Alveolar epithelial fluid clearance is mediated by endogenous catecholamines at birth in guinea pigs

Association of Antenatal Terbutaline and Respiratory Support Requirements in Preterm Neonates

BACKGROUND

Before the advent of antenatal steroids, early non-invasive respiratory support (NIV), and intratracheal surfactant, antenatal terbutaline was also used to improve lung compliance and reduce the incidence of respiratory distress syndrome (RDS).

OBJECTIVES

The objective of this paper was to study the association between antenatal terbutaline and endotracheal intubation (ET) within the first 24 hours of life, RDS, bronchopulmonary dysplasia (BPD), and intraventricular hemorrhage (IVH) in infants with the gestational age (GA) of <32 weeks, and to study the association between antenatal terbutaline, and ET or NIV within the first 24 hours of life, and RDS in infants with the GA of 32 to 36 weeks.

METHOD

This was a retrospective medical record review of preterm infants delivered at a single tertiary care center from October 2016 to December 2020. Multivariable logistic regression was used to explore the association between antenatal terbutaline and neonatal respiratory support.

RESULT

1,794 infants were included, 234 (13.0%) had the GA of <32 weeks and 1,560 (86.9%) had the GA of 32 to 36 weeks. Antenatal terbutaline, corticosteroid, or both agents were administered in 561 (31.3%), 1,461 (81.4%), and 555 (30.9%), respectively. Antenatal terbutaline was significantly associated with a reduction in ET (adjusted odds ratio [aOR] = 0.40, 95% confident interval [CI] 0.19-0.82, p = 0.012) in infants with the GA of <32 weeks, but not in infants with the GA of 32-36 weeks. Antenatal terbutaline was not associated with RDS or BPD but was significantly associated with a reduction in grade III-IV IVH (aOR 0.11, CI 0.01-0.98; p = 0.048), in infants with the GA of <32 weeks.

CONCLUSION

In a state-of-the-art neonatal care setting, antenatal terbutaline was associated with a reduction in ET during the first 24 hours in infants with the GA of <32 weeks. The use of antenatal terbutaline to improve acute neonatal respiratory outcomes merits reconsideration.

KEY POINTS

· The neonatal respiratory benefits of antenatal terbutaline in the era of antenatal corticosteroids were uncertain.. · Terbutaline is associated with a reduction in endotracheal intubation in a modern care setting.. · The role of terbutaline, and potentially other betamimetics, to improve neonatal respiratory outcomes merits reconsideration..

Adenosine A2B receptor activation stimulates alveolar fluid clearance through alveolar epithelial sodium channel via cAMP pathway in endotoxin-induced lung injury

Clinical studies have established that the capacity of removing excess fluid from alveoli is impaired in most patients with acute respiratory distress syndrome. Impaired alveolar fluid clearance (AFC) correlates with poor outcomes. Adenosine A_2B receptor (A_2BAR) has the lowest affinity with adenosine among four adenosine receptors. It is documented that A_2BAR can activate adenylyl cyclase (AC) resulting in elevated cAMP. Based on the understanding that cAMP is a key regulator of epithelial sodium channel (ENaC), which is the limited step in sodium transport, we hypothesized that A_2BAR signaling may affect AFC in acute lung injury (ALI) through regulating ENaC via cAMP, thus attenuating pulmonary edema. To address this, we utilized pharmacological approaches to determine the role of A_2BAR in AFC in rats with endotoxin-induced lung injury and further focused on the mechanisms in vitro. We observed elevated pulmonary A_2BAR level in rats with ALI and the similar upregulation in alveolar epithelial cells exposed to LPS. A_2BAR stimulation significantly attenuated pulmonary edema during ALI, an effect that was associated with enhanced AFC and increased ENaC expression. The regulatory effects of A_2BAR on ENaC-α expression were further verified in cultured alveolar epithelial type II (ATII) cells. More importantly, activation of A_2BAR dramatically increased amiloride-sensitive Na⁺ currents in ATII cells. Moreover, we observed that A_2BAR activation stimulated cAMP accumulation, whereas the cAMP inhibitor abolished the regulatory effect of A_2BAR on ENaC-α expression, suggesting that A_2BAR activation regulates ENaC-α expression via cAMP-dependent mechanism. Together, these findings suggest that signaling through alveolar epithelial A_2BAR promotes alveolar fluid balance during endotoxin-induced ALI by regulating ENaC via cAMP pathway, raising the hopes for treatment of pulmonary edema due to ALI.

Factors relating caesarean section to persistent pulmonary hypertension of the newborn

BACKGROUND

Several studies have clearly demonstrated a significantly higher incidence of persistent pulmonary hypertension of the newborn (PPHN) in neonates delivered by caesarean section (CS) compared to those delivered vaginally. The pathophysiological factors underlying the link between CS and PPHN are still poorly understood. In this review, we describe the mechanisms that could explain the association between CS delivery and subsequent PPHN, as well as potential preventive measures.

DATA SOURCES

A literature search was conducted by electronic scanning of databases such as PubMed and Web of Science using the key words "persistent pulmonary hypertension of the newborn", "caesarean section", "iatrogenic prematurity", "oxidative stress", "late preterm", "labor" and "vasoactive agents".

RESULTS

Iatrogenic prematurity, higher rates of late preterm delivery and lack of physiological changes of labor play an important role in the association between CS and PPHN. CS delivery also results in limited endogenous pulmonary vasodilator synthesis and lower levels of protective anti-oxidants in the neonates. In addition, CS delivery exposes infants to a higher risk of respiratory distress syndrome and its concomitant increase in endothelin-1 levels, which might indirectly lead to a higher risk of developing PPHN. We believe that neonates delivered by CS are exposed to a combination of these pathophysiological events, culminating in an endpoint of respiratory distress, hypoxia, acidosis, and delayed transition and thereby increased risks of PPHN. The use of antenatal corticosteroids prior to elective CS in late preterm deliveries, promoting accurate informedconsent process, delaying elective CS to 39 weeks of gestation or beyond and antenatal maternal anti-oxidant supplementation could potentially mitigate the effects of CS delivery and minimize CS-related PPHN.

CONCLUSIONS

The link between CS delivery and PPHN is complex. In view of the rising rates of CS worldwide, there is an urgent need to further explore the mechanisms linking CS to PPHN and experimentally test therapeutic options in order to allow effective targeted interventions.

Regulation of Lung Epithelial Sodium Channels by Cytokines and Chemokines

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

Albuterol Improves Alveolar-Capillary Membrane Conductance in Healthy Humans

BACKGROUND

Beta-2 adrenergic receptors (β₂ARs) are located throughout the body including airway and alveolar cells. The β₂ARs regulate lung fluid clearance through a variety of mechanisms including ion transport on alveolar cells and relaxation of the pulmonary lymphatics. We examined the effect of an inhaled β₂-agonist (albuterol) on alveolar-capillary membrane conductance (DM) and pulmonary capillary blood volume (V_C) in healthy humans.

METHODS

We assessed the diffusing capacity of the lungs for carbon monoxide (DLCO) and nitric oxide (DLNO) at baseline, 30 minutes, and 60 minutes following nebulized albuterol (2.5 mg, diluted in 3 mL normal saline) in 45 healthy subjects. Seventeen subjects repeated these measures following nebulized normal saline (age = 27 ± 9 years, height = 165 ± 21 cm, weight = 68 ± 12 kg, BMI = 26 ± 9 kg/m²). Cardiac output (Q), heart rate, systemic vascular resistance (SVR), blood pressure, oxygen saturation, forced expiratory volume at one-second (FEV₁), and forced expiratory flow at 50% of forced vital capacity (FEF₅₀) were assessed at baseline, 30 minutes, and 60 minutes following the administration of albuterol or saline.

RESULTS

Albuterol resulted in a decrease in SVR, and an increase in Q, FEV₁, and FEF₅₀ compared to saline controls. Albuterol also resulted in a decrease in V_C at 60 minutes post albuterol. Both albuterol and normal saline resulted in no change in DLCO or DM when assessed alone, but a significant increase was observed in DM when accounting for changes in V_C.

CONCLUSION

These data suggest that nebulized albuterol improves pulmonary function in healthy humans, while nebulization of both albuterol and saline results in an increase in DM/V_C.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

Dopamine treatment does not need speed recovery of newborns from transient tachypnea

OBJECTIVE

Transient tachypnea of the newborn (TTN) results from inadequate neonatal lung fluid clearance. Low-dose dopamine induces natriuresis in the kidneys and it has been assumed that, at this low dosage, dopamine increases renal perfusion in critically ill patients. Medium doses have positive inotropic and chronotropic effects via increased β-receptor activation. Recent studies have demonstrated that dopamine stimulates the clearance of pulmonary edema. Furthermore, β-adrenergic agonists regulate Na+ channels and Na-K-ATPase activity in the pulmonary epithelium. This study investigated the effect of dopamine at different dosages on TTN treatment.

METHODS

A prospective controlled study examined 60 infants with TTN older than 34 weeks of gestation who required at least 24 h of O2 and nasal continuous positive airway pressure (nCPAP) treatment. The infants were randomized into three groups of 20: controls, infants treated with low-dose dopamine (3 μg/kg/min), and infants treated with a medium dose (5 μg/kg/min). The control and study groups were compared in terms of the requirement for mechanical ventilation, and the durations of nCPAP, oxygen requirement, and hospitalization.

RESULTS

The requirement for mechanical ventilation, and durations of nCPAP, oxygen requirement, and hospitalization did not differ significantly among the three groups (P=0.54, 0.16 and 0.11, respectively).

CONCLUSION

Dopamine treatment in low-moderate doses does not improve the outcome in TTN. Thus, further studies in this area are needed.

Glucocorticoids Distinctively Modulate the CFTR Channel with Possible Implications in Lung Development and Transition into Extrauterine Life

During fetal development, the lung is filled with fluid that is secreted by an active Cl- transport promoting lung growth. The basolateral Na⁺,K⁺,2Cl^- cotransporter (NKCC1) participates in Cl^- secretion. The apical Cl^- channels responsible for secretion are unknown but studies suggest an involvement of the cystic fibrosis transmembrane conductance regulator (CFTR). CFTR is developmentally regulated with a high expression in early fetal development and a decline in late gestation. Perinatal lung transition is triggered by hormones that stimulate alveolar Na⁺ channels resulting in fluid absorption. Little is known on how hormones affect pulmonary Cl^- channels. Since the rise of fetal cortisol levels correlates with the decrease in fetal CFTR expression, a causal relation may be assumed. The aim of this study was to analyze the influence of glucocorticoids on pulmonary Cl^- channels. Alveolar cells from fetal and adult rats, A549 cells, bronchial Calu-3 and 16HBE14o- cells, and primary rat airway cells were studied with real-time quantitative PCR and Ussing chambers. In fetal and adult alveolar cells, glucocorticoids strongly reduced Cftr expression and channel activity, which was prevented by mifepristone. In bronchial and primary airway cells CFTR mRNA expression was also reduced, whereas channel activity was increased which was prevented by LY-294002 in Calu-3 cells. Therefore, glucocorticoids strongly reduce CFTR expression while their effect on CFTR activity depends on the physiological function of the cells. Another apical Cl^- channel, anoctamin 1 showed a glucocorticoid-induced reduction of mRNA expression in alveolar cells and an increase in bronchial cells. Furthermore, voltage-gated chloride channel 5 and anoctamine 6 mRNA expression were increased in alveolar cells. NKCC1 expression was reduced by glucocorticoids in alveolar and bronchial cells alike. The results demonstrate that glucocorticoids differentially modulate pulmonary Cl^- channels and are likely causing the decline of CFTR during late gestation in preparation for perinatal lung transition.

The interaction of glucocorticoids and progesterone distinctively affects epithelial sodium transport

PURPOSE

Glucocorticoids and progesterone exert stimulatory effects on epithelial Na(+) transport, including increased mRNA expression of the participating ion transporters (epithelial Na(+) channels [ENaC] and Na,K-ATPases) and their electrophysiological activity. Fetuses threatened by preterm labor may receive high doses of glucocorticoids to stimulate lung maturation and are naturally exposed to high levels of female sex steroids. However, it is still unknown how the combination of both hormones influences the epithelial Na(+) transport, which is crucial for alveolar fluid clearance.

METHODS

Fetal distal lung epithelial cells were incubated in media supplemented with dexamethasone and progesterone. Real-time qPCR and Ussing chamber analysis were used to determine the effects on ENaC mRNA expression and channel activity. In addition, the specific progesterone receptor antagonist (PF-02367982) and the glucocorticoid receptor antagonist mifepristone were used to identify the involved hormone receptors.

RESULTS

Both dexamethasone and progesterone increased ENaC subunit expression and channel activity. However, the combination of dexamethasone and progesterone reduced the α- and γ-ENaC subunit expression compared to the effect of dexamethasone alone. Furthermore, higher dexamethasone concentrations in combination with progesterone also significantly reduced Na(+) transport in Ussing chamber measurements. Hormone receptor antagonists showed that inhibition of the progesterone receptor increased the mRNA expression of α- and γ-ENaC, whereas mifepristone decreased mRNA expression of all ENaC subunits.

CONCLUSION

Glucocorticoids and progesterone individually increase ENaC mRNA expression; however, the combination of both hormones decreases the stimulatory effects of dexamethasone on Na(+) transport and ENaC mRNA expression.

Epinephrine stimulation of anion secretion in the Calu-3 serous cell model

Calu-3 is a well-differentiated human bronchial cell line with the characteristics of the serous cells of airway submucosal glands. The submucosal glands play a major role in mucociliary clearance because they secrete electrolytes that facilitate airway hydration. Given the significance of both long- and short-term β-adrenergic receptor agonists in the treatment of respiratory diseases, it is important to determine the role of these receptors and their ligands in normal physiological function. The present studies were designed to characterize the effect of epinephrine, the naturally occurring β-adrenergic receptor agonist, on electrolyte transport of the airway serous cells. Interestingly, epinephrine stimulated two anion secretory channels, the cystic fibrosis transmembrane conductance regulator and a Ca(2+)-activated Cl(-) channel, with the characteristics of transmembrane protein 16A, thereby potentially altering mucociliary clearance via multiple channels. Consistent with the dual channel activation, epinephrine treatment resulted in increases in both intracellular cAMP and Ca(2+). Furthermore, the present results extend previous reports indicating that the two anion channels are functionally linked.

Chronic elevation of pulmonary microvascular pressure in chronic heart failure reduces bi-directional pulmonary fluid flux

AIMS

Chronic heart failure leads to pulmonary vascular remodelling and thickening of the alveolar-capillary barrier. We examined whether this protective effect may slow resolution of pulmonary oedema consistent with decreased bi-directional fluid flux.

METHODS AND RESULTS

Seven weeks following left coronary artery ligation, we measured both fluid flux during an acute rise in left atrial pressure (n = 29) and intrinsic alveolar fluid clearance (n = 45) in the isolated rat lung. Chronic elevation of pulmonary microvascular pressure prevented pulmonary oedema and decreased lung compliance when left atrial pressure was raised to 20 cmH2O, and was associated with reduced expression of endothelial aquaporin 1 (P = 0.03). However, no other changes were found in mediators of fluid flux or cellular fluid channels. In isolated rat lungs, chronic LV dysfunction (LV end-diastolic pressure and infarct circumference) was also inversely related to alveolar fluid clearance (P ≤ 0.001). The rate of pulmonary oedema reabsorption was estimated by plasma volume expansion in eight patients with a previous clinical history of chronic heart failure and eight without, who presented with acute pulmonary oedema. Plasma volume expansion was reduced at 24 h in those with chronic heart failure (P = 0.03).

CONCLUSIONS

Chronic elevation of pulmonary microvascular pressure in CHF leads to decreased intrinsic bi-directional fluid flux at the alveolar-capillary barrier. This adaptive response defends against alveolar flooding, but may delay resolution of alveolar oedema.

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.

Hypoxia induced changes in lung fluid balance in humans is associated with beta-2 adrenergic receptor density on lymphocytes

BACKGROUND

Previous studies have demonstrated an important role for beta-2 adrenergic receptors (β(2)AR) in lung fluid clearance. The purpose of this investigation was to examine the relationship between β(2)AR density on lymphocytes and indices of lung water in healthy humans exposed to ≈ 17 h of hypoxia (FIO2 = 12.5% in a hypoxia tent).

METHODS

Thirteen adults (mean ± SEM; age=31 ± 3 years, BMI=24 ± 1 kg/m(2), VO2 Peak = 40 ± 2 ml/kg/min ) participated. Pulmonary function, CT derived lung tissue volume (V(tis)-tissue, blood and water), lung diffusing capacity for carbon monoxide (D(CO)) and nitric oxide (D(NO)), alveolar-capillary conductance (D(M)), pulmonary capillary blood volume (V(c)) and lung water (CT V(tis)-V(c)) were assessed before and after ≈ 17 h normobaric hypoxia (FIO2 = 12.5%). β(2)AR density on lymphocytes was measured via radioligand binding. Arterial oxygen saturation (SaO2), cardiac output (Q), right ventricular systolic pressure (RVSP) and blood pressure (BP) were also assessed.

RESULTS

After 17 h hypoxia, SaO2 decreased from 97 ± 1 (normoxia) to 82 ± 4% and RVSP increased from 14 ± 3 (normoxia) to 29 ± 2 mmHg (p<0.05) with little change in Q or BP. V(c) and D(M) both increased with hypoxia with a small increase in D(M)/V(c) ratio (p>0.05). CT V(tis) decreased and lung water was estimated to decline 7 ± 13%, respectively. β(2)AR density averaged 1497 ± 187 receptors/lymphocyte and increased 21 ± 34% with hypoxia (range -31 to +86%). The post-hypoxia increase in β(2)AR density was significantly related to the reduction in lung water (r=-0.64, p<0.05), with the subjects with the greatest increase in density demonstrating the largest decline in lung water.

CONCLUSIONS

Lung water decreases with 17 h normobaric hypoxia are associated with changes in beta adrenergic receptor density on lymphocytes in healthy adults.

Phosphatidylinositol 4,5-bisphosphate stimulates alveolar epithelial fluid clearance in male and female adult rats

Cell membrane phospholipids, like phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2)], can regulate epithelial Na channel (ENaC) activity. Gender differences in lung ENaC expression have also been demonstrated. However, the effects in vivo on alveolar fluid clearance are uncertain. Thus PI(4,5)P(2) effects on alveolar fluid clearance were studied in male and female rats. An isosmolar 5% albumin solution was intrapulmonary instilled; alveolar fluid clearance was studied for 1 h. Female rats had a 37 ± 19% higher baseline alveolar fluid clearance than male rats. Bilateral ovariectomy attenuated this gender difference. Compared with controls, PI(4,5)P(2) instillation (300 μM) increased alveolar fluid clearance by ∼93% in both genders. Amiloride or the specific αENaC small-interfering RNA inhibited baseline and PI(4,5)P(2)-stimulated alveolar fluid clearance in both genders, indicating a dependence on amiloride-sensitive pathways. The fraction of amiloride inhibition was greater in PI(4,5)P(2)-instilled rats (male: 64 ± 10%; female: 70 ± 11%) than in controls (male: 30 ± 6%; female: 44 ± 8%). PI(4,5)P(2) instillation lacked additional alveolar fluid clearance stimulation above that of terbutaline, nor did propranolol inhibit alveolar fluid clearance after PI(4,5)P(2) instillation, indicating that PI(4,5)P(2) stimulation was not secondary to endogenous β-adrenoceptor activation. PI(4,5)P(2) amine instillation resulted in an intermediate alveolar fluid clearance stimulation, suggesting that, to reach maximal alveolar fluid clearance stimulation, PI(4,5)P(2) must reside in cell membranes. In summary, PI(4,5)P(2) instillation upregulated in vivo alveolar fluid clearance similar to short-term β-adrenoceptor upregulation of alveolar fluid clearance. PI(4,5)P(2) stimulation was mediated partly by increased amiloride-sensitive Na transport. There exist important gender-related effects suggesting a female advantage that may have clinical implications for resolution of acute lung injury.

Differential expression of cyclic nucleotide phosphodiesterases 4 in developing rat lung

During the perinatal period, lungs undergo changes to adapt to air breathing. The genes involved in these changes are developmentally regulated by various signaling pathways, including the cyclic nucleotide cAMP. As PDE4s are critical enzymes for regulation of cAMP levels, the objective of this study was to investigate PDE4's ontogeny in developing rat lung during the perinatal period. Pulmonary PDE4 activity, PDE4A-D, PDE4B, and PDE4D variant expression levels, PDE4B and PDE4D protein levels, and PDE4D localization in distal lung were determined. PDE4 activity increased towards term, dropped at birth, and increased thereafter to reach a plateau at the end of the second week of life. PDE4B2 and PDE4D long forms demonstrated a pattern of expression that increased markedly at birth. After birth, PDE4D was expressed in alveolar epithelial and mesenchymal cells. The study, therefore, evidenced striking variations in expression patterns among the PDE4 family that differed from changes in global PDE4 activity.

Claudin-4 augments alveolar epithelial barrier function and is induced in acute lung injury

Intact alveolar barrier function is associated with better outcomes in acute lung injury patients; however, the regulation of alveolar epithelial paracellular transport during lung injury has not been extensively investigated. This study was undertaken to determine whether changes in tight junction claudin expression affect alveolar epithelial barrier properties and to determine the mechanisms of altered expression. In anesthetized mice exposed to ventilator-induced lung injury, claudin-4 was specifically induced among tight junction structural proteins. Real-time PCR showed an eightfold increase in claudin-4 expression in the lung injury model. To examine the role of this protein in barrier regulation, claudin-4 function was inhibited with small interfering RNA (siRNA) and a blocking peptide derived from the binding domain of Clostridium perfringens enterotoxin (CPE(BD)). Inhibition of claudin-4 decreased transepithelial electrical resistance but did not alter macromolecule permeability in primary rat and human epithelial cells. In mice, CPE(BD) decreased air space fluid clearance >33% and resulted in pulmonary edema during moderate tidal volume ventilation that did not induce edema in control peptide-treated mice. In vitro phorbol ester induced a ninefold increase in claudin-4 expression that was dependent on PKC activation and the JNK MAPK pathway. These data establish that changes in alveolar epithelial claudin expression influence paracellular transport, alveolar fluid clearance rates, and susceptibility to pulmonary edema. We hypothesize that increased claudin-4 expression early in acute lung injury represents a mechanism to limit pulmonary edema and that the regulation of alveolar epithelial claudin expression may be a novel target for acute lung injury therapy.

IL-1beta-induced cortisol stimulates lung fluid absorption in fetal guinea pigs via SGK-mediated Nedd4-2 inhibition

We tested the hypothesis that interleukin (IL)-1beta-induced cortisol synthesis stimulates distal lung fluid absorption in fetal guinea pigs via induction of serum- and glucocorticoid-regulated kinase (SGK) and inhibition of neural precursor cell expressed, developmentally downregulated protein 4-2 (Nedd4-2). IL-1beta was subcutaneously administered daily to timed-pregnant guinea pigs over 3 days. Fetuses were obtained by abdominal hysterotomy at gestation day (GD)61 and GD68 and instilled with an isosmolar 5% albumin solution into the lungs. Distal lung fluid movement was measured over 1 h from the change in distal air space protein concentration. Fetal lungs were secreting lung fluid at GD61 while absorbing lung fluid at GD68. Distal lung fluid absorption was induced at GD61 by IL-1beta but unaffected at GD68. Plasma cortisol concentrations were increased by IL-1beta at GD61 and endogenously at GD68. Distal lung fluid absorption was measured and correlated to SGK and Nedd4-2 expression and to alpha-epithelial Na channel (ENaC) expression. SGK was increased by IL-1beta and late during gestation (GD68), while Nedd4-2 was decreased by IL-1beta and late during gestation. alpha-ENaC was induced by IL-1beta at GD61 and increased late during gestation. Thus our study suggests that cortisol-stimulated fetal lung fluid absorption is mediated by increased ENaC expression and may be governed by the SGK/Nedd4-2 pathway. These observations may explain why babies delivered preterm after intrauterine inflammation have a reduced risk of developing severe respiratory distress.

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.

Induction of serum- and glucocorticoid-induced kinase-1 (SGK1) by cAMP regulates increases in alpha-ENaC

Alpha-ENaC expression and activity is regulated by a variety of hormones including beta-adrenergic agonists via the second messenger cAMP. We evaluated the early intermediate pathways involved in the up-regulation of SGK1 by DbcAMP and whether SGK1 is a prerequisite for induction of alpha-ENaC expression. Submandibular gland epithelial (SMG-C6) cells treated with DbcAMP (1 mM) induced both SGK1 mRNA and protein expression. DbcAMP-stimulated SGK1 mRNA expression was decreased by actinomycin D and mRNA and protein expressions were attenuated by PKA inhibitors (H-89 and KT5720). Inhibition of PI3-K with either LY294002 or dominant negative PI3-K reduced DbcAMP-stimulated SGK1 protein and mRNA levels, attenuated the phosphorylation of CREB (a cAMP-activated transcription factor) and decreased alpha-ENaC protein levels and Na(+) transport. In addition, the combination of PKA inhibitors with dominant negative PI3-K synergistically inhibited DbcAMP-induced Na(+) transport. Inhibition of SGK1 expression by siRNA decreased but did not obliterate DbcAMP-induced alpha-ENaC expression. Thus, in a cell line which endogenously exhibits minimal alpha-ENaC expression, induction of SGK1 by DbcAMP occurs via the PI3-K and PKA pathways. Increased alpha-ENaC levels and function are partly dependent upon the early induction of SGK1 expression.

NKCC-1 and ENaC are down-regulated in nitrofen-induced hypoplastic lungs with congenital diaphragmatic hernia

Congenital diaphragmatic hernia (CDH) is accompanied by pulmonary hypoplasia and pulmonary hypertension. Fetal lung growth is dependent on the secretion of lung liquid, which normally is absorbed at partus. The ion channel NKCC-1 is involved in this secretory process, but has recently also been reported to be implicated in absorption. CDH patients show a disturbed transition from secretion to absorption. alpha- and beta-ENaC are essential for lung liquid absorption. Common for all transcellular ion transport is the need for Na/K-ATPase as a primary driving force. The aim of the study was first to map the normal pulmonary expression of the above proteins during late gestation and secondly to see if the expression was affected in a CDH rat model. Pregnant Sprague-Dawley rat dams were given nitrofen on gestational day 9.5 to induce CDH. The fetuses were removed on gestational days E18 and E21. In addition, newborn rats were harvested postpartum on day P2. The fetuses were put into one of two groups: hypoplastic lungs without CDH (N-CDH) and hypoplastic lungs with CDH (N+CDH). The pulmonary expression of NKCC-1, alpha-/beta-ENaC and Na/K-ATPase was then analyzed using Western blot. We found that the protein levels of NKCC-1 on gestational days E18 and E21 were significantly lower among fetuses with N+CDH as well as N-CDH compared to controls. The expression of beta-ENaC was also significantly down-regulated in both the groups on E18 and E21. The protein levels of alpha-ENaC and Na/K-ATPase were not found to be significantly decreased, but both showed a tendency towards down-regulation. The marked down-regulation of NKCC-1 in fetal hypoplastic lungs with CDH indicates a possibly decreased lung liquid production. This may be one of the mechanisms behind the disturbed pulmonary development in CDH. We also show that beta-ENaC is down-regulated. Down-regulation of beta-ENaC may result in abnormal lung liquid absorption, which could be one of the mechanisms behind the respiratory distress seen in CDH patients postpartum.

RNA interference for CFTR attenuates lung fluid absorption at birth in rats

Background

Small interfering RNA (siRNA) against αENaC (α-subunit of the epithelial Na channel) and CFTR (cystic fibrosis transmembrane conductance regulator) was used to explore ENaC and CTFR function in newborn rat lungs.

Methods

Twenty-four hours after trans-thoracic intrapulmonary (ttip) injection of siRNA-generating plasmid DNA (pSi-0, pSi-4, or pSi-C₂), we measured CFTR and ENaC expression, extravascular lung water, and mortality.

Results

αENaC and CFTR mRNA and protein decreased by ~80% and ~85%, respectively, following αENaC and CFTR silencing. Extravascular lung water and mortality increased after αENaC and CFTR-silencing. In pSi-C₂-transfected isolated DLE cells there were attenuated CFTR mRNA and protein. In pSi-4-transfected DLE cells αENaC mRNA and protein were both reduced. Interestingly, CFTR-silencing also reduced αENaC mRNA and protein. αENaC silencing, on the other hand, only slightly reduced CFTR mRNA and protein.

Conclusion

Thus, ENaC and CFTR are both involved in the fluid secretion to absorption conversion around at birth.

Elective cesarean section: its impact on neonatal respiratory outcome

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

Amiloride-insensitive Na+and fluid absorption in the mammalian distal lung

The ability of the distal lung epithelia to actively transport Na+, with Cl−and water following, from the alveolar spaces inversely correlates with morbidity and mortality of infants, children, and adults with alveolar pulmonary edema. It is now recognized, in contrast to many other Na+transporting epithelia, that at least half of this active transport is not sensitive to amiloride, which inhibits the epithelial Na+channel. This paper reviews amiloride-insensitive Na+and fluid transport in the mammalian distal lung unit under basal conditions and speculates on potential explanations for this amiloride-insensitive transport. It also provides new information, using primary cultures of rat fetal distal lung epithelia and alveolar type II cells grown under submersion and air-liquid interface culture conditions, regarding putative blockers of this transport.

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

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

Involvement of {alpha}ENaC and Nedd4-2 in the conversion from lung fluid secretion to fluid absorption at birth in the rat as assayed by RNA interference analysis

To explore interactions between the epithelial Na channel (ENaC) and neural precursor expressed, developmentally downregulated protein 4-2 (Nedd4-2) at the conversion of the rat lung from fluid secretion to absorption at birth, we used small-interfering RNA (siRNA) against alphaENaC and Nedd4-2. siRNA-generating plasmid DNA (pDNA) was administered via trans-thoracic intrapulmonary (ttip) injection 24 h before ENaC and Nedd4-2 expression, extravascular lung water, and mortality were measured. alphaENaC mRNA and protein were specifically reduced by approximately 65% after pSi-4 injection. Nedd4-2 mRNA and protein were reduced by approximately 60% after pSi-N1 injection. Interestingly, alphaENaC and betaENaC mRNA and protein expression were increased after Nedd4-2 silencing. Extravascular lung water was significantly increased after alphaENaC silencing and reduced after Nedd4-2 silencing. alphaENaC silencing resulted in a fourfold increase in newborn mortality, whereas silencing Nedd4-2 did not affect mortality. We also isolated distal lung epithelial (DLE) cells after in vivo alphaENaC or Nedd4-2 silencing and measured alphaENaC or Nedd4-2 expression in freshly isolated DLE cells. In these DLE cells, there were attenuated alphaENaC or Nedd4-2 mRNA and protein, thus demonstrating that alphaENaC and Nedd4-2 silencing occurred in alveolar epithelial cells after ttip injection. We also looked for pDNA by PCR to determine pDNA presence in the lungs and found strong evidence for pDNA presence in both lungs. Thus we provide evidence that ENaC and Nedd4-2 are involved in the transition from lung fluid secretion to fluid absorption near term and at birth.

Expulsion of liquid from the fetal lung during labour in sheep

Effective gas exchange after birth requires clearance of most of the liquid filling the lung during gestation. To date the focus has been on active Na(+) transport from lung lumen to interstitium, but Na(+) transport begins only close to delivery, making it an unlikely mechanism for clearing the bulk of fetal lung liquid. We hypothesised that fetal trunk muscle contractions, known to occur in labour, are involved in lung liquid clearance. We measured maternal uterine contractions, fetal tracheal flow directly and fetal electromyograms in thoracic and abdominal muscles. During labour in five fetal sheep, brief flow pulses were observed in the trachea, most of which expelled a small volume of lung liquid. Tracheal flow pulses were associated with fetal muscle contractions 89% of the time, which were associated on 91% of occasions with uterine contractions. Our results suggest that liquid contained in the fetal lung is cleared before and during labour as a result of fetal muscular effort, perhaps stimulated by uterine contractions.

Stimulation of MAP kinase pathways after maternal IL-1beta exposure induces fetal lung fluid absorption in guinea pigs

BACKGROUND

We tested the hypothesis that maternal interleukin-1beta (IL-1beta) pretreatment and induction of fetal cortisol synthesis activates MAP kinases and thereby affects lung fluid absorption in preterm guinea pigs.

METHODS

IL-1beta was administered subcutaneously daily to timed-pregnant guinea pigs for three days. Fetuses were obtained by abdominal hysterotomy and instilled with isosmolar 5% albumin into the lungs and lung fluid movement was measured over 1 h by mass balance. MAP kinase expression was measured by western blot.

RESULTS

Lung fluid absorption was induced at 61 days (D) gestation and stimulated at 68D gestation by IL-1beta. Maternal IL-1beta pretreatment upregulated ERK and upstream MEK expression at both 61 and 68D gestation, albeit being much more pronounced at 61D gestation. U0126 instillation completely blocked IL-1beta-induced lung fluid absorption as well as IL-1beta-induced/stimulated ERK expression. Cortisol synthesis inhibition by metyrapone attenuated ERK expression and lung fluid absorption in IL-1beta-pretreated fetal lungs. JNK expression after maternal IL-1beta pretreatment remained unaffected at either gestation age.

CONCLUSION

These data implicate the ERK MAP kinase pathway as being important for IL-1beta induction/stimulation of lung fluid absorption in fetal guinea pigs.

Electrical potential difference across the nasal epithelium is reduced in premature infants with chronic lung disease but is not associated with lower airway inflammation

Airway liquid content and insufficient absorptive airway ion transport at birth are potentially important factors in the development and severity of neonatal respiratory disease. The role of deficient absorptive airway ion transport in the development of chronic lung disease of prematurity is unknown. Additionally, lung inflammatory mediators modulate airway ion transport. Their effect on preterm lung ion transport and absorptive capacity is not established. We performed serial nasal potential difference studies and broncho-alveolar lavage in preterm infants born less than 30 wk postmenstrual age over the first four postnatal weeks. Our study aims were to: 1) compare nasal potential difference between preterm infants developing chronic lung disease and babies of similar gestation who do not; and 2) examine for an association between airway inflammation and ion transport parameters. We found that potential difference across the nasal epithelium increased with gestation, remained low and unchanged in infants developing chronic lung disease over the first four postnatal weeks, was significantly lower at four weeks in chronic lung disease infants, and was not associated with lower airway inflammation at any time point. We conclude that infants with chronic lung disease postnatally have a persistently reduced absorptive airway ion transport capacity.

A Noninflammatory Interleukin-1β Fragment Stimulates Fetal Lung Fluid Absorption in Guinea Pigs

We have previously demonstrated that full-length interleukin (IL)-1beta can induce and stimulate lung fluid absorption in near-term guinea pig fetuses via stimulation of fetal cortisol synthesis and release. To develop a potentially clinically useful drug, we tested the hypothesis that maternal administration of a noninflammatory IL-1beta-fragment (IL-1beta(Fr)) induced cortisol synthesis and stimulated lung fluid absorption in preterm fetuses. IL-1beta(Fr) was administered s.c. daily to timed-pregnant guinea pigs for 3 days with and without simultaneous cortisol synthesis inhibition by metyrapone. Fetuses were obtained by abdominal hysterotomy at 61 and 68 days gestation and instilled with isosmolar 5% albumin into the lungs, and lung fluid absorption was measured over 1 h by mass balance. Lung fluid absorption was induced at 61 days and stimulated at 68 days gestation by IL-1beta(Fr), which both were attenuated by cortisol synthesis inhibition. Moreover, induction of labor by oxytocin stimulated lung fluid absorption at 61 days but had no stimulatory effect at 68 days gestation when given with the IL-1beta(Fr). Plasma adrenocorticotropin and cortisol concentrations were increased by IL-1beta(Fr) at 61 days gestation and remained high but unstimulated by IL-1beta(Fr) at 68 days gestation, and metyrapone always reduced cortisol concentrations. Prenatal lung fluid absorption, when present as well as IL-1beta(Fr)-induced, was always propranolol- and amiloride-sensitive, suggesting that beta-adrenoceptor stimulation and the epithelial Na(+) channel (ENaC) were critical for the induced/stimulated lung fluid absorption. ENaC expression was increased by IL-1beta(Fr) and attenuated by cortisol synthesis inhibition. Thus, our results suggest a potential clinical use of IL-1beta(Fr) therapeutically to induce lung fluid absorption in fetuses at risk of preterm delivery.

Respiratory transition in infants delivered by cesarean section

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

Integrating acute lung injury and regulation of alveolar fluid clearance

The acute respiratory distress syndrome (ARDS) is characterized by non-cardiogenic pulmonary edema and flooding of the alveolar air spaces with proteinaceous fluid. ARDS develops in response to inflammatory stresses including sepsis, trauma, and severe pneumonia, and despite aggressive critical care management, it still has a mortality of 30-50%. At the time of its original description in 1967, relatively little was known about the specific mechanisms by which the alveolar epithelium regulated lung fluid balance. Over the last 20 years, substantial advances in our understanding of the alveolar epithelium have provided major new insights into how molecular and cellular mechanisms regulate the active transport of solutes and fluid across the alveolar epithelium under both normal and pathological conditions. Beginning with the elucidation of active sodium transport as a major driving force for the transport of water from the air space to the interstitium, elegant work by multiple investigators has revealed a complex and integrated network of membrane channels and pumps that coordinately regulates sodium, chloride, and water flux in both a cell- and condition-specific manner. At the Experimental Biology Meeting in San Francisco on April 4, 2006, a symposium was held to discuss some of the most recent advances. Although there is still much to learn about the mechanisms that impair normal alveolar fluid clearance under pathological conditions, the compelling experimental findings presented in this symposium raise the prospect that we are now poised to test and develop therapeutic strategies to improve outcome in patients with acute lung injury.

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.

Physiology of fetal lung fluid clearance and the effect of labor

Respiratory morbidity in near term (> or =34 and <37 weeks) infants delivered spontaneously or by elective cesarean section (ECS) has been well documented in the literature, and accounts for a significant number of admissions to intensive care units among these neonates. Given the high rates of near-term deliveries in the USA and worldwide, the public health and economic impact of morbidity in this subgroup is considerable. Causes of respiratory distress include transient tachypnea of the newborn (TTNB), surfactant deficiency, pneumonia, and pulmonary hypertension. There is considerable evidence that physiologic events in the last few weeks of pregnancy coupled with the onset of spontaneous labor are accompanied by changes in the hormonal milieu of the fetus and its mother, resulting in rapid maturation and preparation of the fetus for delivery and neonatal transition. A surge in endogenous steroids and catecholamines accompanies term gestation and spontaneous vaginal delivery, and is responsible for some of the maturational effects. Rapid clearance of fetal lung fluid clearance plays a key role in the transition to air breathing. The bulk of this fluid clearance is mediated by transepithelial sodium reabsorption through amiloride-sensitive sodium channels in the alveolar epithelial cells with only a limited contribution from mechanical factors and Starling forces. Disruption of this process can lead to retention of fluid in air spaces, setting the stage for alveolar hypoventilation. When infants are delivered near-term, especially by cesarean section (repeat or primary) before the onset of spontaneous labor, the fetus is often deprived of these hormonal changes, making the neonatal transition more difficult. This chapter discusses the physiologic mechanisms underlying fetal lung fluid absorption and explores potential strategies for facilitating neonatal transition.

Forskolin-induced cell shrinkage and apical translocation of functional enhanced green fluorescent protein-human alphaENaC in H441 lung epithelial cell monolayers

Elevation of intracellular cAMP increases fluid re-absorption in the lung by raising amiloride-sensitive Na+ transport through the apically localized epithelial, amiloride-sensitive Na+ channel (ENaC). However, the signaling pathways mediating this response are still not fully understood. We show that inhibition of protein-tyrosine kinase (PTK) with Genistein and protein kinase A (PKA) with KT5720, decreased forskolin-stimulated amiloride-sensitive short circuit current (I(sc)) across H441 adult human lung epithelial cell monolayers. KT5720 also decreased basal I(sc). Stable expression of green fluorescent protein (GFP)-labeled human alphaENaC in H441 cells was used to investigate dynamic changes in the cellular localization of this protein in response to forskolin. Reverse transcription-PCR and immunoblotting analysis revealed two clones expressing a truncated (alphaC3-5) and full-length (alphaC3-3) EGFP-halphaENaC protein. Only the alphaC3-3 clone displayed dome formation and exhibited a 50% increase in basal and forskolin-stimulated amiloride-sensitive I(sc) indicating that the full-length protein was required for functional activity. Apical surface biotinylation and real-time confocal microscopy demonstrated that EGFP-halphaENaC (alphaC3-3) translocated to the apical membrane in response to forskolin in a Brefeldin A-sensitive manner. This effect was completely inhibited by Genistein but only partially inhibited by KT5720. Forskolin also induced a reduction in the height of cells within alphaC3-3 monolayers, indicative of cell shrinkage. This effect was inhibited by KT5720 but not by Genistein or Brefeldin A. These data show that forskolin activates PKA-sensitive cell shrinkage in adult human H441 lung epithelial cell monolayers, which induces a PTK-sensitive translocation of EGFP-halphaENaC subunits to the apical membrane and increases amiloride-sensitive Na+ transport.

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.

RNA interference for alpha-ENaC inhibits rat lung fluid absorption in vivo

We used siRNA against the alpha-ENaC (epithelial Na channel) subunit to investigate ENaC involvement in lung fluid absorption in rats by the impermeable tracer technique during baseline and after beta-adrenoceptor stimulation by terbutaline. Terbutaline stimulation of lung fluid absorption increased fluid absorption by 165% in pSi-0-pretreated rat lungs (irrelevant siRNA-generating plasmid). Terbutaline failed to increase lung fluid absorption in rats given the specific alpha-ENaC siRNA-generating plasmid (pSi-4). pSi-4 pretreatment reduced baseline lung fluid absorption by approximately 30%. alpha-ENaC was undetectable in pSi-4-pretreated lungs, regardless of condition but was normal in pSi-0-pretreated lungs. We carried out a dose-response analysis where rats were given 0-200 microg/kg body wt pSi-4, and alpha-ENaC mRNA and protein expressions were analyzed. To reach IC(50) for alpha-ENaC mRNA expression, 32 microg/kg body wt pSi-4 was needed, and to reach IC(50) for alpha-ENaC protein expression, 59 microg/kg body wt pSi-4 was needed. We tested for lung tissue specificity and found no changes in beta-ENaC expression, at either mRNA or protein level, as well as no changes in alpha(1)-Na-K-ATPase protein expression. We isolated alveolar epithelial type II cells 24 h after in vivo pSi-4 pretreatment. In these cells, alpha-ENaC mRNA was undetectable, demonstrating that alveolar epithelial ENaC expression was attenuated after intratracheal alpha-ENaC siRNA-generating plasmid DNA instillation. We tested for organ specificity and found no changes in kidney alpha- and beta-ENaC mRNA and protein expression. Thus we provide conclusive evidence that beta-adrenoceptor stimulation of lung fluid absorption is critically ENaC dependent, whereas baseline lung fluid absorption seemed less ENaC dependent.

Oxytocin-induced labor augments IL-1beta-stimulated lung fluid absorption in fetal guinea pig lungs

We tested the hypothesis that oxytocin-induced labor augmented IL-1beta-induced/-stimulated lung fluid absorption in preterm guinea pig fetuses. IL-1beta was administered subcutaneously daily to timed-pregnant guinea pigs for 3 days with and without simultaneous cortisol synthesis inhibition by metyrapone. At day 3, oxytocin was administered, and fetuses were delivered by abdominal hysterotomy at 61 and by oxytocin-induced birth at 68 days gestation. Delivered fetuses were instilled with isosmolar 5% albumin into the lungs, and lung fluid movement was measured over 1 h by mass balance. Lung fluid absorption was induced in 61-day and stimulated in 68-day gestation lungs by IL-1beta. Labor induction by oxytocin augmented IL-1beta-induced/-stimulated lung fluid absorption. Metyrapone pretreatment did not affect oxytocin-induced/-stimulated lung fluid absorption, while completely blocking IL-1beta-induced/-stimulated fluid absorption. Fetal lung fluid absorption, when present, was always propranolol and amiloride sensitive, suggesting that beta-adrenoceptor stimulation and amiloride-sensitive sodium channels were critical for fluid absorption. Epithelial sodium channel and Na-K-ATPase subunit expressions were both increased by IL-1beta, but not further by oxytocin. Our results indicate that IL-1beta release into the maternal blood circulation positively affects lung maturation due to the IL-1beta-induced release of cortisol and thus prepares the lungs for the epinephrine surge associated with labor.

Modulation of epithelial sodium channel (ENaC) expression in mouse lung infected with Pseudomonas aeruginosa

Background

The intratracheal instillation of Pseudomonas aeruginosa entrapped in agar beads in the mouse lung leads to chronic lung infection in susceptible mouse strains. As the infection generates a strong inflammatory response with some lung edema, we tested if it could modulate the expression of genes involved in lung liquid clearance, such as the α, β and γ subunits of the epithelial sodium channel (ENaC) and the catalytic subunit of Na⁺-K⁺-ATPase.

Methods

Pseudomonas aeruginosa entrapped in agar beads were instilled in the lung of resistant (BalB/c) and susceptible (DBA/2, C57BL/6 and A/J) mouse strains. The mRNA expression of ENaC and Na⁺-K⁺-ATPase subunits was tested in the lung by Northern blot following a 3 hours to 14 days infection.

Results

The infection of the different mouse strains evoked regulation of α and β ENaC mRNA. Following Pseudomonas instillation, the expression of αENaC mRNA decreased to a median of 43% on days 3 and 7 after infection and was still decreased to a median of 45% 14 days after infection (p < 0.05). The relative expression of βENaC mRNA was transiently increased to a median of 241%, 24 h post-infection before decreasing to a median of 43% and 54% of control on days 3 and 7 post-infection (p < 0.05). No significant modulation of γENaC mRNA was detected although the general pattern of expression of the subunit was similar to α and β subunits. No modulation of α₁Na⁺-K⁺-ATPase mRNA, the catalytic subunit of the sodium pump, was recorded. The distinctive expression profiles of the three subunits were not different, between the susceptible and resistant mouse strains.

Conclusions

These results show that Pseudomonas infection, by modulating ENaC subunit expression, could influence edema formation and clearance in infected lungs.

Norepinephrine Increases Alveolar Fluid Reabsorption and Na,K-ATPase Activity

The purpose of this study was to determine whether alpha-adrenergic receptor agonists have a role in alveolar fluid reabsorption, via Na,K-ATPase, in the alveolar epithelium. Alveolar fluid reabsorption increased approximately twofold with increasing concentrations of norepinephrine (NE) as compared with control rats. Treatment with the nonselective alpha-adrenergic receptor agonist, octopamine, and the specific alpha(1) agonist, phenylephrine, increased alveolar fluid reabsorption by 54 and 40%, respectively, as compared with control. The specific alpha(1)-adrenergic receptor antagonist, prazosin, inhibited the stimulatory effects of NE by approximately 30%, whereas alpha(2)-adrenergic antagonist, yohimbine, did not prevent the stimulatory effects of NE. The administration of ouabain, Na,K-ATPase inhibitor, prevented the NE-mediated increase in alveolar fluid reabsorption. In parallel with these changes, NE increased Na,K-ATPase activity and protein abundance in alveolar epithelial type II cells via the alpha(1)- and beta-adrenergic receptor. We report here that NE increased alveolar fluid reabsorption via the activation of both alpha(1)- and beta-adrenergic receptors, but not alpha(2)-adrenergic receptors. These effects are due to increased activity and abundance of the Na,K-ATPase in the basolateral membrane of ATII cells.

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.

Stimulation of distal airspace fluid clearance in guinea pigs involves bumetanide-sensitive ion transport

OBJECTIVE

This study was undertaken to test the hypothesis that beta-adrenoceptor stimulation of fetal lung fluid absorption in near-term guinea pig fetuses involves bumetanide-sensitive ion transport.

STUDY DESIGN

Fetuses were obtained from timed-pregnant guinea pigs at 61 to 69 days' gestation with and without oxytocin-induced preterm labor. The fetuses were placed on continuous positive airway pressure oxygenation, and an isosmolar 5% albumin solution was instilled into the lungs. Distal airspace fluid clearance was measured over 1 hour from the increase in distal airspace protein concentration as fluid was reabsorbed with and without the Cl(-) transport inhibitor bumetanide.

RESULTS

Fetal lungs began to absorb distal airspace fluid at 64 to 66 days' gestation, and at birth, distal airspace fluid clearance rapidly quadrupled. Labor induction by oxytocin stimulated distal airspace fluid clearance. Distal airspace fluid clearance, when present, was sensitive to propranolol-inhibition and depended on beta-adrenoceptor stimulation. Fluid secretion at 61 days' gestation was reduced by bumetanide instillation. Bumetanide addition was only inhibitory when distal airspace fluid clearance was propranolol-sensitive.

CONCLUSION

Beta-adrenoceptor stimulation from endogenous fetal epinephrine increased fetal distal airspace fluid clearance and involved bumetanide-sensitive ion transport.

IL-1β stimulates alveolar fluid absorption in fetal guinea pig lungs via the hypothalamus-pituitary-adrenal gland axis

We tested the hypothesis that interleukin (IL)-1β-induced cortisol synthesis stimulates alveolar fluid clearance in preterm fetuses. IL-1β was administered subcutaneously daily to timed-pregnant guinea pigs for 3 days with and without simultaneous cortisol synthesis inhibition by metyrapone. Fetuses were obtained by abdominal hysterotomy at 61 and 68 days gestation and instilled with isosmolar 5% albumin in the lungs, and alveolar fluid movement was measured over 1 h from the change in alveolar protein concentration. Alveolar fluid clearance was induced at 61 days gestation and stimulated at 68 days gestation by IL-1β, which both were attenuated by cortisol synthesis inhibition. Plasma ACTH and cortisol concentrations were increased by IL-1β at both gestational ages, and metyrapone reduced cortisol concentrations. IL-1β was mostly low or undetectable in both fetal and maternal blood. Prenatal alveolar fluid clearance, when present as well as IL-1β induced, was always propranolol and amiloride sensitive, suggesting that β-adrenoceptor stimulation and amiloride-sensitive Na+channels were critical for fluid absorption. IL-1β increased lung β-adrenoceptor density at gestation day 61, and cortisol synthesis inhibition attenuated the increased β-adrenoceptor density. Epithelial Na+channel and Na+-K+-ATPase subunit expressions were both increased by IL-1β and attenuated by cortisol synthesis inhibition. These results may explain why babies delivered preterm after intrauterine inflammation have a reduced risk of developing severe respiratory distress.

Chronic bronchial allergic inflammation increases alveolar liquid clearance by TNF-alpha -dependent mechanism

Bronchial inflammation in allergic asthma is associated with active exudation from the bronchial tree into the interstitial space of both mucosa and submucosa. The aim of this study was to evaluate epithelial and endothelial permeability as well as alveolar fluid movement in a model of chronic allergic inflammation in Brown-Norway rats sensitized and challenged with ovalbumin (OA). Control groups were challenged with saline solution (C), and rats were immunized by OA but not challenged (Se). Lung sections showed a marked inflammatory infiltrate associated with perivascular and peribronchiolar edema in OA. To measure alveolar liquid clearance, a 5% bovine albumin solution with 1 microCi of (125)I-labeled human albumin was instilled into the air spaces. Alveolar-capillary barrier permeability was evaluated by intravascular injection of 1 microCi of (131)I-labeled albumin. Endothelial permeability was significantly increased in OA, from 0.08 +/- 0.01 in the C group to 0.19 +/- 0.03 in OA group (P < 0.05). Final-to-initial protein ratio was also statistically higher in OA (1.6 +/- 0.05) compared with C (1.38 +/- 0.03, P = 0.01) and Se groups (1.42 +/- 0.03, P = 0.04). Administration of anti-tumor necrosis factor-alpha antibodies within the instillate significantly decreased this ratio (1.32 +/- 0.08, P = 0.003 vs. OA). To conclude, we demonstrated a tumor necrosis factor-alpha-dependent increase in alveolar fluid movement in a model of severe bronchial allergic inflammation associated with endothelial and epithelial leakage.

Invited review: Clearance of lung liquid during the perinatal period

At birth, the distal lung epithelium undergoes a profound phenotypic switch from secretion to absorption in the course of adaptation to air breathing. In this review, we describe the developmental regulation of key membrane transport proteins and the way in which epinephrine, oxygen, glucocorticoids, and thyroid hormones interact to bring about this crucial change in function. Evidence from molecular, transgenic, cell culture, and whole lung studies is presented, and the clinical consequences of the failure of the physiological mechanisms that underlie perinatal lung liquid absorption are discussed.

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.