Effect of epinephrine on alveolar liquid clearance in the rat

Using the sympathetic system, beta blockers and alpha-2 agonists, to address acute respiratory distress syndrome

Acute Respiratory Distress Syndrome (ARDS) manifests as an acute inflammatory lung injury characterized by persistent hypoxemia, featuring a swift onset, high mortality, and predominantly supportive care as the current therapeutic approach, while effective treatments remain an area of active investigation. Adrenergic receptors (AR) play a pivotal role as stress hormone receptors, extensively participating in various inflammatory processes by initiating downstream signaling pathways. Advancements in molecular biology and pharmacology continually unveil the physiological significance of distinct AR subtypes. Interventions targeting these subtypes have the potential to induce specific alterations in cellular and organismal functions, presenting a promising avenue as a therapeutic target for managing ARDS. This article elucidates the pathogenesis of ARDS and the basic structure and function of AR. It also explores the relationship between AR and ARDS from the perspective of different AR subtypes, aiming to provide new insights for the improvement of ARDS.

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.

Adrenaline aggravates lung injury caused by liver ischemia-reperfusion and high-tidal-volume ventilation in rats

BACKGROUND

We often administer adrenaline to improve hypotension of patients undergoing systemic inflammation that is not treated with volume resuscitation. The effects of adrenaline on injured lungs during shock status have not been elucidated. We previously demonstrated that hepatic ischemia-reperfusion followed by high-tidal-volume ventilation-induced systemic inflammation, hypotension, and lung injury in rats. Using this animal model, we investigated the effects of adrenaline on lung injury and hemodynamics.

METHODS

Anesthetized rats were ventilated and underwent hepatic inflow interruption for 15 min twice. After the second liver ischemia-reperfusion, the tidal volume was increased to 24 ml · kg(-1) body weight from 6 ml · kg(-1), and 12 rats in each group were observed for 360 min after reperfusion with or without continuous intravenous adrenaline administration. Extra fluid was administered according to the decline in the arterial blood pressure.

RESULTS

Adrenaline administration significantly reduced the volume of intravenous resuscitation fluid. The wet-to-dry weight ratio of the lungs was higher (7.53 ± 0.37 vs. 4.63 ± 0.35, P < 0.001), the partial oxygen pressure in arterial blood was lower (213 ± 48 vs. 411 ± 33, P = 0.004), and the tumor necrosis factor-α concentration in bronchoalveolar lavage (BAL) fluid was higher (10(2.64) ± 10(0.22) vs. 10(1.91) ± 10(0.27), P = 0.015), with adrenaline. Histopathological examinations revealed marked exudation in the alveolar spaces in rats receiving adrenaline.

CONCLUSIONS

Continuous administration of adrenaline partially prevented a rapid decline in blood pressure but deteriorated lung injury in a rat model of liver ischemia-reperfusion with high-tidal-volume ventilation. A possibility that adrenaline administration aggravate ventilator-induced lung injury during systemic inflammation should be considered.

An inhaled tumor necrosis factor-alpha-derived TIP peptide improves the pulmonary function in experimental lung injury

INTRODUCTION

The lectin-like domain of TNF-α enhances the fluid clearance across the alveolar barrier. For experimental purposes, the lectin-like domain can be mimicked by a synthetic peptide representing the TIP-motif of TNF-α. The present study aims to assess the acute effect of TIP on the pulmonary function in a porcine model of acute respiratory distress syndrome (ARDS).

METHODS

Lung injury was induced in 16 pigs (25-27 kg) by bronchoalveolar lavage followed by injurious ventilation. Following randomisation, either nebulised TIP (1 mg/kg; AP301, APEPTICO, Vienna, Austria) or water for injection (control group) was administered. During 5 h of monitoring, the extravascular lung water index (EVLWI), the quotient of partial pressure of oxygen and inspired oxygen concentration (PaO(2) /FiO(2) ) and the pulmonary shunt fraction were repetitively assessed. The data were evaluated by an analysis of variance including Bonferroni-Holm correction.

RESULTS

Comparable baseline conditions in both groups were achieved. Ventilatory parameters were standardised in both groups. In the TIP group, a significant reduction of the EVLWI and a simultaneous increase in the PaO(2) /FiO(2) ratio was shown (each P < 0.0001). No changes in the control group were observed (EVLWI: P = 0.43, PaO(2) /FiO(2) : P = 0.60). The intergroup comparison demonstrates a significant advantage of TIP inhalation over placebo (EVLWI: P < 0.0001, PaO(2) /FiO(2) : P = 0.004, shunt fraction: P = 0.0005).

CONCLUSIONS

The inhalation of TIP induces an amelioration of clinical surrogate parameters of the lung function in a porcine lung injury model. By mimicking the lectin-like domain, the synthetic TIP peptide AP301 is an innovative approach as supportive therapy in ARDS.

Enhanced alveolar fluid clearance following 72 h of continuous isoproterenol infusion in rats

AIM

We wished to determine the effect of continuous β-receptor stimulation on alveolar fluid clearance and to elucidate the mechanisms behind this effect.

METHODS

Alveolar fluid clearance was measured in anaesthetized rats pretreated for 72 h with the β-agonist isoproterenol (200 μg kg(-1) h(-1) sc) or vehicle. Alveolar fluid clearance in artificially ventilated rats was determined over 1 h by infusion of isotonic Ringer solution containing (125) I-albumin into the lungs. Additionally, alveolar fluid clearance was determined when amiloride or l-cis-diltiazem was added to the solution to block ENaC and cyclic nucleotide-gated (CNG) channels respectively.

RESULTS

Isoproterenol treatment induced a 42% increase in alveolar fluid clearance (18.9 ± 1.4%) vs. vehicle (13.3 ± 3.3%). Addition of amiloride resulted in a net decrease of 8% in both groups, while l-cis-diltiazem caused a net decrease of 12% in isoproterenol-treated animals, but only 5% in vehicle-treated animals. Western blotting showed that isoproterenol treatment increased the abundance of the α-ENaC and β-ENaC subunits (223 ± 51% and 274 ± 55% of vehicle, respectively) but we saw no changes in protein level of the γ-EnaC subunit.

CONCLUSION

Continuous β-adrenoceptor stimulation with isoproterenol enhances alveolar fluid clearance through alternative pathways involving l-cis-diltiazem-sensitive channels.

Nasal potential difference to detect Na+ channel dysfunction in acute lung injury

Pulmonary fluid clearance is regulated by the active transport of Na(+) and Cl(-) through respiratory epithelial ion channels. Ion channel dysfunction contributes to the pathogenesis of various pulmonary fluid disorders including high-altitude pulmonary edema (HAPE) and neonatal respiratory distress syndrome (RDS). Nasal potential difference (NPD) measurement allows an in vivo investigation of the functionality of these channels. This technique has been used for the diagnosis of cystic fibrosis, the archetypal respiratory ion channel disorder, for over a quarter of a century. NPD measurements in HAPE and RDS suggest constitutive and acquired dysfunction of respiratory epithelial Na(+) channels. Acute lung injury (ALI) is characterized by pulmonary edema due to alveolar epithelial-interstitial-endothelial injury. NPD measurement may enable identification of critically ill ALI patients with a susceptible phenotype of dysfunctional respiratory Na(+) channels and allow targeted therapy toward Na(+) channel function.

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.

Pulmonary vascular heterogeneity and the Starling hypothesis

It has generally been assumed that movement of fluid between the pulmonary microvasculature and surrounding tissues is governed by a "Starling" balance of hydrostatic and protein osmotic forces similar to that which prevails in the extremities. However, both recent and older observations suggest that the lungs are more resistant to edema formation than most other organs. Several structural aspects of the lung may account for protection of the airspaces from edema formation. The pulmonary microvasculature, which comprises >70% of the pulmonary circulatory bed, appears to be less permeable to fluid and electrolytes than the endothelium of the pulmonary arteries and veins and other microvascular exchange areas. This arrangement may help explain why early edema is confined to the perivascular and peribronchial regions and why lymphatics do not reach the alveoli. Unlike the peripheral vasculature, which is compressed by edema formation, the extra-alveolar vessels remain tethered open by airway distention, even when interstitial pressures rise above those in the vessels. This may also facilitate return of proteins to the circulation. Ultrafiltration of plasma may lower local protein concentrations in the interstitium, thereby slowing further edema formation. Transendothelial reabsorption of fluid may also be altered by vesicular transport.

Alveolar edema fluid clearance and acute lung injury

Although lung-protective ventilation strategies have substantially reduced mortality of acute lung injury patients there is still a need for new therapies that can further decrease mortality in patients with acute lung injury. Studies of epithelial ion and fluid transport across the distal pulmonary epithelia have provided important new concepts regarding potential new therapies for acute lung injury. Overall, there is convincing evidence that the alveolar epithelium is not only a tight epithelial barrier that resists the movement of edema fluid into the alveoli, but it is also actively involved in the transport of ions and solutes, a process that is essential for edema fluid clearance and the resolution of acute lung injury. The objective of this article is to consider some areas of recent progress in the field of alveolar fluid transport under normal and pathologic conditions. Vectorial ion transport across the alveolar and distal airway epithelia is the primary determinant of alveolar fluid clearance. The general paradigm is that active Na(+) and Cl(-) transport drives net alveolar fluid clearance, as demonstrated in several different species, including the human lung. Although these transport processes can be impaired in severe lung injury, multiple experimental studies suggest that upregulation of Na(+) and Cl(-) transport might be an effective therapy in acute lung injury. We will review mechanisms involved in pharmacological modulation of ion transport in lung injury with a special focus on the use of beta-adrenergic agonists which has generated considerable interest and is a promising therapy for clinical acute lung injury.

Stimulation of alveolar epithelial fluid clearance in human lungs by exogenous epinephrine

OBJECTIVES

Because several experimental studies have demonstrated that cyclic adenosine monophosphate generation following beta-adrenoceptor activation can markedly stimulate alveolar fluid clearance, we determined whether the endogenous levels of catecholamines that occur in the pulmonary edema fluid and plasma of patients with acute lung injury are high enough to stimulate alveolar fluid clearance in the human lung.

DESIGN

Observational clinical study.

SETTING

Academic university hospital and laboratory.

PATIENTS

Twenty-one patients with acute pulmonary edema plus ex vivo human lungs.

INTERVENTIONS

Measurements of catecholamine levels in patient samples and controlled laboratory studies of the effects of these catecholamine levels on the rates of alveolar fluid clearance in ex vivo human lungs.

MEASUREMENTS AND MAIN RESULTS

The concentrations of both epinephrine and norepinephrine in the pulmonary edema fluid and plasma were approximately 10 M (range of 1-8x10 M) in hydrostatic pulmonary edema (n=6) and acute lung injury patients (n=15). We therefore tested whether 10 M epinephrine or norepinephrine stimulated alveolar fluid clearance in isolated human lungs and found that these epinephrine or norepinephrine concentrations did not stimulate alveolar fluid clearance. However, higher concentrations of epinephrine (10 M), but not norepinephrine (10 M), significantly stimulated alveolar fluid clearance by 84% above control. Glibenclamide (10 M) and CFTRinh-172 (10 M), cystic fibrosis transmembrane conductance regulator inhibitors, completely inhibited the epinephrine-induced stimulation of alveolar fluid clearance.

CONCLUSIONS

These results indicate that endogenous catecholamine concentrations in pulmonary edema fluid are probably not sufficient to stimulate alveolar fluid clearance. In contrast, administration of exogenous catecholamines into the distal airspaces can stimulate alveolar fluid clearance in the human lung, an effect that is mediated in part by cystic fibrosis transmembrane conductance regulator. Therefore, exogenous cyclic adenosine monophosphate-dependent stimulation will probably be required to accelerate the resolution of alveolar edema in the lungs of patients with pulmonary 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.

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.

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.

Agonist-mediated airway challenge: cardiopulmonary interactions modulate gas exchange and recovery

Diverse agonists used for airway challenges produce a stereotypic sequence of immediate functional responses (e.g., bronchoconstriction, gas trapping, hypoxemia, etc.) at the time such reactions are triggered. The reaction incorporates both pulmonary and cardiac changes that clearly interact in an orchestrated fashion taking the subject (or animal model) through the response generally to ultimate recovery. We hypothesize that despite differences in the initiation of the response, diverse airway provocations lead to a cascade of events that converge through a common physiologic pathway. To better understand the sequence of events and the counterbalanced cardiopulmonary responses, we examined histamine, methacholine, and ovalbumin (OVA) challenges in the awake guinea pig model and assessed ventilatory and breathing mechanics in the context of associated cardiac parameters. With the histamine response as the prototype, we evaluated the role of beta-adrenoreceptors using propranolol (1.0-10 mg/kg i.p.) and found that beta-adrenoreceptors are critical in reducing challenge-induced gas trapping in the lungs. The disposition of the circulatory response to agonist challenge (the OVA model) was reflected in a significant absolute shunting of blood through poorly ventilated regions of the lung. The methacholine challenge revealed that gasping enhanced lung inflation and reversed the diminished Pa(O2). Moreover, beta-sympathetic function was critical to recovery. Collectively, the response profiles of these disparate models of airway challenge suggest a highly integrated balance to maintain gas exchange among the pulmonary airways and vasculature, modulated in recovery by beta-adrenoreceptors.

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.

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.

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.

Resolution of the oedema associated with allergic pulmonary inflammation in rats assessed noninvasively by magnetic resonance imaging

1. Magnetic resonance imaging (MRI) was used to study noninvasively the effects of compounds to resolve inflammation induced by ovalbumin (OVA) challenge in the lungs of actively sensitised rats. 2. Marked oedematous signals were detected between 24 and 96 h following OVA in vehicle-treated animals. When administered 24 h after OVA, budesonide, a glucocorticosteroid, or 4-(8-benzo[1,2,5]oxadiazol-5-yl-[1,7]naphthyridin-6-yl)-benzoic acid (NVP-ABE171), a selective phosphodiesterase 4 inhibitor, increased the rate of resolution of established oedematous signals detected by MRI. The effect was evident 3 h after drug administration and the signals were nearly fully resolved at 96 h postchallenge. 3. The drug-induced rapid resolution of MRI signals was not accompanied by changes in parameters of inflammation in the bronchoalveolar lavage fluid, but was associated with perivascular oedema detected histologically. 4. In conclusion, the effects of anti-inflammatory drugs on a component of allergic inflammation can be monitored by following with MRI the rate of resolution of the associated oedematous signals.

Catecholamine-induced pulmonary edema and pleural effusion in rats--alpha- and beta-adrenergic effects

We investigated the contribution of alpha- and beta-adrenergic pathways to catecholamine-induced pulmonary edema and the role of pleural effusion in preventing alveolar edema. Female Sprague-Dawley rats received continuous intravenous infusion of norepinephrine and of separate alpha- or beta-adrenergic stimulation over 6-24 h. We performed heart catheterization in vivo and excised post mortem lung tissue for histological analysis. Interleukin (IL)-6 and total protein concentrations were determined in serum, pleural fluid (PF) and bronchoalveolar lavage fluid. alpha-Adrenergic treatment increased right ventricular systolic pressure (RVSP) and total peripheral resistance (TPR) and caused severe alveolar edema associated with IL-6 activation in serum and diffuse pulmonary inflammation. PF amounts were moderate (0.9+/-0.2 ml). beta-Adrenergic stimulation also increased RVSP but decreased TPR. Interstitial but not alveolar edema and focal inflammation without IL-6 activation developed. Large PF amounts (6.2+/-1.5 ml) occurred which were considered to prevent alveolar edema. We conclude that both alpha- and beta-adrenergic stimulation contribute to pulmonary fluid shifts in rats, but alpha-adrenergic pathways cause more acute and more severe lung injury than beta-adrenergic mechanisms.

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.

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

Stimulation of active fluid transport with beta-adrenergic receptor (betaAR) agonists can accelerate the resolution of alveolar edema. However, chronic betaAR-agonist administration may cause betaAR desensitization and downregulation that may impair physiological responsiveness to betaAR-agonist stimulation. Therefore, we measured baseline and terbutaline- (10(-3) M) stimulated alveolar fluid clearance in mice that received subcutaneously (miniosmotic pumps) either saline or albuterol (2 mg. kg(-1). day(-1)) for 1, 3, or 6 days. Continuous albuterol administration increased plasma albuterol levels (10(-5) M), an effect that was associated with 1) a significant decrease in betaAR density and 2) attenuation, but not ablation, of maximal terbutaline-induced cAMP production. Forskolin-mediated cAMP-release was unaffected. Continuous albuterol infusion stimulated alveolar fluid clearance on day 1 but did not increase alveolar fluid clearance on days 3 and 6. However, terbutaline-stimulated alveolar fluid clearance in albuterol-treated mice was not reduced compared with saline-treated mice. Despite significant reductions in betaAR density and agonist-mediated cAMP production by long-term betaAR-agonist exposure, maximal betaAR-agonist-mediated increase in alveolar fluid clearance is not diminished in mice.

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.

Prolonged isoproterenol infusion impairs the ability of beta(2)-agonists to increase alveolar liquid clearance

We determined if prolonged isoproterenol (Iso) infusion in rats impaired the ability of the beta(2)-adrenergic agonist terbutaline to increase alveolar liquid clearance (ALC). We infused rats with Iso (at rates of 4, 40, or 400 microg.kg(-1).h(-1)) or vehicle (0.001 N HCl) for 48 h using subcutaneously implanted miniosmotic pumps. After this time, the rats were anesthetized, and ALC was determined (by mass-balance after instillation of Ringer lactate containing albumin into the lungs) under baseline conditions and after terbutaline administration. Baseline and terbutaline-stimulated ALC in vehicle-infused rats averaged, respectively, 19.6 +/- 1.2% (SE) and 44.7 +/- 1.5%/h. The ability of terbutaline to increase ALC was eliminated at 400 microg.kg(-1).h(-1)Iso, inhibited by 26% at 40 microg.kg(-1).h(-1) Iso, and was not affected by 4 microg.kg(-1).h(-1) Iso. beta-adrenergic receptor (betaAR) density of freshly isolated alveolar epithelial type II (ATII) cells from Iso-infused rats was reduced by the 40 and 400 microg.kg(-1).h(-1) infusion rates. These data demonstrate that prolonged exposure to beta-agonists can impair the ability of beta(2)-agonists to stimulate ALC and produce ATII cell betaAR downregulation.

Effects of hypoxia on alveolar fluid transport capacity in rat lungs

There is little information regarding the effect of hypoxia on alveolar fluid clearance capacity. We measured alveolar fluid clearance, lung water volume, plasma catecholamine concentrations, and serum osmolality in rats exposed to 10% oxygen for up to 120 h and explored the mechanisms responsible for the increase in alveolar fluid clearance. The principal results were 1) alveolar fluid clearance did not change for 48 h and then increased between 72 and 120 h of exposure to hypoxia; 2) although nutritional impairment during hypoxia decreased basal alveolar fluid clearance, endogenous norepinephrine increased net alveolar fluid clearance; 3) the changes of lung water volume and serum osmolality were not associated with those of alveolar fluid clearance; 4) an administration of beta-adrenergic agonists further increased alveolar fluid clearance; and 5) alveolar fluid clearance returned to normal within 24 h of reoxygenation after hypoxia. In conclusion, alveolar epithelial fluid transport capacity increases in rats exposed to hypoxia. It is likely that a combination of endogenous norepinephrine and nutritional impairment regulates alveolar fluid clearance under hypoxic conditions.

Alveolar fluid clearance is impaired in the majority of patients with acute lung injury and the acute respiratory distress syndrome

Because experimental studies have shown that intact alveolar epithelial fluid transport function is critical for resolution of pulmonary edema and acute lung injury, we measured net alveolar fluid clearance in 79 patients with acute lung injury or the acute respiratory distress syndrome. Pulmonary edema fluid and plasma were sampled serially in the first 4 hours after intubation. Net alveolar fluid clearance was calculated from sequential edema fluid protein measurements. Mean alveolar fluid clearance was 6%/h. Of the patients, 56% had impaired alveolar fluid clearance (< 3%/h), 32% had submaximal clearance (> or = 3%/h, < 14%/h), and 13% had maximal clearance (> or = 14%/h). These findings are contrasted to our recent report of 65 patients with hydrostatic pulmonary edema, in whom mean alveolar fluid clearance was 13%/h; only 25% had impaired clearance whereas 75% had submaximal or maximal clearance (J Appl Physiol 1999;87:1301-1312). Acute lung injury with maximal alveolar fluid clearance were more likely to be female (p = 0.03), and less likely to have sepsis (p = 0.01). Endogenous and exogenous catecholamines did not correlate with alveolar fluid clearance. Patients with maximal alveolar fluid clearance had significantly lower mortality and a shorter duration of mechanical ventilation. In summary, in contrast to hydrostatic pulmonary edema, alveolar fluid clearance in patients with acute lung injury and the acute respiratory distress syndrome is impaired in the majority of patients, and maximal alveolar fluid clearance is associated with better clinical outcomes.

Contribution of amiloride-insensitive pathways to alveolar fluid clearance in adult rats

The contributions of amiloride-sensitive and -insensitive fractions of alveolar fluid clearance in adult ventilated rats were studied under control conditions and after beta-adrenergic stimulation. Rats were instilled with a 5% albumin solution containing terbutaline (10(-4) M) or dibutyryl-cGMP (DBcGMP; 10(-4) M) with or without the cyclic nucleotide-gated cation channel inhibitor l-cis-diltiazem (10(-3) M) and/or amiloride (10(-3) M). Alveolar fluid clearance over 1 h was 18 +/- 2% in controls. In controls, amiloride inhibited 46 +/- 15% of alveolar fluid clearance, whereas l-cis-diltiazem had no inhibitory effect. Terbutaline and DBcGMP stimulated alveolar fluid clearance by 85 +/- 3 and 36 +/- 5%, respectively. Amiloride and l-cis-diltiazem inhibited nearly equal fractions of terbutaline-stimulated alveolar fluid clearance when given alone. Amiloride and l-cis-diltiazem given together inhibited a significantly larger fraction of alveolar fluid clearance in terbutaline-stimulated rats and in DBcGMP-stimulated rats. Based on these data, terbutaline stimulation recruited both amiloride-sensitive and l-cis-diltiazem-sensitive pathways. In contrast, DBcGMP mainly recruited l-cis-diltiazem-sensitive pathways. Therefore, the amiloride-insensitive fraction of Na+-driven alveolar fluid clearance may be partly mediated through cyclic nucleotide-gated cation channels and activated by an increase in intracellular cGMP.

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.

Dexamethasone and thyroid hormone pretreatment upregulate alveolar epithelial fluid clearance in adult rats

The in vivo effect of 48-h glucocorticoid and thyroid hormone 3,3', 5-triiodine-L-thyronine (T(3)) pretreatment on alveolar epithelial fluid transport was studied in adult rats. An isosmolar 5% albumin solution was instilled, and alveolar fluid clearance was studied for 1 h. Compared with controls, dexamethasone pretreatment increased alveolar fluid clearance by 80%. T(3) pretreatment stimulated alveolar fluid clearance by 65%, and dexamethasone and T(3) had additive effects (132%). Propranolol did not inhibit alveolar fluid clearance in either group, indicating that stimulation was not secondary to endogenous beta-adrenergic stimulation. With the use of bromodeoxyuridine in vivo labeling, there was no evidence of cell proliferation. Alveolar fluid clearance was partially inhibited by amiloride in all groups. Fractional amiloride inhibition was greater in dexamethasone- and dexamethasone-plus-T(3)-pretreated rats than in control animals, but less in T(3)-pretreated rats. In summary, pretreatment with dexamethasone, T(3), or both in combination upregulate in vivo alveolar fluid clearance similarly to short-term beta-adrenergic stimulation. The effects are mediated partly by increased amiloride-sensitive Na(+) transport, because the stimulated alveolar fluid clearance was more amiloride sensitive than in control rats. These observations may have clinical relevance because glucocorticoid therapy is commonly used with acute lung injury.