Stuart M. Wilson
Lung Membrane Transport Group, Division of Maternal and Child Health Science Ninewells Hospital and Medical School, University of Dundee, Dundee, Scotland

https://doi.org/10.36866/pn.53.33

During fetal life liquid is secreted into the lumen of the developing lung generating a distending pressure that inflates the lung to a volume comparable to the functional residual capacity of the neonatal lung. This process is important to lung morphogenesis as clinical syndromes that disrupt this pressure cause abnormal lung growth. However, this secreted liquid must be entirely removed from the lung by the time of birth as the retention of even a small volume of liquid will impair oxygenation of the blood. Indeed, the postnatal retention of lung liquid is a dangerous feature of neonatal respiratory distress syndrome (RDS), the commonest cause of death amongst newborn and premature infants in the developed world. It was long assumed that liquid was simply squeezed from the lungs during labour and birth but this myth was dispelled by the observation that blockage of the trachea did not prevent a fall in lung liquid volume. The removal of liquid must therefore be an absorptive process dependent upon the lung tissue itself.

Hormonal control of lung liquid absorption

The physiology of lung liquid absorption was investigated by monitoring the volume of liquid in the lungs of fetal lambs. The first such studies explored the effects of adrenoceptor agonists/antagonists as the level of adrenaline in the fetal circulation is known to reach very high levels during labour. As anticipated, lung liquid volume normally rose as liquid was secreted into the lumen, but then fell during labour and birth as it was subsequently reabsorbed. A central role for adrenaline in this process was established by the fact that β­adrenoceptor agonists evoked liquid absorption in mature fetuses whilst antagonists reduced the liquid absorption seen during labour. However, in relatively immature fetuses, β-adrenoceptor antagonists merely slowed the rate of fluid accumulation and so this absorptive response must develop over the last stages of gestation. Moreover, the maturation of this response was blocked by fetal thyroidectomy and, although this effect was reversed by exogenous thyroid hormone (T3), T3 did not cause precocious development of absorptive capacity in immature fetuses unless given with a glucocorticoid. The circulating levels of T3 and glucocorticoids both rise during the perinatal period and so these hormones appear to prime the lung by evoking the development of an absorptive capacity that is then unmasked when the lungs are exposed to high levels of adrenaline during labour.

Pulmonary Na+ transport

Subsequent studies showed that the absorption of fluid from the lung lumen is driven by the active absorption of Na+. At that time, it was known that transepithelial Na+ transport is normally limited by the rate at which Na+ can cross the apical membrane but the channels permitting this Na+ entry were unknown until the early 1990s when Canessa et al. (1993) identified three genes encoding the component subunits of the epithelial Na+ channel (α-, β- and γ-ENaC). Subsequent studies showed that genetic deletion of α-ENaC caused death from severe RDS by blocking the absorption of liquid from the perinatal lung (Hummler et al. 1996). Perinatal lung liquid absorption is thus dependent upon pulmonary Na+ transport and this process has now been extensively studied using isolated rat fetal distal lung epithelial (FDLE) cells, which form Na+ absorbing epithelia when cultured on permeable supports. b-adrenoceptor agonists stimulate Na+ transport in these cells by increasing apical Na+ conductance (GNa+) and this control over GNa+ is dependent upon T3 / glucocorticoids, a finding that explains the importance of these hormones to lung liquid clearance (see above). These observations thus predict that, as well as inhibiting the secretion/synthesis of surfactant, a lack of T3 would impair lung liquid clearance and this may explain why RDS is abnormally prevalent amongst hypothyroid infants. Moreover, premature delivery by Caesarean Section appears to attenuate the perinatal surge in circulating T3 (Baines et al. 2000) and the role of this hormone in the control of GNa+ may explain why RDS is more prevalent in babies delivered in this way.

Role of atmospheric O2

Whilst adrenaline is crucial to the initiation of pulmonary Na+ absorption, other factors must contribute to the maintenance of this phenotype as Na+ absorption normally continues throughout adult life despite a rapid, postnatal fall in circulating adrenaline levels. It is interesting, in this context, that fetal development takes place under hypoxic conditions implying that the distal lung epithelia experience a rise in PO2 as the newborn infant takes its first breaths. The first evidence that this might play a role in the functional maturation of the lung came from studies of fetal rat lung explants, which showed that PO2 could influence pulmonary fluid balance. Subsequent studies of FDLE cells showed clearly that increased PO2 stimulates Na+ transport (Pitkänen et al. 1996) and these data raised the possibility that the rise in alveolar PO2 seen at birth may provide a drive for maintained Na+ transport despite falling adrenaline levels. Interestingly, this effect of O2 was reversible implying that, even in adult life, a fall in alveolar PO2 would reduce the drive for Na+ transport and thus allow liquid to remain in the lungs. Such alveolar flooding often occurs in pulmonary oedema, a condition with many underlying causes including heart failure, septic or haemorrhagic shock and ascent to high altitude and, irrespective of its underlying cause, the resolution of this condition depends upon pulmonary Na+ absorption (Ware & Matthay, 2001). Whilst the alveolar flooding in pulmonary oedema is usually attributed to disturbed pulmonary haemodynamics, there is evidence that hypoxic inhibition of Na+ absorption contributes to the liquid accumulation seen in at least some forms of oedema (Scherrer et al. 1999).

Atmospheric O2 may thus play an important role in pulmonary physiology by stabilising the lungs’ Na+ absorbing phenotype. Despite this potential importance, however, the way in which O2 can control Na+ transport is not well understood. However, raising PO2 increases the abundance of α-, β- and γ-ENaC mRNA, suggesting that the response may involve increased ENaC gene expression. Moreover, the promoter region of the α-ENaC gene contains a binding site for NF-κB (Otulakowski et al. 1999), a transcription factor activated by physiologically relevant increases in PO2 and so it was suggested that O2 may stimulate Na+ transport by evoking NF-κB-dependent ENaC expression (Pitkanen et al. 1996). Although subsequent work showed that increases in PO2 could activate the α-ENaC promoter (Baines et al. 2001), this response was weak and seen only after PO2 was raised for 24–48 h. However, the stimulation of Na+ transport was fully developed by 24 h and cannot, therefore, be a consequence of increased gene expression. However, physiologically relevant increases in PO2 increase the capacity of the basolateral Na+ pump and this response precedes the increased Na+ transport by 3–4 h. This event may well be an important early step in the functional response to increased PO2, whereas increased ENaC expression could be a consequence, rather than the cause, of the increased Na+ transport. The mechanisms that allow the Na+ pump to respond to an increased PO2 are unknown and neither is it clear how this effect is transduced into increased transepithelial Na+ transport.

Conclusions

Pulmonary Na+ transport is important to the integrated functioning of the respiratory tract and the initiation and maintenance of this phenotype is dependent upon the interaction between physiological (circulating hormones) and environmental factors (atmospheric O2). Improved understanding of the way in which these factors control Na+transport may reveal the pathophysiological basis of RDS and certain forms of pulmonary oedema.

References

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Baines DL, Ramminger SJ, Collett A, Haddad JJE, Best OG, Land SC, Olver RE, & Wilson SM (2001). Oxygen-evoked Na+ transport in rat fetal distal lung epithelial cells. J Physiol 532, 105-113

Canessa CM, Horisberger JD & Rossier BC (1993). Epithelial sodium channel related to proteins involved in neurodegeneration. Nature 361, 467-470.

Hummler E, Baker P, Gatzy J, Berrmann F, Verdumo C, Schmidt A, Boucher R & Rossier RC (1996). Early death due to defective neonatal lung liquid clearance in α-ENaC-deficient mice. Nature Genetics 12, 325-328.

Otulakowski G, Raffii B, Bremner HR & O’Brodovich H (1999). Structure and hormone responsiveness of the gene encoding the α­subunit of the rat amiloride-sensitive epithelial sodium channel. Am J Respir Cell Mol Biol 20, 1028-1040

Pitkänen OM, Tanswell AK, Downey G & O’Brodovich H (1996). Increased PO2 alters the bioelectric properties of the fetal distal lung epithelium. Am J Physiol 270, L1060-L1066.

Scherrer U, Sartori C, Lepori M, Allemann Y, Duplain H, Trueb L & Nicod P (1999). High-altitude pulmonary edema: from exaggerated pulmonary hypertension to a defect in transepithelial sodium transport. Adv Exp Med Biol 474, 93-107.

Ware LB & Matthay MA (2001). Alveolar epithelial fluid clearance is impaired in the majority of patients with acute lung injury and acute respiratory distress syndrome. Am J Resp Crit Care Med 163, 1376­1383.