Dopamine activates amiloride-sensitive sodium channels in alveolar type I cells in lung slice preparations

Rescue of alveolar wall liquid secretion blocks fatal lung injury due to influenza-staphylococcal coinfection

Secondary lung infection by inhaled Staphylococcus aureus (SA) is a common and lethal event for individuals infected with influenza A virus (IAV). How IAV disrupts host defense to promote SA infection in lung alveoli, where fatal lung injury occurs, is not known. We addressed this issue using real-time determinations of alveolar responses to IAV in live, intact, perfused lungs. Our findings show that IAV infection blocked defensive alveolar wall liquid (AWL) secretion and induced airspace liquid absorption, thereby reversing normal alveolar liquid dynamics and inhibiting alveolar clearance of inhaled SA. Loss of AWL secretion resulted from inhibition of the cystic fibrosis transmembrane conductance regulator (CFTR) ion channel in the alveolar epithelium, and airspace liquid absorption was caused by stimulation of the alveolar epithelial Na+ channel (ENaC). Loss of AWL secretion promoted alveolar stabilization of inhaled SA, but rescue of AWL secretion protected against alveolar SA stabilization and fatal SA-induced lung injury in IAV-infected mice. These findings reveal a central role for AWL secretion in alveolar defense against inhaled SA and identify AWL inhibition as a critical mechanism of IAV lung pathogenesis. AWL rescue may represent a new therapeutic approach for IAV-SA coinfection.

Ferulic acid ameliorates lipopolysaccharide-induced tracheal injury via cGMP/PKGII signaling pathway

BACKGROUND

Tracheal injury is a common clinical condition that still lacks an effective therapy at present. Stimulation of epithelial sodium channel (ENaC) increases Na⁺ transport, which is a driving force to keep tracheal mucosa free edema fluid during tracheal injury. Ferulic acid (FA) has been proved to be effective in many respiratory diseases through exerting anti-oxidant, anti-inflammatory, and anti-thrombotic effects. However, these studies rarely involve the level of ion transport, especially ENaC.

METHODS

C57BL/J male mice were treated intraperitoneally with normal saline or FA (100 mg/kg) 12 h before, and 12 h after intratracheal administration of lipopolysaccharide (LPS, 5 mg/kg), respectively. The effects of FA on tracheal injury were not only assessed through HE staining, immunofluorescence assay, and protein/mRNA expressions of ENaC located on tracheas, but also evaluated by the function of ENaC in mouse tracheal epithelial cells (MTECs). Besides, to explore the detailed mechanism about FA involved in LPS-induced tracheal injury, the content of cyclic guanosine monophosphate (cGMP) was measured, and Rp-cGMP (cGMP inhibitor) or cGMP-dependent protein kinase II (PKGII)-siRNA (siPKGII) were applied in primary MTECs, respectively.

RESULTS

Histological examination results demonstrated that tracheal injury was obviously attenuated by pretreatment of FA. Meanwhile, FA could reverse LPS-induced reduction of both protein/mRNA expressions and ENaC activity. ELISA assay verified cGMP content was increased by FA, and administration of Rp-cGMP or transfection of siPKGII could reverse the FA up-regulated ENaC protein expression in MTECs.

CONCLUSIONS

Ferulic acid can attenuate LPS-induced tracheal injury through up-regulation of ENaC at least partially via the cGMP/PKGII pathway, which may provide a promising new direction for preventive and therapeutic strategy in tracheal injury.

Phosphodiesterase 4 mediates interleukin-8-induced heterologous desensitization of the β2 -adrenergic receptor

Acute respiratory distress syndrome (ARDS) is a life-threatening illness characterized by decreased alveolar-capillary barrier function, pulmonary edema consisting of proteinaceous fluid, and inhibition of net alveolar fluid transport responsible for resolution of pulmonary edema. There is currently no pharmacotherapy that has proven useful to prevent or treat ARDS, and two trials using beta-agonist therapy to treat ARDS demonstrated no effect. Prior studies indicated that IL-8-induced heterologous desensitization of the beta2-adrenergic receptor (β₂ -AR) led to decreased beta-agonist-induced mobilization of cyclic adenosine monophosphate (cAMP). Interestingly, phosphodiesterase (PDE) 4 inhibitors have been used in human airway diseases characterized by low intracellular cAMP levels and increases in specific cAMP hydrolyzing activity. Therefore, we hypothesized that PDE4 would mediate IL-8-induced heterologous internalization of the β₂ -AR and that PDE4 inhibition would restore beta-agonist-induced functions. We determined that CINC-1 (a functional IL-8 analog in rats) induces internalization of β₂ -AR from the cell surface, and arrestin-2, PDE4, and β₂ -AR form a complex during this process. Furthermore, we determined that cAMP associated with the plasma membrane was adversely affected by β₂ -AR heterologous desensitization. Additionally, we determined that rolipram, a PDE4 inhibitor, reversed CINC-1-induced derangements of cAMP and also caused β₂ -AR to successfully recycle back to the cell surface. Finally, we demonstrated that rolipram could reverse CINC-1-mediated inhibition of beta-agonist-induced alveolar fluid clearance in a murine model of trauma-shock. These results indicate that PDE4 plays a role in CINC-1-induced heterologous internalization of the β₂ -AR; PDE4 inhibition reverses these effects and may be a useful adjunct in particular ARDS patients.

DRD1 downregulation contributes to mechanical stretch-induced lung endothelial barrier dysfunction

Rationale: The lung-protective effects of dopamine and its role in the pathology of ventilator-induced lung injury (VILI) are emerging. However, the underlying mechanisms are still largely unknown.

Objective: To investigate the contribution of dopamine receptor dysregulation in the pathogenesis of VILI and therapeutic potential of dopamine D1 receptor (DRD1) agonist in VILI.

Methods: The role of dopamine receptors in mechanical stretch-induced endothelial barrier dysfunction and lung injury was studied in DRD1 knockout mice, in isolated mouse lung vascular endothelial cells (MLVECs), and in lung samples from patients who underwent pulmonary lobectomy with mechanical ventilation for different time periods.

Measurements and Main Results: DRD1 was downregulated in both surgical patients and mice exposed to mechanical ventilation. Prophylactic administration of dopamine or DRD1 agonist attenuated mechanical stretch-induced lung endothelial barrier dysfunction and lung injury. By contrast, pulmonary knockdown or global knockout of DRD1 exacerbated these effects. Prophylactic administration of dopamine attenuated mechanical stretch-induced α-tubulin deacetylation and subsequent endothelial hyperpermeability through DRD1 signaling. We identified that cyclic stretch-induced glycogen-synthase-kinase-3β activation led to phosphorylation and activation of histone deacetylase 6 (HDAC6), which resulted in deacetylation of α-tubulin. Upon activation, DRD1 signaling attenuated mechanical stretch-induced α-tubulin deacetylation and subsequent lung endothelial barrier dysfunction through cAMP/exchange protein activated by cAMP (EPAC)-mediated inactivation of HDAC6.

Conclusions: This work identifies a novel protective role for DRD1 against mechanical stretch-induced lung endothelial barrier dysfunction and lung injury. Further study of the mechanisms involving DRD1 in the regulation of microtubule stability and interference with DRD1/cAMP/EPAC/HDAC6 signaling may provide insight into therapeutic approaches for VILI.

High-mobility group box-1 increases epithelial sodium channel activity and inflammation via the receptor for advanced glycation end products

Cystic fibrosis (CF) lung disease persists and remains life-limiting for many patients. Elevated high-mobility group box-1 protein (HMGB-1) levels and epithelial sodium channel hyperactivity (ENaC) are hallmark features of the CF lung. The objective of this study was to better understand the pathogenic role of HMGB-1 signaling and ENaC in CF airway cells. We hypothesize that HMGB-1 links airway inflammation [via signaling to the receptor for advanced glycation end products (RAGE)] and airway surface liquid dehydration (via upregulation of ENaC) in the CF lung. We calculated equivalent short-current (I_sc) and single-channel ENaC open probability (P_o) in normal and CF human small airway epithelial cells (SAEC) in the presence and absence of human HMGB-1 peptide (0.5 μg/mL). In normal SAECs, HMGB-1 increased amiloride-sensitive I_sc and elevated ENaC P_o from 0.15 ± 0.03 to 0.28 ± 0.04 (P < 0.01). In CF SAECs, ENaC P_o increased from 0.45 ± 0.06 to 0.73 ± 0.04 (P < 0.01). Pretreatment with 1 μM FPS-ZM1 (a RAGE inhibitor) attenuated all HMGB-1 effects on ENaC current in normal and CF SAECs. Confocal analysis of SAECs indicates that nuclear size and HMBG-1 localization can be impacted by ENaC dysfunction. Masson's trichrome labeling of mouse lung showed that intraperitoneally injected HMGB-1 significantly increased pulmonary fibrosis. Bronchoalveolar lavage fluid from HMGB-1-treated mice showed significant increases in IL-1β, IL-10, IL-6, IL-27, IL-17A, IFN-β, and granulocyte-macrophage colony-stimulating factor compared with vehicle-injected mice (P < 0.05). These studies put forth a new model in which HMGB-1 signaling to RAGE plays an important role in perpetuating ENaC dysfunction and inflammation in the CF lung.

Spatial Network Mapping of Pulmonary Multidrug-Resistant Tuberculosis Cavities Using RNA Sequencing

Rationale: There is poor understanding about protective immunity and the pathogenesis of cavitation in patients with tuberculosis.Objectives: To map pathophysiological pathways at anatomically distinct positions within the human tuberculosis cavity.Methods: Biopsies were obtained from eight predetermined locations within lung cavities of patients with multidrug-resistant tuberculosis undergoing therapeutic surgical resection (n = 14) and healthy lung tissue from control subjects without tuberculosis (n = 10). RNA sequencing, immunohistochemistry, and bacterial load determination were performed at each cavity position. Differentially expressed genes were normalized to control subjects without tuberculosis, and ontologically mapped to identify a spatially compartmentalized pathophysiological map of the cavity. In silico perturbation using a novel distance-dependent dynamical sink model was used to investigate interactions between immune networks and bacterial burden, and to integrate these identified pathways.Measurements and Main Results: The median (range) lung cavity volume on positron emission tomography/computed tomography scans was 50 cm3 (15-389 cm3). RNA sequence reads (31% splice variants) mapped to 19,049 annotated human genes. Multiple proinflammatory pathways were upregulated in the cavity wall, whereas a downregulation "sink" in the central caseum-fluid interface characterized 53% of pathways including neuroendocrine signaling, calcium signaling, triggering receptor expressed on myeloid cells-1, reactive oxygen and nitrogen species production, retinoic acid-mediated apoptosis, and RIG-I-like receptor signaling. The mathematical model demonstrated that neuroendocrine, protein kinase C-θ, and triggering receptor expressed on myeloid cells-1 pathways, and macrophage and neutrophil numbers, had the highest correlation with bacterial burden (r > 0.6), whereas T-helper effector systems did not.Conclusions: These data provide novel insights into host immunity to Mycobacterium tuberculosis-related cavitation. The pathways defined may serve as useful targets for the design of host-directed therapies, and transmission prevention interventions.

Rosiglitazone promotes ENaC-mediated alveolar fluid clearance in acute lung injury through the PPARγ/SGK1 signaling pathway

Background

Pulmonary edema is one of the pathological characteristics of acute respiratory distress syndrome (ARDS). The epithelial sodium channel (ENaC) is thought to be the rate-limiting factor for alveolar fluid clearance (AFC) during pulmonary edema. The peroxisome proliferator-activated receptor γ (PPARγ) agonist rosiglitazone was shown to stimulate ENaC-mediated salt absorption in the kidney. However, its role in the lung remains unclear. Here, we investigated the role of the PPARγ agonist in the lung to find out whether it can regulate AFC during acute lung injury (ALI). We also attempted to elucidate the mechanism for this.

Methods

Our ALI model was established through intratracheal instillation of lipopolysaccharide (LPS) in C57BL/6 J mice. The mice were randomly divided into 4 groups of 10. The control group underwent a sham operation and received an equal quantity of saline. The three experimental groups underwent intratracheal instillation of 5 mg/kg LPS, followed by intraperitoneal injection of 4 mg/kg rosiglitazone, 4 mg/kg rosiglitazone plus 1 mg/kg GW9662, or only equal quantity of saline. The histological morphology of the lung, the levels of TNF-α and IL-1β in the bronchoalveolar lavage fluid (BALF), the level of AFC, and the expressions of αENaC and serum and glucocorticoid-induced kinase-1 (SGK1) were determined. Type 2 alveolar (AT II) cells were incubated with rosiglitazone (15 μM) with or without GW9662 (10 μM). The expressions of αENaC and SGK1 were determined 24 h later.

Results

A mouse model of ALI was successfully established. Rosiglitazone significantly ameliorated the lung injury, decreasing the TNF-α and IL-1β levels in the BALF, enhancing AFC, and promoting the expressions of αENaC and SGK1 in ALI mice, which were abolished by the specific PPARγ blocker GW9662. In vitro, rosiglitazone increased the expressions of αENaC and SGK1. This increase was prevented by GW9662.

Conclusions

Rosiglitazone ameliorated the lung injury and promoted ENaC-mediated AFC via a PPARγ/SGK1-dependent signaling pathway, alleviating pulmonary edema in a mouse model of ALI.

Cytokine-Ion Channel Interactions in Pulmonary Inflammation

The lungs conceptually represent a sponge that is interposed in series in the bodies’ systemic circulation to take up oxygen and eliminate carbon dioxide. As such, it matches the huge surface areas of the alveolar epithelium to the pulmonary blood capillaries. The lung’s constant exposure to the exterior necessitates a competent immune system, as evidenced by the association of clinical immunodeficiencies with pulmonary infections. From the in utero to the postnatal and adult situation, there is an inherent vital need to manage alveolar fluid reabsorption, be it postnatally, or in case of hydrostatic or permeability edema. Whereas a wealth of literature exists on the physiological basis of fluid and solute reabsorption by ion channels and water pores, only sparse knowledge is available so far on pathological situations, such as in microbial infection, acute lung injury or acute respiratory distress syndrome, and in the pulmonary reimplantation response in transplanted lungs. The aim of this review is to discuss alveolar liquid clearance in a selection of lung injury models, thereby especially focusing on cytokines and mediators that modulate ion channels. Inflammation is characterized by complex and probably time-dependent co-signaling, interactions between the involved cell types, as well as by cell demise and barrier dysfunction, which may not uniquely determine a clinical picture. This review, therefore, aims to give integrative thoughts and wants to foster the unraveling of unmet needs in future research.

Regulation of Lung Epithelial Sodium Channels by Cytokines and Chemokines

Acute lung injury leading to acute respiratory distress (ARDS) is a global health concern. ARDS patients have significant pulmonary inflammation leading to flooding of the pulmonary alveoli. This prevents normal gas exchange with consequent hypoxemia and causes mortality. A thin fluid layer in the alveoli is normal. The maintenance of this thin layer results from fluid movement out of the pulmonary capillaries into the alveolar interstitium driven by vascular hydrostatic pressure and then through alveolar tight junctions. This is then balanced by fluid reabsorption from the alveolar space mediated by transepithelial salt and water transport through alveolar cells. Reabsorption is a two-step process: first, sodium enters via sodium-permeable channels in the apical membranes of alveolar type 1 and 2 cells followed by active extrusion of sodium into the interstitium by the basolateral Na⁺, K⁺-ATPase. Anions follow the cationic charge gradient and water follows the salt-induced osmotic gradient. The proximate cause of alveolar flooding is the result of a failure to reabsorb sufficient salt and water or a failure of the tight junctions to prevent excessive movement of fluid from the interstitium to alveolar lumen. Cytokine- and chemokine-induced inflammation can have a particularly profound effect on lung sodium transport since they can alter both ion channel and barrier function. Cytokines and chemokines affect alveolar amiloride-sensitive epithelial sodium channels (ENaCs), which play a crucial role in sodium transport and fluid reabsorption in the lung. This review discusses the regulation of ENaC via local and systemic cytokines during inflammatory disease and the effect on lung fluid balance.

Alveolar nonselective channels are ASIC1a/α-ENaC channels and contribute to AFC

A thin fluid layer in alveoli is normal and results from a balance of fluid entry and fluid uptake by transepithelial salt and water reabsorption. Conventional wisdom suggests the reabsorption is via epithelial Na⁺ channels (ENaC), but if all Na⁺ reabsorption were via ENaC, then amiloride, an ENaC inhibitor, should block alveolar fluid clearance (AFC). However, amiloride blocks only half of AFC. The reason for failure to block is clear from single-channel measurements from alveolar epithelial cells: ENaC channels are observed, but another channel is present at the same frequency that is nonselective for Na⁺ over K⁺, has a larger conductance, and has shorter open and closed times. These two channel types are known as highly selective channels (HSC) and nonselective cation channels (NSC). HSC channels are made up of three ENaC subunits since knocking down any of the subunits reduces HSC number. NSC channels contain α-ENaC since knocking down α-ENaC reduces the number of NSC (knocking down β- or γ-ENaC has no effect on NSC, but the molecular composition of NSC channels remains unclear). We show that NSC channels consist of at least one α-ENaC and one or more acid-sensing ion channel 1a (ASIC1a) proteins. Knocking down either α-ENaC or ASIC1a reduces both NSC and HSC number, and no NSC channels are observable in single-channel patches on lung slices from ASIC1a knockout mice. AFC is reduced in knockout mice, and wet wt-to-dry wt ratio is increased, but the percentage increase in wet wt-to-dry wt ratio is larger than expected based on the reduction in AFC.

Oxidized glutathione (GSSG) inhibits epithelial sodium channel activity in primary alveolar epithelial cells

Amiloride-sensitive epithelial Na(+) channels (ENaC) regulate fluid balance in the alveoli and are regulated by oxidative stress. Since glutathione (GSH) is the predominant antioxidant in the lungs, we proposed that changes in glutathione redox potential (Eh) would alter cell signaling and have an effect on ENaC open probability (Po). In the present study, we used single channel patch-clamp recordings to examine the effect of oxidative stress, via direct application of glutathione disulfide (GSSG), on ENaC activity. We found a linear decrease in ENaC activity as the GSH/GSSG Eh became less negative (n = 21; P < 0.05). Treatment of 400 μM GSSG to the cell bath significantly decreased ENaC Po from 0.39 ± 0.06 to 0.13 ± 0.05 (n = 8; P < 0.05). Likewise, back-filling recording electrodes with 400 μM GSSG reduced ENaC Po from 0.32 ± 0.08 to 0.17 ± 0.05 (n = 10; P < 0.05), thus implicating GSSG as an important regulatory factor. Biochemical assays indicated that oxidizing potentials promote S-glutathionylation of ENaC and irreversible oxidation of cysteine residues with N-ethylmaleimide blocked the effects of GSSG on ENaC Po. Additionally, real-time imaging studies showed that GSSG impairs alveolar fluid clearance in vivo as opposed to GSH, which did not impair clearance. Taken together, these data show that glutathione Eh is an important determinant of alveolar fluid clearance in vivo.

Lung niches for the generation and maintenance of tissue-resident memory T cells

The extent to which tissue-specific viral infections generate memory T cells specifically adapted to and maintained within the target infection site is unknown. Here, we show that respiratory virus-specific memory T cells in mice and humans are generated and maintained in compartmentalized niches in lungs, distinct from populations in lymphoid tissue or circulation. Using a polyclonal mouse model of influenza infection combined with an in vivo antibody labeling approach and confocal imaging, we identify a spatially distinct niche in the lung where influenza-specific T-cell responses are expanded and maintained long term as tissue-resident memory (T(RM)) CD4 and CD8 T cells. Lung T(RM) are further distinguished from circulating memory subsets in lung and spleen based on CD69 expression and persistence independent of lymphoid stores. In humans, influenza-specific T cells are enriched within the lung T(RM) subset, whereas memory CD8 T cells specific for the systemic virus cytomegalovirus are distributed in both lung and spleen, suggesting that the site of infection affects T(RM) generation. Our findings reveal a precise spatial organization to virus-specific T-cell memory, determined by the site of the initial infection, with important implications for the development of targeted strategies to boost immunity at appropriate tissue sites.

Expression of urea transporters and their regulation

UT-A and UT-B families of urea transporters consist of multiple isoforms that are subject to regulation of both acutely and by long-term measures. This chapter provides a brief overview of the expression of the urea transporter forms and their locations in the kidney. Rapid regulation of UT-A1 results from the combination of phosphorylation and membrane accumulation. Phosphorylation of UT-A1 has been linked to vasopressin and hyperosmolality, although through different kinases. Other acute influences on urea transporter activity are ubiquitination and glycosylation, both of which influence the membrane association of the urea transporter, again through different mechanisms. Long-term regulation of urea transport is most closely associated with the environment that the kidney experiences. Low-protein diets may influence the amount of urea transporter available. Conditions of osmotic diuresis, where urea concentrations are low, will prompt an increase in urea transporter abundance. Although adrenal steroids affect urea transporter abundance, conflicting reports make conclusions tenuous. Urea transporters are upregulated when P2Y2 purinergic receptors are decreased, suggesting a role for these receptors in UT regulation. Hypercalcemia and hypokalemia both cause urine concentration deficiencies. Urea transporter abundances are reduced in aging animals and animals with angiotensin-converting enzyme deficiencies. This chapter will provide information about both rapid and long-term regulation of urea transporters and provide an introduction into the literature.

Regulation of ion transport by oxidants

Ion channels perform a variety of cellular functions in lung epithelia. Oxidant- and antioxidant-mediated mechanisms (that is, redox regulation) of ion channels are areas of intense research. Significant progress has been made in our understanding of redox regulation of ion channels since the last Experimental Biology report in 2003. Advancements include: 1) identification of nonphagocytic NADPH oxidases as sources of regulated reactive species (RS) production in epithelia, 2) an understanding that excessive treatment with antioxidants can result in greater oxidative stress, and 3) characterization of novel RS signaling pathways that converge upon ion channel regulation. These advancements, as discussed at the 2013 Experimental Biology Meeting in Boston, MA, impact our understanding of oxidative stress in the lung, and, in particular, illustrate that the redox state has profound effects on ion channel and cellular function.

Cholinergic regulation of epithelial sodium channels in rat alveolar type 2 epithelial cells

We and others have shown that epithelial Na(+) channels (ENaC) in alveolar type 2 (AT2) cells are activated by β2 agonists, steroid hormones, elevated oxygen tension, and by dopamine. Although acetylcholine receptors (AChRs) have been previously described in the lung, there are few reports of whether cholinergic agonists alter sodium transport in the alveolar epithelium. Therefore, we investigated how cholinergic receptors regulate ENaC activity in primary cultures of rat AT2 cells using cell-attached patch-clamp recordings to assess ENaC activity. We found that the muscarinic agonists, carbachol (CCh) and oxotremorine, activated ENaC in a dose-dependent manner but that nicotine did not. CCh-induced activation of ENaC was blocked by atropine. Western blotting and immunohistochemistry suggested that muscarinic M2 and M3 receptors (mAChRs) but not nicotinic receptors were present in AT2 cells. Endogenous RhoA and GTP-RhoA increased in response to CCh and the increase was reduced by pretreatment with atropine. We showed that Y-27632, an inhibitor of Rho-associated protein kinase (ROCK), abolished endogenous ENaC activity and inhibited the activation of ENaC by CCh. We also showed that ROCK signaling was necessary for ENaC stability in 2F3 cells, a model for AT2 cells. Our results showed that muscarinic agonists activated ENaC in rat AT2 cells through M2 and/or M3 mAChRs probably via a RhoA/ROCK signaling pathway.

Chronic alcohol ingestion changes the landscape of the alveolar epithelium

Similar to effects of alcohol on the heart, liver, and brain, the effects of ethanol (EtOH) on lung injury are preventable. Unlike other vital organ systems, however, the lethal effects of alcohol on the lung are underappreciated, perhaps because there are no signs of overt pulmonary disorder until a secondary insult, such as a bacterial infection or injury, occurs in the lung. This paper provides overview of the complex changes in the alveolar environment known to occur following both chronic and acute alcohol exposures. Contemporary animal and cell culture models for alcohol-induced lung dysfunction are discussed, with emphasis on the effect of alcohol on transepithelial transport processes, namely, epithelial sodium channel activity (ENaC). The cascading effect of tissue and phagocytic Nadph oxidase (Nox) may be triggered by ethanol exposure, and as such, alcohol ingestion and exposure lead to a prooxidative environment; thus impacting alveolar macrophage (AM) function and oxidative stress. A better understanding of how alcohol changes the landscape of the alveolar epithelium can lead to improvements in treating acute respiratory distress syndrome (ARDS) for which hospitalized alcoholics are at an increased risk.

Nadph oxidase regulates alveolar epithelial sodium channel activity and lung fluid balance in vivo via O⁻₂ signaling

To define roles for reactive oxygen species (ROS) and epithelial sodium channel (ENaC) in maintaining lung fluid balance in vivo, we used two novel whole animal imaging approaches. Live X-ray fluoroscopy enabled quantification of air space fluid content of C57BL/6J mouse lungs challenged by intratracheal (IT) instillation of saline; results were confirmed by using conventional lung wet-to-dry weight ratios and Evans blue as measures of pulmonary edema. Visualization and quantification of ROS produced in lungs was performed in mice that had been administered a redox-sensitive dye, hydro-Cy7, by IT instillation. We found that inhibition of NADPH oxidase with a Rac-1 inhibitor, NSC23766, resulted in alveolar flooding, which correlated with a decrease in lung ROS production in vivo. Consistent with a role for Nox2 in alveolar fluid balance, Nox2(-/-) mice showed increased retention of air space fluid compared with wild-type controls. Interestingly, fluoroscopic analysis of C57BL/6J lungs IT instilled with LPS showed an acute stimulation of lung fluid clearance and ROS production in vivo that was abrogated by the ROS scavenger tetramethylpiperidine-N-oxyl (TEMPO). Acute application of LPS increased the activity of 20 pS nonselective ENaC channels in rat type 1 cells; the average number of channel and single-channel open probability (NPo) increased from 0.14 ± 0.04 to 0.62 ± 0.23. Application of TEMPO to the same cell-attached recording caused an immediate significant decrease in ENaC NPo to 0.04 ± 0.03. These data demonstrate that, in vivo, ROS has the capacity to stimulate lung fluid clearance by increasing ENaC activity.

Ion transport by pulmonary epithelia

The lung surface of air-breathing vertebrates is formed by a continuous epithelium that is covered by a fluid layer. In the airways, this epithelium is largely pseudostratified consisting of diverse cell types such as ciliated cells, goblet cells, and undifferentiated basal cells, whereas the alveolar epithelium consists of alveolar type I and alveolar type II cells. Regulation and maintenance of the volume and viscosity of the fluid layer covering the epithelium is one of the most important functions of the epithelial barrier that forms the outer surface area of the lungs. Therefore, the epithelial cells are equipped with a wide variety of ion transport proteins, among which Na⁺, Cl⁻, and K⁺ channels have been identified to play a role in the regulation of the fluid layer. Malfunctions of pulmonary epithelial ion transport processes and, thus, impairment of the liquid balance in our lungs is associated with severe diseases, such as cystic fibrosis and pulmonary oedema. Due to the important role of pulmonary epithelial ion transport processes for proper lung function, the present paper summarizes the recent findings about composition, function, and ion transport properties of the airway epithelium as well as of the alveolar epithelium.

Dopamine and renal function and blood pressure regulation

Dopamine is an important regulator of systemic blood pressure via multiple mechanisms. It affects fluid and electrolyte balance by its actions on renal hemodynamics and epithelial ion and water transport and by regulation of hormones and humoral agents. The kidney synthesizes dopamine from circulating or filtered L-DOPA independently from innervation. The major determinants of the renal tubular synthesis/release of dopamine are probably sodium intake and intracellular sodium. Dopamine exerts its actions via two families of cell surface receptors, D1-like receptors comprising D1R and D5R, and D2-like receptors comprising D2R, D3R, and D4R, and by interactions with other G protein-coupled receptors. D1-like receptors are linked to vasodilation, while the effect of D2-like receptors on the vasculature is variable and probably dependent upon the state of nerve activity. Dopamine secreted into the tubular lumen acts mainly via D1-like receptors in an autocrine/paracrine manner to regulate ion transport in the proximal and distal nephron. These effects are mediated mainly by tubular mechanisms and augmented by hemodynamic mechanisms. The natriuretic effect of D1-like receptors is caused by inhibition of ion transport in the apical and basolateral membranes. D2-like receptors participate in the inhibition of ion transport during conditions of euvolemia and moderate volume expansion. Dopamine also controls ion transport and blood pressure by regulating the production of reactive oxygen species and the inflammatory response. Essential hypertension is associated with abnormalities in dopamine production, receptor number, and/or posttranslational modification.

Dopamine D1-like receptor antagonist attenuates Th17-mediated immune response and ovalbumin antigen-induced neutrophilic airway inflammation

Allergic airway inflammation is generally considered a Th2-type immune response. Recent studies, however, demonstrated that Th17-type immune responses also play important roles in this process, especially in the pathogenesis of neutrophilic airway inflammation, a hallmark of severe asthma. We previously reported that dendritic cells release dopamine to naive CD4(+) T cells in Ag-specific cell-cell interaction, in turn inducing Th17 differentiation through dopamine D1-like receptor (D1-like-R). D1-like-R antagonist attenuates Th17-mediated diseases such as experimental autoimmune encephalomyelitis and autoimmune diabetes. However, the effect of antagonizing D1-like-R on Th17-mediated airway inflammation has yet to be studied. In this study, we examined whether D1-like-R antagonist suppresses OVA-induced neutrophilic airway inflammation in OVA TCR-transgenic DO11.10 mice and then elucidated the mechanism of action. DO11.10 mice were nebulized with OVA or PBS, and some mice received D1-like-R antagonist orally before OVA nebulization. D1-like-R antagonist significantly suppressed OVA-induced neutrophilic airway inflammation in DO11.10 mice. It also inhibited the production of IL-17 and infiltration of Th17 cells in the lung. Further, D1-like-R antagonist suppressed the production of IL-23 by lung CD11c(+) APCs. In contrast, D1-like-R antagonist did not increase Foxp3(+) regulatory T cells in the lung. D1-like-R antagonist neither suppressed nonspecific LPS-induced neutrophilic airway inflammation nor OVA-induced eosinophilic airway inflammation. These results indicate that D1-like-R antagonist could suppress Th17-mediated neutrophilic airway inflammation, raising the possibility that antagonizing D1-like-R serves as a promising new strategy for treating neutrophil-dominant severe asthma.

Urea transport in the kidney

Urea transport proteins were initially proposed to exist in the kidney in the late 1980s when studies of urea permeability revealed values in excess of those predicted by simple lipid-phase diffusion and paracellular transport. Less than a decade later, the first urea transporter was cloned. Currently, the SLC14A family of urea transporters contains two major subgroups: SLC14A1, the UT-B urea transporter originally isolated from erythrocytes; and SLC14A2, the UT-A group with six distinct isoforms described to date. In the kidney, UT-A1 and UT-A3 are found in the inner medullary collecting duct; UT-A2 is located in the thin descending limb, and UT-B is located primarily in the descending vasa recta; all are glycoproteins. These transporters are crucial to the kidney's ability to concentrate urine. UT-A1 and UT-A3 are acutely regulated by vasopressin. UT-A1 has also been shown to be regulated by hypertonicity, angiotensin II, and oxytocin. Acute regulation of these transporters is through phosphorylation. Both UT-A1 and UT-A3 rapidly accumulate in the plasma membrane in response to stimulation by vasopressin or hypertonicity. Long-term regulation involves altering protein abundance in response to changes in hydration status, low protein diets, adrenal steroids, sustained diuresis, or antidiuresis. Urea transporters have been studied using animal models of disease including diabetes mellitus, lithium intoxication, hypertension, and nephrotoxic drug responses. Exciting new animal models are being developed to study these transporters and search for active urea transporters. Here we introduce urea and describe the current knowledge of the urea transporter proteins, their regulation, and their role in the kidney.

Amiloride-sensitive sodium channels and pulmonary edema

The development of pulmonary edema can be considered as a combination of alveolar flooding via increased fluid filtration, impaired alveolar-capillary barrier integrity, and disturbed resolution due to decreased alveolar fluid clearance. An important mechanism regulating alveolar fluid clearance is sodium transport across the alveolar epithelium. Transepithelial sodium transport is largely dependent on the activity of sodium channels in alveolar epithelial cells. This paper describes how sodium channels contribute to alveolar fluid clearance under physiological conditions and how deregulation of sodium channel activity might contribute to the pathogenesis of lung diseases associated with pulmonary edema. Furthermore, sodium channels as putative molecular targets for the treatment of pulmonary edema are discussed.

Direct tracheal instillation of solutes into mouse lung

Intratracheal instillations deliver solutes directly into the lungs. This procedure targets the delivery of the instillate into the distal regions of the lung, and is therefore often incorporated in studies aimed at studying alveoli. We provide a detailed survival protocol for performing intratracheal instillations in mice. Using this approach, one can target delivery of test solutes or solids (such as lung therapeutics, surfactants, viruses, and small oligonucleotides) into the distal lung. Tracheal instillations may be the preferred methodology, over inhalation protocols that may primarily target the upper respiratory tract and possibly expose the investigator to potentially hazardous substances. Additionally, in using the tracheal instillation protocol, animals can fully recover from the non-invasive procedure. This allows for making subsequent physiological measurements on test animals, or reinstallation using the same animal. The amount of instillate introduced into the lung must be carefully determined and osmotically balanced to ensure animal recovery. Typically, 30-75 μL instillate volume can be introduced into mouse lung.

Evidence for Na+-glucose cotransporter in type I alveolar epithelium

Functional evidence of Na(+)-glucose cotransport in rat lung has been provided by Basset et al. (J. Physiol. 384:325-345, 1987). By autoradiography [(3)H]phloridzin binding has been found confined to alveolar epithelial type II cells in mouse and rabbit lungs (Boyd, J. Physiol. 422: 44P, 1990). In this research we checked by immunofluorescence whether Na(+)-glucose cotransporter (SGLT1) is also expressed in alveolar type I cells. Lungs of anesthetized rats and lambs were fixed by paraformaldehyde, perfused in pulmonary artery, or instilled into a bronchus, respectively. Tissue blocks embedded in paraffin or frozen were sectioned. Two specific anti-SGLT1 antibodies for rat recognizing aminoacid sequence 402-420, and 546-596 were used in both species. Bound primary antibody was detected by secondary antibody conjugated to fluorescein isothiocianate or Texas red, respectively. In some sections cellular nuclei were also stained. In rats alveolar type I cells were identified by fluorescent Erythrina cristagalli lectin. Sections were examined by confocal laser-scanning microscope. Both in rats and lambs alveolar epithelium was stained by either antibody; no labeling occurred in negative controls. Hence, SGLT1 appears to be also expressed in alveolar type I cells. This is functionally relevant because type I cells provide 95-97% of alveolar surface, and SGLT1, besides contributing to removal of lung liquid under some circumstances, keeps low glucose concentration in lining liquid, which is useful to prevent lung infection.

Airway surface liquid volume regulation determines different airway phenotypes in liddle compared with betaENaC-overexpressing mice

Studies in cystic fibrosis patients and mice overexpressing the epithelial Na(+) channel beta-subunit (betaENaC-Tg) suggest that raised airway Na(+) transport and airway surface liquid (ASL) depletion are central to the pathogenesis of cystic fibrosis lung disease. However, patients or mice with Liddle gain-of-function betaENaC mutations exhibit hypertension but no lung disease. To investigate this apparent paradox, we compared the airway phenotype (nasal versus tracheal) of Liddle with CFTR-null, betaENaC-Tg, and double mutant mice. In mouse nasal epithelium, the region that functionally mimics human airways, high levels of CFTR expression inhibited Liddle epithelial Nat channel (ENaC) hyperfunction. Conversely, in mouse trachea, low levels of CFTR failed to suppress Liddle ENaC hyperfunction. Indeed, Na(+) transport measured in Ussing chambers ("flooded" conditions) was raised in both Liddle and betaENaC-Tg mice. Because enhanced Na(+) transport did not correlate with lung disease in these mutant mice, measurements in tracheal cultures under physiologic "thin film" conditions and in vivo were performed. Regulation of ASL volume and ENaC-mediated Na(+) absorption were intact in Liddle but defective in betaENaC-Tg mice. We conclude that the capacity to regulate Na(+) transport and ASL volume, not absolute Na(+) transport rates in Ussing chambers, is the key physiologic function protecting airways from dehydration-induced lung disease.

Dopamine and G protein-coupled receptor kinase 4 in the kidney: role in blood pressure regulation

Complex interactions between genes and environment result in a sodium-induced elevation in blood pressure (salt sensitivity) and/or hypertension that lead to significant morbidity and mortality affecting up to 25% of the middle-aged adult population worldwide. Determining the etiology of genetic and/or environmentally-induced high blood pressure has been difficult because of the many interacting systems involved. Two main pathways have been implicated as principal determinants of blood pressure since they are located in the kidney (the key organ responsible for blood pressure regulation), and have profound effects on sodium balance: the dopaminergic and renin-angiotensin systems. These systems counteract or modulate each other, in concert with a host of intracellular second messenger pathways to regulate sodium and water balance. In particular, the G protein-coupled receptor kinase type 4 (GRK4) appears to play a key role in regulating dopaminergic-mediated natriuresis. Constitutively activated GRK4 gene variants (R65L, A142V, and A486V), by themselves or by their interaction with other genes involved in blood pressure regulation, are associated with essential hypertension and/or salt-sensitive hypertension in several ethnic groups. GRK4γ 142Vtransgenic mice are hypertensive on normal salt intake while GRK4γ 486V transgenic mice develop hypertension only with an increase in salt intake. GRK4 gene variants have been shown to hyperphosphorylate, desensitize, and internalize two members of the dopamine receptor family, the D(1) (D(1)R) and D(3) (D(3)R) dopamine receptors, but also increase the expression of a key receptor of the renin-angiotensin system, the angiotensin type 1 receptor (AT(1)R). Knowledge of the numerous blood pressure regulatory pathways involving angiotensin and dopamine may provide new therapeutic approaches to the pharmacological regulation of sodium excretion and ultimately blood pressure control.

Rac1-mediated NADPH oxidase release of O2- regulates epithelial sodium channel activity in the alveolar epithelium

We examine whether alveolar cells can control release of O(2)(-) through regulated NADPH oxidase (NOX) 2 (NOX2) activity to maintain lung fluid homeostasis. Using FACS to purify alveolar epithelial cells, we show that type 1 cells robustly express each of the critical NOX components that catalyze the production of O(2)(-) (NOX2 or gp91(phox), p22(phox), p67(phox), p47(phox), and p40(phox) subunits) as well as Rac1 at substantially higher levels than type 2 cells. Immunohistochemical labeling of lung tissue shows that Rac1 expression is cytoplasmic and resides near the apical surface of type 1 cells, whereas NOX2 coimmunoprecipitates with epithelial sodium channel (ENaC). Since Rac1 is a known regulator of NOX2, and hence O(2)(-) release, we tested whether inhibition or activation of Rac1 influenced ENaC activity. Indeed, 1 microM NSC23766 inhibition of Rac1 decreased O(2)(-) output in lung cells and significantly decreased ENaC activity from 0.87 +/- 0.16 to 0.52 +/- 0.16 [mean number of channels (N) and single-channel open probability (P(o)) (NP(o)) +/- SE, n = 6; P < 0.05] in type 2 cells. NSC23766 (10 microM) decreased ENaC NP(o) from 1.16 +/- 0.27 to 0.38 +/- 0.10 (n = 6 in type 1 cells). Conversely, 10 ng/ml EGF (a known stimulator of both Rac1 and O(2)(-) release) increased ENaC NP(o) values in both type 1 and 2 cells. NP(o) values increased from 0.48 +/- 0.21 to 0.91 +/- 0.28 in type 2 cells (P < 0.05; n = 10). In type 1 cells, ENaC activity also significantly increased from 0.40 +/- 0.15 to 0.60 +/- 0.23 following EGF treatment (n = 7). Sequestering O(2)(-) using 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) compound prevented EGF activation of ENaC in both type 1 and 2 cells. In conclusion, we report that Rac1-mediated NOX2 activity is an important component in O(2)(-) regulation of ENaC.

The contribution of epithelial sodium channels to alveolar function in health and disease

Amiloride-sensitive epithelial sodium channels (ENaC) play an important role in lung sodium transport. Sodium transport is closely regulated to maintain an appropriate fluid layer on the alveolar surface. Both alveolar type I and II cells have several different sodium-permeable channels in their apical membranes that play a role in normal lung physiology and pathophysiology. In many epithelial tissues, ENaC is formed from three subunit proteins: alpha, beta, and gamma ENaC. Part of the diversity of sodium-permeable channels in lung arises from assembling different combinations of these subunits to form channels with different biophysical properties and different mechanisms for regulation. Thus, lung epithelium has enormous flexibility to alter the magnitude of salt and water transport. In lung, ENaC is regulated by many transmitter and hormonal agents. Regulation depends upon the type of sodium channel but involves controlling the number of apical channels and/or the activity of individual channels.

Epac regulates UT-A1 to increase urea transport in inner medullary collecting ducts

Urea plays a critical role in the concentration of urine, thereby regulating water balance. Vasopressin, acting through cAMP, stimulates urea transport across rat terminal inner medullary collecting ducts (IMCD) by increasing the phosphorylation and accumulation at the apical plasma membrane of UT-A1. In addition to acting through protein kinase A (PKA), cAMP also activates Epac (exchange protein activated by cAMP). In this study, we tested whether the regulation of urea transport and UT-A1 transporter activity involve Epac in rat IMCD. Functional analysis showed that an Epac activator significantly increased urea permeability in isolated, perfused rat terminal IMCD. Similarly, stimulating Epac by adding forskolin and an inhibitor of PKA significantly increased urea permeability. Incubation of rat IMCD suspensions with the Epac activator significantly increased UT-A1 phosphorylation and its accumulation in the plasma membrane. Furthermore, forskolin-stimulated cAMP significantly increased ERK 1/2 phosphorylation, which was not prevented by inhibiting PKA, indicating that Epac mediated this phosphorylation of ERK 1/2. Inhibition of MEK 1/2 phosphorylation decreased the forskolin-stimulated UT-A1 phosphorylation. Taken together, activation of Epac increases urea transport, accumulation of UT-A1 at the plasma membrane, and UT-A1 phosphorylation, the latter of which is mediated by the MEK-ERK pathway.

Regulation of Epithelial Na+ Channel (ENaC) in the Salivary Cell Line SMG-C6

Glucocorticoids and mineralocorticoids modulate Na+ transport via epithelial Na+ channels (ENaC). The rat submandibular epithelial cell line, SMG-C6, expresses α-ENaC mRNA and protein and exhibits amiloride-sensitive Na+ transport when grown in low-serum (2.5%) defined medium, therefore, we examined the effects of altering the composition of the SMG-C6 cell growth medium on ENaC expression and function. No differences in basal or amiloride-sensitive short-circuit current (Isc) were measured across SMG-C6 monolayers grown in the absence of thyroid hormone, insulin, transferrin, or EGF. In the absence of hydrocortisone, basal and amiloride-sensitive Isc significantly decreased. Similarly, monolayers grown in 10% serum-supplemented medium had lower basal Isc and no response to amiloride. Adding hydrocortisone (1.1 μM) to either the low or 10% serum medium increased basal and amiloride-sensitive Isc, which was blocked by RU486, the glucocorticoid and progesterone receptor antagonist. Aldosterone also induced an increase in α-ENaC expression and Na+ transport, which was also blocked by RU486 but not by the mineralocorticoid receptor antagonist spironolactone. Thus, in the SMG-C6 cell line, hydrocortisone and aldosterone increased ENaC expression and basal epithelial Na+ transport. The absence of endogenous ENaC expression in culture conditions devoid of steroids makes the properties of this cell line an excellent model for investigating pathways regulating ENaC expression and Na+ transport.

Pulmonary neuroendocrine cell-secreted factors may alter fetal lung liquid clearance

The neuroendocrine system is most active at birth and may play a role in the transition from fetal to postnatal life, in particular in the lungs' transition from fluid secretion to fluid absorption. Pulmonary neuroendocrine cells do release dopamine (DA), serotonin, and gastrin-releasing peptide but their effects on lung ion and fluid transport are poorly understood. Therefore, we studied their effects on fetal distal lung explants and primary cultures of fetal distal lung epithelium (FDLE). We show that DA, but neither serotonin nor gastrin-releasing peptide, alters ion and fluid transport, in a dose-dependent manner. DAs effects were abrogated by D1/D2 receptor blockers in FDLE but not in explants. Propranolol abrogated DAs effects in both models. DA increased intracellular cAMP levels in FDLE. Terbutaline, forskolin, and isobutylmethylxanthine did not increase short circuit current (Isc) in DA-treated cells, despite a further increase in cAMP. We conclude that at least one, but not all mediators released by pulmonary neuroendocrine cells alter distal lung epithelial ion transport.

Spiperone, identified through compound screening, activates calcium-dependent chloride secretion in the airway

Cystic fibrosis (CF) is caused by mutations in the gene producing the cystic fibrosis transmembrane conductance regulator (CFTR). CFTR functions as a Cl(-) channel. Its dysfunction limits Cl(-) secretion and enhances Na+ absorption, leading to viscous mucus in the airway. Ca2+-activated Cl(-) channels (CaCCs) are coexpressed with CFTR in the airway surface epithelia. Increases in cytosolic Ca(2+) activate the epithelial CaCCs, which provides an alternative Cl(-) secretory pathway in CF. We developed a screening assay and screened a library for compounds that could enhance cytoplasmic Ca2+, activate the CaCC, and increase Cl(-) secretion. We found that spiperone, a known antipsychotic drug, is a potent intracellular Ca2+ enhancer and demonstrated that it stimulates intracellular Ca2+, not by acting in its well-known role as an antagonist of serotonin 5-HT2 or dopamine D2 receptors, but through a protein tyrosine kinase-coupled phospholipase C-dependent pathway. Spiperone activates CaCCs, which stimulates Cl(-) secretion in polarized human non-CF and CF airway epithelial cell monolayers in vitro and in CFTR-knockout mice in vivo. In conclusion, we have identified spiperone as a new therapeutic platform for correction of defective Cl(-) secretion in CF via a pathway independent of CFTR.

Redox regulation of epithelial sodium channels examined in alveolar type 1 and 2 cells patch-clamped in lung slice tissue

The alveolar surface of the lung is lined by alveolar type 1 (AT1) and type 2 (AT2) cells. Using single channel patch clamp analysis in lung slice preparations, we are able to uniquely study AT1 and AT2 cells separately from intact lung. We report for the first time the Na+ transport properties of type 2 cells accessed in live lung tissue (as we have done in type 1 cells). Type 2 cells in lung tissue slices express both highly selective cation and nonselective cation channels with average conductances of 8.8 +/- 3.2 and 22.5 +/- 6.3 picosiemens, respectively. Anion channels with 10-picosiemen conductance are also present in the apical membrane of type 2 cells. Our lung slice studies importantly verify the use of cultured cell model systems commonly used in lung epithelial sodium channel (ENaC) studies. Furthermore, we identify novel functional differences between the cells that make up the alveolar epithelium. One important difference is that exposure to the nitric oxide (NO) donor, PAPA-NONOate (1.5 microm), significantly decreases average ENaC NPo in type 2 cells (from 1.38 +/- 0.26 to 0.82 +/- 0.16; p < 0.05 and n = 18) but failed to alter ENaC activity in alveolar type 1 cells. Elevating endogenous superoxide (O2.) levels with Ethiolat, a superoxide dismutase inhibitor, prevented NO inhibition of ENaC activity in type 2 cells, supporting the novel hypothesis that O2. and NO signaling plays an important role in maintaining lung fluid balance.

Elective cesarean section: its impact on neonatal respiratory outcome

Physiologic events in the last few weeks of pregnancy coupled with the onset of spontaneous labor are accompanied by changes in the hormonal milieu of the fetus and its mother, resulting in preparation of the fetus for neonatal transition. Rapid clearance of fetal lung fluid is a key part of these changes, and is mediated in large part by transepithelial sodium reabsorption through amiloride-sensitive sodium channels in the alveolar epithelial cells, with only a limited contribution from mechanical factors and Starling forces. This article discusses the respiratory morbidity associated with elective cesarean section, the physiologic mechanisms underlying fetal lung fluid absorption, and potential strategies for facilitating neonatal transition when infants are delivered by elective cesarean section before the onset of spontaneous labor.

Improving survival by increasing lung edema clearance: is airspace delivery of dopamine a solution?

In this issue of Critical Care Chamorro-Marin and coworkers provide new evidence that dopamine instilled into airspaces is beneficial in a rat model of ventilator-induced lung injury. This study is important because it is the first to explore the effects of dopamine on survival, albeit short term. The delivery of dopamine into the airspaces in vivo is also novel and builds upon previous studies describing the mechanisms by which dopamine exerts its effect by upregulating active Na⁺ transport in the lungs. Dopamine appears to increase active Na⁺ transport via activation of amiloride-sensitive sodium channels and the basolateral Na⁺/K⁺-ATPase within minutes, and it has been shown to be effective in normal lungs and several models of lung injury. This information is relevant to current clinical trials exploring the effects of alveolar fluid clearance stimulation in patients with acute lung injury.

Electrophysiological characterization of rat type II pneumocytes in situ

Optimal aeration of the lungs is dependent on an alveolar fluid clearance, a process that is governed by Na+ and Cl- transport. However, the specific contribution of various ion channels in different alveolar cell types under basal or stimulated conditions is not exactly known. We established a novel functional model of rat lung slices suitable for nystatin-perforated whole-cell patch-clamp experiments. Lung slices retained a majority of live cells for up to 72 hours. Type II pneumocytes in situ had a mean capacitance of 8.8 +/- 2.5 pF and a resting membrane potential of -4.4 +/- 1.9 mV. Bath replacement of Na+ with NMDG+ decreased inward whole-cell currents by 70%, 21% and 52% of which were sensitive to 10 microM and 1 mM of amiloride, respectively. Exposure of slices to 0.5 microM dexamethasone for 1 hour did not affect ion currents, while chronic exposure (0.5 microM, 24-72 h) induced an increase in both total Na+-entry currents and amiloride-sensitive currents. Under acute exposure to 100 microM cpt-cAMP, Type II cells in situ rapidly hyperpolarized by 25-30 mV, due to activation of whole-cell Cl- currents sensitive to 0.1 mM of 5-Nitro-2-(3-phenylpropylamino)benzoic acid. In addition, in the presence of cpt-cAMP, total sodium currents and currents sensitive to 10 microM amiloride increased by 32% and 70%, respectively. Thus, in Type II pneumocytes in situ: (1) amiloride-sensitive sodium channels contribute to only half of total Na+-entry and are stimulated by chronic exposure to glucocorticoids; (2) acute increase in cellular cAMP content simultaneously stimulates the entry of Cl- and Na+ ions.

Effects of cardiogenic edema fluid on ion and fluid transport in the adult lung

We have previously shown that cardiogenic pulmonary edema fluid (EF) increases Na+ and fluid transport by fetal distal lung epithelia (FDLE) (Rafii B, Gillie DJ, Sulowski C, Hannam V, Cheung T, Otulakowski G, Barker PM and O'Brodovich H. J Physiol 544: 537–548, 2002). We now report the effect of EF on Na+ and fluid transport by the adult lung. We first studied primary cultures of adult type II (ATII) epithelium and found that overnight exposure to EF increased Na+ transport, and this effect was mainly due to factors other than catecholamines. Plasma did not stimulate Na+ transport in ATII. Purification of EF demonstrated that at least some agent(s) responsible for the amiloride-insensitive component resided within the globulin fraction. ATII exposed to globulins demonstrated a conversion of amiloride-sensitive short-circuit current ( Isc) to amiloride-insensitive Isc with no increase in total Isc. Patch-clamp studies showed that ATII exposed to EF for 18 h had increased the number of highly selective Na+ channels in their apical membrane. In situ acute exposure to EF increased the open probability of Na+-permeant ion channels in ATII within rat lung slices. EF did increase, by amiloride-sensitive pathways, the alveolar fluid clearance from the lungs of adult rats. We conclude that cardiogenic EF increases Na+ transport by adult lung epithelia in primary cell culture, in situ and in vivo.

Alveolar epithelial transport in the adult lung

The alveolar surface comprises >99% of the internal surface area of the lungs. At birth, the fetal lung rapidly converts from a state of net fluid secretion, which is necessary for normal fetal lung development, to a state in which there is a minimal amount of alveolar liquid. The alveolar surface epithelium facing the air compartment is composed of TI and TII cells. The morphometric characteristics of both cell types are fairly constant over a range of mammalian species varying in body weight by a factor of approximately 50,000. From the conservation of size and shape across species, one may infer that both TI and TII cells also have important conserved functions. The regulation of alveolar ion and liquid transport has been extensively investigated using a variety of experimental models, including whole animal, isolated lung, isolated cell, and cultured cell model systems, each with their inherent strengths and weaknesses. The results obtained with different model systems and a variety of different species point to both interesting parallels and some surprising differences. Sometimes it has been difficult to reconcile results obtained with different model systems. In this section, the primary focus will be on aspects of alveolar ion and liquid transport under normal physiologic conditions, emphasizing newer data and describing evolving paradigms of lung ion and fluid transport. We will highlight some of the unanswered questions, outline the similarities and differences in results obtained with different model systems, and describe some of the complex and interweaving regulatory networks.

Respiratory transition in infants delivered by cesarean section

One of the biggest challenges a newborn faces after birth is the task of making a smooth transition to air breathing. This task is complicated by the fact that fetal lungs are full of fluid which must be cleared rapidly to allow for gas exchange. Respiratory morbidity as a result of failure to clear fetal lung fluid is not uncommon, and can be particularly problematic in some infants delivered by elective cesarean delivery (ECS). Given the high rates of cesarean deliveries in the USA and worldwide, the public health and economic impact of morbidity in this subgroup is considerable. Whereas the occurrence of birth asphyxia, trauma, and meconium aspiration is reduced by elective Cesarean delivery, the risk of respiratory distress secondary to transient tachypnea of the newborn, surfactant deficiency, and pulmonary hypertension is increased. It is clear that physiologic events in the last few weeks of pregnancy coupled with the onset of spontaneous labor are accompanied by changes in the hormonal milieu of the fetus and its mother, resulting in preparation of the fetus for neonatal transition. Rapid clearance of fetal lung fluid is a key part of these changes, and is mediated in large part by transepithelial Na reabsorption through amiloride-sensitive Na channels in the alveolar epithelial cells, with only a limited contribution from mechanical factors and Starling forces. This chapter discusses the physiologic mechanisms underlying fetal lung fluid absorption and explores potential strategies for facilitating neonatal transition when infants are delivered by ECS before the onset of spontaneous labor.

DNA MICROARRAY ANALYSIS OF PULMONARY FIBROSIS THREE MONTHS AFTER EXPOSURE TO PARAQUAT IN RATS

Although paraquat (PQ) is known to induce pulmonary fibrosis, how it does so is not entirely clear. To elucidate the mechanisms involved, the profile of gene expression in the lung at three months after exposure to PQ (7 mg/kg, s.c., daily for eight administrations) was investigated in rats using a DNA microarray. Changes in gene expression that were considered to reflect damage to the lung, a change in the balance of electrolytes and fluid, and alveolar remodeling were observed. The products of these genes were: CSF-1 receptor, which is a receptor of inflammatory cytokines that activates monocyte/macrophages; TGF-beta type II receptor, which is a receptor of TGF-betas involved in wound healing and fibrosis; a subunit of Na+/K(+)-ATPase, an amiloride-sensitive cation channel, and a subunit of the potassium channel, all of which regulate the alveolar fluid balance and play a role in clearing lung edema; the adenosine A2a receptor, which has a protective function in the lung and interacts with dopamine D1 and D2 receptors to regulate the function of amiloride-sensitive cation channels; cofilin, which is involved in the depolymerization and cleavage of actin filaments; LIM motif-containing protein kinase 1, which negatively regulates the activity of cofilin; SHPS-1, which regulates the integrin-mediated reorganization of the cytoskeleton; and sodium channel beta 2, which is involved in cell adhesion and migration. These results indicate that PQ-induced pulmonary fibrosis does not merely terminate as cicatrices three months after the discontinuation of PQ treatment, but that dynamic functional change continues in the lung.