed that the direct association between ANO1 and calmodulin at high [Ca(2+)](i) is responsible for changes in anion permeability. Calmodulin physically interacted with ANO1 in a [Ca(2+)](i)-dependent manner, and addition of recombinant calmodulin to the cytosolic side of excised patches reversibly increased P(HCO3)/P(Cl). In addition, the high [Ca(2+)](i)-induced increase in HCO(3)(-) permeability was reproduced in mouse submandibular gland acinar cells, in which ANO1 plays a critical role in fluid secretion. These results indicate that the HCO(3)(-) permeability of ANO1 can be dynamically modulated and that ANO1 may play an important role in cellular HCO(3)(-) transport, especially in transepithelial HCO(3)(-) secretion.

A quantitative analysis of electrolyte exchange in the salivary duct

A healthy salivary gland secretes saliva in two stages. First, acinar cells generate primary saliva, a plasma-like, isotonic fluid high in Na(+) and Cl(-). In the second stage, the ducts exchange Na(+) and Cl(-) for K(+) and HCO(3)(-), producing a hypotonic final saliva with no apparent loss in volume. We have developed a tool that aims to understand how the ducts achieve this electrolyte exchange while maintaining the same volume. This tool is part of a larger multiscale model of the salivary gland and can be used at the duct or gland level to investigate the effects of genetic and chemical alterations. In this study, we construct a radially symmetric mathematical model of the mouse salivary gland duct, representing the lumen, the cell, and the interstitium. For a given flow and primary saliva composition, we predict the potential differences and the luminal and cytosolic concentrations along a duct. Our model accounts well for experimental data obtained in wild-type animals as well as knockouts and chemical inhibitors. Additionally, the luminal membrane potential of the duct cells is predicted to be very depolarized compared with acinar cells. We investigate the effects of an electrogenic vs. electroneutral anion exchanger in the luminal membrane on concentration and the potential difference across the luminal membrane as well as how impairing the cystic fibrosis transmembrane conductance regulator channel affects other ion transporting mechanisms. Our model suggests the electrogenicity of the anion exchanger has little effect in the submandibular duct.

Transepithelial bicarbonate secretion: lessons from the pancreas

Many cystic fibrosis transmembrane conductance regulator (CFTR)-expressing epithelia secrete bicarbonate (HCO(3)(-))-containing fluids. Recent evidence suggests that defects in epithelial bicarbonate secretion are directly involved in the pathogenesis of cystic fibrosis, in particular by building up hyperviscous mucus in the ductal structures of the lung and pancreas. Pancreatic juice is one of the representative fluids that contain a very high concentration of bicarbonate among bodily fluids that are secreted from CFTR-expressing epithelia. We introduce up-to-date knowledge on the basic principles of transepithelial bicarbonate transport by showing the mechanisms involved in pancreatic bicarbonate secretion. The model of pancreatic bicarbonate secretion described herein may also apply to other exocrine epithelia. As a central regulator of bicarbonate transport at the apical membrane, CFTR plays an essential role in both direct and indirect bicarbonate secretion. The major role of CFTR in bicarbonate secretion would be variable depending on the tissue and cell type. For example, in epithelial cells that produce a low concentration of bicarbonate-containing fluid (up to 80 mm), either CFTR-dependent Cl(-)/HCO(3)(-) exchange or CFTR anion channel with low bicarbonate permeability would be sufficient to generate such fluid. However, in cells that secrete high-bicarbonate-containing fluids, a highly selective CFTR bicarbonate channel activity is required. Therefore, understanding the molecular mechanism of transepithelial bicarbonate transport and the role of CFTR in each specific epithelium will provide therapeutic strategies to recover from epithelial defects induced by hyposecretion of bicarbonate in cystic fibrosis.

Deletion of Slc26a6 alters the stoichiometry of apical Cl-/HCO-3 exchange in mouse pancreatic duct

To define the stoichiometry and molecular identity of the Cl(-)/HCO(3)(-) exchanger in the apical membrane of pancreatic duct cells, changes in luminal pH and volume were measured simultaneously in interlobular pancreatic ducts isolated from wild-type and Slc26a6-null mice. Transepithelial fluxes of HCO(3)(-) and Cl(-) were measured in the presence of anion gradients favoring rapid exchange of intracellular HCO(3)(-) with luminal Cl(-) in cAMP-stimulated ducts. The flux ratio of Cl(-) absorption/HCO(3)(-) secretion was ∼0.7 in wild-type ducts and ∼1.4 in Slc26a6(-/-) ducts where a different Cl(-)/HCO(3)(-) exchanger, most likely SLC26A3, was found to be active. Interactions between Cl(-)/HCO(3)(-) exchange and cystic fibrosis transmembrane conductance regulator (CFTR) in cAMP-stimulated ducts were examined by measuring the recovery of intracellular pH after alkali-loading by acetate prepulse. Hyperpolarization induced by luminal application of CFTRinh-172 enhanced HCO(3)(-) efflux across the apical membrane via SLC26A6 in wild-type ducts but significantly reduced HCO(3)(-) efflux in Slc26a6(-/-) ducts. In microperfused wild-type ducts, removal of luminal Cl(-), or luminal application of dihydro-4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid to inhibit SLC26A6, caused membrane hyperpolarization, which was abolished in Slc26a6(-/-) ducts. In conclusion, we have demonstrated that deletion of Slc26a6 alters the apparent stoichiometry of apical Cl(-)/HCO(3)(-) exchange in native pancreatic duct. Our results are consistent with SLC26A6 mediating 1:2 Cl(-)/HCO(3)(-) exchange, and the exchanger upregulated in its absence, most probably SLC26A3, mediating 2:1 exchange.

Molecular mechanism of pancreatic and salivary gland fluid and HCO3 secretion

Fluid and HCO(3)(-) secretion is a vital function of all epithelia and is required for the survival of the tissue. Aberrant fluid and HCO(3)(-) secretion is associated with many epithelial diseases, such as cystic fibrosis, pancreatitis, Sjögren's syndrome, and other epithelial inflammatory and autoimmune diseases. Significant progress has been made over the last 20 years in our understanding of epithelial fluid and HCO(3)(-) secretion, in particular by secretory glands. Fluid and HCO(3)(-) secretion by secretory glands is a two-step process. Acinar cells secrete isotonic fluid in which the major salt is NaCl. Subsequently, the duct modifies the volume and electrolyte composition of the fluid to absorb the Cl(-) and secrete HCO(3)(-). The relative volume secreted by acinar and duct cells and modification of electrolyte composition of the secreted fluids varies among secretory glands to meet their physiological functions. In the pancreas, acinar cells secrete a small amount of NaCl-rich fluid, while the duct absorbs the Cl(-) and secretes HCO(3)(-) and the bulk of the fluid in the pancreatic juice. Fluid secretion appears to be driven by active HCO(3)(-) secretion. In the salivary glands, acinar cells secrete the bulk of the fluid in the saliva that is driven by active Cl(-) secretion and contains high concentrations of Na(+) and Cl(-). The salivary glands duct absorbs both the Na(+) and Cl(-) and secretes K(+) and HCO(3)(-). In this review, we focus on the molecular mechanism of fluid and HCO(3)(-) secretion by the pancreas and salivary glands, to highlight the similarities of the fundamental mechanisms of acinar and duct cell functions, and to point out the differences to meet gland-specific secretions.

Novel role for pendrin in orchestrating bicarbonate secretion in cystic fibrosis transmembrane conductance regulator (CFTR)-expressing airway serous cells

In most HCO(3)(-)-secreting epithelial tissues, SLC26 Cl(-)/HCO(3)(-) transporters work in concert with the cystic fibrosis transmembrane conductance regulator (CFTR) to regulate the magnitude and composition of the secreted fluid, a process that is vital for normal tissue function. By contrast, CFTR is regarded as the only exit pathway for HCO(3)(-) in the airways. Here we show that Cl(-)/HCO(3)(-) anion exchange makes a major contribution to transcellular HCO(3)(-) transport in airway serous cells. Real-time measurement of intracellular pH from polarized cultures of human Calu-3 cells demonstrated cAMP/PKA-activated Cl(-)-dependent HCO(3)(-) transport across the luminal membrane via CFTR-dependent coupled Cl(-)/HCO(3)(-) anion exchange. The pharmacological and functional profile of the luminal anion exchanger was consistent with SLC26A4 (pendrin), which was shown to be expressed by quantitative RT-PCR, Western blot, and immunofluorescence. Pendrin-mediated anion exchange activity was confirmed by shRNA pendrin knockdown (KD), which markedly reduced cAMP-activated Cl(-)/HCO(3)(-) exchange. To establish the relative roles of CFTR and pendrin in net HCO(3)(-) secretion, transepithelial liquid secretion rate and liquid pH were measured in wild type, pendrin KD, and CFTR KD cells. cAMP/PKA increased the rate and pH of the secreted fluid. Inhibiting CFTR reduced the rate of liquid secretion but not the pH, whereas decreasing pendrin activity lowered pH with little effect on volume. These results establish that CFTR predominately controls the rate of liquid secretion, whereas pendrin regulates the composition of the secreted fluid and identifies a critical role for this anion exchanger in transcellular HCO(3)(-) secretion in airway serous cells.

Functional coupling of apical Cl-/HCO3- exchange with CFTR in stimulated HCO3- secretion by guinea pig interlobular pancreatic duct

Pancreatic ductal epithelium produces a HCO(3)(-)-rich fluid. HCO(3)(-) transport across ductal apical membranes has been proposed to be mediated by both SLC26-mediated Cl(-)/HCO(3)(-) exchange and CFTR-mediated HCO(3)(-) conductance, with proportional contributions determined in part by axial changes in gene expression and luminal anion composition. In this study we investigated the characteristics of apical Cl(-)/HCO(3)(-) exchange and its functional interaction with Cftr activity in isolated interlobular ducts of guinea pig pancreas. BCECF-loaded epithelial cells of luminally microperfused ducts were alkalinized by acetate prepulse or by luminal Cl(-) removal in the presence of HCO(3)(-)-CO(2). Intracellular pH recovery upon luminal Cl(-) restoration (nominal Cl(-)/HCO(3)(-) exchange) in cAMP-stimulated ducts was largely inhibited by luminal dihydro-DIDS (H(2)DIDS), accelerated by luminal CFTR inhibitor inh-172 (CFTRinh-172), and was insensitive to elevated bath K(+) concentration. Luminal introduction of CFTRinh-172 into sealed duct lumens containing BCECF-dextran in HCO(3)(-)-free, Cl(-)-rich solution enhanced cAMP-stimulated HCO(3)(-) secretion, as calculated from changes in luminal pH and volume. Luminal Cl(-) removal produced, after a transient small depolarization, sustained cell hyperpolarization of approximately 15 mV consistent with electrogenic Cl(-)/HCO(3)(-) exchange. The hyperpolarization was inhibited by H(2)DIDS and potentiated by CFTRinh-172. Interlobular ducts expressed mRNAs encoding CFTR, Slc26a6, and Slc26a3, as detected by RT-PCR. Thus Cl(-)-dependent apical HCO(3)(-) secretion in pancreatic duct is mediated predominantly by an Slc26a6-like Cl(-)/HCO(3)(-) exchanger and is accelerated by inhibition of CFTR. This study demonstrates functional coupling between Cftr and Slc26a6-like Cl(-)/HCO(3)(-) exchange activity in apical membrane of guinea pig pancreatic interlobular duct.

CFTR functions as a bicarbonate channel in pancreatic duct cells

Pancreatic duct epithelium secretes a HCO(3)(-)-rich fluid by a mechanism dependent on cystic fibrosis transmembrane conductance regulator (CFTR) in the apical membrane. However, the exact role of CFTR remains unclear. One possibility is that the HCO(3)(-) permeability of CFTR provides a pathway for apical HCO(3)(-) efflux during maximal secretion. We have therefore attempted to measure electrodiffusive fluxes of HCO(3)(-) induced by changes in membrane potential across the apical membrane of interlobular ducts isolated from the guinea pig pancreas. This was done by recording the changes in intracellular pH (pH(i)) that occurred in luminally perfused ducts when membrane potential was altered by manipulation of bath K(+) concentration. Apical HCO(3)(-) fluxes activated by cyclic AMP were independent of Cl(-) and luminal Na(+), and substantially inhibited by the CFTR blocker, CFTR(inh)-172. Furthermore, comparable HCO(3)(-) fluxes observed in ducts isolated from wild-type mice were absent in ducts from cystic fibrosis (Delta F) mice. To estimate the HCO(3)(-) permeability of the apical membrane under physiological conditions, guinea pig ducts were luminally perfused with a solution containing 125 mM HCO(3)(-) and 24 mM Cl(-) in the presence of 5% CO(2). From the changes in pH(i), membrane potential, and buffering capacity, the flux and electrochemical gradient of HCO(3)(-) across the apical membrane were determined and used to calculate the HCO(3)(-) permeability. Our estimate of approximately 0.1 microm sec(-1) for the apical HCO(3)(-) permeability of guinea pig duct cells under these conditions is close to the value required to account for observed rates of HCO(3)(-) secretion. This suggests that CFTR functions as a HCO(3)(-) channel in pancreatic duct cells, and that it provides a significant pathway for HCO(3)(-) transport across the apical membrane.

Effect of Slc26a6 deletion on apical Cl-/HCO3- exchanger activity and cAMP-stimulated bicarbonate secretion in pancreatic duct

The role of Slc26a6 (PAT1) on apical Cl-/HCO3- exchange and bicarbonate secretion in pancreatic duct cells was investigated using Slc26a6 null and wild-type (WT) mice. Apical Cl-/HCO3- exchange activity was measured with the pH-sensitive dye BCECF in microperfused interlobular ducts. The HCO3(-)-influx mode of apical [Cl-]i/[HCO3-]o exchange (where brackets denote concentration and subscripts i and o denote intra- and extracellular, respectively) was dramatically upregulated in Slc26a6 null mice (P < 0.01 vs. WT), whereas the HCO3(-)-efflux mode of apical [Cl-]o/[HCO3-]i exchange was decreased in Slc26a6 null mice (P < 0.05 vs. WT), suggesting the unidirectionality of the Slc26a6-mediated HCO3- transport. Fluid secretory rate in interlobular ducts were comparable in WT and Slc26a6 null mice (P > 0.05). In addition, when pancreatic juice was collected from whole animal in basal and secretin-stimulated conditions, neither juice volume nor its pH showed differences between WT and Slc26a6 null mice. Semiquantitative RT-PCR demonstrated more than fivefold upregulation in Slc26a3 (DRA) expression in Slc26a6 knockout pancreas. In conclusion, these results point to the role of Slc26a6 in HCO3- efflux at the apical membrane and also suggest the presence of a robust Slc26a3 compensatory upregulation, which can replace the function of Slc26a6 in pancreatic ducts.

Slc26a6 regulates CFTR activity in vivo to determine pancreatic duct HCO3- secretion: relevance to cystic fibrosis

Fluid and HCO(3)(-) secretion are vital functions of the pancreatic duct and other secretory epithelia. CFTR and Cl(-)/HCO(3)(-) exchange activity at the luminal membrane are required for these functions. The molecular identity of the Cl(-)/HCO(3)(-) exchangers and their relationship with CFTR in determining fluid and HCO(3)(-) secretion are not known. We show here that the Cl(-)/HCO(3)(-) exchanger slc26a6 controls CFTR activity and ductal fluid and HCO(3)(-) secretion. Unexpectedly, deletion of slc26a6 in mice and measurement of fluid and HCO(3)(-) secretion into sealed intralobular pancreatic ducts revealed that deletion of slc26a6 enhanced spontaneous and decreased stimulated secretion. Remarkably, inhibition of CFTR activity with CFTR(inh)-172, knock-down of CFTR by siRNA and measurement of CFTR current in WT and slc26a6(-/-) duct cells revealed that deletion of slc26a6 resulted in dis-regulation of CFTR activity by removal of tonic inhibition of CFTR by slc26a6. These findings reveal the intricate regulation of CFTR activity by slc26a6 in both the resting and stimulated states and the essential role of slc26a6 in pancreatic HCO(3)(-) secretion in vivo.

Pattern of Ca2+ increase determines the type of secretory mechanism activated in dog pancreatic duct epithelial cells

Intracellular calcium concentration ([Ca(2+)](i)) is a key factor controlling secretion from various cell types. We investigated how different patterns of [Ca(2+)](i) signals evoke salt secretion via ion transport mechanisms and mucin secretion via exocytosis in dog pancreatic duct epithelial cells (PDEC). Activation of epithelial P2Y(2) receptors by UTP generated two patterns of [Ca(2+)](i) change: 2-10 microm UTP induced [Ca(2+)](i) oscillations, whereas 100 microm UTP induced a sustained [Ca(2+)](i) increase, both in the micromolar range. As monitored by carbon-fibre amperometry, the sustained [Ca(2+)](i) increase stimulated a larger increase in exocytosis than [Ca(2+)](i) oscillations, despite their similar amplitude. In contrast, patch-clamp recordings revealed that [Ca(2+)](i) oscillations synchronously activated a K(+) current as efficiently as the sustained [Ca(2+)](i) increase. This K(+) current was mediated by intermediate-conductance Ca(2+)-activated K(+) channels (32 pS at -100 mV) which were sensitive to charybdotoxin and resistant to TEA. Activation of these Ca(2+)-dependent K(+) channels hyperpolarized the plasma membrane from a resting potential of -40 mV to -90 mV, as monitored in perforated whole-cell configuration, in turn enhancing Na(+)-independent, Cl(-)-dependent and DIDS-sensitive HCO(3)(-) secretion, as monitored through changes in intracellular pH. PDEC therefore encode concentrations of purinergic agonists as different patterns of [Ca(2+)](i) changes, which differentially stimulate K(+) channels, the Cl(-)-HCO(3)(-) exchanger, and exocytosis. Thus, in addition to amplitude, the temporal pattern of [Ca(2+)](i) increases is an important mechanism for transducing extracellular stimuli into different physiological effects.

Protein kinase C mediates the inhibitory effect of substance P on HCO3- secretion from guinea pig pancreatic ducts

The inhibitory control of pancreatic ductal HCO(3)(-) secretion may be physiologically important in terms of limiting the hydrostatic pressure developed within the ducts and in terms of switching off pancreatic secretion after a meal. Substance P (SP) inhibits secretin-stimulated HCO(3)(-) secretion by modulating a Cl(-)-dependent HCO(3)(-) efflux step at the apical membrane of the duct cell (Hegyi P, Gray MA, and Argent BE. Am J Physiol Cell Physiol 285: C268-C276, 2003). In the present study, we have shown that SP is present in periductal nerves within the guinea pig pancreas, that PKC mediates the effect of SP, and that SP inhibits an anion exchanger on the luminal membrane of the duct cell. Secretin (10 nM) stimulated HCO(3)(-) secretion by sealed, nonperfused, ducts about threefold, and this effect was totally inhibited by SP (20 nM). Phorbol 12,13-dibutyrate (PDBu; 100 nM), an activator of PKC, reduced basal HCO(3)(-) secretion by approximately 40% and totally blocked secretin-stimulated secretion. In addition, bisindolylmaleimide I (1 nM to 1 microM), an inhibitor of PKC, relieved the inhibitory effect of SP on secretin-stimulated HCO(3)(-) secretion and also reversed the inhibitory effect of PDBu. Western blot analysis revealed that guinea pig pancreatic ducts express the alpha-, beta(I)-, delta-, epsilon-, eta-, theta-, zeta-, and mu-isoforms of PKC. In microperfused ducts, luminal H(2)DIDS (0.5 mM) caused intracellular pH to alkalinize and, like SP, inhibited basal and secretin-stimulated HCO(3)(-) secretion. SP did not inhibit secretion further when H(2)DIDS was present in the lumen, suggesting that SP and H(2)DIDS both inhibit the activity of an anion exchanger on the luminal membrane of the duct cell.

Gating of CFTR by the STAS domain of SLC26 transporters

Chloride absorption and bicarbonate secretion are vital functions of epithelia, as highlighted by cystic fibrosis and diseases associated with mutations in members of the SLC26 chloride-bicarbonate exchangers. Many SLC26 transporters (SLC26T) are expressed in the luminal membrane together with CFTR, which activates electrogenic chloride-bicarbonate exchange by SLC26T. However, the ability of SLC26T to regulate CFTR and the molecular mechanism of their interaction are not known. We report here a reciprocal regulatory interaction between the SLC26T DRA, SLC26A6 and CFTR. DRA markedly activates CFTR by increasing its overall open probablity (NP(o)) sixfold. Activation of CFTR by DRA was facilitated by their PDZ ligands and binding of the SLC26T STAS domain to the CFTR R domain. Binding of the STAS and R domains is regulated by PKA-mediated phosphorylation of the R domain. Notably, CFTR and SLC26T co-localize in the luminal membrane and recombinant STAS domain activates CFTR in native duct cells. These findings provide a new understanding of epithelial chloride and bicarbonate transport and may have important implications for both cystic fibrosis and diseases associated with SLC26T.

Basolateral anion transport mechanisms underlying fluid secretion by mouse, rat and guinea-pig pancreatic ducts

Fluid secretion by interlobular pancreatic ducts was determined by using video microscopy to measure the rate of swelling of isolated duct segments that had sealed following overnight culture. The aim was to compare the HCO(3)(-) requirement for secretin-evoked secretion in mouse, rat and guinea-pig pancreas. In mouse and rat ducts, fluid secretion could be evoked by 10 nm secretin and 5 microm forskolin in the absence of extracellular HCO(3)(-). In guinea-pig ducts, however, fluid secretion was totally dependent on HCO(3)(-). Forskolin-stimulated fluid secretion by mouse and rat ducts in the absence of HCO(3)(-) was dependent on extracellular Cl(-) and was completely inhibited by bumetanide (30 microm). It was therefore probably mediated by a basolateral Na(+)-K(+)-2Cl(-) cotransporter. In the presence of HCO(3)(-), forskolin-stimulated fluid secretion was reduced approximately 40% by bumetanide, approximately 50% by inhibitors of basolateral HCO(3)(-) uptake (3 microm EIPA and 500 microm H(2)DIDS), and was totally abolished by simultaneous application of all three inhibitors. We conclude that the driving force for secretin-evoked fluid secretion by mouse and rat ducts is provided by parallel basolateral mechanisms: Na(+)-H(+) exchange and Na(+)-HCO(3)(-) cotransport mediating HCO(3)(-) uptake, and Na(+)-K(+)-2Cl(-) cotransport mediating Cl(-) uptake. The absence or inactivity of the Cl(-) uptake pathway in the guinea-pig pancreatic ducts may help to account for the much higher concentrations of HCO(3)(-) secreted in this species.

Effect of tetramethylpyrazine on exocrine pancreatic and bile secretion

AIM: To investigate the effect of tetramethylpyrazine (ligustrazine, TMP) on the secretion of exocrine pancreas (and biliary).

METHODS: In in vivo study, we investigated the effect of TMP on the secretion of pancreatic-bile juice (PBJ) in rats. Using human pancreatic duct cell line, CAPAN-1, combined with the short-circuit current (I_SC) technique we further studied the effect of TMP on the pancreatic anion secretion.

RESULTS: Administration of TMP (80 mg/kg, ip) significantly increased the secretion of PBJ (P < 0.05), but the pH of PBJ and the secretion of pancreatic protein were not significantly affected. Basolateral addition of TMP produced a dose-dependent increase in I_SC (EC₅₀ = 1.56 mmol/L), which contained a fast transient I_SC response followed by a slow decay. Apical application of Cl^- channel blockers, DPC (1 mmol/L), decreased the response by about 67.1% (P < 0.001), whereas amiloride (100 μmol/L), a epithelial sodium channel blockers, had no effect. Removal of extracellular HCO₃^- abolished TMP-induced increase in I_SC by about 74.4% (P < 0.001), but the removal of external Cl^- did not. Pretreatment with phosphodiesterase inhibitor, IBMX (0.5 mmol/L), decreased the TMP-induced I_SC by 91% (P < 0.001).

CONCLUSION: TMP could stimulate the secretion of PBJ, especially pancreatic ductal HCO₃^- secretion via cAMP or cGMP-dependent pathway. It need further study to investigate the roles of cAMP or cGMP in the effect of TMP on the secretion of exocrine pancreas.

Substance P inhibits bicarbonate secretion from guinea pig pancreatic ducts by modulating an anion exchanger

The stimulatory pathways controlling HCO3- secretion by the pancreatic ductal epithelium are well described. However, only a few data are available concerning inhibitory mechanisms, which may play an important role in the physiological control of the pancreas. The aim of this study was to investigate the cellular mechanism by which substance P (SP) inhibits pancreatic ductal HCO3- secretion. Small intra/interlobular ducts were isolated from the pancreas of guinea pigs. During overnight culture the ducts seal to form a closed sac. Transmembrane HCO3- fluxes were calculated from changes in intracellular pH (measured using the pH-sensitive dye BCECF) and the buffering capacity of the cells. We found that secretin can stimulate HCO3- secretion in guinea pig pancreatic ducts about fivefold and that this effect could be totally blocked by SP. The inhibitory effect of SP was relieved by spantide, an SP receptor antagonist. SP had no effect on the activity of basolateral Na+-HCO3- cotransporters and Na+/H+ exchangers. However, the peptide did inhibit a Cl--dependent HCO3- efflux (secretory) mechanism, most probably the Cl-/HCO3 exchanger on the apical membrane of the duct cell.

Membrane potential and bicarbonate secretion in isolated interlobular ducts from guinea-pig pancreas

The interlobular duct cells of the guinea-pig pancreas secrete HCO(3)(-) across their luminal membrane into a HCO(3)(-)-rich (125 mM) luminal fluid against a sixfold concentration gradient. Since HCO(3)(-) transport cannot be achieved by luminal Cl-/HCO(3)(-) exchange under these conditions, we have investigated the possibility that it is mediated by an anion conductance. To determine whether the electrochemical potential gradient across the luminal membrane would favor HCO(3)(-) efflux, we have measured the intracellular potential (V(m)) in microperfused, interlobular duct segments under various physiological conditions. When the lumen was perfused with a 124 mM Cl- -25 mM HCO(3)(-) solution, a condition similar to the basal state, the resting potential was approximately -60 mV. Stimulation with dbcAMP or secretin caused a transient hyperpolarization (approximately 5 mV) due to activation of electrogenic Na+-HCO(3)(-) cotransport at the basolateral membrane. This was followed by depolarization to a steady-state value of approximately -50 mV as a result of anion efflux across the luminal membrane. Raising the luminal HCO(3)(-) concentration to 125 mM caused a hyperpolarization (approximately 10 mV) in both stimulated and unstimulated ducts. These results can be explained by a model in which the depolarizing effect of Cl- efflux across the luminal membrane is minimized by the depletion of intracellular Cl- and offset by the hyperpolarizing effects of Na+-HCO(3)(-) cotransport at the basolateral membrane. The net effect is a luminally directed electrochemical potential gradient for HCO(3)(-) that is sustained during maximal stimulation. Our calculations indicate that the electrodiffusive efflux of HCO(3)(-) to the lumen via CFTR, driven by this gradient, would be sufficient to fully account for the observed secretory flux of HCO(3)(-).

Chloride transport in microperfused interlobular ducts isolated from guinea-pig pancreas

Isolated interlobular ducts from the guinea-pig pancreas secrete a HCO3--rich fluid in response to secretin. To determine the role of Cl- transporters in this process, intracellular Cl- concentration ([Cl-]i) was measured in ducts loaded with the Cl--sensitive fluoroprobe, 6-methoxy-N-ethylquinolinium chloride (MEQ). [Cl-]i decreased when the luminal Cl- concentration was reduced. This effect was stimulated by forskolin, was not dependent on HCO3- and was not inhibited by application of the anion channel/transporter inhibitor H2DIDS to the luminal membrane. It is therefore attributed to a cAMP-stimulated Cl- conductance, probably the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel. [Cl-]i also decreased when the basolateral Cl- concentration was reduced. This effect was not stimulated by forskolin, was largely dependent on HCO3- and was inhibited by basolateral H2DIDS. It is therefore mediated mainly by Cl-/HCO3- exchange. With high Cl- and low HCO3- concentrations in the lumen, steady-state [Cl-]i was 25-35 mM in unstimulated cells. Stimulation with forskolin caused [Cl-]i to increase by approximately 4 mM due to activation of the luminal anion exchanger. With low Cl- and high HCO3- concentrations in the lumen to simulate physiological conditions, steady-state [Cl-]i was 10-15 mM in unstimulated cells. Upon stimulation with forskolin, [Cl-]i fell to approximately 7 mM due to increased Cl- efflux via the luminal conductance. We conclude that, during stimulation under physiological conditions, [Cl-]i decreases to very low levels in guinea-pig pancreatic duct cells, largely as a result of the limited capacity of the basolateral transporters for Cl- uptake. The resulting lack of competition from intracellular Cl- may therefore favour HCO3- secretion via anion conductances in the luminal membrane, possibly CFTR.