SUMMARY1. In the chronically catheterized fetal lamb, intravenous infusion of adrenaline at 0 5 jug/min produced slowing of the secretion of lung liquid or its absorption, an effect which increased exponentially with advancing gestation. Between 120 and 130 days, the characteristic response was slowing of secretion, whereas after 130 days it was absorption.2. Stimulus-response curves, relating secretion or absorption rate to plasma adrenaline concentration, were obtained by infusing adrenaline into the fetus intravenously at rates between 0-1 and 1.0 #sg/min (0-55-5 5 nmol/min). These curves allowed estimation of the minimum concentration of adrenaline required to inhibit secretion ([Ai]) and this was found to decrease from 0-43 ng/ml. (2-35 nM) at 132-4 days' gestation to 0-029 ng/ml. (0 16 nM) at gestations above 140 days.3. During spontaneous labour there was a slowing of lung liquid secretion in the early stages followed by absorption during the last 50-150 min. The mean concentration of adrenaline in plasma increased from 0-087 ng/ml. (048 nM) in early labour to 6-86 ng/ml. (37 5 nM) in the last 50 min and to 7-17 ng/ml. (39-2 nM) in the early post-natal period. Mean noradrenaline levels at the same times were 1-71 ng/ml.(10-1 nM), 12-14 ng/ml. (71'8 nM) and 9-10 ng/ml. (53 9 nM). The relationship between the plasma adrenaline concentration and the rate of absorption during labour was similar to that found when adrenaline was infused at various rates into the non-labouring fetus of comparable gestational age.4. The upper airway of the fetus was shown to be capable of acting as a one-way valve allowing outflow but not inflow of liquid. Thus withdrawal of liquid at 5-20 ml./hr from the fetal trachea below the larynx caused closure of the upper airway and this result was obtained both when the recurrent laryngeal nerves were intact and when they were divided.

SummaryWe have examined the effect on lung liquid secretion of catecholamines ipfused in chronically catheterized fetal lambs in utero. Isoproterenol and epinephrine inhibited secretion, an effect which increased with gestation and, in fetuses near delivery, caused absorption of lung liquid. In 7 out of 8 experiments nor-epinephrine had no effect on secretion. This pattern of response and the fact that the inhibitory effect could

2. The permeability sequence of the pulmonary epithelium for alkali metals was, Na+ > K+ > Rb+ > Li+ > Cs+ and that for halides I-Br-> C1-. Permeabilities to alkaline earths were lower than for the other ions, no definite sequence being established.3. There was an electrical potential difference of -1 to -10 mV (mean -4.3 mV) between lung liquid and plasma (lung liquid negative). Plasma/lung liquid chemical activity ratios were less than unity for the halides (Cl-, Br-, I-), and for K+ and Rb+, whereas the ratio of oneway fluxes (plasma --lung liquid)/(lung liquid -+ plasma) was in each case greater than unity. From the difference between the measured flux ratios and those predicted from the forces determining passive flux, it was concluded that the halides, K+ and Rb+ were actively transported from plasma to lung liquid, Cl-being quantitatively the most important. Na+ and Ca2+ appeared to move passively down a gradient of electrochemical potential.R. E. OLVER AND L. B. STRANG 4. When alveolar liquid [HCO ] was artificially raised, a net flux of HCO-from lung liquid against a gradient of electrochemical activity was observed, suggesting active transport of that ion out of lung liquid.5. The addition of KCN to lung liquid stopped the secretion of liquid and absorption took place.

SUMMARY1. Adrenaline was infused intravenously at rates of 0 1-10 tg/min into chronically catheterized fetal lambs (125-141 days gestation) to induce slowing of secretion or reabsorption of lung liquid.2. There was an electrical potential difference (p.d.) of -0 3 to -9-5 mV (mean -3.4 mV) between lung liquid and plasma (lung liquid negative) during control lung liquid secretion. In response to adrenaline infusion, the p.d. increased (lung lumen more negative) and this change was greatest (1-8+0±3 mV) in experiments in which reabsorption occurred.3. Measurements were made of bidirectional fluxes of Na+ and Cl-across the pulmonary epithelium during control lung liquid secretion and during adrenaline infusion. Adrenaline-induced reabsorption of lung liquid was associated with an increase in Na+ flux from lung lumen to plasma. Similar but smaller changes occurred when the adrenaline response was slowing of secretion.

The developing distal lung epithelium displays an evolving liquid transport phenotype, reflecting a changing and dynamic balance between Cl- ion secretion and Na+ ion absorption, which in turn reflects changing functional requirements. Thus in the fetus, Cl--driven liquid secretion predominates throughout gestation and generates a distending pressure to stretch the lung and stimulate growth. Increasing Na+ absorptive capacity develops toward term, anticipating the switch to an absorptive phenotype at birth and beyond. There is some empirical evidence of ligand-gated regulation of Cl- transport and of regulation via changes in the driving force for Cl- secretion. Epinephrine, O2, glucocorticoid, and thyroid hormones interact to stimulate Na+ absorption by increasing Na+ pump activity and apical Na+ conductance (GNa+) to bring about the switch from net secretion to net absorption as lung liquid is cleared from the lung at birth. Postnatally, the lung lumen contains a small Cl--based liquid secretion that generates a surface liquid layer, but the lung retains a large absorptive capacity to prevent alveolar flooding and clear edema fluid. This review explores the mechanisms underlying the functional development of the lung epithelium and draws upon evidence from classic integrative physiological studies combined with molecular physiology approaches.

SUMMARY1. Two sets of experiments were performed on intact foetal lambs exteriorized at Caesarean section; in one set radioactively labelled test substances (inulin, sucrose, mannitol, erythritol, urea) were injected I.V. either singly or in pairs and then followed in plasma, lung lymph and alveolar liquid; in the other set labelled test substances (inulin, sucrose, mannitol, erythritol, D-serine, L-serine, D-a-alanine, urea, water, thiourea, N-ethylthiourea) were introduced singly, in pairs, or sequentially into alveolar liquid and their concentration followed in alveolar liquid and plasma.2. Inulin was found to cross lung capillary walls but not alveolar walls. Measurements of its concentration following injection into alveolar liquid were used to determine the volume of foetal alveolar liquid (mean = 30 ml./kg) and its rate of formation (mean = 0-036 ml./min. kg). The volume of the lung interstitial space was determined from previous experiments in which [1251]PVP had been injected I.v. then measured after 2 hr in lung tissue and lung lymph (mean = 10 4 % foetal lung weight after withdrawal of liquid; . 20 % wet lung tissue weight).3. Transfer constants (min-') for lung capillaries (Kc) and alveoli (KO) were obtained from the experimental results by compartmental analysis. Permeability constants (PC and Po, cm/sec) were derived from them using estimates for capillary and alveolar areas. For lipid insoluble molecules Pc and PO both increased with decreasing molecular radius, the effect being much greater for PO than Pc. Po was also shown to increase with lipid solubility of the test molecule even though molecular size increased with lipid solubility in the series tested (urea, thiourea, N-ethylthiourea).

A regulated apical Na ϩ conductance in dexamethasone-treated H441 airway epithelial cells. with dexamethasone raised the abundance of mRNA encoding the epithelial Na ϩ channel ␣-and ␤-subunits and increased transepithelial ion transport (measured as short-circuit current, I sc) from Ͻ4 A ⅐ cm Ϫ2 to 10 -20 A ⅐ cm Ϫ2 . This dexamethasone-stimulated ion transport was blocked by amiloride analogs with a rank order of potency of benzamil Ն amiloride Ͼ EIPA and can thus be attributed to active Na ϩ absorption. Studies of apically permeabilized cells showed that this increased transport activity did not reflect a rise in Na ϩ pump capacity, whereas studies of basolateral permeabilized cells demonstrated that dexamethasone increased apical Na ϩ conductance (GNa) from a negligible value to 100 -200 S ⅐ cm Ϫ2 . Experiments that explored the ionic selectivity of this dexamethasone-induced conductance showed that it was equally permeable to Na ϩ and Li ϩ and that the permeability to these cations was approximately fourfold greater than to K ϩ . There was also a small permeability to N-methyl-Dglucammonium, a nominally impermeant cation. Forskolin, an agent that increases cellular cAMP content, caused an ϳ60% increase in I sc, and measurements made after these cells had been basolaterally permeabilized demonstrated that this response was associated with a rise in G Na. This cAMP-dependent control over GNa was disrupted by brefeldin A, an inhibitor of vesicular trafficking. Dexamethasone thus stimulates Na ϩ transport in H441 cells by evoking expression of an amiloride-sensitive apical conductance that displays moderate ionic selectivity and is subject to acute control via a cAMP-dependent pathway. airway epithelium; epithelial sodium channel; Ussing chambers; glucocorticoids; apical membrane GLUCOCORTICOID HORMONES CONTRIBUTE to the development and maintenance of the distal airway epithelia's capacity to absorb Na ϩ from the overlying film of surface liquid (see Refs. 37,41,50,52), a process that is vital to the integrated functioning of the respiratory tract (see Ref. 6). In most tissues, the absorption of Na ϩ is clearly dependent on epithelial Na ϩ channels (ENaC), transport proteins composed of three subunits (␣-, ␤-, and ␥-ENaC) that form a highly selective, amiloride-sensitive Na ϩ conductance if coexpressed in Xenopus oocytes or mammalian systems (8,9,16,17). Because glucocorticoids can regulate the expression of at least some members of this gene family (37,41,52), it is tempting to attribute glucocorticoidevoked pulmonary Na ϩ transport to increased expression of such selective Na ϩ channels, but, despite this, many authors have been unable to identify selective Na ϩ channels in distal airway epithelia. Indeed, the majority of studies suggests that regulated Na ϩ influx in these cells occurs via nonselective cation channels that discriminate very poorly between Na ϩ and K ϩ (24, 29 -32, 36, 49). Moreover, there is evidence that these channels may be formed when ␣-ENaC is expressed independently of the ␤-and ␥-subunit...

Monolayer cultures of rat fetal distal lung epithelial (FDLE) cells generated larger spontaneous short circuit currents (ISC) when maintained (48 h) at neonatal alveolar PO2 (100 mmHg) than at fetal PO2 (23 mmHg). When cells were shifted between these atmospheres in order to impose a rise in PO2 equivalent to that seen at birth, no rise in ISC was seen after 6 h but the response was fully established by 24 h. Studies of basolaterally permeabilised cells revealed a small rise in apical Na+ conductance (GNa) 6 h after PO2 was raised but no further change had occurred by 24 h. A substantial rise was, however, seen after 48 h. Reporter gene assays showed that no activation of the α‐ENaC (epithelial Na+ channel α‐subunit) promoter was discernible 24 h after PO2 was raised but increased transcriptional activity was seen at 48 h. Studies of apically permeabilised cells showed that a small rise in Na+ pump capacity was evident 6 h after PO2 was raised and, in common with the rise in ISC, this effect was fully established by 24 h. The rise in ISC thus develops 6‐24 h after PO2 is raised and is due, primarily, to increased Na+ pump capacity. The increase in GNa thus coincides with activation of the α‐ENaC promoter but these effects occur after the rise in ISC is fully established and so cannot underlie this physiological response. The increased transcription may be an adaptation to increased Na+ transport and not its cause.