Mammalian airways normally regulate the volume of a thin liquid layer, the periciliary liquid (PCL), to facilitate the mucus clearance component of lung defense. Studies under standard (static) culture conditions revealed that normal airway epithelia possess an adenosine-regulated pathway that blends Na ؉ absorption and Cl ؊ secretion to optimize PCL volume. In cystic fibrosis (CF), the absence of CF transmembrane conductance regulator results in a failure of adenosine regulation of PCL volume, which is predicted to initiate mucus stasis and infection. However, under conditions that mimic the phasic motion of the lung in vivo, ATP release into PCL was increased, CF ion transport was rebalanced, and PCL volume was restored to levels adequate for lung defense. This ATP signaling system was vulnerable, however, to insults that trigger CF bacterial infections, such as viral (respiratory syncitial virus) infections, which up-regulated extracellular ATPase activity and abolished motion-dependent ATP regulation of CF PCL height. These studies demonstrate (i) how the normal coordination of opposing ion transport pathways to maintain PCL volume is disrupted in CF, (ii) the hitherto unknown role of phasic motion in regulating key aspects of normal and CF innate airways defense, and (iii) that maneuvers directed at increasing motion-induced nucleotide release may be therapeutic in CF patients.The lung must continually defend itself against bacteria that deposit on airway surfaces during normal tidal breathing. It appears that mechanical clearance of bacteria mediated by mucus transport is the principal innate defense mechanism of mammalian airways (1-4). Recent data have shown that a critical component of this defense system is the thin (ϳ7) m liquid layer lining airway surfaces, the periciliary liquid (PCL), 2 that provides a low viscosity solution for ciliary beating and acts a lubricant layer for mucus transport (5, 6). In cystic fibrosis (CF) lung disease, it appears that the primary pathophysiologic defect is the depletion of PCL volume, resulting in a failure of mucus clearance of bacteria and persistent airways infection (7,8).However, questions have been raised as to the relevance of PCL depletion to CF pathogenesis in vivo (9). For example, whereas in vitro data from standard (static) culture systems describe rapid depletion of PCL height and a complete failure of mucus transport (7), young CF patients exhibit reduced but measurable rates of mucus clearance in vivo (10). This inconsistency suggests that mechanisms for PCL height regulation operating in vivo are absent from standard static culture systems. In addition, clinical observations suggest that CF lung disease exacerbates intermittently and is heterogeneous. Often, viral infections trigger these disease exacerbations (11, 12), but no links between viral infection and PCL regulation have been reported.To investigate these questions, we used a well differentiated airway epithelial culture system that exhibits PCL volume regulation and mucus transport (7). Ba...

In the lungs, the first line of defence against bacterial infection is the thin layer of airway surface liquid (ASL) lining the airway surface. The superficial airway epithelium exhibits complex regulatory pathways that blend ion transport to adjust ASL volume to maintain proper mucociliary clearance (MCC). We hypothesized that stresses generated by airflow and transmural pressures during breathing govern ASL volume by regulating the rate of epithelial ATP release. Luminal ATP, via interactions with apical membrane P2-purinoceptors, regulates the balance of active ion secretion versus absorption to maintain ASL volume at optimal levels for MCC. In this study we tested the hypothesis that cyclic compressive stress (CCS), mimicking normal tidal breathing, regulates ASL volume in airway epithelia. Polarized tracheobronchial epithelial cultures from normal and cystic fibrosis (CF) subjects responded to a range of CCS by increasing the rate of ATP release. In normal airway epithelia, the CCS-induced increase in ASL ATP concentration was sufficient to induce purinoceptor-mediated increases in ASL height and MCC, via inhibition of epithelial Na + -channel-mediated Na + absorption and stimulation of Cl − secretion through CFTR and the Ca 2+ -activated chloride channels. In contrast, static, non-oscillatory stress did not stimulate ATP release, ion transport or MCC, emphasizing the importance of rhythmic mechanical stress for airway defence. In CF airway cultures, which exhibit basal ASL depletion, CCS was partially effective, producing less ASL volume secretion than in normal cultures, but a level sufficient to restore MCC. The present data suggest that CCS may (1) regulate ASL volume in the normal lung and (2) improve clearance in the lungs of CF patients, potentially explaining the beneficial role of exercise in lung defence.

Dinucleoside polyphosphates act as agonists on purinergic P2Y receptors to mediate a variety of cellular processes. Symmetrical, naturally occurring purine dinucleotides are found in most living cells and their actions are generally known. Unsymmetrical purine dinucleotides and all pyrimidine containing dinucleotides, however, are not as common and therefore their actions are not well understood. To carry out a thorough examination of the activities and specificities of these dinucleotides, a robust method of synthesis was developed to allow manipulation of either nucleoside of the dinucleotide as well as the phosphate chain lengths. Adenosine containing dinucleotides exhibit some level of activity on P2Y 1 while uridine containing dinucleotides have some level of agonist response on P2Y 2 and P2Y 6 . The length of the linking phosphate chain determines a different specificity; diphosphates are most accurately mimicked by dinucleoside triphosphates and triphosphates most resemble dinucleoside tetraphosphates. The pharmacological activities and relative metabolic stabilities of these dinucleotides are reported with their potential therapeutic applications being discussed.

-(2Ј-deoxycytidine 5Ј)tetraphosphate, tetrasodium salt] is a deoxycytidine-uridine dinucleotide with agonist activity at the P2Y 2 receptor. In primate lung tissues, the P2Y 2 receptor mRNA was located by in situ hybridization predominantly in epithelial cells and not in smooth muscle or stromal tissue. The pharmacologic profile of INS37217 parallels that of UTP, leading to increased chloride and water secretion, increased cilia beat frequency, and increased mucin release. The combined effect of these actions was confirmed in an animal model of tracheal mucus velocity that showed that a single administration of INS37217 significantly enhanced mucus transport for at least 8 h after dosing. This extended duration of action is consistent with the ability of INS37217 to resist metabolism by airway cells and sputum enzymes. The enhanced metabolic stability and resultant increased duration of improved mucociliary clearance may confer significant advantages to INS37217 over other P2Y 2 agonists in the treatment of diseases such as cystic fibrosis.Cystic fibrosis (CF) is a recessive genetic disease, characterized by pulmonary and reproductive tract dysfunctions, which affects more than 30,000 people in the United States . CF is caused by mutations in the CF transmembrane regulator (CFTR) gene, which encodes for an apical membrane epithelial protein that functions both as a cAMP-regulated chloride channel and a regulator of the epithelial sodium channel (Boucher, 1994). A defective CFTR leads to abnormal fluid and solute transport across epithelia, which contributes to the formation of viscous, dehydrated mucus in airways. The resulting mucostasis leads to progressive loss of ventilatory function and severe inflammatory responses to chronic bacterial infection (Mickle and Cutting, 1998). Most deaths of patients with CF occur as a consequence of pulmonary disease. Although improved treatment of lung disease has increased longevity, the median age for survival is still only 32 years, and patients have significant morbidity, including frequent hospitalizations, throughout their lives (Ramsey, 1996). Current therapies for CF include inhaled antibiotics, bronchodilators, mucolytics, and physiotherapy. Clearly, new therapeutic approaches are needed for the prevention and treatment of CF lung disease.An emerging therapeutic paradigm for the treatment of Article, publication date, and citation information can be found at

The concentration of ATP in airway surface liquid is in a range that is relevant for the activation of airway nucleotide receptors. However, despite this finding that suggests endogenous nucleotides may be important for the regulation of mucociliary clearance, our data do not support a role for CFTR in regulating extracellular nucleotide concentrations on airway surfaces.

In human airways, extracellular adenosine regulates epithelial functions supporting mucociliary clearance, an important airway defense mechanism against bacterial infection. Thus, defining the mechanisms of adenosine generation is critical for elucidating the role of this nucleoside in airway homeostasis. In this study, we identified the source of adenosine on the mucosal surface of human airway epithelia. Polarized primary cultures of human nasal or bronchial epithelial cells were assayed for transepithelial transport, cytosolic and cell surface adenosine production. Ussing chamber experiments indicated that serosal 1 M ). In bronchial cultures and tissues, ecto 5-NT accounted for >80% of total activity toward 0.01 mM AMP, compared with <15% for 5 mM AMP. The proximal airway AP isoform was identified as nonspecific AP (NS AP) by levamisole sensitivity and mRNA expression. The two ectoenzymes presented opposite airway distributions, ecto 5-NT and NS AP mRNA dominating in higher and lower airways, respectively. Collectively, these experiments support a major role for extracellular nucleotide catalysis and for ecto 5-NT and NS AP in the regulation of adenosine concentrations on airway surfaces. Mucociliary clearance (MCC)1 constitutes an essential component of airway defense against the development of infectious lung diseases (1). Several epithelial functions involved in MCC are regulated by extracellular nucleotides. For instance, P2Y 2 receptor activation by ATP or UTP-stimulated Ca 2ϩ -dependent Cl Ϫ channels (I CA ) (2, 3), cilia beating frequency (CBF) (4, 5), and mucin secretion from goblet cells and submucosal glands (6, 7). Two members of the P2X receptor subfamily were recently identified in human airway epithelial cultures: P2X 4 and P2X 5 (8). These ATP-gated cationic channels increased Ca 2ϩ -dependent Cl Ϫ secretion (8) and CBF (9) in airway epithelia. Interestingly, extracellular adenosine was found to be responsible for regulating the post-peak sustained phase of increased CBF induced by ATP on human nasal explants (4). In a human bronchial cell line lacking P2Y 2 receptors (Calu-3; Ref. 10), the channel activity of the cystic fibrosis transmembrane regulator was inhibited by 8-p-sulfophenyltheophylline, a nonspecific blocker of cell surface adenosine receptors (11). Subsequently, adenosine was shown to regulate CBF (4, 5) and ion transport (12-15) through activation of cell surface A 2B receptors.The importance of adenosine receptor-mediated regulation of MCC remains unclear because of the lack of information on endogenous sources of extracellular adenosine on the mucosal surface of airway epithelia. Adenosine could originate from the interstitial compartment and penetrate airway epithelial tight junctions to reach the lumen. The nucleoside could also be generated intracellularly by the cytosolic AMP-specific 5Ј-nucleotidase (CN-I) (16,17) and reach the mucosal surface through nucleoside transporters (for review see Ref. 18). Alternatively, ATP release and cell surface conversion into adenosine has...

There is controversy over whether abnormalities in the salt concentration or volume of airway surface liquid (ASL) initiate cystic fibrosis (CF) airway disease. In vivo studies of CF mouse nasal epithelia revealed an increase in goblet cell number that was associated with decreased ASL volume rather than abnormal [Cl(-)]. Aerosolization of osmolytes in vivo failed to raise ASL volume. In vitro studies revealed that osmolytes and pharmacological agents were effective in producing isotonic volume responses in human airway epithelia but were typically short acting and less effective in CF cultures with prolonged volume hyperabsorption and mucus accumulation. These data show that (1) therapies can be designed to normalize ASL volume, without producing deleterious compositional changes in ASL, and (2) therapeutic efficacy will likely depend on development of long-acting pharmacologic agents and/or an increased efficiency of osmolyte delivery.

5'-Nucleoside triphosphates (NTP) are present in the liquid covering airway surfaces and mediate important physiologic events through their interaction with P2-nucleotide receptors. Activation of airway P2Y(2) receptors, for example, stimulates ciliary beat frequency, chloride/liquid secretion, and goblet cell degranulation. We, therefore, have studied the metabolic pathways that regulate the concentration of nucleotides on airway surfaces. Stimulation of submucosal gland secretion in the nose was previously found to decrease the concentration of 5'-adenosine triphosphate (ATP) in nasal lavage samples due to the presence of a secreted 5'-nucleoside triphosphatase (NTPase). In this study, gland secretions were further studied and found to also contain adenylate kinase (AK) and nucleoside diphosphokinase (NDPK) activities. Ecto-AK and ecto-NDPK activities were also detected in well-differentiated cultures of superficial nasal epithelia, which reflected a combination of cell-associated and released (into culture media) AK and NDPK activities. This study demonstrates that "ecto-kinases" on airway surfaces (1) emanate from different enzyme families, including both AK and NDPK; (2) are expressed at superficial epithelial surfaces and in submucosal glands; and (3) may be important regulators of nucleotide concentrations on airway surfaces.