Ca(2+)-dependent stimulation of alveolar fluid clearance in near-term fetal guinea pigs

Association of Labor With Neonatal Respiratory Outcomes at 36-40 Weeks of Gestation

OBJECTIVE

To evaluate whether labor is associated with lower odds of respiratory morbidity among neonates born from 36 to 40 weeks of gestation and to assess whether this association varies by gestational age and maternal diabetic status.

METHODS

We conducted a secondary analysis of women in the Assessment of Perinatal Excellence obstetric cohort who delivered across 25 U.S. hospitals over a 3-year period. Women with a singleton liveborn nonanomalous neonate who delivered from 36 to 40 weeks of gestation were included in our analysis. Those who received antenatal corticosteroids, underwent amniocentesis for fetal lung maturity, or did not meet dating criteria were excluded. Our primary outcome was composite neonatal respiratory morbidity, which included respiratory distress syndrome, ventilator support, continuous positive airway pressure, or neonatal death. Maternal characteristics and neonatal outcomes between women who labored and those who did not were compared. Multivariable logistic regression models were used to evaluate the association between labor and the primary outcome. Interactions between labor and diabetes mellitus and labor and gestational age were tested.

RESULTS

Our analysis included 63,187 women who underwent labor and 10,629 who did not. There was no interaction between labor and diabetes mellitus (P=.90). However, there was a significant interaction between labor and gestational age (P=.01). In the adjusted model, labor was associated with lower odds of neonatal respiratory morbidity compared with no labor for neonates delivered from 36-39 weeks of gestation. A 1-week increase in gestational age was associated with a 1.2 times increase in the adjusted odds ratio for the neonatal outcome comparing labor and no labor.

CONCLUSION

Labor was associated with lower odds of the composite outcome among neonates delivered from 36-39 weeks of gestation. The magnitude of this association varied by gestational age. The association was similar for women with or without diabetes mellitus.

Neonatal Lung Disorders: Pattern Recognition Approach to Diagnosis

OBJECTIVE

Lung disease is a common indication for neonates to require medical attention, and neonatal chest radiographs are among the most common studies interpreted by pediatric radiologists. Radiographic features of many neonatal lung disorders overlap, and it may be difficult to differentiate among conditions.

CONCLUSION

This review presents an up-to-date practical approach to the radiologic diagnosis of neonatal lung disorders, with a focus on pattern recognition and consideration of clinical history, patient age, and symptoms.

Novel mechanisms for crotonaldehyde-induced lung edema

Background

Crotonaldehyde is a highly noxious α,β-unsaturated aldehyde in cigarette smoke that causes edematous acute lung injury.

Objective

To understand how crotonaldehyde impairs lung function, we examined its effects on human epithelial sodium channels (ENaC), which are major contributors to alveolar fluid clearance.

Methods

We studied alveolar fluid clearance in C57 mice and ENaC activity was examined in H441 cells. Expression of α- and γ-ENaC was measured at protein and mRNA levels by western blot and real-time PCR, respectively. Intracellular ROS levels were detected by the dichlorofluorescein assay. Heterologous αβγ-ENaC activity was observed in an oocyte model.

Results

Our results showed that crotonaldehyde reduced transalveolar fluid clearance in mice. Furthermore, ENaC activity in H441 cells was inhibited by crotonaldehyde dose-dependently. Expression of α- and γ-subunits of ENaC was decreased at the protein and mRNA level in H441 cells exposed to crotonaldehyde, which was probably mediated by the increase in phosphorylated extracellular signal-regulated protein kinases 1 and 2. ROS levels increased time-dependently in cells exposed to crotonaldehyde. Heterologous αβγ-ENaC activity was rapidly eliminated by crotonaldehyde.

Conclusion

Our findings suggest that crotonaldehyde causes edematous acute lung injury by eliminating ENaC activity at least partly via facilitating the phosphorylation of extracellular signal-regulated protein kinases 1 and 2 signal molecules. Long-term exposure may decrease the expression of ENaC subunits and damage the cell membrane integrity, as well as increase the levels of cellular ROS products.

1,25-Dihydroxyvitamin D Enhances Alveolar Fluid Clearance by Upregulating the Expression of Epithelial Sodium Channels

Vitamin D is implicated in the pathogenesis of asthma, acute lung injury, and other respiratory diseases. 1,25-Dihydroxyvitamin D (1,25(OH)2D3), the hormonal form of vitamin D, has been shown to reduce vascular permeability and ameliorate lung edema. Therefore, we speculate that 1,25(OH)2D3 may regulate alveolar Na(+) transport via targeting epithelial Na(+) channels (ENaC), a crucial pathway for alveolar fluid clearance. In vivo total alveolar fluid clearance was 39.4 ± 3.8% in 1,25(OH)2D3-treated mice, significantly greater than vehicle-treated controls (24.7 ± 1.9 %, n = 10, p < 0.05). 1,25(OH)2D3 increased amiloride-sensitive short-circuit currents in H441 monolayers, and whole-cell patch-clamp data confirmed that ENaC currents in single H441 cell were enhanced in 1,25(OH)2D3-treated cells. Western blot showed that the expression of α-ENaC was significantly elevated in 1,25(OH)2D3-treated mouse lungs and 1,25(OH)2D3-treated H441 cells. These observations suggest that vitamin D augments transalveolar fluid clearance, and vitamin D therapy may potentially be used to ameliorate pulmonary edema.

Physiological effect of protein kinase C on ENaC-mediated lung liquid regulation in the adult rat lung

Tight control of lung liquid (LL) regulation is vital for pulmonary function. The aim of this work was to determine whether PKC activation is involved in the physiological regulation of LL volume in a whole lung preparation. Rat lungs were perfused with a modified Ringer solution, and the lumen was filled with the same solution without glucose. LL volume was measured during a control period and after modulating drugs were administered, and net LL transepithelial movement (J(v)) was calculated. When the PKC activator PMA (10(-5) M) and the Ca(2+) ionophore ionomycin (10(-6) M) were instilled into the lung together, J(v) was significantly reduced (P = 0.03). This reduction was blocked by the PKC inhibitor chelerythrine chloride (10(-6) M; P = 0.56) and by a second PKC inhibitor GF109203X (10(-5) M; P = 0.98). When PMA and ionomycin were added with the β-adrenergic agonist terbutaline, the terbutaline-induced increase in J(v) was abolished. Addition of PMA and ionomycin with the epithelial Na(+) channel (ENaC) blocker amiloride had no additional inhibitory effect. Together, these results suggest that PKC is likely to be involved in LL absorption, and the ability of PMA/ionomycin to block the terbutaline-induced increase in J(v) suggests that the downstream target of PKC is ENaC.

Inhaled beta-2 agonist salbutamol for the treatment of transient tachypnea of the newborn

OBJECTIVE

To evaluate the efficacy of inhaled salbutamol, a beta-2 adrenergic agonist, for the treatment of transient tachypnea of the newborn (TTN) and to determine whether inhaled salbutamol is safe in newborn infants.

STUDY DESIGN

Inhaled salbutamol or normal saline solution was administered to 54 infants with gestational ages ranging from 34 to 39 weeks and TTN. The response to salbutamol therapy was evaluated by determining respiratory rate, clinical score of TTN, level of respiratory support, and fraction of inspired oxygen before and at 30 minutes and 1 and 4 hours after salbutamol nebulization.

RESULTS

Among the 54 infants with TTN, 32 received salbutamol and 22 received normal saline solution. After one dose, the salbutamol group showed significant improvements in respiratory rate, clinical score of TTN, fraction of inspired oxygen, and level of respiratory support (P < .05). After treatment, the mean pH, partial pressure of arterial oxygen, and partial pressure of arterial carbon dioxide values were better in the salbutamol group when compared with the placebo group (P < .05). Duration of hospitalization in the neonatal intensive care unit was also shorter for the salbutamol group (P < .05).

CONCLUSION

Inhaled salbutamol treatment was effective with respect to both clinical and laboratory findings of TTN and without adverse events.

K+ channel openers restore verapamil-inhibited lung fluid resolution and transepithelial ion transport

Background

Lung epithelial Na⁺ channels (ENaC) are regulated by cell Ca²⁺ signal, which may contribute to calcium antagonist-induced noncardiogenic lung edema. Although K⁺ channel modulators regulate ENaC activity in normal lungs, the therapeutical relevance and the underlying mechanisms have not been completely explored. We hypothesized that K⁺ channel openers may restore calcium channel blocker-inhibited alveolar fluid clearance (AFC) by up-regulating both apical and basolateral ion transport.

Methods

Verapamil-induced depression of heterologously expressed human αβγ ENaC in Xenopus oocytes, apical and basolateral ion transport in monolayers of human lung epithelial cells (H441), and in vivo alveolar fluid clearance were measured, respectively, using the two-electrode voltage clamp, Ussing chamber, and BSA protein assays. Ca²⁺ signal in H441 cells was analyzed using Fluo 4AM.

Results

The rate of in vivo AFC was reduced significantly (40.6 ± 6.3% of control, P < 0.05, n = 12) in mice intratracheally administrated verapamil. K_Ca3.1 (1-EBIO) and K_ATP (minoxidil) channel openers significantly recovered AFC. In addition to short-circuit current (Isc) in intact H441 monolayers, both apical and basolateral Isc levels were reduced by verapamil in permeabilized monolayers. Moreover, verapamil significantly altered Ca²⁺ signal evoked by ionomycin in H441 cells. Depletion of cytosolic Ca²⁺ in αβγ ENaC-expressing oocytes completely abolished verapamil-induced inhibition. Intriguingly, K_V (pyrithione-Na), K _Ca3.1 (1-EBIO), and K_ATP (minoxidil) channel openers almost completely restored the verapamil-induced decrease in Isc levels by diversely up-regulating apical and basolateral Na⁺ and K⁺ transport pathways.

Conclusions

Our observations demonstrate that K⁺ channel openers are capable of rescuing reduced vectorial Na⁺ transport across lung epithelial cells with impaired Ca²⁺ signal.

Risk factors and management of transient tachypnea of the newborn

Transient tachypnea of the newborn (TTN) is the consequence of delayed clearance of fetal lung liquid in the newborn. With recognition of the increased risk in babies born by Cesarean sections, epidemiologic association with maternal asthma and increasing research on the possible role of genetic polymorphisms of ion-channel subunits, our understanding of the pathophysiology of this condition has vastly improved. We now know that the late-preterm infant, born at 34–36 weeks gestation, is at increased risk for both TTN and respiratory distress syndrome due to surfactant deficiency. As the incidence of Cesarean sections rises, there is likelihood of increased respiratory morbidity in newborns that will necessitate additional medical interventions and exposure to complications of intensive care. This review focuses on the risk factors that are associated with the development of TTN and the treatment strategies that are employed for the management of this condition.

Amniotic fluid water dynamics

Water arrives in the mammalian gestation from the maternal circulation across the placenta. It then circulates between the fetal water compartments, including the fetal body compartments, the placenta and the amniotic fluid. Amniotic fluid is created by the flow of fluid from the fetal lung and bladder. A major pathway for amniotic fluid resorption is fetal swallowing; however in many cases the amounts of fluid produced and absorbed do not balance. A second resorption pathway, the intramembranous pathway (across the amnion to the fetal circulation), has been proposed to explain the maintenance of normal amniotic fluid volume. Amniotic fluid volume is thus a function both of the amount of water transferred to the gestation across the placental membrane, and the flux of water across the amnion. Membrane water flux is a function of the water permeability of the membrane; available data suggests that the amnion is the structure limiting intramembranous water flow. In the placenta, the syncytiotrophoblast is likely to be responsible for limiting water flow across the placenta. In human tissues, placental trophoblast membrane permeability increases with gestational age, suggesting a mechanism for the increased water flow necessary in late gestation. Membrane water flow can be driven by both hydrostatic and osmotic forces. Changes in both osmotic/oncotic and hydrostatic forces in the placenta my alter maternal-fetal water flow. A normal amniotic fluid volume is critical for normal fetal growth and development. The study of amniotic fluid volume regulation may yield important insights into the mechanisms used by the fetus to maintain water homeostasis. Knowledge of these mechanisms may allow novel treatments for amniotic fluid volume abnormalities with resultant improvement in clinical outcome.

H89, an inhibitor of protein kinase A (PKA), stimulates Na+ transport by translocating an epithelial Na+ channel (ENaC) in fetal rat alveolar type II epithelium

The present study was performed to clarify the effect of H89, an inhibitor of cAMP-activated protein kinase (protein kinase A; PKA), on Na(+) absorption in fetal rat alveolar type II epithelium. H89 stimulated the Na(+) absorption by increasing the open probability (Po) and number of a nonselective cation (NSC) channel composed of four alpha subunits of epithelial Na(+) channel (ENaC). Brefeldin A (BFA), an inhibitor of intracellular protein translocation, blocked the stimulatory action of H89 on the Na(+) absorption by interrupting the action of H89 on the Po and number of the NSC channel. H85, an inactive form of H89, showed an effect similar to H89, suggesting that H89 does not show its effect by inhibiting PKA, but acts on the channel depending the structure. These observations indicate that: (1) the H89 induced increase in number of the channel at the apical membrane is due to translocation of alpha subunit of ENaC to the apical membrane, (2) the elevation of Po of the channel is mediated through translocation of a protein activating alpha subunit of ENaC, and (3) the effect of H89 is dependent on its structure without any relation to PKA.

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.