Dafydd V. Walters

Developmental regulation of lumenal lung fluid and electrolyte transport

In the fetus, there is a net secretion of liquid (LL) by the lung as a result of active transport of chloride ions. The rate of secretion and the resulting volume of LL are vital for normal lung growth but how volume is sensed and how secretion may be regulated are still unknown. Towards term under the influence of thyroid and adrenocorticoid hormones, the epithelial sodium channel (ENaC) is increasingly expressed in the pulmonary epithelium. Adrenaline released by the fetus during labour activates ENaC and produces rapid absorption of liquid in preparation for air breathing; absence of ENaC is incompatible with survival. There may be other mechanisms involved in aiding liquid clearance including changes in epithelial permeability, an effect of oxygen on both ENaC and Na/K ATPase and perhaps the influence of additional hormones on ENaC activity. Some time after birth there are further developmental changes with the appearance of other cation channels (CNG1 and perhaps NSCC) which contribute to the liquid absorptive side of the balance existing across the epithelium between secretion and absorption to produce essentially almost no net liquid movement in the postnatal lung. The evidence for these processes is discussed and areas of uncertainty indicated.

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.

Hydrogen sulfide decreases β-adrenergic agonist-stimulated lung liquid clearance by inhibiting ENaC-mediated transepithelial sodium absorption

In pulmonary epithelia, β-adrenergic agonists regulate the membrane abundance of the epithelial sodium channel (ENaC) and, thereby, control the rate of transepithelial electrolyte absorption. This is a crucial regulatory mechanism for lung liquid clearance at birth and thereafter. This study investigated the influence of the gaseous signaling molecule hydrogen sulfide (H2S) on β-adrenergic agonist-regulated pulmonary sodium and liquid absorption. Application of the H2S-liberating molecule Na2S (50 μM) to the alveolar compartment of rat lungs in situ decreased baseline liquid absorption and abrogated the stimulation of liquid absorption by the β-adrenergic agonist terbutaline. There was no additional effect of Na2S over that of the ENaC inhibitor amiloride. In electrophysiological Ussing chamber experiments with native lung epithelia (Xenopus laevis), Na2S inhibited the stimulation of amiloride-sensitive current by terbutaline. β-adrenergic agonists generally increase ENaC abundance by cAMP formation and activation of PKA. Activation of this pathway by forskolin and 3-isobutyl-1-methylxanthine increased amiloride-sensitive currents in H441 pulmonary epithelial cells. This effect was inhibited by Na2S in a dose-dependent manner (5-50 μM). Na2S had no effect on cellular ATP concentration, cAMP formation, and activation of PKA. By contrast, Na2S prevented the cAMP-induced increase in ENaC activity in the apical membrane of H441 cells. H441 cells expressed the H2S-generating enzymes cystathionine-β-synthase, cystathionine-γ-lyase, and 3-mercaptopyruvate sulfurtransferase, and they produced H2S amounts within the employed concentration range. These data demonstrate that H2S prevents the stimulation of ENaC by cAMP/PKA and, thereby, inhibits the proabsorptive effect of β-adrenergic agonists on lung liquid clearance.

Why babies don't drown at birth?

The paper of Aslan et al. (1) in this issue, demonstrating a link between certain β-adrenoceptor polymorphisms and the development of transient tachypnoea of the newborn, is the latest to address the question of the physiological mechanisms controlling liquid movement across the respiratory epithelium at birth – a question that has long exercised respiratory physiologists (see reference 2 for a recent review of the developmental regulation of lung fluid transport). The abrupt removal of the liquid present within the lung at birth is perhaps the most dramatic demonstration of the importance of lung transport mechanisms that operate throughout life, and it is consequently not surprising that paediatricians (including paediatric pathologists) and obstetricians have been at the forefront of attempts to understand the key physiological mechanisms underlying these processes. Knowledge of the fluid absorptive capacity of the lung is more than 150 years old (3) but until the late 1970s it was believed that the removal of lung liquid at birth was a passive process due to the compression of the chest during its passage down the birth canal. However, there are several lines of evidence which argue against this simple explanation (see 2), not the least of which is the observation that occlusion of the trachea in experimental animals does not impede lung liquid absorption. It had already been established that lung liquid is secreted by a process involving ‘active’ chloride transport (see 2) when Enhorning and co-workers (4) and Walters and Olver (5) demonstrated the key role of β-adrenoceptor stimulation in the rapid switch in the direction of net liquid movement across the lung epithelium at birth. Enhorning et al. (4) noted the dehydrating effect of the synthetic β-adrenoceptor

The role of Cl− in the regulation of ion and liquid transport in the intact alveolus during β‐adrenergic stimulation

•  What is the central question of this study? The co‐ordinated regulation of ion transport across the pulmonary epithelium determines the volume of the thin liquid layer lining the normal lung. However, the integrated homeostasis of the liquid transport physiology of the intact alveolus remains elusive. •  What is the main finding and its importance? Using a whole‐lung preparation, we explored the mechanism of the β‐agonist‐mediated upregulation of Na+ and alveolar liquid clearance, leading to resolution of pulmonary oedema. We demonstated the functional presence of secretory rather than absorptive Cl− channels, which implies that this upregulation is mediated via a direct effect on Na+ conductance and not through transepithelial Cl− absorption and an increased driving force for Na+.

Dapagliflozin‐lowered blood glucose reduces respiratory Pseudomonas aeruginosa infection in diabetic mice

Hyperglycaemia increases glucose concentrations in airway surface liquid and increases the risk of pulmonary Pseudomonas aeruginosa infection. We determined whether reduction of blood and airway glucose concentrations by the anti‐diabetic drug dapagliflozin could reduce P. aeruginosa growth/survival in the lungs of diabetic mice.