Lubiprostone stimulates secretion from tracheal submucosal glands of sheep, pigs, and humans

Lubiprostone, a putative ClC-2 chloride channel opener, has been investigated for its effects on airway epithelia (tracheas). Lubiprostone is shown to increase submucosal gland secretion in pigs, sheep, and humans and to increase short-circuit current (SCC) in the surface epithelium of pigs and sheep. Use of appropriate blocking agents and ion-substitution experiments shows anion secretion is the driving force for fluid formation in both glands and surface epithelium. From SCC concentration-response relations, it is shown that for apical lubiprostone K(d) = 10.5 nM with a Hill slope of 1.08, suggesting a single type of binding site and, from the speed of the response, close to the apical surface, confirmed the rapid blockade by Cd ions. Responses to lubiprostone were reversible and repeatable, responses being significantly larger with ventral compared with dorsal epithelium. Submucosal gland secretion rates following basolateral lubiprostone were, respectively, 0.2, 0.5, and 0.8 nl gl(-1) min(-1) in humans, sheep, and pigs. These rates dwarf any contribution surface secretion adds to the accumulation of surface liquid under the influence of lubiprostone. Lubiprostone stimulated gland secretion in two out of four human cystic fibrosis (CF) tissues and in two of three disease controls, chronic obstructive pulmonary disease and idiopathic pulmonary fibrosis (COPD/IPF), but in neither type of tissue was the increase significant. Lubiprostone was able to increase gland secretion rates in normal human tissue in the continuing presence of a high forskolin concentration. Lubiprostone had no spasmogenic activity on trachealis muscle, making it a potential agent for increasing airway secretion that may have therapeutic utility.

Synergistic airway gland mucus secretion in response to vasoactive intestinal peptide and carbachol is lost in cystic fibrosis

Cystic fibrosis (CF) is caused by dysfunction of the CF transmembrane conductance regulator (CFTR), an anion channel whose dysfunction leads to chronic bacterial and fungal airway infections via a pathophysiological cascade that is incompletely understood. Airway glands, which produce most airway mucus, do so in response to both acetylcholine (ACh) and vasoactive intestinal peptide (VIP). CF glands fail to secrete mucus in response to VIP, but do so in response to ACh. Because vagal cholinergic pathways still elicit strong gland mucus secretion in CF subjects, it is unclear whether VIP-stimulated, CFTR-dependent gland secretion participates in innate defense. It was recently hypothesized that airway intrinsic neurons, which express abundant VIP and ACh, are normally active and stimulate low-level gland mucus secretion that is a component of innate mucosal defenses. Here we show that low levels of VIP and ACh produced significant mucus secretion in human glands via strong synergistic interactions; synergy was lost in glands of CF patients. VIP/ACh synergy also existed in pig glands, where it was CFTR dependent, mediated by both Cl(-) and HCO(3) (-), and clotrimazole sensitive. Loss of "housekeeping" gland mucus secretion in CF, in combination with demonstrated defects in surface epithelia, may play a role in the vulnerability of CF airways to bacterial infections.

Parasympathetic control of airway submucosal glands: central reflexes and the airway intrinsic nervous system

Airway submucosal glands produce the mucus that lines the upper airways to protect them against insults. This review summarizes evidence for two forms of gland secretion, and hypothesizes that each is mediated by different but partially overlapping neural pathways. Airway innate defense comprises low level gland secretion, mucociliary clearance and surveillance by airway-resident phagocytes to keep the airways sterile in spite of nearly continuous inhalation of low levels of pathogens. Gland secretion serving innate defense is hypothesized to be under the control of intrinsic (peripheral) airway neurons and local reflexes, and these may depend disproportionately on non-cholinergic mechanisms, with most secretion being produced by VIP and tachykinins. In the genetic disease cystic fibrosis, airway glands no longer secrete in response to VIP alone and fail to show the synergy between VIP, tachykinins and ACh that is observed in normal glands. The consequent crippling of the submucosal gland contribution to innate defense may be one reason that cystic fibrosis airways are infected by mucus-resident bacteria and fungi that are routinely cleared from normal airways. By contrast, the acute (emergency) airway defense reflex is centrally mediated by vagal pathways, is primarily cholinergic, and stimulates copious volumes of gland mucus in response to acute, intense challenges to the airways, such as those produced by very vigorous exercise or aspiration of foreign material. In cystic fibrosis, the acute airway defense reflex can still stimulate the glands to secrete large amounts of mucus, although its properties are altered. Importantly, treatments that recruit components of the acute reflex, such as inhalation of hypertonic saline, are beneficial in treating cystic fibrosis airway disease. The situation for recipients of lung transplants is the reverse; transplanted airways retain the airway intrinsic nervous system but lose centrally mediated reflexes. The consequences of this for gland secretion and airway defense are poorly understood, but it is possible that interventions to modify submucosal gland secretion in transplanted lungs might have therapeutic consequences.

Mucus secretion by single tracheal submucosal glands from normal and cystic fibrosis transmembrane conductance regulator knockout mice

Submucosal glands line the cartilaginous airways and produce most of the antimicrobial mucus that keeps the airways sterile. The glands are defective in cystic fibrosis (CF), but how this impacts airway health remains uncertain. Although most CF mouse strains exhibit mild airway defects, those with the C57Bl/6 genetic background have increased airway pathology and susceptibility to Pseudomonas. Thus, they offer the possibility of studying whether, and if so how, abnormal submucosal gland function contributes to CF airway disease. We used optical methods to study fluid secretion by individual glands in tracheas from normal, wild-type (WT) mice and from cystic fibrosis transmembrane conductance regulator (CFTR) knockout mice (Cftr(m1UNC)/Cftr(m1UNC); CF mice). Glands from WT mice qualitatively resembled those in humans by responding to carbachol and vasoactive intestinal peptide (VIP), although the relative rates of VIP- and forskolin-stimulated secretion were much lower in mice than in large mammals. The pharmacology of mouse gland secretion was also similar to that in humans; adding bumetanide or replacement of HCO(3)(-) by Hepes reduced the carbachol response by approximately 50%, and this inhibition increased to 80% when both manoeuvres were performed simultaneously. It is important to note that glands from CFTR knockout mice responded to carbachol but did not secrete when exposed to VIP or forskolin, as has been shown previously for glands from CF patients. Tracheal glands from WT and CF mice both had robust secretory responses to electrical field stimulation that were blocked by tetrodotoxin. It is interesting that local irritation of the mucosa using chili pepper oil elicited secretion from WT glands but did not stimulate glands from CF mice. These results clarify the mechanisms of murine submucosal gland secretion and reveal a novel defect in local regulation of glands lacking CFTR which may also compromise airway defence in CF patients.

Acinar origin of CFTR-dependent airway submucosal gland fluid secretion

Cystic fibrosis (CF) airway disease arises from defective innate defenses, especially defective mucus clearance of microorganisms. Airway submucosal glands secrete most airway mucus, and CF airway glands do not secrete in response to VIP or forskolin. CFTR, the protein that is defective in CF, is expressed in glands, but immunocytochemistry finds the highest expression of CFTR in either the ciliated ducts or in the acini, depending on the antibodies used. CFTR is absolutely required for forskolin-mediated gland secretion; we used this finding to localize the origin of forskolin-stimulated, CFTR-dependent gland fluid secretion. We tested the hypothesis that secretion to forskolin might originate from the gland duct rather than or in addition to the acini. We ligated gland ducts at various points, stimulated the glands with forskolin, and monitored the regions of the glands that swelled. The results supported an acinar rather than ductal origin of secretion. We tracked particles in the mucus using Nomarski time-lapse imaging; particles originated in the acini and traveled toward the duct orifice. Estimated bulk flow accelerated in the acini and mucus tubules, consistent with fluid secretion in those regions, but was constant in the unbranched duct, consistent with a lack of fluid secretion or absorption by the ductal epithelium. We conclude that CFTR-dependent gland fluid secretion originates in the serous acini. The failure to observe either secretion or absorption from the CFTR and epithelial Na+ channel (ENaC)-rich ciliated ducts is unexplained, but may indicate that this epithelium alters the composition rather than the volume of gland mucus.

Hyposecretion, not hyperabsorption, is the basic defect of cystic fibrosis airway glands

Human airways and glands express the anion channel cystic fibrosis transmembrane conductance regulator, CFTR, and the epithelial Na(+) channel, ENaC. Cystic fibrosis (CF) airway glands fail to secrete mucus in response to vasoactive intestinal peptide or forskolin; the failure was attributed to loss of CFTR-mediated anion and fluid secretion. Alternatively, CF glands might secrete acinar fluid via CFTR-independent pathways, but the exit of mucus from the glands could be blocked by hyperabsorption of fluid in the gland ducts. This could occur because CFTR loss can disinhibit ENaC, and ENaC activity can drive absorption. To test these two hypotheses, we measured single gland mucus secretion optically and applied ENaC inhibitors to determine whether they augmented secretion. Human CF glands were pretreated with benzamil and then stimulated with forskolin in the continued presence of benzamil. Benzamil did not rescue the lack of secretion to forskolin (50 glands, 6 CF subjects) nor did it increase the rate of cholinergically mediated mucus secretion from CF glands. Finally, neither benzamil nor amiloride increased forskolin-stimulated mucus secretion from porcine submucosal glands (75 glands, 7 pigs). One possible explanation for these results is that ENaC within the gland ducts was not active in our experiments. Consistent with that possibility, we discovered that human airway glands express Kunitz-type and non-Kunitz serine protease inhibitors, which might prevent proteolytic activation of ENaC. Our results suggest that CF glands do not display excessive, ENaC-mediated fluid absorption, leaving defective, anion-mediated fluid secretion as the most likely mechanism for defective mucus secretion from CF glands.

Submucosal glands and airway defense

Most airway mucus is produced by submucosal glands in response to neural signals. Gland mucus traps microbes, inhibits their replication, and clears them from the airways. In cystic fibrosis mucus clearance is compromised, allowing pathogens to persist in static mucus. These trigger an influx of inflammatory cells, but optimal effectiveness of inflammation, and especially its resolution, also requires effective mucus clearance. Our objective is to understand the basis for defective mucus clearance in cystic fibrosis. We discovered that in subjects with cystic fibrosis, submucosal gland secretion in response to agents that elevate intracellular cyclic AMP level is completely lost and mucus stimulated by elevating intracellular Ca2+ level is thicker. We hypothesize that loss of functional cystic fibrosis transmembrane conductance regulator from gland serous cells renders them unable to secrete anions and fluid in response to any stimulus, resulting in thickened gland mucus that can be tethered to the gland ducts. In primary ciliary dyskinesias, mucus is normal, but the dysfunctional cilia lining the gland ducts may also lead to inadequate clearance of mucus from glands. Thus, understanding of lung pathology in each disease may require that an improved understanding of gland structure and function be added to our rapidly growing understanding of surface epithelia.

An inwardly rectifying potassium channel in apical membrane of Calu-3 cells

Patch clamp methods and reverse transcription-polymerase chain reaction (RT-PCR) were used to characterize an apical K+ channel in Calu-3 cells, a widely used model of human airway gland serous cells. In cell-attached and excised apical membrane patches, we found an inwardly rectifying K+ channel (Kir). The permeability ratio was PNa/PK = 0.058. In 30 patches with both cystic fibrosis transmembrane conductance regulator and Kir present, we observed 79 cystic fibrosis transmembrane conductance regulator and 58 Kir channels. The average chord conductance was 24.4 +/- 0.5 pS (n = 11), between 0 and -200 mV, and was 9.6 +/- 0.7 pS (n = 8), between 0 and 50 mV; these magnitudes and their ratio of approximately 2.5 are most similar to values for rectifying K+ channels of the Kir4.x subfamilies. We attempted to amplify transcripts for Kir4.1, Kir4.2, and Kir5.1; of these only Kir4.2 was present in Calu-3 lysates. The channel was only weakly activated by ATP and was relatively insensitive to internal pH. External Cs+ and Ba2+ blocked the channel with Kd values in the millimolar range. Quantitative modeling of Cl- secreting epithelia suggests that secretion rates will be highest and luminal K+ will rise to 16-28 mm if 11-25% of the total cellular K+ conductance is placed in the apical membrane (Cook, D. I., and Young, J. A. (1989) J. Membr. Biol. 110, 139-146). Thus, we hypothesize that the K+ channel described here optimizes the rate of secretion and is involved in K+ recycling for the recently proposed apical H+ -K+ -ATPase in Calu-3 cells.

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.

Regulation of Antiprotease and Antimicrobial Protein Secretion by Airway Submucosal Gland Serous Cells

Airway submucosal gland serous cells express the cystic fibrosis transmembrane conductance regulator (CFTR) and secrete antimicrobial, anti-inflammatory, and antioxidant molecules. In cystic fibrosis, diminished gland secretion may impair innate airway host defenses. We used Calu-3 cells as a serous cell model to study the types of proteins released, the pathways that release them, and the possible involvement of CFTR activity in protein release. Many proteins were secreted constitutively into the apical fluid and showed increased release to agonists. We identified some of them by high pressure liquid chromatography-mass spectrometry and reverse transcriptase PCR, including lysozyme, siderocalin (the protein NGAL), which inhibits bacterial growth by binding iron-containing siderophores, HSC-71, which is thought to have anti-inflammatory properties, and the serine protease inhibitors alpha-1-antitrypsin and alpha-1-antichymotrypsin, which may function as antimicrobials as well as play a potential role in diminishing the activation of epithelial Na(+) channels by serine proteases. We used an enzyme-linked immunosorbent assay to quantify lysozyme secretion by Calu-3 cells in response to various agonists and inhibitors. Forskolin increased the lysozyme secretion rate (J(lyz)) from 32 to 77 ng/hr/cm(2) (n = 36, p < 0.005). Thapsigargin increased J(lyz) from 40 to 63 ng/h/cm(2) (n = 16, p < 0.005), and forskolin plus thapsigargin further increased the forskolin-stimulated J(lyz) by 48% (n = 9, p < 0.05). 1-Ethyl-benzimidazolinone and carbachol were less effective. Glibenclamide inhibited basal and stimulated J(lyz), but clotrimazole was without effect. CFTR(inh)172 caused a small (15%) but significant inhibition of forskolin-stimulated J(lyz) without affecting basal J(lyz). Thus, Calu-3 cells secrete diverse proteins that in aggregate would be expected to suppress microbial growth, protect the airways from damage, and limit the activation of epithelial Na(+) channels via serine proteases.

Acid and base secretion in the Calu-3 model of human serous cells

Submucosal glands are the primary source of airway mucus, a critical component of lung innate defenses. Airway glands are defective in cystic fibrosis (CF), showing a complete absence of secretion to vasoactive intestinal peptide or forskolin, which increase intracellular cAMP concentration. This defect is attributed to gland serous cells, which express the cystic fibrosis transmembrane conductance regulator. Calu-3 cells, which mimic many features of serous cells, secrete Cl(-) and HCO(3)(-), with HCO(3)(-) secretion predominating for forskolin stimulation and Cl(-) secretion predominating for stimuli that open basolateral K(+) channels to hyperpolarize the cells. We used pH stat and ion substitution experiments to clarify the mechanisms and consequences of these two modes of secretion. We confirm that Calu-3 cells secrete primarily HCO(3)(-) in response to forskolin. Unexpectedly, HCO(3)(-) secretion continued in response to K(+) channel openers, with Cl(-) secretion being added to it. Secretion of HCO(3)(-) from hyperpolarized cells occurs via the conversion of CO(2) to HCO(3)(-) and is reduced by approximately 50% with acetazolamide. A gap between the base equivalent current and short-circuit current was observed in all experiments and was traced to secretion of H(+) via a ouabain-sensitive, K(+)-dependent process (possibly H(+)-K(+)-ATPase), which partially neutralized the secreted HCO(3)(-). The conjoint secretion of HCO(3)(-) and H(+) may help explain the puzzling finding that mucus secreted from normal and CF glands has the same acidic pH as does mucus from glands stimulated with forskolin or ACh. It may also help explain how human airway glands produce mucus that is hypotonic.

Absent secretion to vasoactive intestinal peptide in cystic fibrosis airway glands

We are testing the hypothesis that the malfunctioning of airway gland serous cells is a component of cystic fibrosis (CF) airway disease. CF is caused by mutations that disrupt CF transmembrane conductance regulator, an anion channel essential for proper fluid secretion in some epithelia. Submucosal glands supply most of the mucus in upper airways, and gland serous cells are the primary site of CF transmembrane conductance regulator expression in airways. We have discovered a major defect in CF glands by in situ optical monitoring of secretions from single human airway glands. CF glands did not secrete to agents that elevated [cAMP](i) (0 responses/450 glands, 8 subjects), whereas glands were responsive in all donor tracheas (605/827 glands, 15 subjects) and in bronchi from subjects who were transplanted because of other lung diseases (148/166 glands, n = 10). CF glands secreted to cholinergic stimulation, and serous cells were abundant in glands from all CF subjects. The complete absence of secretion to agents that elevate [cAMP](i) suggests that altered secretion of gland mucus could contribute to CF lung disease.

Mucus secretion from single submucosal glands of pig. Stimulation by carbachol and vasoactive intestinal peptide

Secretion rates of >700 individual glands in isolated tracheal mucosa from 56 adult pigs were monitored optically. "Basal" secretion of 0.7 +/- 0.1 nl x min(-1) gland(-1) was observed 1-9 h post-harvest but was near zero on day 2. Secretion to carbachol (10 microm) peaked at 2-3 min and then declined to a sustained phase. Peak secretion was 12.4 +/- 1.1 nl x min(-1) gland(-1); sustained secretion was approximately one-third of peak secretion. Thapsigargin (1 microm) increased secretion from 0.1 +/- 0.05 to 0.7 +/- 0.2 nl x min(-1) gland(-1); thapsigargin did not cause contraction of the trachealis muscles. Isoproterenol and phenylephrine (10 microm each) were ineffective, but vasoactive intestinal peptide (1 microm) and forskolin (10 microm) each produced sustained secretion of 1.0 +/- 0.5 and 1.7 +/- 0.2 nl x min(-1) gland(-1), respectively. The density of actively secreting glands was 1.3/mm(2). Secretion to either carbachol or forskolin was inhibited (approximately 50%) by either bumetanide or HCO(3)(-) removal and inhibited approximately 90% by the combined treatments. Mucus secreted in response to carbachol or forskolin was acidic by approximately 0.2 pH units relative to the bath and remained acidic by approximately 0.1 pH units after bumetanide. The strong secretory response to vasoactive intestinal peptide, the acidity of [cAMP](i)-stimulated mucus, and its inhibition by bumetanide were unexpected.

A phase II, double-blind, randomized, placebo-controlled clinical trial of tgAAVCF using maxillary sinus delivery in patients with cystic fibrosis with antrostomies

tgAAVCF, an adeno-associated cystic fibrosis transmembrane conductance regulator (CFTR) viral vector/gene construct, was administered to 23 patients in a Phase II, double-blind, randomized, placebo-controlled clinical trial. For each patient, a dose of 100,000 replication units of tgAAVCF was administered to one maxillary sinus, while the contralateral maxillary sinus received a placebo treatment, thereby establishing an inpatient control. Neither the primary efficacy endpoint, defined as the rate of relapse of clinically defined, endoscopically diagnosed recurrent sinusitis, nor several secondary endpoints (sinus transepithelial potential difference [TEPD], histopathology, sinus fluid interleukin [IL]-8 measurements) achieved statistical significance when comparing treated to control sinuses within patients. One secondary endpoint, measurements of the anti-inflammatory cytokine IL-10 in sinus fluid, was significantly (p < 0.03) increased in the tgAAVCF-treated sinus relative to the placebo-treated sinus at day 90 after vector instillation. The tgAAVCF administration was well tolerated, without adverse respiratory events, and there was no evidence of enhanced inflammation in sinus histopathology or alterations in serum-neutralizing antibody titer to adeno-associated virus (AAV) capsid protein after vector administration. In summary, this Phase II trial confirms the safety of tgAAVCF but provides little support of its efficacy in the within-patient controlled sinus study. Various potentially confounding factors are discussed.

Evidence that CFTR channels can regulate the open duration of other CFTR channels: cooperativity

CFTR channels mediate secretion and absorption in epithelia, and cystic fibrosis is caused by their malfunction. CFTR proteins are members of the ABC transporter family and are complexly regulated by phosphorylation and nucleosides; they also influence other channel activity. Do CFTR molecules also influence one another? Cooperativity has been observed among other channels and has been suggested for CFTR. Therefore, we looked for evidence of cooperativity among CFTR channels using three independent approaches. All three methods provided evidence for cooperativity in CFTR gating. We estimated mean open times, independent of the number of channels in the patch, in multi-channel patches and showed that, on average, they increased as channel number increased. We observed many trials having larger than expected variances, consistent with cooperative gating. We also measured deviations from binomial statistics, which revealed cooperativity and further indicated that its magnitude is underestimated to an unknown extent because of masking that occurs when CFTR channel populations within a single patch have heterogeneous open probabilities. Simulations showed that the observed departures from binomial statistics were too large to have arisen by chance. The evidence that CFTR P(o) increases with channel density has important functional implications.

Optical method for quantifying rates of mucus secretion from single submucosal glands

We describe an optical method to quantify single- gland secretion. Isolated tracheal mucosa were mounted at the air-Krebs interface and coated with oil. Gland secretions formed spherical bubbles that were digitally imaged at intervals, allowing rates of secretion to be calculated. We monitored 340 glands in 54 experiments with 12 sheep. Glands secreted basally at low rates (0.57 +/- 0.04 nl x min(-1) x gland(-1), 123 glands) in tissues up to 9 h postharvest and at lower rates for up to 3 days. Carbachol (10 microM) stimulated secretion with an early transient and a sustained or oscillating phase. Peak secretion was 15.7 +/- 1.2 nl x min(-1) x gland(-1) (60 glands); sustained secretion was 4.5 +/- 0.5 nl x min(-1) x gland(-1) (10 glands). Isoproterenol and phenylephrine (10 microM each) stimulated only small, transient responses. We confirmed that cats have a large secretory response to phenylephrine (11.6 +/- 3.7 nl x min(-1) x gland(-1), 12 glands), but pigs, sheep, and humans all have small responses (<2 nl x min(-1)m x gland(-1)). Carbachol-stimulated peak secretion was inhibited 56% by bumetanide, 67% by HCO replacement with HEPES, and 92% by both. The distribution of secretion rates was nonnormal, suggesting the existence of subpopulations of glands.

Submucosal gland secretions in airways from cystic fibrosis patients have normal [Na(+)] and pH but elevated viscosity

Fluid and macromolecule secretion by submucosal glands in mammalian airways is believed to be important in normal airway physiology and in the pathophysiology of cystic fibrosis (CF). An in situ fluorescence method was applied to measure the ionic composition and viscosity of freshly secreted fluid from airway glands. Fragments of human large airways obtained at the time of lung transplantation were mounted in a humidified perfusion chamber and the mucosal surface was covered by a thin layer of oil. Individual droplets of secreted fluid were microinjected with fluorescent indicators for measurement of [Na(+)], [Cl(-)], and pH by ratio imaging fluorescence microscopy and viscosity by fluorescence recovery after photobleaching. After carbachol stimulation, 0.1--0.5 microl of fluid accumulated in spherical droplets at gland orifices in approximately 3--5 min. In gland fluid from normal human airways, [Na(+)] was 94 +/- 8 mM, [Cl(-)] was 92 +/- 12 mM, and pH was 6.97 +/- 0.06 (SE, n = 7 humans, more than five glands studied per sample). Apparent fluid viscosity was 2.7 +/- 0.3-fold greater than that of saline. Neither [Na(+)] nor pH differed in gland fluid from CF airways, but viscosity was significantly elevated by approximately 2-fold compared to normal airways. These results represent the first direct measurements of ionic composition and viscosity in uncontaminated human gland secretions and indicate similar [Na(+)], [Cl(-)], and pH to that in the airway surface liquid. The elevated gland fluid viscosity in CF may be an important factor promoting bacterial colonization and airway disease.

HCO3- transport in relation to mucus secretion from submucosal glands

The role of HCO(3)(-) transport in relation to fluid secretion by submucosal glands is being studied in sheep, pigs, cats and humans. Optical methods have been developed to measure secretion rates of mucus volume from single glands with sufficient temporal resolution to detect differences in minute-by-minute secretion rates among glands. The ionic composition and viscoelastic properties of the uncontaminated gland mucus are measured with a combination of ratiometric fluorescent indicators, ion-selective microelectrodes, FRAP, and a miniaturized, magnetic force viscometer. Sheep glands secreted basally at low rates, showed small, transient responses to alpha- and beta-adrenergic agonists, and large responses to a cholinergic agonist, carbachol. Peak rates and temporal patterns of responses to carbachol differed markedly among glands. To assess the contribution of HCO(3)(-) transport to gland secretion, we either inhibited Na(+)/K(+)/2Cl(-) cotransporter (NKCC) with bumetanide or replaced HCO(3)(-) with HEPES and gassed with O(2). Bumetanide caused a small, non-significant inhibition of basal secretion, but removal of HCO(3)(-)/CO(2) significantly reduced basal secretion almost by half. Both bumetanide and removal of HCO(3)(-)/CO(2) reduced carbachol-stimulated secretion significantly, with HCO(3)(-) removal having the larger effect: a reduction to 33% of control (P<0.01). The remaining secretory response to carbachol was nearly eliminated by bumetanide. Sheep mucus pH measured with ion selective electrodes was about 0.4 log more acidic than the bath. In humans, we observed the same pattern of responses to agonists and antagonists as in sheep, and observed a mucus pH of 7.0 using 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein (BCECF). We hypothesize that HCO(3)(-) transport is important in the formation of mucus secretion, but that most HCO(3)(-) is scavenged before the final mucus appears at the duct opening. Cystic fibrosis transmembrane conductance regulator's (CFTR) best understood function is as an anion channel, but increasing attention has been given to its role in HCO(3)(-) transport. By analogy with organ-specific CFTR effects on Cl(-) transport, it seems likely that the relative importance of CFTR in HCO(3)(-) transport will also vary across organs. Because lung disease is by far the greatest cause of mortality among people with cystic fibrosis, it is important to determine how loss of CFTR function causes lung disease. We are testing the hypothesis that loss of CFTR alters serous cell secretion in the lungs, and the corollary that such loss contributes to cystic fibrosis (CF) lung disease. CFTR is highly expressed in serous cells of submucosal glands and the Calu-3 serous cell model secretes HCO(3)(-). Human gland serous cells grown in culture and tested for fluid secretion under open circuit conditions showed reduced fluid secretion to all mediators. However, submucosal glands are complex organs containing at least 4 distinct regions and at least that many cell types, making it difficult to predict the consequences on whole-organ function from experiments with individual cell types. Therefore, we have resurrected long-neglected methods for studying whole-gland function, and have attempted to improve them in a variety of ways. We are refining these methods and increasing our understanding of gland function by studying tracheal glands from sheep, pigs and cats. As human tissues become available, they are studied with the best methods presently available. The key questions now being asked are: Is mucus secretion from submucosal glands altered in cystic fibrosis? If so, how is it altered and how does it contribute to CF lung disease? Answering the last question will require an understanding of how glands interact with other regions of the lung. In the context of this meeting, we present preliminary data on the role of HCO(3)(-) in gland mucus secretion.

Cystic fibrosis: the 'bicarbonate before chloride' hypothesis

The specific effects of some mutations that cause cystic fibrosis suggest that reduced HCO(3)(-) transport is the key to understanding cystic fibrosis pathology. But there is a puzzling discrepancy between measures of CFTR-mediated chloride conductance in expression systems and the sweat chloride values of patients.

Cystic fibrosis transmembrane conductance regulator gating requires cytosolic electrolytes

Cystic fibrosis transmembrane conductance regulator (CFTR), which causes cystic fibrosis when nonfunctional, is an anion channel and a member of the ATP binding cassette superfamily. After phosphorylation, CFTR gates by binding and hydrolyzing ATP. We show that CFTR open probability (P(o)) also depends on the electrolyte concentration of the cytosol. Inside-out patches from Calu-3 cells were transiently exposed to solutions of 160 mm salt or solutions in which up to 90% of the salt was replaced by nonionic osmolytes such as sucrose. In lowered salt solutions, CFTR P(o) declined within 1 s to a stable lower value that depended on the electrolyte concentration, (K(1/2) approximately 80 mm NaCl). P(o) was rapidly restored in normal salt concentrations without regard to the electrolyte species. Reducing external electrolytes did not affect CFTR P(o). The same results were obtained when CFTR was stably phosphorylated with adenosine 5'-O-(thiotriphosphate). The decrease in P(o) resulted entirely from an increase in mean closed time. Increasing ATP levels up to 20-fold did not counteract the effect of low electrolytes. The same effect was observed for CFTR expressed in C127 cells but not for a different species of anion channel. Cytosolic electrolytes are an unsuspected, essential cofactor for CFTR gating.