Selected contribution: long-term effects of beta(2)-adrenergic receptor stimulation on alveolar fluid clearance in mice

Mechanisms of impaired alveolar fluid clearance

Impaired alveolar fluid clearance (AFC) is an important cause of alveolar edema fluid accumulation in patients with acute respiratory distress syndrome (ARDS). Alveolar edema leads to insufficient gas exchange and worse clinical outcomes. Thus, it is important to understand the pathophysiology of impaired AFC in order to develop new therapies for ARDS. Over the last few decades, multiple experimental studies have been done to understand the molecular, cellular, and physiological mechanisms that regulate AFC in the normal and the injured lung. This review provides a review of AFC in the normal lung, focuses on the mechanisms of impaired AFC, and then outlines the regulation of AFC. Finally, we summarize ongoing challenges and possible future research that may offer promising therapies for ARDS.

Effect of β2-adrenergic receptor stimulation on lung fluid in stable heart failure patients

BACKGROUND

The purpose of this study was to determine: (1) whether stable heart failure patients with reduced ejection fraction (HFrEF) have elevated extravascular lung water (EVLW) when compared with healthy control subjects; and (2) the effect of acute β₂-adrenergic receptor (β₂AR) agonist inhalation on lung fluid balance.

METHODS

Twenty-two stable HFrEF patients and 18 age- and gender-matched healthy subjects were studied. Lung diffusing capacity for carbon monoxide (DLCO), alveolar-capillary membrane conductance (Dm_CO), pulmonary capillary blood volume (V_c) (via re-breathe) and lung tissue volume (V_tis) (via computed tomography) were assessed before and within 30 minutes after administration of nebulized albuterol. EVLW was derived as V_tis - V_c.

RESULTS

Before administration of albuterol, V_tis and EVLW were higher in HFrEF vs control (998 ± 200 vs 884 ± 123 ml, p = 0.041; and 943 ± 202 vs 802 ± 133 ml, p = 0.015, respectively). Albuterol decreased V_tis and EVLW in HFrEF patients (-4.6 ± 7.8%, p = 0.010; -4.6 ± 8.8%, p = 0.018) and control subjects (-2.8 ± 4.9%, p = 0.029; -3.0 ± 5.7%, p = 0.045). There was an inverse relationship between pre-albuterol values and pre- to post-albuterol change for EVLW (r² = -0.264, p = 0.015) and Dm_CO (r² = -0.343, p = 0.004) in HFrEF only.

CONCLUSION

Lung fluid is elevated in stable HFrEF patients relative to healthy subjects. Stimulation of β₂ARs may cause fluid removal in HFrEF, especially in patients with greater evidence of increased lung water at baseline.

Alveolar Epithelial Type I Cells Express β2-Adrenergic Receptors and G-protein Receptor Kinase 2

β2-Adrenergic agonists stimulate alveolar epithelial sodium (Na+) transport and lung fluid clearance. Alveolar type II (AT2) cells have been reported to express β2-adrenergic receptors (β2AR). Given the large surface area covered by alveolar type I (AT1) cells and their potential role in alveolar fluid removal, we were interested in learning if AT1 cells express β2AR as well. Because β2AR is potentially susceptible to desensitization by G-protein-coupled receptor kinase 2 (GRK2), we also undertook localization of GRK2. β2AR and GRK2 expression was evaluated in whole lung, isolated alveolar epithelial cells (AECs), and AECs in primary culture, and was localized to specific AEC phenotypes by immunofluorescence techniques. β2AR is highly expressed in AT1 cells. β2AR mRNA increases with time in culture as AT2 cells transdifferentiate towards the AT1 cell phenotype. Immunoreactive GRK2 is seen in both AT1 and AT2 cells in similar amounts. These data suggest that both AT1 and AT2 cells may contribute to the increased alveolar Na+ and water clearance observed after exposure to β2 adrenergic agents. Both cell types also express GRK2, suggesting that both may undergo desensitization of β2AR with subsequent decline in the stimulatory effects of β2-adrenergic agonists over time.

Evolution, Development, and Function of the Pulmonary Surfactant System in Normal and Perturbed Environments

Surfactant lipids and proteins form a surface active film at the air-liquid interface of internal gas exchange organs, including swim bladders and lungs. The system is uniquely positioned to meet both the physical challenges associated with a dynamically changing internal air-liquid interface, and the environmental challenges associated with the foreign pathogens and particles to which the internal surface is exposed. Lungs range from simple, transparent, bag-like units to complex, multilobed, compartmentalized structures. Despite this anatomical variability, the surfactant system is remarkably conserved. Here, we discuss the evolutionary origin of the surfactant system, which likely predates lungs. We describe the evolution of surfactant structure and function in invertebrates and vertebrates. We focus on changes in lipid and protein composition and surfactant function from its antiadhesive and innate immune to its alveolar stability and structural integrity functions. We discuss the biochemical, hormonal, autonomic, and mechanical factors that regulate normal surfactant secretion in mature animals. We present an analysis of the ontogeny of surfactant development among the vertebrates and the contribution of different regulatory mechanisms that control this development. We also discuss environmental (oxygen), hormonal and biochemical (glucocorticoids and glucose) and pollutant (maternal smoking, alcohol, and common "recreational" drugs) effects that impact surfactant development. On the adult surfactant system, we focus on environmental variables including temperature, pressure, and hypoxia that have shaped its evolution and we discuss the resultant biochemical, biophysical, and cellular adaptations. Finally, we discuss the effect of major modern gaseous and particulate pollutants on the lung and surfactant system.

Synergistic Inhibition of β2-adrenergic Receptor-mediated Alveolar Epithelial Fluid Transport by Interleukin-8 and Transforming Growth Factor-β

BACKGROUND

Patients with acute respiratory distress syndrome who retain maximal alveolar fluid clearance (AFC) have better clinical outcomes. The release of endogenous catecholamines associated with shock or the administration of β2-adrenergic receptor (β2AR) agonists enhances AFC via a 3'-5'-cyclic adenosine monophosphate-dependent mechanism. The authors have previously reported that transforming growth factor-β1 (TGF-β1) and interleukin-8 (IL-8), two major mediators of alveolar inflammation associated with the early phase of acute respiratory distress syndrome, inhibit AFC upregulation by β2AR agonists via a phosphoinositol-3-kinase (PI3K)-dependent mechanism. However, whether TGF-β1 and IL-8 cause an additive or synergistic inhibition of AFC is unclear. Thus, the central hypothesis of the study was to determine whether they synergistically inhibit the β2AR-stimulated AFC by activating two different isoforms of PI3K.

METHODS

The effects of TGF-β1 or IL-8 on β2AR agonist-stimulated net alveolar fluid transport were studied using short-circuit current studies. Molecular pathways of inhibition were confirmed by pharmacologic inhibitors and Western blotting of p-Akt, G-protein-coupled receptor kinase 2, protein kinase C-ζ, and phospho-β2AR. Finally, our observations were confirmed by an in vivo model of AFC.

RESULTS

Combined exposure to TGF-β1 and IL-8/cytokine-induced neutrophil chemoattractant-1 caused synergistic inhibition of β2AR agonist-stimulated vectorial Cl across alveolar epithelial type II cells (n = 12 in each group). This effect was explained by activation of different isoforms of PI3K by TGF-β1 and IL-8/cytokine-induced neutrophil chemoattractant-1 (n = 12 in each group). Furthermore, the inhibitory effect of TGF-β1 on 3'-5'-cyclic adenosine monophosphate-stimulated alveolar epithelial fluid transport required the presence of IL-8/cytokine-induced neutrophil chemoattractant-1 (n = 12 in each group). Inhibition of cytokine-induced neutrophil chemoattractant-1 prevented TGF-β1-mediated heterologous β2AR downregulation and restored physiologic β2AR agonist-stimulated AFC in rats (n = 6 in each group).

CONCLUSIONS

TGF-β1 and IL-8 have a synergistic inhibitory effect on β2AR-mediated stimulation of pulmonary edema removal by the alveolar epithelium. This result may, in part, explain why a large proportion of the patients with acute respiratory distress syndrome have impaired AFC.

Effects of the ß2-adrenoceptor agonist, albuterol, in a mouse model of anti-MuSK myasthenia gravis

The β2-adrenergic receptor agonist, albuterol, has been reported beneficial in treating several forms of congenital myasthenia. Here, for the first time, we examined the potential benefit of albuterol in a mouse model of anti-Muscle Specific Kinase (MuSK) myasthenia gravis. Mice received 15 daily injections of IgG from anti-MuSK positive patients, which resulted in whole-body weakness. At neuromuscular junctions in the tibialis anterior and diaphragm muscles the autoantibodies caused loss of postsynaptic acetylcholine receptors, and reduced the amplitudes of the endplate potential and spontaneous miniature endplate potential in the diaphragm muscle. Treatment with albuterol (8 mg/kg/day) during the two-week anti-MuSK injection series reduced the degree of weakness and weight loss, compared to vehicle-treated mice. However, the compound muscle action potential recorded from the gastrocnemius muscle displayed a decremental response in anti-MuSK-injected mice whether treated with albuterol or vehicle. Ongoing albuterol treatment did not increase endplate potential amplitudes compared to vehicle-treated mice nor did it prevent the loss of acetylcholine receptors from motor endplates. On the other hand, albuterol treatment significantly reduced the degree of fragmentation of endplate acetylcholine receptor clusters and increased the extent to which the remaining receptor clusters were covered by synaptophysin-stained nerve terminals. The results provide the first evidence that short-term albuterol treatment can ameliorate weakness in a robust mouse model of anti-MuSK myasthenia gravis. The results also demonstrate that it is possible for albuterol treatment to reduce whole-body weakness without necessarily reversing myasthenic impairment to the structure and function of the neuromuscular junction.

IL-8 inhibits cAMP-stimulated alveolar epithelial fluid transport via a GRK2/PI3K-dependent mechanism

Patients with acute lung injury (ALI) who retain maximal alveolar fluid clearance (AFC) have better clinical outcomes. Experimental and small clinical studies have shown that β2-adrenergic receptor (β2AR) agonists enhance AFC via a cAMP-dependent mechanism. However, two multicenter phase 3 clinical trials failed to show that β2AR agonists provide a survival advantage in patients with ALI. We hypothesized that IL-8, an important mediator of ALI, directly antagonizes the alveolar epithelial response to β2AR agonists. Short-circuit current and whole-cell patch-clamping experiments revealed that IL-8 or its rat analog CINC-1 decreases by 50% β2AR agonist-stimulated vectorial Cl(-) and net fluid transport across rat and human alveolar epithelial type II cells via a reduction in the cystic fibrosis transmembrane conductance regulator activity and biosynthesis. This reduction was mediated by heterologous β2AR desensitization and down-regulation (50%) via the G-protein-coupled receptor kinase 2 (GRK2)/PI3K signaling pathway. Inhibition of CINC-1 restored β2AR agonist-stimulated AFC in an experimental model of ALI in rats. Finally, consistent with the experimental results, high pulmonary edema fluid levels of IL-8 (>4000 pg/ml) were associated with impaired AFC in patients with ALI. These results demonstrate a novel role for IL-8 in inhibiting β2AR agonist-stimulated alveolar epithelial fluid transport via GRK2/PI3K-dependent mechanisms.-Roux, J., McNicholas, C. M., Carles, M., Goolaerts, A., Houseman, B. T., Dickinson, D. A., Iles, K. E., Ware, L. B., Matthay, M. A., Pittet, J.-F. IL-8 inhibits cAMP-stimulated alveolar epithelial fluid transport via a GRK2/PI3K-dependent mechanism.

S-nitrosylation: integrator of cardiovascular performance and oxygen delivery

Delivery of oxygen to tissues is the primary function of the cardiovascular system. NO, a gasotransmitter that signals predominantly through protein S-nitrosylation to form S-nitrosothiols (SNOs) in target proteins, operates coordinately with oxygen in mammalian cellular systems. From this perspective, SNO-based signaling may have evolved as a major transducer of the cellular oxygen-sensing machinery that underlies global cardiovascular function. Here we review mechanisms that regulate S-nitrosylation in the context of its essential role in "systems-level" control of oxygen sensing, delivery, and utilization in the cardiovascular system, and we highlight examples of aberrant S-nitrosylation that may lead to altered oxygen homeostasis in cardiovascular diseases. Thus, through a bird's-eye view of S-nitrosylation in the cardiovascular system, we provide a conceptual framework that may be broadly applicable to the functioning of other cellular systems and physiological processes and that illuminates new therapeutic promise in cardiovascular medicine.

Randomized, placebo-controlled clinical trial of an aerosolized β₂-agonist for treatment of acute lung injury

RATIONALE

β₂-Adrenergic receptor agonists accelerate resolution of pulmonary edema in experimental and clinical studies.

OBJECTIVES

This clinical trial was designed to test the hypothesis that an aerosolized β₂-agonist, albuterol, would improve clinical outcomes in patients with acute lung injury (ALI).

METHODS

We conducted a multicenter, randomized, placebo-controlled clinical trial in which 282 patients with ALI receiving mechanical ventilation were randomized to receive aerosolized albuterol (5 mg) or saline placebo every 4 hours for up to 10 days. The primary outcome variable for the trial was ventilator-free days.

MEASUREMENTS AND MAIN RESULTS

Ventilator-free days were not significantly different between the albuterol and placebo groups (means of 14.4 and 16.6 d, respectively; 95% confidence interval for the difference, -4.7 to 0.3 d; P = 0.087). Rates of death before hospital discharge were not significantly different between the albuterol and placebo groups (23.0 and 17.7%, respectively; 95%confidence interval for the difference,-4.0 to 14.7%;P = 0.30). In the subset of patients with shock before randomization, the number of ventilator-free days was lower with albuterol, although mortality was not different. Overall, heart rates were significantly higher in the albuterol group by approximately 4 beats/minute in the first 2 days after randomization, but rates of new atrial fibrillation (10% in both groups) and other cardiac dysrhythmias were not significantly different.

CONCLUSIONS

These results suggest that aerosolized albuterol does not improve clinical outcomes in patients with ALI. Routine use of β₂-agonist therapy in mechanically ventilated patients with ALI cannot be recommended. Clinical trial registered with www.clinicaltrials.gov (NCT 00434993).

β2-Adrenergics in hypoxia desensitize receptors but blunt inhibition of reabsorption in rat lungs

Alveolar edema and decreased inspired Po(2) decrease the oxygen supply to alveolar epithelia, impairing β(2)-adrenergic receptor (β2AR) signaling and alveolar reabsorption. β2AR agonists potently stimulate alveolar reabsorption. Thus, hypoxia impairs a major defense mechanism that provides protection from alveolar edema. Because in vivo data on the combined effects of prolonged hypoxia and β2AR agonist treatment on β2AR signaling are sparse, we tested whether in vivo hypoxia augments the inactivation of β2AR during prolonged stimulation. Rats were exposed to normoxia (N) and hypoxia (8% O(2); H), and were also treated with terbutaline (T; 2.5 mg/kg, intraperitoneal, twice daily) or saline (S) for 4 days. β2AR signaling was studied in alveolar epithelial (ATII) cells and in whole-lung tissue from treated rats. The terbutaline-stimulated formation of cyclic adenosine monophosphate was decreased by approximately 40% in whole lung and in ATII cells of NT, HS, and HT. The effects were not additive. The β2AR number was increased in HS, but decreased in NT and HT. Treatment increased the G-protein-coupled receptor kinase 2 protein in the plasma membranes of ATII cells, but did not affect G proteins. In vivo hypoxia significantly decreased total and amiloride-sensitive alveolar fluid reabsorption, which was prevented by acute alveolar treatment and 4 days of systemic terbutaline treatment. The αENaC (subunit of epithelial Na channels) protein in plasma membranes was increased in HT, without effects on mRNA. These results indicate that prolonged alveolar hypoxia and treatment with terbutaline impaired β2AR signaling in alveolar epithelia and in whole lungs, and this signaling was not further impaired by hypoxia. Despite impaired β2AR signaling, treatment with terbutaline for 4 days prevented the inhibition of alveolar reabsorption caused by in vivo hypoxia.

Regulation of T cells in airway disease by beta-agonist

It is widely recognized that Th2 cytokines derived from T cells play a major role in the development of allergic lung inflammation that causes most asthma. Beta-agonists are important rescue and maintenance therapies for asthma, yet our understanding of beta-agonist effects on T cell biology is surprisingly poor. Recent studies using both cell culture and more integrative models are beginning to reveal beta-agonist regulation of T cell signaling and function that may be important in the pathogenesis and treatment of asthma and possibly other inflammatory diseases. Here we provide a comprehensive review of the literature concerning beta-agonist effects on T cells, and discuss the relevance of emerging paradigms of beta-adrenergic receptor signaling to T cell function.

Long-term terbutaline exposure stimulates alpha1-Na+-K+-ATPase expression at posttranscriptional level in rat fetal distal lung epithelial cells

Transepithelial Na(+) transport through epithelial Na(+) channels (ENaC) on the apical membrane and Na(+)-K(+)-ATPase activity on the basolateral membrane of distal lung epithelial cells are critical for alveolar fluid clearance. Acute exposure to beta-adrenergic agonists stimulates lung fluid clearance by increasing Na(+) transport. We investigated the effects of chronic exposure to the beta(2)-adrenergic agonist terbutaline on the transepithelial Na(+) transport in rat fetal distal lung epithelia (FDLE). FDLE monolayers exposed to 10(-4) M terbutaline for 48 h had significantly increased propanolol-blockable transepithelial total and amiloride-sensitive short-circuit current (I(sc)); however, when these chronically exposed monolayers were acutely exposed to additional beta-agonists and intracellular cAMP upregulators, there was no further increase in I(sc). Monolayers exposed to terbutaline for >48 h had I(sc) similar to control cells. Ouabain-sensitive Na(+)-K(+)-ATPase activity was increased in 48-h terbutaline-exposed FDLE whose apical membranes were permeabilized with nystatin. In contrast, terbutaline did not increase amiloride-sensitive apical membrane I(sc) in FDLE whose basolateral membranes were permeabilized with nystatin. Terbutaline treatment did not affect alpha-, beta-, or gamma-ENaC mRNA or alpha-ENaC protein steady-state levels, but increased total cellular levels and rate of synthesis of alpha(1)-Na(+)-K(+)-ATPase protein in FDLE in the absence of any change in alpha(1)-Na(+)-K(+)-ATPase mRNA. Total cellular beta(1)-Na(+)-K(+)-ATPase mRNA and protein levels were not affected by terbutaline. These data suggest that FDLE have different responses from adult type II epithelial cells when chronically exposed to terbutaline, and their increased transepithelial Na(+) transport occurs via a posttranscriptional increase in alpha(1)-Na(+)-K(+)-ATPase expression.

Terbutaline improves ischemia-reperfusion injury after left-sided orthotopic rat lung transplantation

Beta2-agonists have been shown to increase alveolar fluid reabsorption, and at least part of their effect depends on active sodium transport from the alveolus into the epithelial cell by the amiloride-sensitive epithelial sodium channel (ENaC). Few data exist on their effect in the injured lung. The authors therefore investigated the effect of intrabronchially administered terbutaline pretransplantation by measuring outcome 1 day after experimental donor lung transplantation with severe injury due to prolonged ischemia. Orthotopic single left-sided lung allotransplantation was performed in female rats (Wistar to Wistar) after a total ischemic time of 20 hours. Graft PaO2/FiO2 in 6 recipients treated with 10(-4) M terbutaline in 500 microL NaCl 0.9% was superior 24 hours after transplantation, with a PaO2 of 329 (111 [SD]) mm Hg versus 5 vehicle controls with 44 (15) mm Hg (P = .002). The beneficial effect of 10(-4) M terbutaline was abrogated by 10(-4) M of the sodium channel blocker amiloride to 71 (34) mm Hg in 3 recipients (P = .028 versus terbutaline 10(-4) M). Ten recipients receiving 10(-5) M terbutaline in 500 microL NaCl 0.9% showed inconsistent improvements of gas exchange, with a PaO2 of 158 (+/- 153) mm Hg (P = .058). Terbutaline at a high dose significantly improved the transplanted rat lung function at 24 hours after transplantation. Part of it may be via activating epithelial sodium transport, thus suggesting an important role of alveolar fluid transport in such a model of acute lung injury.

Inflammatory processes enhance cAMP-mediated uterus relaxation in the pregnant rat: the role of TNF-alpha

The objective of this study was to assess the in vitro uterus relaxing potency of beta(2)-adrenergic receptor (beta(2)-AR) agonists in pregnant rats after in utero administration of the bacterial lipopolysaccharide, Escherichia coli endotoxin (LPS). The LPS (100 microg/kg) was injected into the uterine lumen on day 16 of pregnancy. The effects of beta(2)-AR agonist terbutaline was tested in vitro, in isolated uterine rings precontracted by electric field stimulation. Uterine beta(2)-AR densities were detected by radioligand binding assay, the activated G-protein levels were investigated by a radiolabelled GTP binding assay. Uterine cAMP accumulation and the serum tumor necrosis factor-alpha (TNF-alpha) levels were measured by enzyme immunoassay. The endotoxin-evoked preterm delivery occurred on day 21. Higher pD(2) values of terbutaline (p < 0.001) were detected in endotoxin-treated rats: 9.14 +/- 0.36 vs. 7.71 +/- 0.12 compared with sham-operated rats. The densities or the equilibrium dissociation constants of beta(2)-ARs were not different (p > 0.05) in LPS-treated vs. control animals. Serum TNF-alpha level rose threefold after LPS treatment, but this rise was abolished by thalidomide. In LPS + thalidomide-treated rats, the effect of terbutaline became similar to that in sham-operated controls. By the measurement of myometrial cAMP levels, we documented that the concentration-response curve of terbutaline on cAMP accumulation was shifted to the left in the LPS-treated rats, with a significant rise in the pD(2). We concluded that in the case of uterine inflammation, the in vitro uterus-relaxing potency of beta(2)-agonists enhances, which is possibly mediated by TNF-alpha and uterine cAMP levels and that may serve as a rationale for the use of beta(2)-AR agonists in the attenuation of preterm uterine contractions on an inflammatory basis.

Expression and regulation of epithelial Na+ channels by nucleotides in pleural mesothelial cells

Pleural effusions are commonly clinical disorders, resulting from the imbalance between pleural fluid turnover and reabsorption. The mechanisms underlying pleural fluid clearance across the mesothelium remain to be elucidated. We hypothesized that epithelial Na(+) channel (ENaC) is expressed and forms the molecular basis of the amiloride-sensitive resistance in human mesothelial cells. Our RT-PCR results showed that three ENaC subunits, namely, alpha, beta, gamma, and two delta ENaC subunits, are expressed in human primary pleural mesothelial cells, a human mesothelioma cell line (M9K), and mouse pleural tissue. In addition, Western blotting and immunofluorescence microscopy studies revealed that alpha, beta, gamma, and delta ENaC subunits are expressed in primary human mesothelial cells and M9K cells at the protein level. An amiloride-inhibitable short-circuit current was detected in M9K monolayers and mouse pleural tissues when mounted in Ussing chambers. Whole-cell patch clamp recordings showed an ENaC-like channel with an amiloride concentration producing 50% inhibition of 12 microM in M9K cells. This cation channel has a high affinity for extracellular Na+ ions (K(m): 53 mM). The ion selectivity of this channel to cations follows the same order as ENaC: Li+ > Na+ > K+. The unitary Li(+) conductance was 15 pS in on-cell patches. Four ENaC subunits form a functional Na+ channel when coinjected into Xenopus oocytes. Furthermore, we found that both forskolin and cGMP increased the short-circuit currents in mouse pleural tissues. Taken together, our data demonstrate that the ENaC channels are biochemically and functionally expressed in human pleural mesothelial cells, and can be up-regulated by cyclic AMP and cyclic GMP.

Patterns of alveolar fluid clearance in heart failure

Alveolar fluid clearance (AFC) is important in keeping the airspaces free of edema. This process is accomplished via passive and active transport of Na(+) across the alveolo-capillary barrier mostly by apical Na(+) channels and basolateral Na,K-ATPases, respectively. Patterns of alveolar fluid clearance were found to be decreased in acutely elevated left atrial pressures, possibly due to the inhibition of alveolar epithelial active sodium transport. On the other hand, chronic elevation of pulmonary capillary pressure, such as seen in experimental and clinical congestive heart failure, increases alveolar fluid clearance most likely secondary to upregulation of active sodium transport.

Electrolyte and Fluid Transport in Mesothelial Cells

Mesothelial cells are specialized epithelial cells, which line the pleural, pericardial, and peritoneal cavities. Accumulating evidence suggests that the monolayer of mesothelial cells is permeable to electrolyte and fluid, and thereby govern both fluid secretion and re-absorption in the serosal cavities. Disorders in these salt and fluid transport systems may be fundamental in the pathogenesis of pleural effusion, pericardial effusion, and ascites. In this review, we discuss the location, physiological function, and regulation of active transport (Na(+)-K(+)-ATPase) systems, cation and anion channels (Na(+), K(+), Cl(-), and Ca(2+) channels), antiport (exchangers) systems, and symport (co-transporters) systems, and water channels (aquaporins). These secretive and absorptive pathways across mesothelial monolayer cells for electrolytes and fluid may provide pivotal therapeutical targets for novel clinical intervention in edematous diseases of serous cavities.

Protein kinase A and mitogen-activated protein kinase pathways mediate cAMP induction of alpha-epithelial Na+ channels (alpha-ENaC)

A major mechanism for Na+ transport across epithelia occurs through epithelial Na+ channels (ENaC). ENaC is a multimeric channel consisting of three subunits (alpha, beta, and gamma). The alpha-subunit is critical for ENaC function. In specific culture conditions, the rat submandibular gland epithelial cell line (SMG-C6) demonstrates minimal Na+ transport properties and exposure to dibutyryl cAMP (DbcAMP) for up to 48 h caused an elevation of alpha-ENaC mRNA and protein expression and amiloride-sensitive short-circuit current (I(SC)). Here we examined the early signaling pathways evoked by DbcAMP which contribute to the eventual increase in Na+ transport is present. Treatment with either of the protein kinase A (PKA) inhibitors KT5720 or H-89 followed by exposure to 1 mM DbcAMP for 24 h markedly attenuated DbcAMP-induced alpha-ENaC protein formation and I(SC). Exposure of SMG-C6 cells to 1 mM DbcAMP induced a rapid, transient phosphorylation of the cAMP response element binding protein (CREB). This response was attenuated in the presence of either KT5720 or H-89. Dominant-negative CREB decreased DbcAMP-induced alpha-ENaC expression. Suppression of the extracellular signal-regulated protein kinase (ERK 1,2) with PD98059 or the p38 mitogen-activated protein kinase (MAPK) pathway with SB203580 reduced DbcAMP-induced alpha-ENaC protein levels in SMG-C6 cells. DbcAMP-induced phosphorylation of CREB was markedly attenuated by PD98059 or SB203580. DbcAMP-induced activation of the either the p38 or the ERK 1,2 MAPK pathways was abolished by either of the PKA inhibitors, H-89 or KT5720. Cross talk between these signaling pathways induced by DbcAMP via the activation of CREB appears to contribute to increased levels of alpha-ENaC observed after 24 h of treatment in SMG-C6 epithelial cells.

Alveolar epithelial beta2-adrenergic receptors

beta(2)-adrenergic receptors are present throughout the lung, including the alveolar airspace, where they play an important role for regulation of the active Na(+) transport needed for clearance of excess fluid out of alveolar airspace. beta(2)-adrenergic receptor signaling is required for up-regulation of alveolar epithelial active ion transport in the setting of excess alveolar edema. The positive, protective effects of beta(2)-adrenergic receptor signaling on alveolar active Na(+) transport in normal and injured lungs provide substantial support for the use of beta-adrenergic agonists to accelerate alveolar fluid clearance in patients with cardiogenic and noncardiogenic pulmonary edema. In this review, we summarize the role of beta(2)-adrenergic receptors in the alveolar epithelium with emphasis on their role in the regulation of alveolar active Na(+) transport in normal and injured lungs.

Genetic variation of the β2-adrenergic receptor is associated with differences in lung fluid accumulation in humans

The β2-adrenergic receptors (β2AR) play an important role in lung fluid regulation. Previous research has suggested that subjects homozygous for arginine at amino acid 16 of the β2AR (Arg16) may have attenuated receptor function relative to subjects homozygous for glycine at the same amino acid (Gly16). We sought to determine if the Arg16Gly polymorphism of the β2AR influenced lung fluid balance in response to rapid saline infusion. We hypothesized that subjects homozygous for Arg at amino acid 16 ( n = 14) would have greater lung fluid accumulation compared with those homozygous for Gly ( n = 15) following a rapid intravenous infusion of isotonic saline (30 ml/kg over 17 min). Changes in lung fluid were determined using measures of lung density and tissue volume (computerized tomography imaging) and measures of pulmonary capillary blood volume (Vc) and alveolar-capillary conductance (DM, determined from the simultaneous assessment of the diffusing capacities of the lungs for carbon monoxide and nitric oxide). The saline infusion resulted in elevated catecholamines in both genotype groups (Arg16 283 ± 117% vs. Gly16 252 ± 118%, P > 0.05). The Arg16 group had a larger decrease in DMand increase in lung tissue volume and lung water after saline infusion relative to the Gly16 group (DM−13 ± 14 vs. 0 ± 26%, P < 0.05; lung tissue volume 13 ± 11 vs. 3 ± 11% and lung water +90 ± 66 vs. +48 ± 144 ml, P = 0.10, P < 0.05, for Arg vs. Gly16, respectively, means ± SD). These data suggest that subjects homozygous for Arg at amino acid 16 of the β2AR have a greater susceptibility for lung fluid accumulation relative to subjects homozygous for Gly at this position.

The alveolar-epithelial barrier: a target for potential therapy

During acute lung injury (ALI), the alveolar-capillary barrier is damaged, resulting in the accumulation of fluid and protein in the alveolar space characteristic of the acute respiratory distress syndrome (ARDS). Disordered epithelial repair may contribute to the development of fibrosis and worsen outcomes in patients who have lung injury. This article discusses novel emerging therapies based on these mechanisms that are designed to preserve the function and promote the repair of the alveolar epithelium in patients who have ALI/ARDS.

Reduced rate of alveolar-capillary recruitment and fall of pulmonary diffusing capacity during exercise in patients with heart failure

BACKGROUND

Patients with chronic heart failure (CHF) have reduced pulmonary diffusing capacity for carbon monoxide (DLCO). Acute pulmonary congestion also causes reduction of DLCO, which is reversible. We hypothesized for patients with CHF that the rate of rise of exercise DLCO is reduced compared to healthy controls and falls near end-exercise consistent with progressive interstitial edema.

METHODS AND RESULTS

DLCO and pulmonary blood flow (QC)) were measured by a rebreathe technique in CHF subjects (n = 11) and controls (n = 8) at rest, during constant workload exercise, and after exercise. DLCO of CHF subjects was less than controls at rest (16.5 +/- 1 vs. 21.9 +/- 2 mL/min/mm Hg, P < .01). CHF subjects exercised 11 +/- 2 minutes to 90% peak VO2, whereas controls exercised 17 +/- 2 minutes, reaching 88% peak VO2. In CHF subjects, DLCO increased to 19 +/- 2 mL/min/mm Hg and for controls to 38 +/- 3 mL/min/mm Hg. During the final 3 minutes of exercise, DLCO increased 5% in controls while decreasing 5% in CHF subjects (DLCO/Q(C)) was lower in CHF subjects at rest and progressively lower throughout exercise (P < .01).

CONCLUSION

In patients with CHF, DLCO has reduced rate of rise with exercise and falls near end-exercise consistent with limitation of alveolar-capillary recruitment and progressive interstitial edema.

Congenital diaphragmatic hernia prevents absorption of distal air space fluid in late-gestation rat fetuses

We hypothesized that congenital diaphragmatic hernia (CDH) may decrease distal air space fluid absorption due to immaturity of alveolar epithelial cells from a loss of the normal epithelial Na+ transport, as assessed by amiloride and epithelial Na+ channel (ENaC) and Na-K-ATPase expression, as well as failure to respond to endogenous epinephrine as assessed by propranolol. Timed-pregnant dams were gavage fed 100 mg of nitrofen at 9.5-day gestation to induce CDH in the fetuses, and distal air space fluid absorption experiments were carried out on 22-day gestation (term) fetuses. Controls were nitrofen-exposed fetuses without CDH. Absorption of distal air space fluid was measured from the increase in 131I-albumin concentration in an isosmolar, physiological solution instilled into the developing lungs. In controls, distal air space fluid absorption was rapid and mediated by β-adrenoceptors as demonstrated by reversal to fluid secretion after propranolol. Normal lung fluid absorption was also partially inhibited by amiloride. In contrast, CDH fetuses continued to show lung fluid secretion, and this secretion was not affected by either propranolol or amiloride. CDH lungs showed a 67% reduction in α-ENaC and β-ENaC expression, but no change in α1-Na-K-ATPase expression. These studies demonstrate: 1) CDH delays lung maturation with impaired distal air space fluid absorption secondary to inadequate Na+ uptake by the distal lung epithelium that results in fluid-filled lungs at birth with reduced capacity to establish postnatal breathing, and 2) the main stimulus to lung fluid absorption in near-term control fetuses, elevated endogenous epinephrine levels, is not functional in CDH fetuses.

Impaired alveolar liquid clearance after 48-h isoproterenol infusion spontaneously recovers by 96 h of continuous infusion

We previously demonstrated that 48-h isoproterenol (Iso) infusion in rats impaired the ability of beta-adrenoceptor (beta-AR) agonists to increase alveolar liquid clearance (ALC). In this study, we determined whether this impairment persisted over longer time periods by infusing 400 mug.kg(-1).h(-1) Iso by osmotic minipump for 24-144 h (n = 6-7/group). ALC in control rats was 19.0 +/- 2.4 (SD)% of instilled volume absorbed per hour. In Iso-infused rats, ALC was elevated at 24 h (34.9 +/- 2.4%) and decreased at 48 h (15.2 +/- 4.4%) and had recovered to 24 h values at 96 h (37.3 +/- 3.8%) and 144 h (35.2 +/- 3.3%). Plasma Iso concentrations remained elevated at all Iso infusion times. Peripheral lung beta(2)-AR expression exhibited a parallel time course, with a reduction in expression observed at 48 h, followed by an increase to 24 h values at 96 and 144 h. Propranolol prevented the increase in ALC observed at 96 and 144 h, indicating that the recovery in ALC was mediated by a recovery of beta-AR function and beta-AR signaling. ALC at 96 and 144 h could not be further increased by terbutaline, indicating that ALC was maximally stimulated. These data indicate that recovery of beta-AR-stimulated ALC can occur in the continued presence of Iso and is mediated by a recovery of the ability of the distal lung epithelium to respond to beta-AR stimulation.

Is there a role for beta-adrenoceptor agonists in the management of acute lung injury and the acute respiratory distress syndrome?

Despite improvements in general supportive care and ventilatory strategies designed to limit lung injury, no specific pharmacological therapy has yet proven to be efficacious in the management of acute lung injury (ALI) and the acute respiratory distress syndrome (ARDS). Based on experimental studies, as well as studies of the ex-vivo human lung, pulmonary edema fluid clearance from the alveolar space can be augmented by both inhaled and systemic beta2-adrenoceptor agonists (beta2-agonists). Additionally, in the presence of lung injury, beta2-agonists may reduce lung vascular permeability. Treatment with beta2-agonists may also increase the secretion of surfactant and have anti-inflammatory effects. In view of these potentially beneficial effects, beta2-agonist therapy should be evaluated for the treatment of lung injury in humans, particularly because they are already in wide clinical use and do not seem to have serious adverse effects in critically ill patients.

Mechanisms of pulmonary edema clearance

The mechanisms of pulmonary edema resolution are different from those regulating edema formation. Absorption of excess alveolar fluid is an active process that involves vectorial transport of Na+out of alveolar air spaces with water following the Na+osmotic gradient. Active Na+transport across the alveolar epithelium is regulated via apical Na+and chloride channels and basolateral Na-K-ATPase in normal and injured lungs. During lung injury, mechanisms regulating alveolar fluid reabsorption are inhibited by yet unclear pathways and can be upregulated by pharmacological means. Better understanding of the mechanisms that regulate edema clearance may lead to therapeutic interventions to improve the ability of lungs to clear fluid, which is of clinical significance.

Decreased expression of both the alpha1- and alpha2-subunits of the Na-K-ATPase reduces maximal alveolar epithelial fluid clearance

Impaired epithelial sodium channel function predisposes to delayed resorption of pulmonary edema and more severe experimental lung injury, whereas even a small fraction of the normal Na-K-ATPase activity is thought to be sufficient to maintain normal ion transport. However, direct proof is lacking. Therefore, we studied baseline and cAMP stimulated alveolar fluid clearance (AFC) in mice with a 50% decrease in lung protein expression of the alpha(1)- and/or alpha(2)-subunit of the Na-K-ATPase. There was no difference in basal and stimulated AFC in alpha(1)(+/-) or alpha(2)(+/-) mice compared with wild-type littermates. Also, the compound heterozygous mice (alpha(1)(+/-)/alpha(2)(+/-)) had normal basal AFC. However, the combined alpha(1)(+/-)/alpha(2)(+/-) mice showed a significant decrease in cAMP-stimulated AFC compared with wild-type littermates (11.1 +/- 1.0 vs. 14.9 +/- 1.8%/30 min, P < 0.001). When exposed to 96 h of >95% hyperoxia, the decrease in stimulated AFC in the alpha(1)(+/-)/alpha(2)(+/-) mice was not associated with more lung edema compared with wild-type littermates (lung wet-to-dry weight ratio 6.6 +/- 0.9 vs. 5.9 +/- 1.1, respectively; P = not significant). Thus a 50% decrease in protein expression of the alpha(1)- or alpha(2)-subunits of the Na-K-ATPase does not impair basal or stimulated AFC. However, a 50% protein reduction in both the alpha(1)- and alpha(2)-subunits of the Na-K-ATPase produces a submaximal stimulated AFC, suggesting a synergistic role for alpha(1)- and alpha(2)-subunits in cAMP-dependent alveolar epithelial fluid clearance.

Adrenoceptor subtypes in the control of burn-induced plasma extravasation

Burn trauma is known to induce a significant rise in circulating catecholamine levels and despite catecholamines being potent endogenous vasoactive agents with known actions on microvascular permeability, their effect on burn edema has been poorly investigated. The present study in rats investigated the role and importance of adrenergic receptor subtypes in the regulation of basal capillary permeability in normal skin and hyperpermeability in partial- and full-thickness skin burns. Edema was quantified by spectrophotometric analysis of extravasated Evans blue-albumin. Evaluation was based on intravenous administration of the following adrenergic agonists and antagonists: l-phenylephrine (alpha(1)-receptor agonist), prazosin (alpha(1)-receptor antagonist), clonidine (alpha(2)-receptor agonist), yohimbine (alpha(2)-receptor antagonist), prenalterol (beta(1)-receptor agonist), terbutaline (beta(2)-receptor agonist), or propranolol (beta(1)- and beta(2)-receptor antagonist). Results showed increased capillary permeability in normal skin following administration of terbutaline (p<0.01) and yohimbine (p<0.01). In partial-thickness burns, clonidine significantly (p<0.05) reduced edema formation, whereas in full-thickness burns edema was significantly reduced by clonidine (p<0.05) and l-phenylephrine (p<0.01). In conclusion, the inhibition of postburn edema induced by stimulation of alpha(1)-receptors (l-phylephrine) and alpha(2)-receptors (clonidine) could be secondary to increased vascular resistance and reduced tissue perfusion pressure and/or suppressed inflammatory reaction in the burn injury. In the treatment of burn patients, clonidine is particularly interesting since the agent has previously been proven to induce potent analgesia in thermally injured.

Upregulation of alveolar epithelial active Na+ transport is dependent on beta2-adrenergic receptor signaling

Alveolar epithelial beta-adrenergic receptor (betaAR) activation accelerates active Na+ transport in lung epithelial cells in vitro and speeds alveolar edema resolution in human lung tissue and normal and injured animal lungs. Whether these receptors are essential for alveolar fluid clearance (AFC) or if other mechanisms are sufficient to regulate active transport is unknown. In this study, we report that mice with no beta1- or beta2-adrenergic receptors (beta1AR-/-/beta2AR-/-) have reduced distal lung Na,K-ATPase function and diminished basal and amiloride-sensitive AFC. Total lung water content in these animals was not different from wild-type controls, suggesting that betaAR signaling may not be required for alveolar fluid homeostasis in uninjured lungs. Comparison of isoproterenol-sensitive AFC in mice with beta1- but not beta2-adrenergic receptors to beta1AR-/-/beta2AR-/- mice indicates that the beta2AR mediates the bulk of beta-adrenergic-sensitive alveolar active Na+ transport. To test the necessity of betaAR signaling in acute lung injury, beta1AR-/-/beta2AR-/-, beta1AR+/+/beta2AR-/-, and beta1AR+/+/beta2AR+/+ mice were exposed to 100% oxygen for up to 204 hours. beta1AR-/-/beta2AR-/- and beta1AR+/+/beta2AR-/- mice had more lung water and worse survival from this form of acute lung injury than wild-type controls. Adenoviral-mediated rescue of beta2-adrenergic receptor (beta2AR) function into the alveolar epithelium of beta1AR-/-/beta2AR-/- and beta1AR+/+/beta2AR-/- mice normalized distal lung beta2AR function, alveolar epithelial active Na+ transport, and survival from hyperoxia. These findings indicate that betaAR signaling may not be necessary for basal AFC, and that beta2AR is essential for the adaptive physiological response needed to clear excess fluid from the alveolar airspace of normal and injured lungs.

Fluid transport across cultured rat alveolar epithelial cells: a novel in vitro system

Previous studies have used fluid-instilled lungs to measure net alveolar fluid transport in intact animal and human lungs. However, intact lung studies have two limitations: the contribution of different distal lung epithelial cells cannot be studied separately, and the surface area for fluid absorption can only be approximated. Therefore, we developed a method to measure net vectorial fluid transport in cultured rat alveolar type II cells using an air-liquid interface. The cells were seeded on 0.4-microm microporous inserts in a Transwell system. At 96 h, the transmembrane electrical resistance reached a peak level (1,530 +/- 115 Omega.cm(2)) with morphological evidence of tight junctions. We measured net fluid transport by placing 150 microl of culture medium containing 0.5 microCi of (131)I-albumin on the apical side of the polarized cells. Protein permeability across the cell monolayer, as measured by labeled albumin, was 1.17 +/- 0.34% over 24 h. The change in concentration of (131)I-albumin in the apical fluid was used to determine the net fluid transported across the monolayer over 12 and 24 h. The net basal fluid transport was 0.84 microl.cm(-2).h(-1). cAMP stimulation with forskolin and IBMX increased fluid transport by 96%. Amiloride inhibited both the basal and stimulated fluid transport. Ouabain inhibited basal fluid transport by 93%. The cultured cells retained alveolar type II-like features based on morphologic studies, including ultrastructural imaging. In conclusion, this novel in vitro system can be used to measure net vectorial fluid transport across cultured, polarized alveolar epithelial cells.

Transforming growth factor-beta1 decreases expression of the epithelial sodium channel alphaENaC and alveolar epithelial vectorial sodium and fluid transport via an ERK1/2-dependent mechanism

Acute lung injury (ALI) is characterized by the flooding of the alveolar airspaces with protein-rich edema fluid and diffuse alveolar damage. We have previously reported that transforming growth factor-beta1 (TGF-beta1) is a critical mediator of ALI after intratracheal administration of bleomycin or Escherichia coli endotoxin, at least in part due to effects on lung endothelial and alveolar epithelial permeability. In the present study, we hypothesized that TGF-beta1 would also decrease vectorial ion and water transport across the distal lung epithelium. Therefore, we studied the effect of active TGF-beta1 on 22Na+ uptake across monolayers of primary rat and human alveolar type II (ATII) cells. TGF-beta1 significantly reduced the amiloride-sensitive fraction of 22Na+ uptake and fluid transport across monolayers of both rat and human ATII cells. TGF-beta1 also significantly decreased alphaENaC mRNA and protein expression and inhibited expression of a luciferase reporter downstream of the alphaENaC promoter in lung epithelial cells. The inhibitory effect of TGF-beta1 on sodium uptake and alphaENaC expression in ATII cells was mediated by activation of the MAPK, ERK1/2. Consistent with the in vitro results, TGF-beta1 inhibited the amiloride-sensitive fraction of the distal airway epithelial fluid transport in an in vivo rat model at a dose that was not associated with any change in epithelial protein permeability. These data indicate that increased TGF-beta1 activity in the distal airspaces during ALI promotes alveolar edema by reducing distal airway epithelial sodium and fluid clearance. This reduction in sodium and fluid transport is attributable in large part to a reduction in apical membrane alphaENaC expression mediated through an ERK1/2-dependent inhibition of the alphaENaC promoter activity.

Postreceptor defects in alveolar epithelial beta-adrenergic signaling after prolonged isoproterenol infusion

We previously found that prolonged isoproterenol (Iso) infusion in rats impaired the ability of beta-adrenoceptor (beta-AR) agonists to increase alveolar liquid clearance (ALC). Here, we determined if postreceptor defects in beta-AR signaling contribute to this impairment. Iso was infused using subcutaneous miniosmotic pumps (4, 40, or 400 microg. kg-1. h-1) in rats for 48 h. At this time, forskolin-stimulated ALC was measured by mass balance. Forskolin-stimulated ALC [33.4 +/- 2.1%/h (mean +/- SE) in vehicle-infused rats] was reduced by 25 and 38%, respectively, after the 40 and 400 microg. kg-1. h-1 Iso infusions. The ability of forskolin to increase cAMP was reduced by 70% in alveolar type II (ATII) cells isolated from rats infused with 400 microg. kg-1. h-1 Iso. Additionally, the ability of the stable cAMP analog 8-bromoadenosine-3',5'-cyclic monophosphorothioate, Sp-isomer, to increase ALC (48.7 +/- 3.0% in vehicle-infused rats) was reduced by 25 and 51%, respectively, after the 40 and 400 microg. kg-1. h-1 infusions. Finally, the ability of cAMP to increase protein kinase A activity was eliminated in ATII cells isolated from rats infused with Iso at 400 microg. kg-1. h-1. These data demonstrate that prolonged beta-AR agonist exposure can impair alveolar epithelial beta-AR signaling downstream of the beta-AR.