Ion transport by cultures of human tracheobronchial submucosal glands

Acidic Submucosal Gland pH and Elevated Protein Concentration Produce Abnormal Cystic Fibrosis Mucus

In response to respiratory insults, airway submucosal glands secrete copious mucus strands to increase mucociliary clearance and protect the lung. However, in cystic fibrosis, stimulating submucosal glands has the opposite effect, disrupting mucociliary transport. In cystic fibrosis (CF) pigs, loss of cystic fibrosis transmembrane conductance regulator (CFTR) anion channels produced submucosal gland mucus that was abnormally acidic with an increased protein concentration. To test whether these variables alter mucus, we produced a microfluidic model of submucosal glands using mucus vesicles from banana slugs. Acidic pH and increased protein concentration decreased mucus gel volume and increased mucus strand elasticity and tensile strength. However, once mucus strands were formed, changing pH or protein concentration largely failed to alter the biophysical properties. Likewise, raising pH or apical perfusion did not improve clearance of mucus strands from CF airways. These findings reveal mechanisms responsible for impaired mucociliary transport in CF and have important implications for potential treatments.

Airway Gland Structure and Function

Submucosal glands contribute to airway surface liquid (ASL), a film that protects all airway surfaces. Glandular mucus comprises electrolytes, water, the gel-forming mucin MUC5B, and hundreds of different proteins with diverse protective functions. Gland volume per unit area of mucosal surface correlates positively with impaction rate of inhaled particles. In human main bronchi, the volume of the glands is ∼ 50 times that of surface goblet cells, but the glands diminish in size and frequency distally. ASL and its trapped particles are removed from the airways by mucociliary transport. Airway glands have a tubuloacinar structure, with a single terminal duct, a nonciliated collecting duct, then branching secretory tubules lined with mucous cells and ending in serous acini. They allow for a massive increase in numbers of mucus-producing cells without replacing surface ciliated cells. Active secretion of Cl(-) and HCO3 (-) by serous cells produces most of the fluid of gland secretions. Glands are densely innervated by tonically active, mutually excitatory airway intrinsic neurons. Most gland mucus is secreted constitutively in vivo, with large, transient increases produced by emergency reflex drive from the vagus. Elevations of [cAMP]i and [Ca(2+)]i coordinate electrolyte and macromolecular secretion and probably occur together for baseline activity in vivo, with cholinergic elevation of [Ca(2+)]i being mainly responsive for transient increases in secretion. Altered submucosal gland function contributes to the pathology of all obstructive diseases, but is an early stage of pathogenesis only in cystic fibrosis.

Chloride secretion by cultures of pig tracheal gland cells

Malfunction of airway submucosal glands contributes to the pathology of cystic fibrosis (CF), and cell cultures of CF human airway glands show defects in Cl(-) and water transport. Recently, a transgenic pig model of CF (the CF pig) has been developed. Accordingly, we have developed cell cultures of pig airway gland epithelium for use in investigating alterations in gland function in CF. Our cultures form tight junctions (as evidenced by high transepithelial electrical resistance) and show high levels of active anion secretion (measured as amiloride-insensitive short-circuit current). In agreement with recent results on human airway glands, neurohumoral agents that elevate intracellular Ca(2+) potently stimulated anion secretion, while elevation of cAMP was comparatively ineffective. Our cultures express lactoferrin and lysozyme (serous gland cell markers) and MUC5B (the main mucin of airway glands). They are, therefore, potentially useful in determining if CF-related alterations in anion transport result in altered secretion of serous cell antimicrobial agents or mucus.

Cystic fibrosis and the relationship between mucin and chloride secretion by cultures of human airway gland mucous cells

We investigated how cystic fibrosis (CF) alters the relationship between Cl(-) and mucin secretion in cultures of non-CF and CF human tracheobronchial gland mucous (HTGM and CFTGM, respectively) cells. Biochemical studies showed that HTMG cells secreted typical airway mucins, and immunohistochemical studies showed that these cells expressed MUC1, MUC4, MUC5B, MUC8, MUC13, MUC16, and MUC20. Effects of cumulative doses of methacholine (MCh), phenylephrine (Phe), isoproterenol (Iso), and ATP on mucin and Cl(-) secretion were studied on HTGM and CFTGM cultures. Baseline mucin secretion was not significantly altered in CFTGM cells, and the increases in mucin secretion induced by mediators were unaltered (Iso, Phe) or slightly decreased (MCh, ATP). Across mediators, there was no correlation between the maximal increases in Cl(-) secretion and mucin secretion. In HTGM cells, the Cl(-) channel blocker, diphenylamine-2-carboxylic acid, greatly inhibited Cl(-) secretion but did not alter mucin release. In HTGM cells, mediators (10(-5) M) increased mucin secretion in the rank order ATP > Phe = Iso > MCh. They increased Cl(-) secretion in the sequence ATP > MCh ≈ Iso > Phe. The responses in Cl(-) secretion to MCh, ATP, and Phe were unaltered by CF, but the response to Iso was greatly reduced. We conclude that mucin secretion by cultures of human tracheobronchial gland cells is independent of Cl(-) secretion, at baseline, and is unaltered in CF; that the ratio of Cl(-) secretion to mucus secretion varies markedly depending on mediator; and that secretions induced by stimulation of β-adrenergic receptors will be abnormally concentrated in CF.

Cultures of human tracheal gland cells of mucous or serous phenotype

There are two main epithelial cell types in the secretory tubules of mammalian glands: serous and mucous. The former is believed to secrete predominantly water and antimicrobials, the latter mucins. Primary cultures of human airway gland epithelium have been available for almost 20 yr, but they are poorly differentiated and lack clear features of either serous or mucous cells. In this study, by varying growth supports and media, we have produced cultures from human airway glands that in terms of their ultrastructure and secretory products resemble either mucous or serous cells. Of four types of porous-bottomed insert tested, polycarbonate filters (Transwells) most strongly promoted the mucous phenotype. Coupled with the addition of epidermal growth factor (EGF), this growth support produced “mucous” cells that contained the large electron-lucent granules characteristic of native mucous cells, but lacked the small electron-dense granules characteristic of serous cells. Furthermore, they showed high levels of mucin secretion and low levels of release of lactoferrin and lysozyme (markers of native serous cells). By contrast, growth on polyethylene terephthalate filters (Cyclopore) in medium lacking EGF produced “serous” cells in which small electron-dense granules replaced the electron-lucent ones, and the cells had high levels of lactoferrin and lysozyme but low levels of mucins. Measurements of transepithelial resistance and short-circuit current showed that both “serous” and “mucous” cell cultures possessed tight junctions, had become polarized, and were actively secreting Cl.

Action of N-acylated ambroxol derivatives on secretion of chloride ions in human airway epithelia

We report the effects of new N-acylated ambroxol derivatives (TEI-588a, TEI-588b, TEI-589a, TEI-589b, TEI-602a and TEI-602b: a, aromatic amine-acylated derivative; b, aliphatic amine-acylated derivative) induced from ambroxol (a mucolytic agent to treat human lung diseases) on Cl(-) secretion in human submucosal serous Calu-3 cells under a Na(+)/K(+)/2Cl(-) cotransporter-1 (NKCC1)-mediated hyper-secreting condition. TEI-589a, TEI-589b and TEI-602a diminished hyper-secretion of Cl(-) by diminishing the activity of NKCC1 without blockade of apical Cl(-) channel (TEI-589a>TEI-602a>TEI-589b), while any other tested compounds including ambroxol had no effects on Cl(-) secretion. These indicate that the inhibitory action of an aromatic amine-acylated derivative on Cl(-) secretion is stronger that that of an aliphatic amine-acylated derivative, and that 3-(2,5-dimethyl)furoyl group has a strong action in inhibition of Cl(-) secretion than cyclopropanoyl group. We here indicate that TEI-589a, TEI-589b and TEI-602a reduce hyper-secretion to an appropriate level in the airway, providing a possibility that the compound can be an effective drug in airway obstructive diseases including COPD by reducing the airway resistance under a hyper-secreting condition.

Enhancement of P2Y6-induced Cl- secretion by IL-13 and modulation of SK4 channels activity in human bronchial cells

Expression of functional P2Y(6) receptors was demonstrated in primary cultures of human bronchial cells (NHBE cells). P2Y(6) receptors were located only on the apical membranes of NHBE cells. Their stimulation by UDP induced a chloride secretion (short-circuit current) reflected by the development of two I(sc) components (I(fast) and I(late)). A pharmacological characterization of those two I(sc) components showed the involvement of CaCC and CFTR channel activity in I(fast) and I(late) respectively. I(fast) was also found to be under control of basolateral SK4 channels. Indeed, inhibition of SK4 channels opening by clotrimazole dramatically reduced I(fast) amplitude. The epithelial ion transporting phenotype depends on the cellular state of differentiation. As previously reported, we observed that Ultroser G increased the epithelial tightness and Na(+)-transport capacity while IL-13 switch the epithelial ion transport phenotype from a Na(+)-absorbing to a Cl(-)-secreting one. In our study, we report for the first time a change in the K(+) cell permeability associated to IL-13-induced cell differentiation. IL-13 treatment increased the-resting K(+) permeability as well as the Ca(2+)-dependent K(+) permeability stimulated by UDP or ionomycin. SK4 channels activity, underlying the Ca(2+)-dependent K(+) permeability was in particular increased by IL-13. The on/off effect of IL-13 on P2Y(6)-induced Cl-secretion may help to identify the molecular determinants responsible for the CaCC channel activity.

Optical imaging of Ca2+-evoked fluid secretion by murine nasal submucosal gland serous acinar cells

Airway submucosal glands are sites of high expression of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel and contribute to fluid homeostasis in the lung. However, the molecular mechanisms of gland ion and fluid transport are poorly defined. Here, submucosal gland serous acinar cells were isolated from murine airway, identified by immunofluorescence and gene expression profiling, and used in physiological studies. Stimulation of isolated acinar cells with carbachol (CCh), histamine or ATP was associated with marked decreases in cell volume (20 +/- 2% within 62 +/- 5 s) that were tightly correlated with increases in cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) as revealed by simultaneous DIC and fluorescent indicator dye microscopy. Simultaneous imaging of cell volume and the Cl(-)-sensitive fluorophore SPQ indicated that the 20% shrinkage was associated with a fall of [Cl(-)](i) from 65 mm to 28 mm, reflecting loss of 67% of cell Cl(-) content, accompanied by parallel efflux of K(+). Upon agonist removal, [Ca(2+)](i) relaxed and the cells swelled back to resting volume via a bumetanide-sensitive Cl(-) influx pathway, likely to be NKCC1. Accordingly, agonist-induced serous acinar cell shrinkage and swelling are caused by activation of solute efflux and influx pathways, respectively, and cell volume reflects the secretory state of these cells. In contrast, elevation of cAMP failed to elicit detectible volume responses, or enhance those induced by submaximal [CCh], because the magnitude of the changes were likely to be below the threshold of detection using optical imaging. Finally, when stimulated with cholinergic or cAMP agonists, cells from mice that lacked CFTR, as well as wild-type cells treated with a CFTR inhibitor, exhibited identical rates and magnitudes of shrinkage and Cl(-) efflux compared with control cells. These results provide insights into the molecular mechanisms of salt and water secretion by lung submucosal glands, and they suggest that while murine submucosal gland fluid secretion in response to cholinergic stimulation can originate from CFTR-expressing serous acinar cells, it is not dependent upon CFTR function.

Action of neltenexine on anion secretion in human airway epithelia

Neltenexine has been applied to human lung diseases such as chronic obstructive pulmonary disease (COPD) as a mucolytic agent. However, we have no information on the neltenexine action in bronchial epithelial cells. We studied the neltenexine action on the ion transport in human submucosal serous Calu-3 cells. Under a hyper-secreting condition caused by terbutaline (a beta2-adrenergic agonist), neltenexine diminished anion secretion by inhibiting the Cl- and HCO3- uptake via Na+/K+/2Cl- cotransporter and Na+/HCO3- cotransporter without blockade of the cystic fibrosis transmembrane conductance regulator (CFTR) channel, and also diminished anion secretion via stimulation of Cl-/HCO3- exchanger, which facilitates the extrusion of more CFTR-permeant anion, Cl-, with the uptake of less CFTR-permeant anion, HCO3-. Thus, neltenexine reduced the hyper-secretion to keep an appropriate fluid level in the airway, providing a possibility that neltenexine can be an effective drug in airway obstructive diseases by decreasing the airway resistance under a hyper-secreting condition.

Culture of Calu-3 cells at the air interface provides a representative model of the airway epithelial barrier

PURPOSE

The aim of this study was to compare the effect of liquid-covered culture (LCC) and air-interfaced culture (AIC) on Calu-3 cell layer morphology and permeability, thus assessing the fitness of these culture systems as models of airway epithelium barrier function.

METHODS

Cell layers were grown on 0.33 cm2 Transwell polyester cell culture supports. Cell layers grown using LCC and AIC were evaluated by using light and electron microscopy, transepithelial electrical resistance (TER), and permeability to the transepithelial flux of fluorescein sodium (flu-Na), and by varying molecular weight dextrans labeled with fluorescein isothiocyanate (FITC-dex). The tight junction protein, zona occludens protein-1 (ZO-1), was visualized by confocal microscopy and apical glycoprotein secretions were identified by using alcian blue.

RESULTS

Cells grown via AIC produced a more columnar epithelium with a more rugged apical topography and greater glycoprotein secretion compared to cells grown via LCC. Apical protrusions appearing to be cilia-like structures were observed on occasional cells using AIC, but typical airway ciliated cell phenotypes were not produced under either condition. Secretory granules were observed in cells cultured under both conditions. Cells cultured using LCC exhibited higher levels of ZO-1 protein than the AIC counterpart. The maximal TER of cells using LCC, 1,086 +/- 113 ohms cm2 at 11-16 days, was significantly greater than the TER of cells cultured using AIC, 306 +/- 53 ohms cm2 at 11-13 days. Apparent permeability (P(app)) values for the transport of flu-Na using LCC and AIC were 1.48 +/- 0.19x10(-7) and 3.36 +/- 0.47x10(-7) cm s(-1), respectively. Transport rates of flu-Na and FITC-dex were inversely proportional to molecular weight, and were significantly lower (p < 0.05) in cell layers grown using LCC than AIC. Renkin analysis fitted the data to single pore populations of radii 7.7 and 11.0 nm for LCC and AIC, respectively.

CONCLUSION

Distinct differences in morphology and permeability result when Calu-3 cells are grown using AIC or LCC. Cells cultured using AIC generate a model more morphologically representative of the airway epithelium than cells cultured using LCC.

Ambroxol-induced modification of ion transport in human airway Calu-3 epithelia

Ambroxol is often used as a mucolytic agent in various lung diseases. However, it is unclear how ambroxol acts on bronchial epithelial cells. To clarify the action of ambroxol, we studied the effects of ambroxol on the ion transport in human Calu-3 cells, a human submucosal serous cell line, measuring the transepithelial short-circuit current and conductance across monolayers of Calu-3 cells. Ambroxol of 100 microM diminished the terbutaline (a beta2-adrenergic agonist)-stimulated Cl-/HCO3(-)-dependent secretion without any decreases in the conductance of cystic fibrosis transmembrane conductance regulator (CFTR) channel locating on the apical membrane. On the other hand, under the basal (unstimulated) condition ambroxol increased the Cl(-)-dependent secretion with no significant change in the apical CFTR channel conductance and decreased the HCO3- secretion associated with a decrease in the apical CFTR channel conductance. Ambroxol had no major action on the epithelial Na+ channel (ENaC) or the ENaC-mediated Na+ absorption. These results indicate that in Calu-3 cells: (1) under the basal (unstimulated) condition ambroxol increases Cl- secretion by stimulating the entry step of Cl- and decreases HCO3- secretion by diminishing the activity of the CFTR channel and/or the Na+/HCO3(-)-dependent cotransporter, (2) under the adrenergic agonist-stimulated condition, ambroxol decreases Cl- secretion by acting on the Cl-/HCO3- exchanger, and (3) ambroxol has a more powerful action than the adrenergic agonist on the Cl-/HCO3- exchanger, leading fluid secretion to a moderately stimulated level from a hyper-stimulated level.

A “virtual gland” method for quantifying epithelial fluid secretion

We developed a new apparatus, the virtual gland (VG), for measuring the rate of fluid secretion ( Jv), its composition, and the transepithelial potential (TEP) in cultured epithelial cells under open circuit. The VG creates a 10-μl chamber above the apical surface of epithelial cells on a Costar filter with a small hole leading to an oil-filled reservoir. After the chamber is primed with a fluid of choice, secreted fluid is forced through the hole into the oil, where it forms a bubble that is monitored optically to determine Jv and collected for analysis. Calu-3 cells were mounted in the VG with a basolateral bath consisting of Krebs-Ringer bicarbonate buffer at 37°C. Basal Jv was 2.7 ± 0.1 μl·cm−2·h−1 ( n = 42), and TEP was −9.2 ± 0.6 mV ( n = 33); both measures were reduced to zero by ouabain ( n = 6). Jv and TEP were stimulated 64 and 59%, respectively, by 5 μM forskolin ( n = 10), 173 and 101% by 1 mM 1-ethyl-2-benzimidazolinone ( n = 5), 213 and 122% by 333 nM thapsigargin ( n = 5), and 520 and 240% by forskolin + thapsigargin ( n = 6). Basal Jv and TEP were inhibited to 82 and 63%, respectively, with 10 μM bumetanide ( n = 5), 71 and 82% with 100 μM acetazolamide ( n = 5), and 47 and 56% with 600 μM glibenclamide ( n = 4). Basal Jv and TEP were 52 and 89% of control values, respectively, after HCO3− replacement with HEPES ( n = 16). The net HCO3− concentration of the secreted fluid was close to that of the bath (25 mM), except when stimulated with forskolin or VIP, when it increased (∼80 mM). These results validate the use of the VG apparatus and provide the first direct measures of Jv in Calu-3 cells.

Cyclic ADP-ribose, a putative Ca2+-mobilizing second messenger, operates in submucosal gland acinar cells

Cyclic ADP-ribose (cADPR), a putative Ca(2+)-mobilizing second messenger, has been reported to operate in several mammalian cells. To investigate whether cADPR is involved in electrolyte secretion from airway glands, we used a patch-clamp technique, the measurement of microsomal Ca(2+) release, quantification of cellular cADPR, and RT-PCR for CD38 mRNA in human and feline tracheal glands. cADPR (>6 microM), infused into the cell via the patch pipette, caused ionic currents dependent on cellular Ca(2+). Infusions of lower concentrations (2-4 microM) of cADPR or inositol 1,4,5-trisphosphate (IP(3)) alone were without effect on the baseline current, but a combined application of cADPR and IP(3) mimicked the cellular response to low concentrations of acetylcholine (ACh). Microsomes derived from the isolated glands released Ca(2+) in response to both IP(3) and cADPR. cADPR released Ca(2+) from microsomes desensitized to IP(3) or those treated with heparin. The mRNA for CD38, an enzyme protein involved in cADPR metabolism, was detected in human tissues, including tracheal glands, and the cellular content of cADPR was increased with physiologically relevant concentrations of ACh. We conclude that cADPR, in concert with IP(3), operates in airway gland acinar cells to mobilize Ca(2+), resulting in Cl(-) secretion.

Role of airway surface liquid and submucosal glands in cystic fibrosis lung disease

Cystic fibrosis (CF) is caused by mutations in the CF transmembrane conductance regulator (CFTR) protein, an epithelial chloride channel expressed in the airways, pancreas, testis, and other tissues. A central question is how defective CFTR function in CF leads to chronic lung infection and deterioration of lung function. Several mechanisms have been proposed to explain lung disease in CF, including abnormal airway surface liquid (ASL) properties, defective airway submucosal gland function, altered inflammatory response, defective organellar acidification, loss of CFTR regulation of plasma membrane ion transporters, and others. This review focuses on the physiology of the ASL and submucosal glands with regard to their proposed role in CF lung disease. Experimental evidence for defective ASL properties and gland function in CF is reviewed, and deficiencies in understanding ASL/gland physiology are identified as areas for further investigation. New model systems and measurement technologies are being developed to make progress in establishing lung disease mechanisms in CF, which should facilitate mechanism-based design of therapies for CF.

Effects of ion transport inhibitors on MCh-mediated secretion from porcine airway submucosal glands

Submucosal glands secrete macromolecules and liquid that are essential for normal airway function. To determine the mechanisms responsible for airway gland secretion and the interaction between gland secretion and epithelial ion transport, studies were performed in porcine tracheal epithelia by using the hillocks and Ussing techniques. No significant baseline gland fluid flux (J(G)) was measured by the hillocks technique after 3 min, and the epithelia had an average potential difference of 7.5 +/- 0.5 mV (lumen negative) with a short-circuit current of 73 +/- 4 microA/cm(2), as measured by the Ussing technique. The secretagogue methacholine induced concentration-dependent increases in J(G) after 3 min from 0.003 microl. min(-1). cm(-2) at 0.1 microM to 0.41 +/- 0.04 microl. min(-1). cm(-2) at 1,000 microM, with a 0.9 +/- 0.1 mV hyperpolarization of the epithelium at 1,000 microM. When the epithelium was pretreated for 3 min with the sodium channel blocker amiloride, the methacholine (1,000 microM)-induced J(G) increased to 0.67 +/- 0.09 microl. min(-1). cm(-2), and the hyperpolarization increased to 2.2 +/- 0.5 mV over the amiloride-pretreated level. When pretreated for 3 min with the chloride channel blocker diphenylamine-2-carboxylic acid, the methacholine (1,000 microM)-induced J(G) was inhibited to 0.20 +/- 0.06 microl. min(-1). cm(-2), and the methacholine-induced hyperpolarization was abolished. These data indicate that, in porcine airways, methacholine-induced J(G) may be increased by inhibition of sodium absorption and decreased by inhibition of chloride secretion.

Characterization of basolateral K+ channels underlying anion secretion in the human airway cell line Calu-3

Transepithelial anion secretion in many tissues depends upon the activity of basolateral channels. Using monolayers of the Calu-3 cell line, a human submucosal serous cell model mounted in an Ussing chamber apparatus, we investigated the nature of the K+ channels involved in basal, cAMP- and Ca2+-stimulated anion secretion, as reflected by the transepithelial short circuit current (I(sc)). The non-specific K+ channel inhibitor Ba2+ inhibited the basal I(sc) by either 77 or 16 % when applied directly to the basolateral or apical membranes, respectively, indicating that a basolateral K+ conductance is required for maintenance of basal anion secretion. Using the K+ channel blockers clofilium and clotrimazole, we found basal I(sc) to be sensitive to clofilium, with a small clotrimazole-sensitive component. By stimulating the cAMP and Ca2+ pathways, we determined that cAMP-stimulated anion secretion was almost entirely abolished by clofilium, but insensitive to clotrimazole. In contrast, the Ca2+-stimulated response was sensitive to both clofilium and clotrimazole. Thus, pharmacologically distinct basolateral K+ channels are differentially involved in the control of anion secretion under different conditions. Isolation of the basolateral K+ conductance in permeabilized monolayers revealed a small basal and forskolin-stimulated I(sc). Finally, using the reverse transcriptase-polymerase chain reaction, we found that Calu-3 cells express the K+ channel genes KCNN4 and KCNQ1 and the subunits KCNE2 and KCNE3. We conclude that while KCNN4 contributes to Ca2+-activated anion secretion by Calu-3 cells, basal and cAMP-activated secretion are more critically dependent on other K+ channel types, possibly involving one or more class of KCNQ1-containing channel complexes.

Functional and molecular characterization of an anion exchanger in airway serous epithelial cells

Serous cells secrete Cl(-) and HCO(3)(-) and play an important role in airway function. Recent studies suggest that a Cl(-)/HCO(3)(-) anion exchanger (AE) may contribute to Cl(-) secretion by airway epithelial cells. However, the molecular identity, the cellular location, and the contribution of AEs to Cl(-) secretion in serous epithelial cells in tracheal submucosal glands are unknown. The goal of the present study was to determine the molecular identity, the cellular location, and the role of AEs in the function of serous epithelial cells. To this end, Calu-3 cells, a human airway cell line with a serous-cell phenotype, were studied by RT-PCR, immunoblot, and electrophysiological analysis to examine the role of AEs in Cl(-) secretion. In addition, the subcellular location of AE proteins was examined by immunofluorescence microscopy. Calu-3 cells expressed mRNA and protein for AE2 as determined by RT-PCR and Western blot analysis, respectively. Immunofluorescence microscopy identified AE2 in the basolateral membrane of Calu-3 cells in culture and rat tracheal serous cells in situ. In Cl(-)/HCO(3)(-)/Na(+)-containing media, the 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate (CPT-cAMP)-stimulated short-circuit anion current (I(sc)) was reduced by basolateral but not by apical application of 4, 4'-diisothiocyanostilbene-2,2'-disulfonic acid (50 microM) and 4, 4'-dinitrostilbene-2,2'-disulfonic acid [DNDS (500 microM)], inhibitors of AEs. In the absence of Na(+) in the bath solutions, to eliminate the contributions of the Na(+)/HCO(3)(-) and Na(+)/K(+)/2Cl(-) cotransporters to I(sc), CPT-cAMP stimulated a small DNDS-sensitive I(sc). Taken together with previous studies, these observations suggest that a small component of cAMP-stimulated I(sc) across serous cells may be referable to Cl(-) secretion and that uptake of Cl(-) across the basolateral membrane may be mediated by AE2.

Fluid transport across cultures of human tracheal glands is altered in cystic fibrosis

1. There is evidence that defective submucosal gland secretion contributes to the airway pathology of cystic fibrosis (CF). Using a capacitance probe technique, we have compared fluid transport across submucosal gland cultures from individuals with and without CF. 2. Under baseline conditions, approximately 60% of non-CF cultures secreted fluid; the rest absorbed. In secreting tissues, amiloride increased secretion, whereas in absorbing tissues it reduced or reversed absorption. 5-Nitro-2(3-phenylpropylamino)-benzoate (NPPB) a blocker of the CF transmembrane conductance regulator (CFTR), converted secretion to absorption. Thus, the direction and magnitude of baseline fluid movement depended on a balance between active absorption of Na+ and cAMP-dependent secretion of Cl-. 3. 8-(4-Chlorophenylthio)-adenosine 3',5'-cyclic monophosphate (CPT-cAMP), methacholine and luminal uridine 5'-triphosphate (UTP) all induced or increased fluid secretion across non-CF cultures. Results with NPPB and with 4,4'-diisothiocyanatostilbene-2,2'-disulphonate (DIDS), a blocker of Ca(2+)-activated Cl- channels, suggested that fluid secretion induced by CPT-cAMP was mediated primarily by CFTR; UTP acted entirely via Ca(2+)-activated Cl- channels, and methacholine activated both pathways. 4. All CF cultures showed baseline fluid absorption, which was abolished by amiloride. 5. CF cultures showed a normal secretory response to UTP, a reduced response to methacholine, and no response to CPT-cAMP. 6. Thus, the absorptive processes of airway glands are retained in CF, but the cAMP-dependent secretory process is lost. This would markedly reduce the water content of gland secretions. The resulting change in viscosity would contribute to the accumulation of airway mucus which is characteristic of this disease.

Effect of acetyl choline on ion transport in sheep tracheal epithelium

The action of acetylcholine (ACh) on sheep tracheal epithelium has been investigated. ACh increases transiently the short-circuit current (ISC). The same response is obtained in tissues in which the apical membrane has been permeabilized with amphotericin B in the presence of a potassium gradient. Microelectrode studies show that the majority of tracheal epithelial cells depolarize as the apical fractional resistance decreases on application of ACh. These results, together with the finding that bumetanide decreases the initial ACh-induced ISC increase, are consistent with an initial activation by ACh of apical Cl- channels and basolateral K+ channels. Following the initial increase, ISC declines to values lower than in control conditions both in untreated and in amphotericin-permeabilized tissues, suggesting that the basolateral K+ conductance falls during this phase. The late decrease in ISC induced by ACh is significantly reduced in tissues pretreated with amiloride, suggesting that the apical Na+ channels are also involved in this response. ACh abolishes the net Na+ absorption by decreasing the mucosal to serosal Na+ flux. This effect is possibly a result of a down-regulation of apical Na+ channels and basolateral K+ channels.

Characterization of immortal cystic fibrosis tracheobronchial gland epithelial cells

Tracheobronchial glands were isolated and cultured from a patient with cystic fibrosis (CF). Cultured epithelial cells were transformed with pSVori-. All transformed cell lines express cytokeratin filaments and at early passages express the junctional complex molecule cell CAM 120/80, indicating their epithelial origin. Several gland cell lines express antigens that localize to secretory cells in vivo. Cl- transport measured by 36Cl efflux shows that CF gland epithelial cells, like CF surface airway and nasal polyp epithelial cells, are unable to respond to increases in intracellular cAMP. However, they do produce an increase in intracellular cAMP after treatment with isoproterenol or forskolin. One CF gland cell line shows increased intracellular calcium in response to a number of agents and increased Cl- efflux comparable to that observed in a non-CF airway surface epithelial cell line after addition of calcium ionophore. All cell lines express CF transmembrane conductance regulator mRNA, as measured by PCR amplification of first-strand cDNA. The CF tracheobronchial gland cell lines described here are compound heterozygotes, having a single copy of the delta F508 mutation.