Alveolar epithelial fluid clearance mechanisms are intact after moderate hyperoxic lung injury in rats

Physiological complexity of acute traumatic brain injury in patients treated with a brain oxygen protocol: utility of symbolic regression in predictive modeling of a dynamical system

Predictive modeling of emergent behavior, inherent to complex physiological systems, requires the analysis of large complex clinical data streams currently being generated in the intensive care unit. Brain tissue oxygen protocols have yielded outcome benefits in traumatic brain injury (TBI), but the critical physiological thresholds for low brain oxygen have not been established for a dynamical patho-physiological system. High frequency, multi-modal clinical data sets from 29 patients with severe TBI who underwent multi-modality neuro-clinical care monitoring and treatment with a brain oxygen protocol were analyzed. The inter-relationship between acute physiological parameters was determined using symbolic regression (SR) as the computational framework. The mean patient age was 44.4±15 with a mean admission GCS of 6.6±3.9. Sixty-three percent sustained motor vehicle accidents and the most common pathology was intra-cerebral hemorrhage (50%). Hospital discharge mortality was 21%, poor outcome occurred in 24% of patients, and good outcome occurred in 56% of patients. Criticality for low brain oxygen was intracranial pressure (ICP) ≥22.8 mm Hg, for mortality at ICP≥37.1 mm Hg. The upper therapeutic threshold for cerebral perfusion pressure (CPP) was 75 mm Hg. Eubaric hyperoxia significantly impacted partial pressure of oxygen in brain tissue (PbtO2) at all ICP levels. Optimal brain temperature (Tbr) was 34-35°C, with an adverse effect when Tbr≥38°C. Survivors clustered at [Formula: see text] Hg vs. non-survivors [Formula: see text] 18 mm Hg. There were two mortality clusters for ICP: High ICP/low PbtO2 and low ICP/low PbtO2. Survivors maintained PbtO2 at all ranges of mean arterial pressure in contrast to non-survivors. The final SR equation for cerebral oxygenation is: [Formula: see text]. The SR-model of acute TBI advances new physiological thresholds or boundary conditions for acute TBI management: PbtO2≥25 mmHg; ICP≤22 mmHg; CPP≈60-75 mmHg; and Tbr≈34-37°C. SR is congruous with the emerging field of complexity science in the modeling of dynamical physiological systems, especially during pathophysiological states. The SR model of TBI is generalizable to known physical laws. This increase in entropy reduces uncertainty and improves predictive capacity. SR is an appropriate computational framework to enable future smart monitoring devices.

Inhibition of NADPH oxidase prevents acute lung injury in obese rats following severe trauma

Lung capillary filtration coefficient (Kf) and impacts of oxidative stress have not been determined in the setting of severe trauma, especially in obese patients who exhibit increased lung injury. We hypothesized that severe trauma leads to a greater increase in lung Kf in obesity due to exacerbated production of and/or vulnerability to oxidative stress. Severe trauma was induced in lean and obese Zucker rats by muscle injury, fibula fracture, and bone component injection to both hindlimbs, with or without 24-h treatments of apocynin, a NADPH oxidase (NOX) inhibitor. Lung wet/dry weight ratios, lung vascular Kf, lung neutrophil counts, lung NOX and myeloperoxidase (MPO) activity, and plasma IL-6 levels were measured 24 h after trauma. In an additional study, lungs were isolated from nontrauma lean and obese rats to determine the acute effect of phenazime methosulfate, a superoxide donor, on pulmonary vascular Kf. After trauma, compared with lean rats, obese rats exhibited greater increases in lung capillary Kf, neutrophil accumulation, NOX and MPO activity, and plasma IL-6. The lung wet/dry weight ratio was increased in obese rats but not in lean rats. Apocynin treatment decreased lung Kf, neutrophil counts, NOX and MPO activities, wet/dry weight ratio, and plasma IL-6 in obese rats. Phenazime methosulfate treatment resulted in a greater increase in lung Kf in nontrauma obese rats compared with nontrauma lean rats. These results suggest that obese rats are susceptible to lung injury following severe trauma due to increased production of and responsiveness to pulmonary oxidative stress.

ENaC-mediated alveolar fluid clearance and lung fluid balance depend on the channel-activating protease 1

Sodium transport via epithelial sodium channels (ENaC) expressed in alveolar epithelial cells (AEC) provides the driving force for removal of fluid from the alveolar space. The membrane-bound channel-activating protease 1 (CAP1/Prss8) activates ENaC in vitro in various expression systems. To study the role of CAP1/Prss8 in alveolar sodium transport and lung fluid balance in vivo, we generated mice lacking CAP1/Prss8 in the alveolar epithelium using conditional Cre-loxP-mediated recombination. Deficiency of CAP1/Prss8 in AEC induced in vitro a 40% decrease in ENaC-mediated sodium currents. Sodium-driven alveolar fluid clearance (AFC) was reduced in CAP1/Prss8-deficient mice, due to a 48% decrease in amiloride-sensitive clearance, and was less sensitive to β₂-agonist treatment. Intra-alveolar treatment with neutrophil elastase, a soluble serine protease activating ENaC at the cell surface, fully restored basal AFC and the stimulation by β₂-agonists. Finally, acute volume-overload increased alveolar lining fluid volume in CAP1/Prss8-deficient mice. This study reveals that CAP1 plays a crucial role in the regulation of ENaC-mediated alveolar sodium and water transport and in mouse lung fluid balance.

Beta-adrenoceptor stimulation of alveolar fluid clearance is increased in rats with heart failure

The alveolar epithelium plays a critical role in resolving pulmonary edema. We thus hypothesized that its function might be upregulated in rats with heart failure, a condition that severely challenges the lung's ability to maintain fluid balance. Heart failure was induced by left coronary artery ligation. Echocardiographic and cardiovascular hemodynamics confirmed its development at 16 wk postligation. At that time, alveolar fluid clearance was measured by an increase in protein concentration over 1 h of a 5% albumin solution instilled into the lungs. Baseline alveolar fluid clearance was similar in heart failure and age-matched control rats. Terbutaline was added to the instillate to determine whether heart failure rats responded to beta-adrenoceptor stimulation. Alveolar fluid clearance in heart failure rats was increased by 194% after terbutaline stimulation compared with a 153% increase by terbutaline in control rats. To determine the mechanisms responsible for this accelerated alveolar fluid clearance, we measured ion transporter expression (ENaC, Na-K- ATPase, CFTR). No significant upregulation was observed for these ion transporters in the heart failure rats. Lung morphology showed significant alveolar epithelial type II cell hyperplasia in heart failure rats. Thus, alveolar epithelial type II cell hyperplasia is the likely explanation for the increased terbutaline-stimulated alveolar fluid clearance in heart failure rats. These data provide evidence for previously unrecognized mechanisms that can protect against or hasten resolution of alveolar edema in heart failure.

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.

Visualizing water clearance in the lung with MRI

Current indirect measurements of alveolar fluid clearance (AFC) suggest that the rate of fluid clearance correlates with morbidity and mortality in patients with pulmonary edema. In a traditional AFC-measurement, fluid laced with a tracer macromolecule is instilled into the lung and thereafter repeated samples of the instilled fluid are extracted from the lung's fluid-filled airspaces. The change in concentration of the tracer molecule indicates the AFC-rate. In this work, a new MRI technique was developed to image lung water clearance by adding Gadolinium-DTPA to the instilled fluid. As fluid is absorbed by the animal, the concentration of gadolinium will increase, reducing the T(1) relaxation time. By repeatedly measuring the T(1) relaxation time, the AFC can be tracked over time with high spatial resolution. The new technique was tested both in phantoms and 10 Yorkshire piglets.

Low expression of the beta-ENaC subunit impairs lung fluid clearance in the mouse

Transepithelial alveolar sodium (Na+) transport mediated by the amiloride-sensitive epithelial sodium channel (ENaC) constitutes the driving force for removal of fluid from the alveolar space. To define the role of the beta-ENaC subunit in vivo in the mature lung, we studied a previously established mouse strain harboring a disruption of the beta-ENaC gene locus resulting in low levels of beta-ENaC mRNA expression. Real-time RT-PCR experiments confirmed that beta-ENaC mRNA levels were decreased by >90% in alveolar epithelial cells from homozygous mutant (m/m) mice. beta-ENaC protein was undetected in lung homogenates from m/m mice by Western blotting, but alpha- and gamma-ENaC proteins were increased by 83% and 45%, respectively, compared with wild-type (WT) mice. At baseline, Na+-driven alveolar fluid clearance (AFC) was significantly reduced by 32% in m/m mice. Amiloride at the concentration 1 mM inhibited AFC by 75% and 34% in WT and m/m mice, respectively, whereas a higher concentration (5 mM) induced a 75% inhibition of AFC in both groups. The beta2-agonist terbutaline significantly increased AFC in WT but not in m/m mice. These results show that despite the compensatory increase in alpha- and gamma-ENaC protein expression observed in mutant mouse lung, low expression of beta-ENaC results in a moderate impairment of baseline AFC and in decreased AFC sensitivity to amiloride, suggesting a possible change in the stoichiometry of ENaC channels. Finally, adequate beta-ENaC expression appears to be required for AFC stimulation by beta2-agonists.

Role of the endothelium in pulmonary arterial hypertension

Pulmonary hypertension represents a significant disease burden in both the developed and developing worlds. Certain forms of pulmonary hypertension are more common in some countries than others but people of all races, all ages and both sexes are affected. Treatment options are limited and expensive. The development of new therapies will be determined by improved understanding of endothelial cell biology.

Pharmacotherapy of acute lung injury and the acute respiratory distress syndrome

Acute lung injury and the acute respiratory distress syndrome are common syndromes with a high mortality rate that affect both medical and surgical patients. Better understanding of the pathophysiology of acute lung injury and the acute respiratory distress syndrome and advances in supportive care and mechanical ventilation have led to improved clinical outcomes since the syndrome was first described in 1967. Although several promising pharmacological therapies, including surfactant, nitric oxide, glucocorticoids and lysofylline, have been studied in patients with acute lung injury and the acute respiratory distress syndrome, none of these pharmacological treatments reduced mortality. This article provides an overview of pharmacological therapies of acute lung injury and the acute respiratory distress syndrome tested in clinical trials and current recommendations for their use as well as a discussion of potential future pharmacological therapies including beta(2)-adrenergic agonist therapy, keratinocyte growth factor, and activated protein C.

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.

Benzamil, a blocker of epithelial Na(+) channel-induced upregulation of artery oxygen pressure level in acute lung injury rabbit ventilated with high frequency oscillation

The epithelial Na(+) transport via an epithelial Na(+) channel (ENaC) expressed in the lung epithelium would play a key role in recovery from lung edema at acute lung injury by removing the fluid in lung luminal space. The lung edema causes dysfunction of gas exchange, decreasing oxygen pressure level of artery [P(aO(2))]. To study if ENaC plays a key role in recovering P(aO(2)) from a decreased level to a normal one in acute lung injury, we applied benzamil (20microM, a specific blocker of ENaC) to the lung luminal space in acute lung injury treated with high frequency oscillation ventilation (HFOV) that is a lung-protective ventilation with a lower tidal volume and a smaller pressure swing than conventional mechanical ventilation (CMV). Benzamil facilitated the recovery of P(aO(2)) in acutely injured lung with HFOV but not CMV. The observation suggests that in acutely injured lung treated with HFOV an ENaC blocker, benzamil, can be applied as a therapeutic drug for acute lung injury combing with HFOV.

Bench-to-bedside review: the role of the alveolar epithelium in the resolution of pulmonary edema in acute lung injury

Clearance of pulmonary edema fluid is accomplished by active ion transport, predominantly by the alveolar epithelium. Various ion pumps and channels on the surface of the alveolar epithelial cell generate an osmotic gradient across the epithelium, which in turn drives the movement of water out of the airspaces. Here, the mechanisms of alveolar ion and fluid clearance are reviewed. In addition, many factors that regulate the rate of edema clearance, such as catecholamines, steroids, cytokines, and growth factors, are discussed. Finally, we address the changes to the alveolar epithelium and its transport processes during acute lung injury (ALI). Since relevant clinical outcomes correlate with rates of edema clearance in ALI, therapies based on our understanding of the mechanisms and regulation of fluid transport may be developed.

Defective respiratory amiloride-sensitive sodium transport predisposes to pulmonary oedema and delays its resolution in mice

Pulmonary oedema results from an imbalance between the forces driving fluid into the airspace and the biological mechanisms for its removal. In mice lacking the alpha-subunit of the amiloride-sensitive sodium channel (alphaENaC(-/-)), impaired sodium transport-mediated lung liquid clearance at birth results in neonatal death. Transgenic expression of alphaENaC driven by a cytomegalovirus (CMV) promoter (alphaENaC(-/-)Tg+) rescues the lethal pulmonary phenotype, but only partially restores respiratory sodium transport in vitro. To test whether this may also be true in vivo, and to assess the functional consequences of this defect on experimental pulmonary oedema, we measured respiratory transepithelial potential difference (PD) and alveolar fluid clearance (AFC), and quantified pulmonary oedema during experimental acute lung injury in these mice. Both respiratory PD and AFC were roughly 50% lower (P < 0.01) in alphaENaC(-/-)Tg+ than in control mice. This impairment was associated with a significantly larger increase of the wet/dry lung weight ratio in alphaENaC(-/-)Tg+ than in control mice, both after exposure to hyperoxia and thiourea. Moreover, the rate of resolution of thiourea-induced pulmonary oedema was more than three times slower (P < 0.001) in alphaENaC(-/-)Tg+ mice. alphaENaC(-/-)Tg+ mice represent the first model of a constitutively impaired respiratory transepithelial sodium transport, and provide direct evidence that this impairment facilitates pulmonary oedema in conscious freely moving animals. These data in mice strengthen indirect evidence provided by clinical studies, suggesting that defective respiratory transepithelial sodium transport may also facilitate pulmonary oedema in humans.

Clinically relevant concentrations of beta2-adrenergic agonists stimulate maximal cyclic adenosine monophosphate-dependent airspace fluid clearance and decrease pulmonary edema in experimental acid-induced lung injury

OBJECTIVE

To determine whether clinically relevant airspace concentrations of beta2-adrenergic agonists stimulated maximal alveolar fluid clearance rates and to determine whether beta2 agonist therapy decreased pulmonary edema in experimental acute lung injury.

DESIGN

Prospective randomized laboratory investigation.

SETTING

University-affiliated laboratory.

SUBJECTS

Sprague Dawley rats.

INTERVENTIONS

Dibutyryl cyclic adenosine monophosphate (cAMP), salmeterol, albuterol, and isoproterenol in normal rat lung. Salmeterol in a rat model of acid-induced lung injury.

MEASUREMENTS AND MAIN RESULTS

Basal alveolar fluid clearance was 7.6 +/- 2.2 %/hr. Maximal cAMP-dependent alveolar fluid clearance rate was 32.9 +/- 10.9 %/hr (p <.05). Racemic albuterol 10(-5) M, salmeterol 10(-6) M, and isoproterenol 10(-6) M each stimulated alveolar fluid clearance to a level comparable to maximal cAMP-dependent alveolar fluid clearance. Compared with basal rates, alveolar fluid clearance was increased by both racemic albuterol 10(-6) M (14.5 +/- 3.0%, p <.05) and R-enantiomer 10(-6) M (15.0 +/- 4.6%, p <.05), but there was no difference between the two groups. Intra-alveolar salmeterol 10 (-6) M attenuated the degree of pulmonary edema following acid-induced lung injury. Extravascular lung water increased to only 180 +/- 30 microL with salmeterol treatment, compared with 296 +/- 65 microL in saline-treated rats 4 hrs after acid injury (p <.05). This decrease in lung water was accompanied by a 2.4-fold increase in the rate of alveolar fluid clearance at 4 hrs in the salmeterol-treated group. Lung endothelial permeability, expressed as extravascular plasma equivalents, was reduced to 64 +/- 9 microL with salmeterol compared with 119 +/- 51 microL in saline-treated rats 4 hrs after acid injury (p <.05).

CONCLUSIONS

Clinically relevant airspace concentrations of beta2-adrenergic agonists a) stimulate maximal cAMP-dependent airspace fluid clearance in normal lungs and b) reduce pulmonary edema in acid aspiration-induced lung injury by increasing alveolar fluid clearance and decreasing endothelial permeability. Clinical studies are required to determine whether beta2-adrenergic agonists improve outcome in patients with acute lung injury.

Mechanisms of beta-receptor stimulation-induced improvement of acute lung injury and pulmonary edema

Acute lung injury (ALI) and the acute respiratory distress syndrome are complex syndromes because both inflammatory and coagulation cascades cause lung injury. Transport of salt and water, repair and remodeling of the lung, apoptosis, and necrosis are additional important mechanisms of injury. Alveolar edema is cleared by active transport of salt and water from the alveoli into the lung interstitium by complex cellular mechanisms. Beta-2 agonists act on the cellular mechanisms of pulmonary edema clearance as well as other pathways relevant to repair in ALI. Numerous studies suggest that the beneficial effects of beta-2 agonists in ALI include at least enhanced fluid clearance from the alveolar space, anti-inflammatory actions, and bronchodilation. The purposes of the present review are to consider the effects of beta agonists on three mechanisms of improvement of lung injury: edema clearance, anti-inflammatory effects, and bronchodilation. This update reviews specifically the evidence on the effects of beta-2 agonists in human ALI and in models of ALI. The available evidence suggests that beta-2 agonists may be efficacious therapy in ALI. Further randomized controlled trials of beta agonists in pulmonary edema and in acute lung injury are necessary.

Malnutrition impairs alveolar fluid clearance in rat lungs

Inadequate nutrition complicates the clinical course of critically ill patients, and many of these patients develop pulmonary edema. However, little is known about the effect of malnutrition on the mechanisms that resolve alveolar edema. Therefore, we studied the mechanisms responsible for the decrease in alveolar fluid clearance in rats exposed to malnutrition. Rats were allowed access to water, but not to food, for 120 h. Then, the left and right lungs were isolated for the measurement of lung water volume and alveolar fluid clearance, respectively. The rate of alveolar fluid clearance was measured by the progressive increase in the concentration of Evans blue dye that was instilled into the distal air spaces with an isosmolar 5% albumin solution over 1 h. Malnutrition decreased alveolar fluid clearance by 38% compared with controls. Amiloride (10−3M) abolished alveolar fluid clearance in malnourished rats. Either refeeding for 120 h following nutritional deprivation for 120 h or an oral supply of sodium glutamate during nutritional deprivation for 120 h restored alveolar fluid clearance to 91 and 86% of normal, respectively. Dibutyryl-cGMP, a cyclic nucleotide-gated cation channel agonist, increased alveolar fluid clearance in malnourished rats supplied with sodium glutamate. Terbutaline, a β2-adrenergic agonist, increased alveolar fluid clearance in rats under all conditions (control, malnutrition, refeeding, and glutamate-treated). These results indicate that malnutrition impairs primarily amiloride-insensitive and dibutyryl-cGMP-sensitive alveolar fluid clearance, but this effect is partially reversible by refeeding, treatment with sodium glutamate, or β-adrenergic agonist therapy.

Bench-to-bedside review: beta2-Agonists and the acute respiratory distress syndrome

The acute respiratory distress syndrome (ARDS) is a devastating constellation of clinical, radiological and pathological signs characterized by failure of gas exchange and refractory hypoxia. Despite nearly 30 years of research, no specific pharmacological therapy has yet proven to be efficacious in manipulating the pathophysiological processes that underlie this condition. Several in vitro and in vivo animal or human studies suggest a potential role for β₂-agonists in the treatment of ARDS. These agents have been shown to reduce pulmonary neutrophil sequestration and activation, accelerate alveolar fluid clearance, enhance surfactant secretion, and modulate the inflammatory and coagulation cascades. They are also used widely in clinical practice and are well tolerated in critically ill patients. The present review examines the evidence supporting a role for β₂-agonists as a specific pharmacological intervention in patients with ARDS.

Alveolar liquid clearance and sodium channel expression are decreased in transplanted canine lungs

To determine the impact of transplantation-associated injury on the clearance mechanisms of pulmonary edema, we created a canine single lung transplant model. After 3 hours of preservation and 4 hours of reperfusion, right native lungs and left transplanted lungs were used to measure alveolar liquid clearance (ALC) in ex vivo liquid-filled lung preparations. We also examined the role of the pulmonary circulation in edema clearance in in vivo liquid-filled lungs between 4 and 8 hours of reperfusion. To study molecular modifications in ALC, we also measured expression levels of the epithelial sodium channel (ENaC) and sodium-potassium-adenosine triphosphatase (ATPase). We found that ALC was significantly lower in transplanted than in right native lungs ex vivo (p < 0.05) and that transplanted lungs did not respond to the beta-adrenergic agonist terbutaline. Our in vivo study confirmed the ex vivo results. Molecular analyses revealed that ENaC messenger RNA but not sodium-potassium-ATPase was significantly decreased in transplanted lungs (p < 0.01). Furthermore, there was a significant decrease in ENaC protein expression. Therefore, we conclude that the current investigation indicates that the lung injury caused by lung preservation and transplantation significantly reduces the edema clearance ability of transplanted lungs.

Mechanical ventilation-induced pneumoprotein CC-16 vascular transfer in rats: effect of KGF pretreatment

After air-blood barrier injury, "pneumoproteins" specific to lung epithelial distal airspaces reaching the bloodstream are putative markers of lung hyperpermeability. The contribution of mechanical ventilation (MV) to this leakage is unknown. To explore this issue, 16-kDa Clara cell protein (CC-16) concentration was quantified in bronchoalveolar lavages (BALFs) and/or sera of rats first exposed either to ambient air or to 48 h of hyperoxia-induced acute lung injury and then ventilated for 2 h according to one of the following strategies: 1) spontaneous ventilation (SV), 2) very-low-volume high PEEP (VLVHP, where PEEP is positive end-expiratory pressure), 3) low-volume zero PEEP, 4) moderate-volume low PEEP, and 5) high-volume zero PEEP (HVZP). Results show that total proteins in BALFs increased with time and MV, with little impact from hyperoxia preexposure. CC-16 content decreased in BALFs but increased in the bloodstream during MV, suggesting intravascular leakage. Lung overdistension may result either from high-volume (HVZP) or high-PEEP (VLVHP) MV, and it was the most potent inducer of CC-16 leakage (P < 0.05 vs. SV). In the VLVHP group, pretreatment with keratinocyte growth factor was efficient in reducing blood CC-16 transfer.

Lung edema clearance: 20 years of progress: invited review: alveolar edema fluid clearance in the injured lung

Resolution of pulmonary edema involved active transepithelial sodium transport. Although several of the cellular and molecular mechanisms involved are relatively well understood, it is only recently that the regulation of these mechanisms in injured lung are being evaluated. Interestingly, in mild-to-moderate lung injury, alveolar edema fluid clearance is often preserved. This preserved or enhanced alveolar fluid clearance is mediated by catecholamine-dependent or -independent mechanisms. This stimulation of alveolar liquid clearance is related to activation or increased expression of sodium transport molecules such as the epithelial sodium channel or the Na(+)-K(+)-ATPase pump and may also involve the cystic fibrosis transmembrane conductance regulator. When severe lung injury occurs, the decrease in alveolar liquid clearance may be related to changes in alveolar permeability or to changes in activity or expression of sodium or chloride transport molecules. Multiple pharmacological tools such as beta-adrenergic agonists, vasoactive drugs, or gene therapy may prove effective in stimulating the resolution of alveolar edema in the injured lung.

Alveolar fluid clearance in patients with ARDS: does it make a difference?

Experimental methods to quantify alveolar fluid clearance have been adapted for our studies in patients with acute lung injury (ALI) or ARDS. We recently completed a study of 79 patients with ALI/ARDS that was designed to examine alveolar fluid clearance in the setting of alveolar epithelial injury from ALI/ARDS. Pulmonary edema fluid and plasma were sampled serially in the first 4 h after endotracheal intubation and the initiation of positive-pressure ventilation. Net alveolar fluid clearance was calculated from sequential edema fluid protein measurements. Patients with maximal alveolar fluid clearance had a significantly lower mortality rate and a shorter duration of mechanical ventilation. Several mechanisms may account for the decrease in the rate of alveolar fluid clearance in ALI/ARDS patients, including hypoxia, reactive oxygen species, reactive nitrogen species, and the loss of an intact epithelial barrier in the distal airspaces of the lung. Despite the epithelial injury in patients with ALI/ARDS, some experimental studies have suggested that alveolar fluid clearance could be increased with therapy using cyclic adenosine monophosphate agonists or other pharmacologic agents.

Invited review: lung edema clearance: role of Na(+)-K(+)-ATPase

Acute hypoxemic respiratory failure is a consequence of edema accumulation due to elevation of pulmonary capillary pressures and/or increases in permeability of the alveolocapillary barrier. It has been recognized that lung edema clearance is distinct from edema accumulation and is largely effected by active Na(+) transport out of the alveoli rather than reversal of the Starling forces, which control liquid flux from the pulmonary circulation into the alveolus. The alveolar epithelial Na(+)-K(+)-ATPase has an important role in regulating cell integrity and homeostasis. In the last 15 yr, Na(+)-K(+)-ATPase has been localized to the alveolar epithelium and its contribution to lung edema clearance has been appreciated. The importance of the alveolar epithelial Na(+)-K(+)-ATPase function is reflected in the changes in the lung's ability to clear edema when the Na(+)-K(+)-ATPase is inhibited or increased. An important focus of the ongoing research is the study of the mechanisms of Na(+)-K(+)-ATPase regulation in the alveolar epithelium during lung injury and how to accelerate lung edema clearance by modulating Na(+)-K(+)-ATPase activity.

Lung epithelial fluid transport and the resolution of pulmonary edema

The discovery of mechanisms that regulate salt and water transport by the alveolar and distal airway epithelium of the lung has generated new insights into the regulation of lung fluid balance under both normal and pathological conditions. There is convincing evidence that active sodium and chloride transporters are expressed in the distal lung epithelium and are responsible for the ability of the lung to remove alveolar fluid at the time of birth as well as in the mature lung when pathological conditions lead to the development of pulmonary edema. Currently, the best described molecular transporters are the epithelial sodium channel, the cystic fibrosis transmembrane conductance regulator, Na+-K+-ATPase, and several aquaporin water channels. Both catecholamine-dependent and -independent mechanisms can upregulate isosmolar fluid transport across the distal lung epithelium. Experimental and clinical studies have made it possible to examine the role of these transporters in the resolution of pulmonary edema.

Transepithelial sodium and water transport in the lung. Major player and novel therapeutic target in pulmonary edema

Active transepithelial transport of sodium from the airspaces to the lung interstitium is a primary mechanism driving alveolar fluid clearance. This mechanism depends on sodium uptake by amiloride-sensitive sodium channels on the apical membrane of alveolar type II cells followed by extrusion of sodium on the basolateral surface by the Na-K-ATPase. Injury to the alveolar epithelium can disrupt the integrity of the alveolar barrier or downregulate ion transport pathways thus reducing net alveolar fluid reabsorption, and enhancing the extent of alveolar edema. Endogenous catecholamines upregulate alveolar fluid clearance in several experimental models of acute lung injury, but this upregulation is short-term and often not sufficient to counterbalance alveolar flooding. There is new evidence, however, that pharmacological treatment with beta-adrenergic agonists and/or epithelial growth factors may induce a more sustained stimulation of alveolar fluid reabsorption and in turn facilitate recovery from experimental pulmonary edema. Similar results have been achieved experimentally by gene transfer enhancing the abundance of sodium transporters in the alveolar epithelium. Clinical studies show that impaired alveolar fluid transport mechanisms contribute to the development, severity and outcome of pulmonary edema in humans. Very recent data suggest that mechanisms that augment transepithelial sodium transport and enhance the clearance of alveolar edema may lead to more effective prevention or treatment for pulmonary edema and acute lung injury.

Salmeterol for the prevention of high-altitude pulmonary edema

BACKGROUND

Pulmonary edema results from a persistent imbalance between forces that drive water into the air space and the physiologic mechanisms that remove it. Among the latter, the absorption of liquid driven by active alveolar transepithelial sodium transport has an important role; a defect of this mechanism may predispose patients to pulmonary edema. Beta-adrenergic agonists up-regulate the clearance of alveolar fluid and attenuate pulmonary edema in animal models.

METHODS

In a double-blind, randomized, placebo-controlled study, we assessed the effects of prophylactic inhalation of the beta-adrenergic agonist salmeterol on the incidence of pulmonary edema during exposure to high altitudes (4559 m, reached in less than 22 hours) in 37 subjects who were susceptible to high-altitude pulmonary edema. We also measured the nasal transepithelial potential difference, a marker of the transepithelial sodium and water transport in the distal airways, in 33 mountaineers who were prone to high-altitude pulmonary edema and 33 mountaineers who were resistant to this condition.

RESULTS

Prophylactic inhalation of salmeterol decreased the incidence of high-altitude pulmonary edema in susceptible subjects by more than 50 percent, from 74 percent with placebo to 33 percent (P=0.02). The nasal potential-difference value under low-altitude conditions was more than 30 percent lower in the subjects who were susceptible to high-altitude pulmonary edema than in those who were not susceptible (P<0.001).

CONCLUSIONS

Prophylactic inhalation of a beta-adrenergic agonist reduces the risk of high-altitude pulmonary edema. Sodium-dependent absorption of liquid from the airways may be defective in patients who are susceptible to high-altitude pulmonary edema. These findings support the concept that sodium-driven clearance of alveolar fluid may have a pathogenic role in pulmonary edema in humans and therefore represent an appropriate target for therapy.

Clinical Acute Lung Injury and Acute Respiratory Distress Syndrome

This article provides a description of the clinical disorders associated with the development of acute noncardiogenic pulmonary edema, better known as clinical acute lung injury (ALI) or the acute respiratory distress syndrome (ARDS). Much has been learned about the mechanisms by which the lung is injured in patients with sepsis, pneumonia, aspiration of gastric contents, and following major trauma. In the last 5 years, major progress has been made in the treatment of patients with ALI/ARDS. A lung protective ventilatory strategy with a low tidal volume (6 mL/kg/predicted body weight) in conjunction with a plateau pressure limit of 30 cm H(2)0 attenuated the severity of clinical lung injury and reduced mortality by 22%. Ironically, after years of searching for anti-inflammatory treatments for ALI/ARDS, it turns out that a lung protective ventilatory strategy has proven to be the most efficacious anti-inflammatory treatment ever discovered for ALI/ARDS. However, it is still possible that pharmacologic treatments also may enhance survival. For example, a recent report that activated protein C reduces mortality in patients with sepsis raises hope that the incidence and severity of sepsis-induced ALI/ARDS may be reduced by treatment with this agent that has both anti-inflammatory and anticoagulant properties. Also, therapy directed at hastening the resolution of lung injury by increasing the functional recovery of the alveolar epithelium may be of value, both in diminishing the fibroproliferative phase of ALI/ARDS as well as accelerating the resolution of alveolar edema.

Treatment of ARDS

Improved understanding of the pathogenesis of acute lung injury (ALI)/ARDS has led to important advances in the treatment of ALI/ARDS, particularly in the area of ventilator-associated lung injury. Standard supportive care for ALI/ARDS should now include a protective ventilatory strategy with low tidal volume ventilation by the protocol developed by the National Institutes of Health ARDS Network. Further refinements of the protocol for mechanical ventilation will occur as current and future clinical trials are completed. In addition, novel modes of mechanical ventilation are being studied and may augment standard therapy in the future. Although results of anti-inflammatory strategies have been disappointing in clinical trials, further trials are underway to test the efficacy of late corticosteroids and other approaches to modulation of inflammation in ALI/ARDS.

Hyperoxia-induced changes in mouse lung mechanics: forced oscillations vs. barometric plethysmography

Hyperoxia-induced lung damage was investigated via airway and respiratory tissue mechanics measurements with low-frequency forced oscillations (LFOT) and analysis of spontaneous breathing indexes by barometric whole body plethysmography (WBP). WBP was performed in the unrestrained awake mice kept in room air ( n = 12) or in 100% oxygen for 24 ( n = 9), 48 ( n = 8), or 60 ( n = 9) h, and the indexes, including enhanced pause (Penh) and peak inspiratory and expiratory flows, were determined. The mice were then anesthetized, paralyzed, and mechanically ventilated. Airway resistance, respiratory system resistance at breathing frequency, and tissue damping and elastance were identified from the LFOT impedance data by model fitting. The monotonous decrease in airway resistance during hyperoxia correlated best with the increasing peak expiratory flow. Respiratory system resistance and tissue damping and elastance were unchanged up to 48 h of exposure but were markedly elevated at 60 h, with associated decreases in peak inspiratory flow. Penh was increased at 24 h and sharply elevated at 60 h. These results indicate no adverse effect of hyperoxia on the airway mechanics in mice, whereas marked parenchymal damage develops by 60 h. The inconsistent relationships between LFOT parameters and WBP indexes suggest that the changes in the latter reflect alterations in the breathing pattern rather than in the mechanical properties. It is concluded that, in the presence of diffuse lung disease, Penh is inadequate for characterization of the mechanical status of the respiratory system.

Beta1-adrenergic agonist is a potent stimulator of alveolar fluid clearance in hyperoxic rat lungs

Because it was still uncertain whether a stimulation of beta1-adrenoceptors accelerated alveolar fluid clearance in hyperoxic lung injury, the effect of denopamine, a selective beta1-adrenergic agonist, on alveolar fluid clearance was determined in rats exposed to 93% oxygen for 48 and 56 h. Alveolar fluid clearance was measured by the progressive increase in the concentration of Evans blue labeled albumin instilled into the alveolar spaces over 1 h at 37 degrees C in isolated rat lungs. The principle results were as follows: 1) Although lung water volume increased in rats exposed to hyperoxia for 48 and 56 h, basal alveolar fluid clearance did not change for up to 56 h; 2) Denopamine increased alveolar fluid clearance in rats exposed to hyperoxia as well as in rats without exposure to hyperoxia; 3) Denopamine primarily increased amiloride-insensitive alveolar fluid clearance in rats exposed to hyperoxia; 4) The potency of denopmaine was similar to that of terbutaline, a selective beta2-adrenergic agonist. In summary, denopamine is a potent stimulator of alveolar fluid clearance in rats exposed to hyperoxia.

Beta-adrenergic agonist therapy accelerates the resolution of hydrostatic pulmonary edema in sheep and rats

To determine whether beta-adrenergic agonist therapy increases alveolar liquid clearance during the resolution phase of hydrostatic pulmonary edema, we studied alveolar and lung liquid clearance in two animal models of hydrostatic pulmonary edema. Hydrostatic pulmonary edema was induced in sheep by acutely elevating left atrial pressure to 25 cmH(2)O and instilling 6 ml/kg body wt isotonic 5% albumin (prepared from bovine albumin) in normal saline into the distal air spaces of each lung. After 1 h, sheep were treated with a nebulized beta-agonist (salmeterol) or nebulized saline (controls), and left atrial pressure was then returned to normal. beta-Agonist therapy resulted in a 60% increase in alveolar liquid clearance over 3 h (P < 0.001). Because the rate of alveolar fluid clearance in rats is closer to human rates, we studied beta-agonist therapy in rats, with hydrostatic pulmonary edema induced by volume overload (40% body wt infusion of Ringer lactate). beta-Agonist therapy resulted in a significant decrease in excess lung water (P < 0.01) and significant improvement in arterial blood gases by 2 h (P < 0.03). These preclinical experimental studies support the need for controlled clinical trials to determine whether beta-adrenergic agonist therapy would be of value in accelerating the resolution of hydrostatic pulmonary edema in patients.

Mechanisms of repair and remodeling following acute lung injury

At present, we largely lack the ability to correlate the clinical course of ARDS patients with potential factors involved in the biochemical and cellular basis of lung repair. This requires very large patient databases with measurement of many biochemical parameters. Important mechanistic determinants during the repair phase can be sought by correlation with late outcomes, but a large-scale cooperative effort among multiple centers with sharing of follow-up data and patient specimens is essential. We also lack detailed human histologic material from many phases of ARDS and, particularly, know little of the long-term morphologic impact of ARDS in survivors. Establishment of a national registry that follows ARDS survivors and that would seek their cooperation in advance in obtaining autopsy specimens when they die of other causes would be very valuable. Correlating the pathology with their pulmonary function during recovery would give important insights into the reasons for the different patterns of abnormal pulmonary functions. The factors that determine the success of repair are of critical importance in testing new ARDS treatment strategies. Would accelerating the resolution of alveolar edema alter the course of subsequent fibrosis and inflammation? Does surfactant replacement therapy--a costly proposition in adults with ARDS--lead to better long-term outcomes in survivors? How much should we worry about the use of high levels of oxygen for support of arterial partial pressure of oxygen? Is it better to accept hyperoxia to avoid pressure or volume trauma induced by mechanical ventilation with higher minute ventilations? These major management issues all may affect the success of the late repair and recovery process. Intervention trials need to examine the long-term physiologic and functional outcomes.

Alveolar epithelial barrier. Role in lung fluid balance in clinical lung injury

Several studies have established that transport of sodium from the air spaces to the lung interstitium is a primary mechanism driving alveolar fluid clearance, although further work is needed to determine the role of chloride in vectorial fluid transport across the alveolar epithelium. Although there are significant differences among species in the basal rates of sodium and fluid transport, the basic mechanism seems to depend on sodium uptake by channels on the apical membrane of alveolar type II cells, followed by extrusion of sodium on the basolateral surface by Na,K-ATPase. This process can be upregulated by several catecholamine-dependent and independent mechanisms. The identification of water channels expressed in lung, together with the high water permeabilities, suggest a potential role for channel-mediated water movement between the air space and capillary compartments, although definitive evidence will depend on the results of transgenic mouse knock-out studies. The application of this new knowledge regarding salt and water transport in alveolar epithelium in relation to pathologic conditions has been successful in clinically relevant experimental studies, as well as in a few clinical studies. The studies of exogenous and endogenous catecholamine regulation of alveolar fluid clearance are a good example of how new insights into the basic mechanisms of alveolar sodium and fluid transport can be translated to clinically relevant experimental studies. Exogenous catecholamines can increase the rate of alveolar fluid clearance in several species, including the human lung, and it is also apparent that release of endogenous catecholamines can upregulate alveolar fluid clearance in animals with septic or hypovolemic shock. It is possible that therapy with beta-adrenergic agonists might be useful to accelerate the resolution of alveolar edema in some patients. In some patients, the extent of injury to the alveolar epithelial barrier may be too severe for beta-adrenergic agonists to enhance the resolution of alveolar edema, although some experimental studies indicate that alveolar fluid clearance can be augmented in the presence of moderately severe lung injury. A longer-term upregulation of alveolar epithelial fluid transport might be achieved by strategies that accelerate the proliferation of alveolar type II cells repopulating the injured epithelium in clinical lung injury. More clinical research is needed to evaluate the strategies that can upregulate alveolar epithelial fluid transport with both short-term therapy (i.e., beta-agonists) and more sustained, longer-term effects of epithelial mitogens such as keratinocyte growth factor. These approaches may be useful in reducing mortality in the acute respiratory distress syndrome.

beta-adrenergic stimulation restores rat lung ability to clear edema in ventilator-associated lung injury

Mechanical ventilation with high tidal volume (HVT) causes lung injury and decreases the lung's ability to clear edema in rats. beta-adrenergic agonists increase active Na(+) transport and lung edema clearance in normal rat lungs by stimulating apical Na(+) channels and basolateral Na,K-ATPase in alveolar epithelial cells. We studied whether beta-adrenergic agonists could restore lung edema clearance in rats ventilated with HVT (40 ml/kg, peak airway pressure of 35 cm H(2)O) for 40 min. The ability of rat lungs to clear edema decreased by approximately 50% after 40 min of HVT ventilation. Terbutaline (TERB) and isoproterenol (ISO) increased lung edema clearance in control nonventilated rats (from 0.50 +/- 0. 02 ml/h to 0.81 +/- 0.04 ml/h and 0.99 +/- 0.05 ml/h, respectively) and restored the lung's ability to clear edema in HVT ventilated rats (from 0.25 +/- 0.03 ml/h to 0.64 +/- 0.02 ml/h and 0.88 +/- 0. 09 ml/h, respectively). Disruption of cell microtubular transport system by colchicine inhibited the stimulatory effects of ISO in HVT ventilated rats, whereas beta-lumicolchicine did not affect beta-adrenergic stimulation. The Na,K-ATPase alpha(1)- and beta(1)-subunit mRNA steady state levels were not affected by incubation with ISO for 60 min in alveolar type II cells isolated from control and HVT ventilated rats. The data suggest that beta-adrenergic agonists increased alveolar fluid reabsorption in rats ventilated with HVT by promoting recruitment of ion-transporting proteins from intracellular pools to the plasma membrane of alveolar epithelial cells.

Alveolar sodium and liquid transport in mice

We have developed a simple isolated lung preparation for measurement of liquid and solute fluxes across mouse alveolar epithelium. Liquid instilled into air spaces was absorbed at the rate (J(w)) of 3.7 +/- 0.32 ml x h(-1) x g dry lung wt(-1) x J(w) was significantly depressed by ouabain (P < 0.001) and amiloride (P < 0.001). Omission of glucose from the instillate or addition of the Na(+)-glucose cotransport inhibitor phloridzin did not affect J(w). However, the low epithelial lining fluid glucose concentration (one-third that of plasma), the larger-than-mannitol permeability of methyl-alpha-D-glucopyranoside, and the presence of Na(+)-glucose cotransporter SGLT1 mRNA in mouse lung tissue suggest that there is a Na(+)-glucose cotransporter in the mouse alveolar-airway barrier. Isoproterenol stimulated J(w) (6.5 +/- 0.45 ml x h(-1) x g dry lung wt(-1); P < 0.001), and this effect was blocked by amiloride, benzamil, ouabain, and the specific beta(2)-adrenergic antagonist ICI-118551 but not by atenolol. Similar stimulation was obtained with terbutaline (6.4 +/- 0.46 ml x h(-1) x g dry lung wt(-1)). Na(+) unidirectional fluxes out of air spaces varied in agreement with J(w) changes. Thus alveolar liquid absorption in mice follows Na(+) transport via the amiloride-sensitive pathway, with little contribution from Na(+)-glucose cotransport, and is stimulated by beta(2)-adrenergic agonists.

Alveolar epithelial fluid transport can be simultaneously upregulated by both KGF and beta-agonist therapy

Although keratinocyte growth factor (KGF) protects against experimental acute lung injury, the mechanisms for the protective effect are incompletely understood. Therefore, the time-dependent effects of KGF on alveolar epithelial fluid transport were studied in rats 48-240 h after intratracheal administration of KGF (5 mg/kg). There was a marked proliferative response to KGF, measured both by in vivo bromodeoxyuridine staining and by staining with an antibody to a type II cell antigen. In controls, alveolar liquid clearance (ALC) was 23 +/- 3%/h. After KGF pretreatment, ALC was significantly increased to 30 +/- 2%/h at 48 h, to 39 +/- 2%/h at 72 h, and to 36 +/- 3%/h at 120 h compared with controls (P < 0.05). By 240 h, ALC had returned to near-control levels (26 +/- 2%/h). The increase in ALC was explained primarily by the proliferation of alveolar type II cells, since there was a good correlation between the number of alveolar type II cells and the increase in ALC (r = 0.92, P = 0.02). The fraction of ALC inhibited by amiloride was similar in control rats (33%) as in 72-h KGF-pretreated rats (38%), indicating that there was probably no major change in the apical pathways for Na uptake in the KGF-pretreated rats at this time point. However, more rapid ALC at 120 h, compared with 48 h after KGF treatment, may be explained by greater maturation of alpha-epithelial Na channel, since its expression was greater at 120 than at 48 h, whereas the number of type II cells was the same at these two time points. beta-Adrenergic stimulation with terbutaline 72 h after KGF pretreatment further increased ALC to 50 +/- 7%/h (P < 0.5). In summary, KGF induced a sustained increase over 120 h in the fluid transport capacity of the alveolar epithelium. This impressive upregulation in fluid transport was further enhanced with beta-adrenergic agonist therapy, thus providing evidence that two different treatments can simultaneously increase the fluid transport capacity of the alveolar epithelium.

Isoproterenol improves ability of lung to clear edema in rats exposed to hyperoxia

Exposure of adult rats to 100% O(2) results in lung injury and decreases active sodium transport and lung edema clearance. It has been reported that beta-adrenergic agonists increase lung edema clearance in normal rat lungs by upregulating alveolar epithelial Na(+)-K(+)-ATPase function. This study was designed to examine whether isoproterenol (Iso) affects lung edema clearance in rats exposed to 100% O(2) for 64 h. Active Na(+) transport and lung edema clearance decreased by approximately 44% in rats exposed to acute hyperoxia. Iso (10(-6) M) increased the ability of the lung to clear edema in room-air-breathing rats (from 0.50 +/- 0.02 to 0.99 +/- 0. 05 ml/h) and in rats exposed to 100% O(2) (from 0.28 +/- 0.03 to 0. 86 +/- 0.09 ml/h; P < 0.001). Disruption of intracellular microtubular transport of ion-transporting proteins by colchicine (0. 25 mg/100 g body wt) inhibited the stimulatory effects of Iso in hyperoxia-injured rat lungs, whereas the isomer beta-lumicolchicine, which does not affect microtubular transport, did not inhibit active Na(+) transport stimulated by Iso. Accordingly, Iso restored the lung's ability to clear edema after hyperoxic lung injury, probably by stimulation of the recruitment of ion-transporting proteins (Na(+)-K(+)-ATPase) from intracellular pools to the plasma membrane in rat alveolar epithelium.

KGF prevents hyperoxia-induced reduction of active ion transport in alveolar epithelial cells

We evaluated the effects of acute hyperoxic exposure on alveolar epithelial cell (AEC) active ion transport and on expression of Na+ pump (Na+-K+-ATPase) and rat epithelial Na+ channel subunits. Rat AEC were cultivated in minimal defined serum-free medium (MDSF) on polycarbonate filters. Beginning on day 5, confluent monolayers were exposed to either 95% air-5% CO2 (normoxia) or 95% O2-5% CO2 (hyperoxia) for 48 h. Transepithelial resistance (Rt) and short-circuit current (Isc) were determined before and after exposure. Na+ channel alpha-, beta-, and gamma-subunit and Na+-K+-ATPase alpha1- and beta1-subunit mRNA levels were quantified by Northern analysis. Na+ pump alpha1- and beta1-subunit protein abundance was quantified by Western blotting. After hyperoxic exposure, Isc across AEC monolayers decreased by approximately 60% at 48 h relative to monolayers maintained under normoxic conditions. Na+ channel beta-subunit mRNA expression was reduced by hyperoxia, whereas alpha- and gamma-subunit mRNA expression was unchanged. Na+ pump alpha1-subunit mRNA was unchanged, whereas beta1-subunit mRNA was decreased approximately 80% by hyperoxia in parallel with a reduction in beta1-subunit protein. Because keratinocyte growth factor (KGF) has recently been shown to upregulate AEC active ion transport and expression of Na+-K+-ATPase under normoxic conditions, we assessed the ability of KGF to prevent hyperoxia-induced changes in active ion transport by supplementing medium with KGF (10 ng/ml) from day 2. The presence of KGF prevented the effects of hyperoxia on ion transport (as measured by Isc) relative to normoxic controls. Levels of beta1 mRNA and protein were relatively preserved in monolayers maintained in MDSF and KGF compared with those cultivated in MDSF alone. These results indicate that AEC net active ion transport is decreased after 48 h of hyperoxia, likely as a result of a decrease in the number of functional Na+ pumps per cell. KGF largely prevents this decrease in active ion transport, at least in part, by preserving Na+ pump expression.

Sodium channels in alveolar epithelial cells: molecular characterization, biophysical properties, and physiological significance

At birth, fetal distal lung epithelial (FDLE) cells switch from active chloride secretion to active sodium (Na+) reabsorption. Sodium ions enter the FDLE and alveolar type II (ATII) cells mainly through apical nonselective cation and Na(+)-selective channels, with conductances of 4-26 pS (picoSiemens) in FDLE and 20-25 pS in ATII cells. All these channels are inhibited by amiloride with a 50% inhibitory concentration of < 1 microM, and some are also inhibited by [N-ethyl-N-isopropyl]-2'-4'-amiloride (50% inhibitory concentration of < 1 microM). Both FDLE and ATII cells contain the alpha-, beta-, and gamma-rENaC (rat epithelial Na+ channels) mRNAs; reconstitution of an ATII cell Na(+)-channel protein into lipid bilayers revealed the presence of 25-pS Na+ single channels, inhibited by amiloride and [N-ethyl-N-isopropyl]-2'-4'-amiloride. A variety of agents, including cAMP, oxygen, glucocorticoids, and in some cases Ca2+, increased the activity and/or rENaC mRNA levels. The phenotypic properties of these channels differ from those observed in other Na(+)-absorbing epithelia. Pharmacological blockade of alveolar Na+ transport in vivo, as well as experiments with newborn alpha-rENaC knock-out mice, demonstrate the importance of active Na+ transport in the reabsorption of fluid from the fetal lung and in reabsorbing alveolar fluid in the injured adult lung. Indeed, in a number of inflammatory diseases, increased production of reactive oxygen-nitrogen intermediates, such as peroxynitrite (ONOO-), may damage ATII and FDLE Na+ channels, decrease Na+ reabsorption in vivo, and thus contribute to the formation of alveolar edema.

Dopamine restores lung ability to clear edema in rats exposed to hyperoxia

Exposure to hyperoxia causes lung injury, decreases active sodium transport and lung edema clearance in rats. Dopamine (DA) increases lung edema clearance by stimulating vectorial Na+ flux and Na, K-ATPase function in rat alveolar epithelium. This study was designed to test whether DA (10(-)5 M) would increase lung edema clearance in rats exposed to 100% O2 for 64 h. Active Na+ transport and lung edema clearance decreased by approximately 44% in rats exposed to acute hyperoxia (p < 0.001). DA increased lung edema clearance in room air breathing rats (from 0.50 +/- 0.02 to 0.75 +/- 0.06 ml/h) and in rats exposed to 100% O2 (from 0.28 +/- 0.03 to 0. 67 +/- 0.03 ml/h). Disruption of cell microtubular transport system by colchicine blocked the stimulatory effect of DA on active Na+ transport in control and hyperoxic rats, whereas the isomer beta-lumicolchicine, which does not affect cell microtubular transport, did not inhibit the stimulatory effects of dopamine. The Na,K-ATPase alpha1-subunit protein abundance increased in the basolateral membranes of alveolar type II (ATII) cells incubated with 10(-)5 M DA for 15 min, probably by recruiting Na+ pumps from intracellular pools. Colchicine, but not beta-lumicolchicine, prevented the recruitment of alpha1 subunits to the plasma membrane by DA. Accordingly, DA restored lung ability to clear edema in hyperoxic-injured rat lungs. Conceivably, dopamine induces recruitment of Na+ pumps from intracellular pools to the plasma membrane of alveolar epithelial cells and thus increases lung edema clearance.

Upregulation of alveolar epithelial fluid transport after subacute lung injury in rats from bleomycin

Alveolar epithelial fluid transport was studied 10 days after subacute lung injury had been induced with intratracheal bleomycin (0.75 U). An isosmolar Ringer lactate solution with 5% bovine serum albumin and 125I-labeled albumin as the alveolar protein tracer was instilled into the right lung; the rats were then studied for either 1 or 4 h. Alveolar fluid clearance was increased in bleomycin-injured rats by 110% over 1 h and by 75% over 4 h compared with control rats (P < 0.05). The increase in alveolar fluid clearance was partially inhibited by amiloride (10(-3) M). Alveolar fluid clearance decreased toward normal levels in rats that were studied 60 days after bleomycin instillation. Remarkably, the measured increase in net alveolar fluid clearance occurred in the presence of a significant increase in alveolar epithelial permeability to protein. Moreover, the increase in alveolar epithelial fluid clearance occurred even though the mRNA for the alpha-subunit of the epithelial sodium channel was decreased in alveolar epithelial type II cells isolated from these rats. In addition, 22Na uptake by isolated alveolar epithelial type II cells from rats treated with bleomycin demonstrated a 52% decrease in uptake compared with type II cells from control rats. Morphological results demonstrated a significant hyperplasia of alveolar type II epithelial cells 10 days after bleomycin injury. Thus, these results provide evidence that proliferation of alveolar epithelial type II cells after acute lung injury may upregulate the transport capacity of the alveolar epithelium, even though the expression of epithelial sodium channels is reduced and the uptake of 22Na per cell is also reduced. These results may have clinical relevance for the resolution of alveolar edema in the subacute phase of lung injury.

Alveolar liquid clearance in the anesthetized ventilated guinea pig

Alveolar liquid clearance was examined in ventilated, anesthetized guinea pigs. An isosmolar 5% albumin solution was instilled into the lungs. Alveolar liquid clearance was studied over 1 h and was measured from the increase in alveolar protein concentration as water was reabsorbed. Basal alveolar liquid clearance was 38% of instilled volume. The high basal alveolar liquid clearance was not secondary to endogenous catecholamine release. Compared with control animals, epinephrine and the general beta-adrenergic agonist isoproterenol increased alveolar liquid clearance to approximately 50% of instilled volume (P < 0.05), whereas the beta 2-adrenergic agonist terbutaline was without effect. The stimulation of alveolar liquid clearance by epinephrine or isoproterenol was completely inhibited by the addition of the general beta-adrenergic inhibitor propranolol or the beta 1-adrenergic inhibitor atenolol. Alveolar liquid clearance was inhibited by the sodium-channel inhibitor amiloride by 30-40% in control animals and in animals treated with epinephrine or isoproterenol. Isoproterenol and epinephrine, but not terbutaline, increased adenosine 3',5'-cyclic monophosphate in in vitro incubated lung tissue. The results suggest that alveolar liquid clearance in guinea pigs is mediated partly through amiloride-sensitive sodium channels and that alveolar liquid clearance can be increased by stimulation of primarily beta 1-adrenergic receptors.