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.

A Mathematical Model of Salivary Gland Duct Cells

Saliva is produced in two stages in the salivary glands: the secretion of primary saliva by the acinus and the modification of saliva composition to final saliva by the intercalated and striated ducts. In order to understand the saliva modification process, we develop a mathematical model for the salivary gland duct. The model utilises the realistic 3D structure of the duct reconstructed from an image stack of gland tissue. Immunostaining results show that TMEM16A and aquaporin are expressed in the intercalated duct cells and that ENaC is not. Based on this, the model predicts that the intercalated duct does not absorb Na\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^+$$\end{document}+ and Cl\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^-$$\end{document}- like the striated duct but secretes a small amount of water instead. The input to the duct model is the time-dependent primary saliva generated by an acinar cell model. Our duct model produces final saliva output that agrees with the experimental measurements at various stimulation levels. It also shows realistic biological features such as duct cell volume, cellular concentrations and membrane potentials. Simplification of the model by omission of all detailed 3D structures of the duct makes a negligible difference to the final saliva output. This shows that saliva production is not sensitive to structural variation of the duct.

Salivary Gland Function, Development and Regeneration

Salivary glands produce and secrete saliva, which is essential for maintaining oral health and overall health. Understanding both the unique structure and physiological function of salivary glands, as well as how they are affected by disease and injury will direct the development of therapy to repair and regenerate them. Significant recent advances, particularly in the OMICS field, increase our understanding of how salivary glands develop at the cellular, molecular and genetic levels; the signaling pathways involved, the dynamics of progenitor cell lineages in development, homeostasis and regeneration and the role of the extracellular matrix microenvironment. These provide a template for cell and gene therapies as well as bioengineering approaches to repair or regenerate salivary function.

A di-functional and label-free carbon-based chem-nanosensor for real-time monitoring of pH fluctuation and quantitative determining of Curcumin

A nitrogen and sulfur dual-doped carbon nanodots (N,S-CNDs) sample was fabricated by an one-step hydrothermal treatment of acid fuchsin. The obtained N,S-CNDs possess yellow fluorescent, excellent water solubility, high fluorescence stability and religious biocompatibility, which can be used as a di-functional and label-free chem-nanosensor for the determination of pH and curcumin (Cur). When the pH is decreased from 8.0 to 4.4, the N,S-CNDs reveals an extraordinary emission escalation at emission wavelength (λ_em) of 543 nm, and the pK_a value of the N,S-CNDs was calculated as 6.06. The N,S-CNDs displays a favourable linear relationship in the physiological pH range of 5.0-7.4, which is meaningful for the research of near-neutral cytosolic pH. Furthermore, the laser scanning confocal microscopic images of intracellular distribution and the determination of pH in HeLa cells were successfully carried out, implying that the N,S-CNDs possess excellent cell membrane permeability and are capable of being further applied to real-time pH fluctuations monitoring in live cells with negligible autofluorescence. More importantly, the fluorescence of N,S-CNDs may be dramatically quenched by Cur via a combination of electrostatic and hydrogen-bond interaction and fluorescence inner filter effect (FIFE). The limit of detection is as low as 81 nmol/L Cur, and the linearity range is 0.5-20 μmol/L Cur. Ultimately, a satisfactory result was obtained when applying the as-constructed fluorescent chem-nanosensing system for the analyses of Cur in practical samples.

Chloride Influx of Anion Exchanger 2 Was Modulated by Calcium-Dependent Spinophilin in Submandibular Glands

Secretory glands including salivary glands by many hormonal inputs produce and secrete biological fluids determined by variety of ion transporters. Spinophilin is a multifunctional scaffolding protein, which involved in receptor signaling and regulation of anion exchangers AE2 activity. We found that spinophilin expressed in salivary glands. The role of salivary spinophilin on the modulation of chloride/bicarbonate exchange remains unknown. The spinophilin enhanced AE2 activity and associated with a STE20/SPS1-related kinase and showed an additive effect on the modulation of the activity of AE2. The cholinergic stimulation and subsequent intracellular Ca²⁺ increase was required for the interaction with AE2 and spinophilin and abrogated the enhanced effect of spinophilin on Cl⁻ transporting activity. Ductal chloride/bicarbonate exchange activity was increased in pretreatment with carbachol. The CaMKII inhibitor KN-93 suppressed the chloride/bicarbonate exchange activity of ducts, suggesting that CaMKII was required for ductal chloride/bicarbonate exchange activity. Additionally, microtubule destabilization by nocodazole attenuated the interaction of AE2 and spinophilin and almost abolished the ductal chloride/bicarbonate exchange activity. The treatment of siRNA-spinophilin on the isolated salivary ducts also reduced the ductal chloride/bicarbonate exchange activity. Therefore, role of salivary spinophilin on AE2 may facilitate the Cl⁻ influx from basolateral in salivary glands in response to cholinergic inputs.

Association of serum electrolytes and smoking with salivary gland stone formation

To further define potential factors that may contribute to stone formation in salivary glands (sialolithiasis), a retrospective chart review was performed of patients diagnosed with sialolithiasis between March 1, 1998 and February 29, 2012. Information on salivary gland stone number, location and size, medical history, medications, and serum electrolyte levels were collected. Associations between electrolyte levels and stone characteristics (such as stone number and size) were examined. Fifty-nine patients were identified; their median age was 58 years (range 25-89 years) and most were male (95%). Salivary stones were most commonly located in the submandibular glands (83%). Thirty-five patients (59%) had a smoking history, with 16 (27%) reported as current smokers. There was a significant association between current smoker status and stone size (mean largest stone size 12.4±8.8mm vs. 7.5±4.8mm in current smokers vs. non-smokers; P=0.03). Serum sodium levels (r=0.32, P=0.014) and serum potassium levels (r=0.31, P=0.017) showed significant positive correlations with stone size. While the aetiology of sialolithiasis remains unclear, smoking (which can contribute to reduced saliva flow) and higher serum sodium levels (which can reflect volume depletion) are associated with larger salivary stones.

The fluorescence detection of autophagosomes in live cells under starvation using core-substituted naphthalenediimide probes

We developed NDI-C probe for the analysis of the autophagosomes in live cells. A new flow cytometric method was also developed for the analysis of cell population on the basis of intracellular pH.

Single Probe for Imaging and Biosensing of pH, Cu(2+) Ions, and pH/Cu(2+) in Live Cells with Ratiometric Fluorescence Signals

It is very essential to disentangle the complicated inter-relationship between pH and Cu in the signal transduction and homeostasis. To this end, reporters that can display distinct signals to pH and Cu are highly valuable. Unfortunately, there is still no report on the development of biosensors that can simultaneously respond to pH and Cu(2+), to the best of our knowledge. In this work, we developed a single fluorescent probe, AuNC@FITC@DEAC (AuNC, gold cluster; FITC, fluorescein isothiocyanate; DEAC, 7-diethylaminocoumarin-3-carboxylic acid), for biosensing of pH, Cu(2+), and pH/Cu(2+) with different ratiometric fluorescent signals. First, 2,2',2″-(2,2',2″-nitrilotris(ethane-2,1-diyl)tris((pyridin-2-yl-methyl)azanediyl))triethanethiol (TPAASH) was designed for specific recognition of Cu(2+), as well as for organic ligand to synthesize fluorescent AuNCs. Then, pH-sensitive molecule, FITC emitting at 518 nm, and inner reference molecule, DEAC with emission peak at 472 nm, were simultaneously conjugated on the surface of AuNCs emitting at 722 nm, thus, constructing a single fluorescent probe, AuNC@FITC@DEAC, to sensing pH, Cu(2+), and pH/Cu(2+) excited by 405 nm light. The developed probe exhibited high selectivity and accuracy for independent determination of pH and Cu(2+) against reactive oxygen species (ROS), other metal ions, amino acids, and even copper-containing proteins. The AuNC-based inorganic-organic probe with good cell-permeability and high biocompatibility was eventually applied in monitoring both pH and Cu(2+) and in understanding the interplaying roles of Cu(2+) and pH in live cells by ratiometric multicolor fluorescent imaging.

Intracellular Cl- as a signaling ion that potently regulates Na+/HCO3- transporters

Cl(-) is a major anion in mammalian cells involved in transport processes that determines the intracellular activity of many ions and plasma membrane potential. Surprisingly, a role of intracellular Cl(-) (Cl(-) in) as a signaling ion has not been previously evaluated. Here we report that Cl(-) in functions as a regulator of cellular Na(+) and HCO3 (-) concentrations and transepithelial transport through modulating the activity of several electrogenic Na(+)-HCO3 (-) transporters. We describe the molecular mechanism(s) of this regulation by physiological Cl(-) in concentrations highlighting the role of GXXXP motifs in Cl(-) sensing. Regulation of the ubiquitous Na(+)-HCO3(-) co-transport (NBC)e1-B is mediated by two GXXXP-containing sites; regulation of NBCe2-C is dependent on a single GXXXP motif; and regulation of NBCe1-A depends on a cryptic GXXXP motif. In the basal state NBCe1-B is inhibited by high Cl(-) in interacting at a low affinity GXXXP-containing site. IP3 receptor binding protein released with IP3 (IRBIT) activation of NBCe1-B unmasks a second high affinity Cl(-) in interacting GXXXP-dependent site. By contrast, NBCe2-C, which does not interact with IRBIT, has a single high affinity N-terminal GXXP-containing Cl(-) in interacting site. NBCe1-A is unaffected by Cl(-) in between 5 and 140 mM. However, deletion of NBCe1-A residues 29-41 unmasks a cryptic GXXXP-containing site homologous with the NBCe1-B low affinity site that is involved in inhibition of NBCe1-A by Cl(-) in. These findings reveal a cellular Cl(-) in sensing mechanism that plays an important role in the regulation of Na(+) and HCO3 (-) transport, with critical implications for the role of Cl(-) in cellular ion homeostasis and epithelial fluid and electrolyte secretion.

Mechanism and synergism in epithelial fluid and electrolyte secretion

A central function of epithelia is the control of the volume and electrolyte composition of bodily fluids through vectorial transport of electrolytes and the obligatory H2O. In exocrine glands, fluid and electrolyte secretion is carried out by both acinar and duct cells, with the portion of fluid secreted by each cell type varying among glands. All acinar cells secrete isotonic, plasma-like fluid, while the duct determines the final electrolyte composition of the fluid by absorbing most of the Cl(-) and secreting HCO3 (-). The key transporters mediating acinar fluid and electrolyte secretion are the basolateral Na(+)/K(+) /2Cl(-) cotransporter, the luminal Ca(2+)-activated Cl(-) channel ANO1 and basolateral and luminal Ca(2+)-activated K(+) channels. Ductal fluid and HCO3 (-) secretion are mediated by the basolateral membrane Na(+)-HCO3 (-) cotransporter NBCe1-B and the luminal membrane Cl(-)/HCO3 (-) exchanger slc26a6 and the Cl(-) channel CFTR. The function of the transporters is regulated by multiple inputs, which in the duct include major regulation by the WNK/SPAK pathway that inhibit secretion and the IRBIT/PP1 pathway that antagonize the effects of the WNK/SPAK pathway to both stimulate and coordinate the secretion. The function of these regulatory pathways in secretory glands acinar cells is yet to be examined. An important concept in biology is synergism among signaling pathways to generate the final physiological response that ensures regulation with high fidelity and guards against cell toxicity. While synergism is observed in all epithelial functions, the molecular mechanism mediating the synergism is not known. Recent work reveals a central role for IRBIT as a third messenger that integrates and synergizes the function of the Ca(2+) and cAMP signaling pathways in activation of epithelial fluid and electrolyte secretion. These concepts are discussed in this review using secretion by the pancreatic and salivary gland ducts as model systems.

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.

Extracellular Ca(2+) sensing in salivary ductal cells

Ca(2+) is secreted from the salivary acinar cells as an ionic constituent of primary saliva. Ions such as Na(+) and Cl(-) get reabsorbed whereas primary saliva flows through the salivary ductal system. Although earlier studies have shown that salivary [Ca(2+)] decreases as it flows down the ductal tree into the oral cavity, ductal reabsorption of Ca(2+) remains enigmatic. Here we report a potential role for the G protein-coupled receptor, calcium-sensing receptor (CSR), in the regulation of Ca(2+) reabsorption by salivary gland ducts. Our data show that CSR is present in the apical region of ductal cells where it is co-localized with transient receptor potential canonical 3 (TRPC3). CSR is activated in isolated salivary gland ducts as well as a ductal cell line (SMIE) by altering extracellular [Ca(2+)] or by aromatic amino acid, L-phenylalanine (L-Phe, endogenous component of saliva), as well as neomycin. CSR activation leads to Ca(2+) influx that, in polarized cells grown on a filter support, is initiated in the luminal region. We show that TRPC3 contributes to Ca(2+) entry triggered by CSR activation. Further, stimulation of CSR in SMIE cells enhances the CSR-TRPC3 association as well as surface expression of TRPC3. Together our findings suggest that CSR could serve as a Ca(2+) sensor in the luminal membrane of salivary gland ducts and regulate reabsorption of [Ca(2+)] from the saliva via TRPC3, thus contributing to maintenance of salivary [Ca(2+)]. CSR could therefore be a potentially important protective mechanism against formation of salivary gland stones (sialolithiasis) and infection (sialoadenitis).

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.

IRBIT: it is everywhere

IRBIT (IP(3)Rs binding protein released with IP(3)) is a protein originally identified by the Mikoshiba group as an inhibitor of IP(3) receptors function. Subsequently it was found to have multiple functions and regulate the activity of diverse proteins, including regulation of HCO(3)(-) transporters to coordinate epithelial HCO(3)(-) secretion and to determine localization of the Fip1 subunit of the CPSF complex to regulate mRNA processing. This review highlights the remarkably divers functions of IRBIT that are likely only a fraction of all the potential functions of this protein.

Channels and transporters in salivary glands

According to the two-stage hypothesis, primary saliva, a NaCl-rich plasma-like isotonic fluid is secreted by salivary acinar cells and its ionic composition becomes modified in the duct system. The ducts secrete K(+) and HCO (3) (-) and reabsorb Na(+) and Cl(-) without any water movement, thus establishing a hypotonic final saliva. Salivary secretion depends on the coordinated action of several channels and transporters localized in the apical and basolateral membrane of acinar and duct cells. Early functional studies in perfused glands, followed by the molecular cloning of several transport proteins and the subsequent analysis of mutant mice, have greatly contributed to our understanding of salivary fluid and the electrolyte secretion process. With a few exceptions, most of the key channels and transporters involved in salivary secretion have now been identified and characterized. However, the picture that has emerged from all these studies is one of a complex molecular network characterized by redundancy for several transport proteins, compensatory mechanisms, and adaptive changes in health and disease. Current research is directed to the molecular interactions between the determinants and the ways in which they are regulated by extracellular signals and intracellular mediators. This review focuses on the functionally and molecularly best-characterized channels and transporters that are considered to be involved in transepithelial fluid and electrolyte transport in salivary glands.

A near-infrared neutral pH fluorescent probe for monitoring minor pH changes: imaging in living HepG2 and HL-7702 cells

A near-neutral pH near-infrared (NIR) fluorescent probe utilizing a fluorophore-spacer- receptor molecular framework that can modulate the fluorescence emission intensity through a fast photoinduced electron-transfer process was developed. Our strategy was to choose tricarbocyanine (Cy), a NIR fluorescent dye with high extinction coefficients, as a fluorophore, and 4'-(aminomethylphenyl)-2,2':6',2''-terpyridine (Tpy) as a receptor. The pH titration indicated that Tpy-Cy can monitor the minor physiological pH fluctuations with a pK(a) of approximately 7.10 near physiological pH, which is valuable for intracellular pH researches. The probe responds linearly and rapidly to minor pH fluctuations within the range of 6.70-7.90 and exhibits strong dependence on pH changes. As expected, the real-time imaging of cellular pH and the detection of pH in situ was achieved successfully in living HepG2 and HL-7702 cells by this probe. It is shown that the probe effectively avoids the influence of autofluorescence and native cellular species in biological systems and meanwhile exhibits high sensitivity, good photostability, and excellent cell membrane permeability.

Apical maxi-K (KCa1.1) channels mediate K+ secretion by the mouse submandibular exocrine gland

The exocrine salivary glands of mammals secrete K+ by an unknown pathway that has been associated with HCO3(-) efflux. However, the present studies found that K+ secretion in the mouse submandibular gland did not require HCO3(-), demonstrating that neither K+/HCO3(-) cotransport nor K+/H+ exchange mechanisms were involved. Because HCO3(-) did not appear to participate in this process, we tested whether a K channel is required. Indeed, K+ secretion was inhibited >75% in mice with a null mutation in the maxi-K, Ca2+-activated K channel (KCa1.1) but was unchanged in mice lacking the intermediate-conductance IKCa1 channel (KCa3.1). Moreover, paxilline, a specific maxi-K channel blocker, dramatically reduced the K+ concentration in submandibular saliva. The K+ concentration of saliva is well known to be flow rate dependent, the K+ concentration increasing as the flow decreases. The flow rate dependence of K+ secretion was nearly eliminated in KCa1.1 null mice, suggesting an important role for KCa1.1 channels in this process as well. Importantly, a maxi-K-like current had not been previously detected in duct cells, the theoretical site of K+ secretion, but we found that KCa1.1 channels localized to the apical membranes of both striated and excretory duct cells, but not granular duct cells, using immunohistochemistry. Consistent with this latter observation, maxi-K currents were not detected in granular duct cells. Taken together, these results demonstrate that the secretion of K+ requires and is likely mediated by KCa1.1 potassium channels localized to the apical membranes of striated and excretory duct cells in the mouse submandibular exocrine gland.

Multi-photon microscopic imaging of rat parotid ducts demonstrates cellular heterogeneity in Ca2+ responsiveness

The heterogeneity of salivary ductal cells, with regard to their sensitivity to Ca(2+)-mobilizing agonists, was visualized by multi-photon microscopy. Stimulation of isolated parotid ducts with 0.1 and 1 microM epinephrine (Epi) elevated the intracellular Ca(2+) levels ([Ca(2+)](i)) in approximately 30% and >90% of the ductal cells, respectively. Of the 0.1 microM Epi-responsive cells, 80% responded rapidly to subsequent stimulation with 1 microM Epi. Similarly, threshold concentrations (0.5 or 1 microM) of phenylephrine (PhL), carbachol (CCh) or ATP, induced responses in approximately 20% of the ductal cells, and subsequent stimulations with 10 microM of the same agonist activated approximately 80% of ductal cells. These observations indicate that parotid ducts contain a certain subpopulation of cells, which exhibits particularly high sensitivity to these Ca(2+)-mobilizing agonists, compared to the remaining ductal cells. Sequential stimulation with threshold concentrations of PhL, CCh, and ATP induced Ca(2+) responses in approximately 33% of ductal cells. Of these responsive cells, the majority (69%) could only respond to one of the three agonists; while a small minority (9%) were capable of responding to all three agonists. These results indicate that low concentrations of PhL, CCh, and ATP activate different subpopulations of parotid ductal cells.

Fluid secretion and the Na+-K+-2Cl- cotransporter in mouse exorbital lacrimal gland

We have previously suggested that fluid flow in the mouse exorbital lacrimal gland is driven by the opening of apical Cl- and K+ channels. These ions move into the lumen of the gland and water follows by osmosis. In many tissues, the Na+-K+-2Cl- cotransporter (NKCC1) replaces the Cl- and K+ ions that move into the lumen. We hypothesize that mouse exorbital lacrimal glands would have NKCC1 co-transporters and that they would be important in fluid transport by this gland. We used immunocytochemistry to localize NKCC1-like immunoreactivity to the membranes of the acinar cells as well as to the basolateral membranes of the duct cells. We developed a method to measure tear flow and its composition from mouse glands in situ. Stimulation with the acetylcholine agonist carbachol produced a peak flow followed by a plateau. Ion concentration measurements of this stimulated fluid showed it was high in K+ and Cl-. Treatment of the gland with furosemide, a blocker of the NKCC1 cotransporter, reduced the plateau phase of fluid flow by approximately 30%. Isolated cells exposed to a hypertonic shock shrank by approximately 20% and then showed a regulatory volume increase (RVI). Both the RVI and swelling were blocked by treatment with furosemide. Cells isolated from these glands shrink by approximately 10% in the presence of carbachol. Blocking NKCC1 with furosemide reduced the amount of shrinkage by approximately 50%. These data suggest that NKCC1 plays an important role in fluid secretion by the exorbital gland of mice.

NaBC1 is a ubiquitous electrogenic Na+ -coupled borate transporter essential for cellular boron homeostasis and cell growth and proliferation

Boron is a vital micronutrient in plants and may be essential for animal growth and development. Whereas the role of boron in the life cycle of plants is well documented, nothing is known about boron homeostasis and function in animal cells. NaBC1, the mammalian homolog of AtBor1, is a borate transporter. In the absence of borate, NaBC1 conducts Na(+) and OH(-) (H(+)), while in the presence of borate, NaBC1 functions as an electrogenic, voltage-regulated, Na(+)-coupled B(OH)(4)(-) transporter. At low concentrations, borate activated the MAPK pathway to stimulate cell growth and proliferation, and at high concentrations, it was toxic. Accordingly, overexpression of NaBC1 shifted both effects of borate to the left, whereas knockdown of NaBC1 halted cell growth and proliferation. These findings may reveal a previously unrecognized role for NaBC1 in borate homeostasis and open the way to better understanding of the many presumed physiological roles of borate in animals.

Subpopulation of store-operated Ca2+ channels regulate Ca2+-induced Ca2+ release in non-excitable cells

Ca2+-induced Ca2+ release (CICR) is a well characterized activity in skeletal and cardiac muscles mediated by the ryanodine receptors. The present study demonstrates CICR in the non-excitable parotid acinar cells, which resembles the mechanism described in cardiac myocytes. Partial depletion of internal Ca2+ stores leads to a minimal activation of Ca2+ influx. Ca2+ influx through this pathway results in an explosive mobilization of Ca2+ from the majority of the stores by CICR. Thus, stimulation of parotid acinar cells in Ca2+ -free medium with 0.5 microm carbachol releases approximately 5% of the Ca2+ mobilizable by 1 mm carbachol. Addition of external Ca2+ induced the same Ca2+ release observed in maximally stimulated cells. Similar results were obtained by a short treatment with 2.5-10 microm cyclopiazonic acid, an inhibitor of the sarco/endoplasmic reticulum Ca2+ ATPase pump. The Ca2+ release induced by the addition of external Ca2+ was largely independent of IP(3)Rs because it was reduced by only approximately 30% by the inhibition of the inositol 1,4,5-trisphosphate receptors with caffeine or heparin. Measurements of Ca2+ -activated outward current and [Ca2+](i) suggested that most CICR triggered by Ca2+ influx occurred away from the plasma membrane. Measurement of the response to several concentrations of cyclopiazonic acid revealed that Ca2+ influx that regulates CICR is associated with a selective portion of the internal Ca2+ pool. The minimal activation of Ca2+ influx by partial store depletion was confirmed by the measurement of Mn2+ influx. Inhibition of Ca2+ influx with SKF96365 or 2-aminoethoxydiphenyl borate prevented activation of CICR observed on addition of external Ca2+. These findings provide evidence for activation of CICR by Ca2+ influx in non-excitable cells, demonstrate a previously unrecognized role for Ca2+ influx in triggering CICR, and indicate that CICR in non-excitable cells resembles CICR in cardiac myocytes with the exception that in cardiac cells Ca2+ influx is mediated by voltage-regulated Ca2+ channels whereas in non-excitable cells Ca2+ influx is mediated by store-operated channels.

Guanylin and functional coupling proteins in the human salivary glands and gland tumors : expression, cellular localization, and target membrane domains

Cystic fibrosis transmembrane conductance regulator (CFTR)-mediated secretion of an electrolyte-rich fluid is a major but incompletely understood function of the salivary glands. We provide molecular evidence that guanylin, a bioactive intestinal peptide involved in the CFTR-regulated secretion of electrolyte/water in the gut epithelium, is highly expressed in the human parotid and submandibular glands and in respective clinically most relevant tumors. Moreover, in the same organs we identified expression of the major components of the guanylin signaling pathway, ie, guanylin-receptor guanylate cyclase-C, cGKII, and CFTR, as well as of the epithelial Cl(-)/HCO(3)(-) anion exchanger type 2 (AE2). At the cellular level, guanylin is localized to epithelial cells of the ductal system that, based on its presence in the saliva, is obviously released into the salivary gland ducts. The guanylin-receptor guanylate cyclase-C, cGKII, CFTR, and AE2 are all confined exclusively to the apical membrane of the same duct cells. These findings implicate guanylin as intrinsic regulator of electrolyte secretion in the salivary glands. We assume that duct epithelial cells synthesize and release guanylin into the saliva to regulate electrolyte secretion in the ductal system by an intraductal luminocrine signaling pathway. Moreover, the high expression of guanylin in pleomorphic adenoma and Warthin tumors (cystadenolymphoma), the most common neoplasms of salivary glands, predicts guanylin as a significant marker in tumor pathology.

Modulation of the maitotoxin response by intracellular and extracellular cations

The aim of the present study was to characterize the role played by intracellular and extracellular calcium and sodium on the maitotoxin (MTX) response in Chinese hamster ovary (CHO) cells. The results presented here indicated that: (1) MTX activates calcium and sodium influx in a concentration-dependent manner; (2) extracellular calcium is required for the sodium influx; (3) removal of the extracellular sodium did not prevent the MTX-induced calcium influx; (4) elevation in the intracellular calcium concentration potentiates the MTX response; and (5) MTX, at the concentrations tested, did not compromise cell viability.

Cytosolic Ca(2+) and Ca(2+)-activated Cl(-) current dynamics: insights from two functionally distinct mouse exocrine cells

The dynamics of Ca(2+) release and Ca(2+)-activated Cl(-) currents in two related, but functionally distinct exocrine cells, were studied to gain insight into how the molecular specialization of Ca(2+) signalling machinery are utilized to produce different physiological endpoints: in this case, fluid or exocytotic secretion. Digital imaging and patch-clamp methods were used to monitor the temporal and spatial properties of changes in cytosolic Ca(2+) concentration ([Ca(2+)](c)) and Cl(-) currents following the controlled photolytic release of caged-InsP(3) or caged-Ca(2+). In parotid and pancreatic acinar cells, changes in [Ca(2+)](c) and activation of a Ca(2+)-activated Cl(-) current occurred with close temporal coincidence. In parotid, a rapid global Ca(2+) signal was invariably induced, even with low-level photolytic release of threshold amounts of InsP(3). In pancreas, threshold stimulation generated an apically delimited [Ca(2+)](c) signal, while a stronger stimulus induced a global [Ca(2+)](c) signal which exhibited characteristics of a propagating wave. InsP(3) was more effective in parotid, where [Ca(2+)](c) signals initiated with shorter latency and exhibited a faster time-to-peak than in pancreas. The increased potency of InsP(3) in parotid probably results from a four-fold higher number of InsP(3) receptors as measured by radiolabelled InsP(3) binding and western blot analysis. The Ca(2+) sensitivity of the Cl(-) channels in parotid and pancreas was determined from the [Ca(2+)]-current relationship measured during a dynamic 'Ca(2+) ramp' produced by the continuous, low-level photolysis of caged-Ca(2+). In addition to a greater number of InsP(3) receptors, the Cl(-) current density of parotid acinar cells was more than four-fold greater than that of pancreatic cells. Whereas activation of the current was tightly coupled to increases in Ca(2+) in both cell types, local Ca(2+) clearance was found to contribute substantially to the deactivation of the current in parotid. These data reveal specializations of common modules of Ca(2+)-release machinery and subsequent effector activation that are specifically suited to the distinct functional roles of these two related cell types.

Characterization of the maitotoxin-activated cationic current from human skin fibroblasts

The maitotoxin (MTX)-induced cationic current (I(mtx)) from human skin fibroblasts was characterized using the patch-clamp technique in whole-cell configuration. Under resting conditions (absence of MTX), the main current observed is produced by an outwardly rectifying K(+) channel which is inhibited by 1 mM TEA. The current reversal potential was -86 mV (n = 12). MTX (500 pM) activated a current with a linear current-voltage relationship and a reversal potential of -10 mV (n = 10). Replacing the extracellular Na(+) and K(+) with N-methyl-D-glucamine (NMDG) caused a shift of the reversal potential to a value below -100 mV, indicating that Na(+) and K(+), but not NMDG, carry I(mtx). Further ion selectivity experiments showed that Ca(2+) carries I(mtx) also. The resulting permeability sequence obtained with the Goldman-Hodgkin-Katz equation yielded Na(+) (1) approximately equal to K(+) (1) > Ca(2+) (0.87). The I(mtx) activation time course reflected the changes in intracellular Ca(2+) and Na(+) measured with the fluorescent indicators fura-2 and SBFI, respectively, suggesting that the activation of I(mtx) brings about an increment in intracellular Ca(2+) and Na(+). Reducing the extracellular Ca(2+) concentration below 1.8 mM prevented the activation of I(mtx) and the increment in intracellular Na(+) induced by MTX. Mn(2+) and Mg(2+) could not replace Ca(2+), but Ba(2+) could replace Ca(2+). MTX activation of current in 10 mM Ba(2+) was approximately 50 % of that induced in the presence of 1.8 mM Ca(2+). When 5 mM of the Ca(2+) chelator BAPTA was included in the patch pipette, MTX either failed to activate the current or induced a small current (less than 15 % of the control), indicating that intracellular Ca(2+) is also required for the activation of I(mtx). Intracellular Ba(2+) can replace Ca(2+) as an activator of I(mtx). However, in the presence of 10 mM Ba(2+) the activation by MTX of the current was 50 % less than the activation with nM concentrations of free intracellular Ca(2+).

Synthesis and photophysical properties of new fluorinated benzo[c]xanthene dyes as intracellular pH indicators

Two new fluorinated benzo[c]xanthene dyes were synthesized by reaction of fluorinated 1,6-dihydroxynaphthalenes with 2,4- (and 2,5)-dicarboxy-3'-dimethylamino-2'-hydroxybenzophenone. The two critical fluorinated 1,6-dihydroxynaphthalene intermediates were prepared via a regioselective route. The fluorinated benzo[c]xanthene dyes exhibit desired lower pK(a) values (6.4 and 7.2, respectively) than their parent compound (pK(a)=7.5) while the pH-dependent dual-emission characteristics are well retained. Their cell-permeable esters have been prepared for intracellular applications.

HCO3- salvage mechanisms in the submandibular gland acinar and duct cells

In the present work, we characterized H(+) and HCO3- transport mechanisms in the submandibular salivary gland (SMG) ducts of wild type, NHE2-/-, NHE3-/-, and NHE2-/-;NHE3-/- double knock-out mice. The bulk of recovery from an acid load across the luminal membrane (LM) of the duct was mediated by a Na(+)-dependent HOE and ethyl-isopropyl-amiloride (EIPA)-inhibitable and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS)-insensitive mechanism. HCO3- increased the rate of luminal Na(+)-dependent pH(i) recovery but did not change inhibition by HOE and EIPA or the insensitivity to DIDS. Despite expression of NHE2 and NHE3 in the LM of the duct, the same activity was observed in ducts from wild type and all mutant mice. Measurements of Na(+)-dependent OH(-) and/or HCO3- cotransport (NBC) activities in SMG acinar and duct cells showed separate DIDS-sensitive/EIPA-insensitive and DIDS-insensitive/EIPA-sensitive NBC activities in both cell types. Functional and immunocytochemical localization of these activities in the perfused duct indicated that pNBC1 probably mediates the DIDS-sensitive/EIPA-insensitive transport in the basolateral membrane, and splice variants of NBC3 probably mediate the DIDS-insensitive/EIPA-sensitive NBC activity in the LM of duct and acinar cells. Notably, the acinar cell NBC3 variants transported HCO3- but not OH(-). By contrast, duct cell NBC3 transported both OH(-) and HCO3-. Accordingly, reverse transcription-polymerase chain reaction analysis revealed that both cell types expressed mRNA for pNBC1. However, the acini expressed mRNA for the NBC3 splice variants NBCn1C and NBCn1D, whereas the ducts expressed mRNA for NCBn1B. Based on these findings we propose that the luminal NBCs in the HCO3- secreting SMG acinar and duct cells function as HCO3- salvage mechanisms at the resting state. These studies emphasize the complexity but also begin to clarify the mechanism of HCO3- homeostasis in secretory epithelia.

Mapping of five new putative anion transporter genes in human and characterization of SLC26A6, a candidate gene for pancreatic anion exchanger

A second distinct family of anion transporters, in addition to the classical SLC4 (or AE) family, has recently been delineated. Members of the SLC26 family are structurally well conserved and can mediate the electroneutral exchange of Cl(-) for HCO(-)(3) across the plasma membrane of mammalian cells like members of the SLC4 family. Three human transporter proteins have been functionally characterized: SLC26A2 (DTDST), SLC26A3 (CLD or DRA), and SLC26A4 (PDS) can transport with different specificities the chloride, iodine, bicarbonate, oxalate, and hydroxyl anions, whereas SLC26A5 (prestin) was suggested to act as the motor protein of the cochlear outer hair cell. We report the expansion of the SLC26 family with five new members in chromosomes 3, 6, 8, 12, and 17 and mapping of SLC26A1 to 4p16.3. We have characterized one of them, SLC26A6, in more detail. It maps to chromosome 3p21.3, encodes a predicted 738-amino-acid transmembrane protein, and is most abundantly expressed in the kidney and pancreas. Pancreatic ductal cell lines Capan-1 and Capan-2 express SLC26A6, and immunohistochemistry localizes SLC26A6 protein to the apical surface of pancreatic ductal cells, suggesting it as a candidate for a luminal anion exchanger. The functional characterization of the novel members of this tissue-specific gene family may provide new insights into anion transport physiology in different parts of the body.

Immunolocalization of anion exchanger AE2 and Na(+)-HCO(-)(3) cotransporter in rat parotid and submandibular glands

Salivary glands secrete K(+) and HCO(-)(3) and reabsorb Na(+) and Cl(-), but the identity of transporters involved in HCO(-)(3) transport remains unclear. We investigated localization of Cl(-)/HCO(-)(3) exchanger isoform AE2 and of Na(+)-HCO(-)(3) cotransporter (NBC) in rat parotid gland (PAR) and submandibular gland (SMG) by immunoblot and immunocytochemical techniques. Immunoblotting of PAR and SMG plasma membranes with specific antibodies against mouse kidney AE2 and rat kidney NBC revealed protein bands at approximately 160 and 180 kDa for AE2 and approximately 130 kDa for NBC, as expected for the AE2 full-length protein and consistent with the apparent molecular mass of NBC in several tissues other than kidney. Immunostaining of fixed PAR and SMG tissue sections revealed specific basolateral staining of PAR acinar cells for AE2 and NBC, but in SMG acinar cells only basolateral AE2 labeling was observed. No AE2 expression was detected in any ducts. Striated, intralobular, and main duct cells of both glands showed NBC expression predominantly at basolateral membranes, with some cells being apically stained. In SMG duct cells, NBC staining exhibited a gradient of distribution from basolateral localization in more proximal parts of the ductal tree to apical localization toward distal parts of the ductal tree. Both immunoblotting signals and immunostaining were abolished in preabsorption experiments with the respective antigens. Thus the mechanisms of fluid and anion secretion in salivary acinar cells may be different between PAR and SMG, and, because NBC was detected in acinar and duct cells, it may play a more important role in transport of HCO(-)(3) by rat salivary duct cells than previously believed.

Cystic fibrosis transmembrane conductance regulator regulates luminal Cl-/HCO3- exchange in mouse submandibular and pancreatic ducts

We have demonstrated previously the regulation of Cl-/HCO3- exchange activity by the cystic fibrosis transmembrane conductance regulator (CFTR) in model systems of cells stably or transiently transfected with CFTR (Lee, M. G., Wigley, W. C., Zeng, W., Noel, L. E., Marino, C. R., Thomas, P. J., and Muallem, S. (1999) J. Biol. Chem. 274, 3414-3421). In the present work we examine the significance of this regulation in cells naturally expressing CFTR. These include the human colonic T84 cell line and the mouse submandibular gland and pancreatic ducts, tissues that express high levels of CFTR in the luminal membrane. As in heterologous expression systems, stimulation of T84 cells with forskolin increased the Cl-/HCO3- exchange activity independently of CFTR Cl- channel activity. Freshly isolated submandibular gland ducts from wild type mice showed variable Cl-/HCO3- exchange activity. Measurement of [Cl-]i revealed that this was largely the result of variable steady-state [Cl-]i. Membrane depolarization with 5 mM Ba2+ or 100 mM K+ increased and stabilized [Cl-]i. Under depolarized conditions wild type and DeltaF/DeltaF mice had comparable basal Cl-/HCO3- exchange activity. Notably, stimulation with forskolin increased Cl-/HCO3- exchange activity in submandibular gland ducts from wild type but not DeltaF/DeltaF mice. Microperfusion of the main pancreatic duct showed Cl-/HCO3- exchange activity in both the basolateral and luminal membranes. Stimulation of ducts from wild type animals with forskolin had no effect on basolateral but markedly stimulated luminal Cl-/HCO3- exchange activity. By contrast, forskolin had no effect on either basolateral or luminal Cl-/HCO3- exchange activity of ducts from DeltaF/DeltaF animals. We conclude that CFTR regulates luminal Cl-/HCO3- exchange activity in CFTR-expressing cells, and we discuss the possible physiological significance of these findings regarding cystic fibrosis.

Regulation of Cl-/ HCO3- exchange by cystic fibrosis transmembrane conductance regulator expressed in NIH 3T3 and HEK 293 cells

A central function of cystic fibrosis transmembrane conductance regulator (CFTR)-expressing tissues is the secretion of fluid containing 100-140 mM HCO3-. High levels of HCO3- maintain secreted proteins such as mucins (all tissues) and digestive enzymes (pancreas) in a soluble and/or inactive state. HCO3- secretion is impaired in CF in all CFTR-expressing, HCO3--secreting tissues examined. The mechanism responsible for this critical problem in CF is unknown. Since a major component of HCO3- secretion in CFTR-expressing cells is mediated by the action of a Cl-/HCO3- exchanger (AE), in the present work we examined the regulation of AE activity by CFTR. In NIH 3T3 cells stably transfected with wild type CFTR and in HEK 293 cells expressing WT and several mutant CFTR, activation of CFTR by cAMP stimulated AE activity. Pharmacological and mutagenesis studies indicated that expression of CFTR in the plasma membrane, but not the Cl- conductive function of CFTR was required for activation of AE. Furthermore, mutations in NBD2 altered regulation of AE activity by CFTR independent of their effect on Cl- channel activity. At very high expression levels CFTR modified the sensitivity of AE to 4,4'-diisothiocyanatostilbene-2, 2'-disulfonate. The novel finding of regulation of Cl-/HCO3- exchange by CFTR reported here may have important physiological implications and explain, at least in part, the impaired HCO3- secretion in CF.

Role of HCO3- ions in cytosolic pH regulation in rat mast cells: evidence for a new Na+-independent, HCO3--dependent alkalinizing mechanism

The role of external HCO3- in pHi regulation in rat mast cells was studied with BCECF. In a HCO3--free medium cells undergo an alkalinization after the addition of 40 mM HCO3Na. This alkalinization is unaffected by pH. In a Na+-free medium, the addition of 20 mM HCO3Na induced a higher alkalinization than 20 mM HCO3K. Amiloride (1 mM), a Na+/H+ exchanger inhibitor, inhibited by 45% the alkalinization induced by HCO3Na, but it did not change that induced by HCO3K. The anion exchanger inhibitor DIDS reduced 20% the alkalinization induced by both salts. An alkalinization of 0.085 units is observed after the addition of 20 mM HCO3K, even when these exchangers are inhibited (in the absence of Na+ and presence of DIDS). We conclude that the Na+/H+ exchanger and the Cl-/HCO3- exchangers are alkalinizing mechanisms that regulate pHi in these cells. Also, there is some HCO3--dependent, Na+-independent mechanism responsible for the intracellular alkalinization.

Membrane-limited expression and regulation of Na+-H+ exchanger isoforms by P2 receptors in the rat submandibular gland duct

1. Cell-specific reverse transcriptase-polymerase chain reaction (RT-PCR), immunolocalization and microspectrofluorometry were used to identify and localize the Na+-H+ exchanger (NHE) isoforms expressed in the submandibular gland (SMG) acinar and duct cells and their regulation by basolateral and luminal P2 receptors in the duct. 2. The molecular and immunofluorescence analysis showed that SMG acinar and duct cells expressed NHE1 in the basolateral membrane (BLM). Duct cells also expressed NHE2 and NHE3 in the luminal membrane (LM). 3. Expression of NHE3 was unequivocally established by the absence of staining in SMG from NHE3 knockout mice. NHE3 was expressed in the LM and in subluminal regions of the duct. 4. Measurement of the inhibition of NHE activity by the amiloride analogue HOE 694 (HOE) suggested expression of NHE1-like activity in the BLM and NHE2-like activity in the LM of the SMG duct. Several acute and chronic treatments tested failed to activate NHE activity with low affinity for HOE as expected for NHE3. Hence, the physiological function and role of NHE3 in the SMG duct is not clear at present. 5. Activation of P2 receptors resulted in activation of an NHE-independent, luminal H+ transport pathway that markedly and rapidly acidified the cells. This pathway could be blocked by luminal but not basolateral Ba2+. 6. Stimulation of P2U receptors expressed in the BLM activated largely NHE1-like activity, and stimulation of P2Z receptors expressed in the LM activated largely NHE2-like activity. 7. The interrelation between basolateral and luminal NHE activities and their respective regulation by P2U and P2Z receptors can be used to co-ordinate membrane transport events in the LM and BLM during active Na+ reabsorption by the SMG duct.

Immunolocalization of vacuolar-type H+-ATPase in rat submandibular gland and adaptive changes induced by acid-base disturbances

Using antibodies against the 31-kD and 70-kD subunits of vacuolar type H+-ATPase (V-ATPase) and light microscopic immunocytochemistry, we have demonstrated the presence of this V-ATPase in rat submandibular gland. We have also investigated the adaptive changes of this transporter during acid-base disturbances such as acute and chronic metabolic acidosis or alkalosis. Our results show intracellularly distributed V-ATPase in striated, granular, and main excretory duct cells in controls, but no V-ATPase immunoreaction in acinar cells. Both acute and chronic metabolic acidosis caused a shift in V-ATPase away from diffuse distribution towards apical localization in striated and granular duct cells, suggesting that a V-ATPase could be involved in the regulation of acid-base homeostasis. In contrast, during acidosis the main excretory duct cells showed no changes in the V-ATPase distribution compared to controls. With acute and chronic metabolic alkalosis, no changes in the V-ATPase distribution occurred. (J Histochem Cytochem 46:91-100, 1998)

Membrane-specific regulation of Cl- channels by purinergic receptors in rat submandibular gland acinar and duct cells

Measurement of [Cl-]i and the Cl- current in the rat salivary submandibular gland (SMG) acinar and duct cells was used to evaluate the role of Cl- channels in the regulation of [Cl-]i during purinergic stimulation. Under resting conditions [Cl-]i averaged 56 +/- 8 and 26 +/- 7 mM in acinar and duct cells, respectively. In both cells, stimulation with 1 mM ATP resulted in Cl- efflux and subsequent influx. Inhibition of NaKCl2 cotransport had no effect on [Cl-]i changes in duct cells and inhibited only about 50% of Cl- uptake in acinar cells. Accordingly, low levels of expression of NaKCl2 cotransporter protein were found in duct cells. Acinar cells expressed high levels of the cotransporter. Measurement of Cl- current under selective conditions revealed that acinar and duct cells express at least five distinct Cl- channels; a ClCO-like, volume-sensitive, inward rectifying, Ca2+-activated and CFTR-like Cl- currents. ATP acting on both cell types activated at least two channels, the Ca2+-activated Cl- channel and a Ca2+-independent glibenclamide-sensitive Cl--current, possibly cystic fibrosis transmembrane regulator (CFTR). Of the many nucleotides tested only 2'-3'-benzoylbenzoyl (Bz)-ATP and UTP activated Cl- channels in SMG cells. Despite their relative potency in increasing [Ca2+]i, BzATP in both SMG cell types largely activated the Ca2+-independent, glibenclamide-sensitive Cl- current, whereas UTP activated only the Ca2+-dependent Cl- current. We interpret this to suggest that BzATP and UTP largely activate Cl- channels residing in the membrane expressing the receptor for the active nucleotide. The present studies reveal a potentially new mechanism for transcellular Cl- transport in a CFTR-expressing tissue, the SMG. Coordinated action of the P2z (luminal) and P2u (basolateral) receptors can mediate part of the transcellular Cl- transport by acinar and duct cells to determine the final electrolyte composition of salivary fluid.

Characterization and localization of P2 receptors in rat submandibular gland acinar and duct cells

[Ca2+]i and the Cl- current were measured in isolated submandibular gland acinar and duct cells to characterize and localize the purinergic receptors expressed in these cells. In both cell types 2'-3'-benzoylbenzoyl (Bz)-ATP and ATP increased [Ca2+]i mainly by activation of Ca2+ influx. UTP had only minimal effect on [Ca2+]i at concentrations between 0.1 and 1 mM. However, a whole cell current recording showed that all nucleotides effectively activated Cl- currents. Inhibition of signal transduction through G proteins by guanyl-5'-beta-thiophosphate revealed that the effect of ATP on Cl- current was mediated in part by activation of a G protein-coupled and in part by a G protein-independent receptor. BzATP activated exclusively the G protein-independent portion, whereas UTP activated only the G protein-dependent portion of the Cl- current. Measurement of [Ca2+]i in the microperfused duct showed that ATP stimulated a [Ca2+]i increase when applied to the luminal or the basolateral sides. BzATP increased [Ca2+]i only when applied to the luminal side, whereas UTP at 100 microM increased -Ca2+-i only when applied to the basolateral side. The combined results suggest that duct and possibly acinar cells express P2z receptors in the luminal and P2u receptors in the basolateral membrane.

H+ transporters in the main excretory duct of the mouse mandibular salivary gland

1. We used microspectrofluorimetry with the pH-sensitive fluoroprobe 2',7'-bis(2-carboxyethyl)-5(and-6)-carboxyfluorescein (BCECF) to study the regulation of cytosolic pH (pHi) in the isolated, perfused main excretory duct of the mouse mandibular gland. 2. In nominally HCO3(-)-free solutions, removal of Na+ from the lumen alone caused pHi to decline whereas removing it from the bath alone did not. 3. Readmission of Na+ to the lumen of ducts studied under zero-Na+ conditions caused pHi to recover fully. This recovery was blocked by 5-(N-ethyl-N-isopropyl)-amiloride (EIPA) with a half-maximum concentration of 0.5 mumol l-1, indicating the presence of an apical Na(+)-H+ exchanger. 4. Readmission of Na+ to the bath of ducts studied under zero-Na+ conditions also caused pHi to recover. This recovery was blocked by 100 mumol l-1 EIPA, indicating the presence of a basolateral Na(+)-H+ exchanger. 5. Measurements of H+ fluxes indicated that the apical Na(+)-H+ exchanger was approximately four times more active than the basolateral Na(+)-H+ exchanger. 6. In three sets of experiments (in the absence of Na+, in the presence of Na+, and in the presence of Na+ plus 100 mumol l-1 EIPA), the effects of changing luminal K+ concentration on pHi were examined. We found no evidence for the presence of K(+)-H+ exchange or Na(+)-coupled K(+)-H+ exchange in the apical membranes of duct cells. 7. pHi recovery under nominally HCO3(-)-free conditions following acidification with an NH4Cl pulse was abolished by removal of Na+ from the bath and luminal solutions, indicating that no Na(+)-independent systems such as H(+)-ATPases were present. 8. A repeat of the above experiments in the presence of 25 mmol l-1 HCO3- plus 5% CO2 did not reveal any additional H+ transport systems. The removal of luminal Cl-, however, caused a small rise in pHi. This latter effect was blocked by 500 mumol l-1 4,4'-diisothiocyanatodihydrostilbene-2,2'-disulphonic acid (H2-DIDS), suggesting that a Cl(-)-HCO3- exchanger in the apical membrane might contribute in a minor way to pHi regulation. 9. We conclude that the predominant H+ transport systems in the mouse mandibular main excretory duct are Na(+)-H+ exchangers in the apical and the basolateral membranes. The model we postulate to account for electrolyte transport across the main duct in the mouse mandibular gland is quite different from that previously developed for the rat duct but is similar to that developed for the rabbit duct. The difference is in concordance with the known ability of the mandibular gland of the rat, but not the rabbit or the mouse, to secrete a HCO3(-)-rich final saliva.

nNOS and Ca2+ influx in rat pancreatic acinar and submandibular salivary gland cells

Regulation of agonist-activated Ca2+ influx by the NOS pathway through generation of cGMP is being found in an increasing number of cell types. In the present work, we examined the role of the NOS pathway in agonist-evoked [Ca2+]i oscillations and attempted to identify the NOS isoform most likely to regulate Ca2+ influx. For this, we first show that two Ca(2+)-mobilizing agonists acting on pancreatic acinar cells, bombesin (BS) and the cholecystokinin analog CCK-JMV-180 (CCKJ), evokes different type of [Ca2+]i oscillations. The BS-evoked [Ca2+]i oscillations rapidly became acutely dependent on the presence of extracellular Ca2+, whereas the CCKJ-evoked oscillations continue for long periods of time in the absence of Ca2+ influx. This differential behavior allowed us to isolate Ca2+ influx and study its regulation while controlling for non specific effects on all other Ca2+ transporting events involved in generating [Ca2+]i oscillations. Inhibitors of selective steps in the NOS pathway inhibited agonist-induced cGMP production. The inhibitors were then used to show that scavenging NO with reduced hemoglobin, inhibition of guanylyl cyclase with 1H-[1,2,4] oxadiazolo[4,3-a] quinoxaline-1-one (ODQ) and inhibition of protein kinase G with Rp-8-pCPT-cGMPS inhibited [Ca2+]i oscillations evoked by BS but not those evoked by CCKJ. These findings were extended to duct and acinar cells of the SMG. In these cells, Ca(2+)-mobilizing agonists stimulate large Ca2+ influx, which was inhibited by all inhibitors of the NOS pathway. Western blot analysis and immunolocalization revealed that the cells did not express iNOS, eNOS was expressed only in blood vessels and capillaries whereas nNOS was expressed at high levels next to the plasma membrane of all cells. Accordingly, the nNOS inhibitor 7-nitroindazole (7-NI) inhibited BS- but not CCKJ-evoked [Ca2+]i oscillations and Ca2+ influx into SMG acinar and duct cells. Thus, together, our findings favor nNOS as the isoform activated by the Ca2+ released from internal stores to generate cGMP and regulate Ca2+ influx.

Sodium Green as a potential probe for intracellular sodium imaging based on fluorescence lifetime

We characterized the use of the fluorescent probe Sodium Green for measurements of intracellular free sodium using frequency-domain, phase-modulation fluorometry. The intensity decays were found to be strongly Na+ dependent, with mean lifetime increasing from 1.13 ns in the absence of Na+ to 2.39 ns in the presence of 140 mM Na+. Detailed analysis of the intensity decays in the presence of Na+ and K+ in the concentration range from 0 to 500 mM is provided. Sodium sensing using data measured at a single modulation frequency is described. Phase and modulation data showed high sensitivity to Na+ and substantially lower sensitivity to K+. Additionally, exposure of Sodium Green to intense illumination indicated that Sodium Green is much more photostable than its precursor, fluorescein. These results indicate that lifetime-based measurements with Sodium Green can be used for imaging of intracellular free [Na+] in the range from about 0.5 to 50 mM with high accuracy.

Increased fluid secretion after adenoviral-mediated transfer of the aquaporin-1 cDNA to irradiated rat salivary glands

A replication-deficient, recombinant adenovirus encoding human aquaporin-1 (hAQP1), the archetypal water channel, was constructed. This virus, AdhAQP1, directed hAQP1 expression in several epithelial cell lines in vitro. In polarized MDCK cell monolayers, hAQP1 was localized in the apical and basolateral plasma membranes. Fluid movement across monolayers infected by AdhAQP1 in response to an osmotic gradient was approximately 4-fold that seen with uninfected monolayers or monolayers infected by a control virus. When AdhAQP1 was administered to rat submandibular glands by retrograde ductal instillation, significant hAQP1 expression was observed by Western blot analysis in crude plasma membranes and by immunohistochemical staining in both acinar and ductal cells. Three or four months after exposure to a single radiation dose (17.5 or 21 Gy, respectively), AdhAQP1 administration to rat submandibular glands led to a two- to threefold increase in salivary secretion compared with secretion from glands administered a control virus. These results suggest that hAQP1 gene transfer may have potential as an unique approach for the treatment of postradiation salivary hypofunction.