Cation currents in human airway epithelial cells induced by infection with influenza A virus

Influenza A viruses cause lung disease via an incompletely understood mechanism that involves the accumulation of liquid within the lungs. The accumulation of lung liquid is normally prevented by epithelial Na(+) absorption, a transport process regulated via several pathways including phosphoinositide-3-kinase (PI3K). Since the influenza A virus encodes a non-structural protein (NS1) that can activate this kinase, we now explore the effects of NS1 upon the biophysical properties of human airway epithelial cells. Transient expression of NS1 depolarized electrically isolated cells maintained in glucocorticoid-free medium by activating a cation conductance identical to the glucocorticoid-induced conductance seen in single cells. This response involved PI3K-independent and PI3K-dependent mechanisms. Infecting glucocorticoid-deprived cells with influenza A virus disrupted the normal electrical coupling between neighbouring cells, but also activated a conductance identical to that induced by NS1. This response to virus infection was only partially dependent upon NS1-mediated activation of PI3K. The presence of NS1 allows influenza A to modify the biophysical properties of infected cells by activating a Na(+)-permeable conductance. Whilst the activation of Na(+)-permeable channels may be expected to increase the rate of Na(+) absorption and thus reduce the volume of liquid in the lung, liquid does normally accumulate in influenza A-infected lungs. The overall effect of influenza A on lung liquid volume may therefore reflect a balance between the activation and inhibition of Na(+)-permeable channels.

Ion transport and water flow in the mammalian lung

The coupling of bulk water flow to active ion transport has been described in various epithelia; evidence presented here suggests that this is also a feature of the mammalian lung. Measurements of the ionic composition of lung liquid and its rate of formation in the fetal lamb in vivo have made it possible to estimate the net flux of each ion and, with water tracer measurements of ion one-way fluxes, to calculate flux ratios. When these are compared with the ratios predicted by the Ussing flux ratio equation it is clear that the secretion of lung liquid is linked to active transport of Cl- from plasma; sodium moves passively. In addition there is an apparent uphill transfer of HCO2- out of lung liquid. In an in vitro preparation of adult canine trachea Cl- is actively transported towards the lumen and is associated with a small net flux of Na+ in the opposite direction. Addition of acetylcholine increases the net Cl- flux towards the lumen but reverses the orientation of the net Na+ flux. Changes such as these may be important determinants of bulk liquid flow in vivo as well as in vitro.

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Academic paediatrics is an exciting and rewarding career path but is not immune to the problems of recruitment and retention currently affecting most branches of medicine. The Modernising Medical Careers initiative, with its explicit academic training path, offers an unparalleled opportunity to develop novel schemes that promote recruitment and retention. Coordinated action is required to define, publicise and support the new academic training programmes and to attract the best trainees into them.

Adenosine-evoked Na+ transport in human airway epithelial cells

BACKGROUND AND PURPOSE

Absorptive epithelia express apical receptors that allow nucleotides to inhibit Na(+) transport but ATP unexpectedly stimulated this process in an absorptive cell line derived from human bronchiolar epithelium (H441 cells) whilst UTP consistently caused inhibition. We have therefore examined the pharmacological basis of this anomalous effect of ATP.

EXPERIMENTAL APPROACH

H441 cells were grown on membranes and the short circuit current (I(SC)) measured in Ussing chambers. In some experiments, [Ca(2+)](i) was measured fluorimetrically using Fura -2. mRNAs for adenosine receptors were determined by the polymerase chain reaction (PCR).

KEY RESULTS

Cross desensitization experiments showed that the inhibitory response to UTP was abolished by prior exposure to ATP whilst the stimulatory response to ATP persisted in UTP-pre-stimulated cells. Apical adenosine evoked an increase in I(SC) and this response resembled the stimulatory component of the response to ATP, and could be mimicked by adenosine receptor agonists. Pre-stimulation with adenosine abolished the stimulatory component of the response to ATP. mRNA encoding A(1), A(2A) and A(2B) receptor subtypes, but not the A(3) subtype, was detected in H441 cells and adenosine receptor antagonists could abolish the ATP-evoked stimulation of Na(+) absorption.

CONCLUSIONS AND IMPLICATIONS

The ATP-induced stimulation of Na(+) absorption seems to be mediated via A(2A/B) receptors activated by adenosine produced from the extracellular hydrolysis of ATP. The present data thus provide the first description of adenosine-evoked Na(+) transport in airway epithelial cells and reveal a previously undocumented aspect of the control of this physiologically important ion transport process.

Expression of intermediate-conductance, Ca2+-activated K+ channel (KCNN4) in H441 human distal airway epithelial cells

Electrophysiological studies of H441 human distal airway epithelial cells showed that thapsigargin caused a Ca(2+)-dependent increase in membrane conductance (G(Tot)) and hyperpolarization of membrane potential (V(m)). These effects reflected a rapid rise in cellular K(+) conductance (G(K)) and a slow fall in amiloride-sensitive Na(+) conductance (G(Na)). The increase in G(Tot) was antagonized by Ba(2+), a nonselective K(+) channel blocker, and abolished by clotrimazole, a KCNN4 inhibitor, but unaffected by other selective K(+) channel blockers. Moreover, 1-ethyl-2-benzimidazolinone (1-EBIO), which is known to activate KCNN4, increased G(K) with no effect on G(Na). RT-PCR-based analyses confirmed expression of mRNA encoding KCNN4 and suggested that two related K(+) channels (KCNN1 and KCNMA1) were absent. Subsequent studies showed that 1-EBIO stimulates Na(+) transport in polarized monolayers without affecting intracellular Ca(2+) concentration ([Ca(2+)](i)), suggesting that the activity of KCNN4 might influence the rate of Na(+) absorption by contributing to G(K). Transient expression of KCNN4 cloned from H441 cells conferred a Ca(2+)- and 1-EBIO-sensitive K(+) conductance on Chinese hamster ovary cells, but this channel was inactive when [Ca(2+)](i) was <0.2 microM. Subsequent studies of amiloride-treated H441 cells showed that clotrimazole had no effect on V(m) despite clear depolarizations in response to increased extracellular K(+) concentration ([K(+)](o)). These findings thus indicate that KCNN4 does not contribute to V(m) in unstimulated cells. The present data thus establish that H441 cells express KCNN4 and highlight the importance of G(K) to the control of Na(+) absorption, but, because KCNN4 is quiescent in resting cells, this channel cannot contribute to resting G(K) or influence basal Na(+) absorption.

A regulated apical Na(+) conductance in dexamethasone-treated H441 airway epithelial cells

Treating H441 cells with dexamethasone raised the abundance of mRNA encoding the epithelial Na(+) channel alpha- and beta-subunits and increased transepithelial ion transport (measured as short-circuit current, I(sc)) from <4 microA.cm(-2) to 10-20 microA.cm(-2). This dexamethasone-stimulated ion transport was blocked by amiloride analogs with a rank order of potency of benzamil >or= amiloride > EIPA and can thus be attributed to active Na(+) absorption. Studies of apically permeabilized cells showed that this increased transport activity did not reflect a rise in Na(+) pump capacity, whereas studies of basolateral permeabilized cells demonstrated that dexamethasone increased apical Na(+) conductance (G(Na)) from a negligible value to 100-200 microS.cm(-2). Experiments that explored the ionic selectivity of this dexamethasone-induced conductance showed that it was equally permeable to Na(+) and Li(+) and that the permeability to these cations was approximately fourfold greater than to K(+). There was also a small permeability to N-methyl-d-glucammonium, a nominally impermeant cation. Forskolin, an agent that increases cellular cAMP content, caused an approximately 60% 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 G(Na) was disrupted by brefeldin A, an inhibitor of vesicular trafficking. Dexamethasone thus stimulates Na(+) 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.

Secretion of acid and base equivalents by intact distal airways

Secretion of HCO(3)(-) by airway submucosal glands is essential for normal liquid and mucus secretion. Because the liquid bathing the airway surface (ASL) is acidic, it has been proposed that the surface epithelium may acidify HCO(3)(-)-rich glandular fluid. The aim of this study was to investigate the mechanisms by which intact distal bronchi, which contain both surface and glandular epithelium, modify pH of luminal fluid. Distal bronchi were isolated from pig lungs, cannulated in a bath containing HCO(3)(-)-buffered solution, and perfused continually with an aliquot of similar, lightly buffered solution (LBS) in which NaCl replaced NaHCO(3)(-) (pH 7 with NaOH). The pH of this circulating LBS initially acidified (by 0.053 +/- 0.0053 pH units) and transepithelial potential difference (PD) depolarized. The magnitude of acidification was increased when pH(LBS) was higher. This acidification was unaffected by luminal dimethylamiloride (DMA, 100 microM) but was inhibited by 100 nM bafilomycin A(1) (by 76 +/- 13%), suggesting involvement of vacuolar-H(+) ATPase. Addition of ACh (10 microM) evoked alkalinization of luminal LBS and hyperpolarization of transepithelial PD. The alkalinization was inhibited in HCO(3)(-)-free solutions containing acetazolamide (1 mM) and by DMA and was enhanced by bumetanide (100 microM), an inhibitor of Cl(-) secretion. The hyperpolarization was unaffected by these maneuvers. The anion channel blocker 5-nitro-2-(3-phenylpropylamino)benzoate (300 microM) and combined treatment with DMA and bumetanide blocked both the alkalinization and hyperpolarization responses to ACh. These results are consistent with earlier studies showing that ACh evokes glandular secretion of HCO(3)(-) and Cl(-). Isolated distal airways thus secrete both acid and base equivalents.

Hormonal modulation of Na(+) transport in rat fetal distal lung epithelial cells

Isolated rat fetal distal lung epithelial (FDLE) cells were cultured (approximately 48 h) on permeable supports in medium devoid of hormones and growth factors whilst P(O2) was maintained at the level found in either the fetal (23 mmHg) or the postnatal (100 mmHg) alveolar regions. The cells became incorporated into epithelial layers that generated a basal short-circuit current (I(SC)) attributable to spontaneous Na(+) absorption. Cells at neonatal P(O2) generated larger currents than did cells at fetal P(O2), indicating that this Na(+) transport process is oxygen sensitive. Irrespective of P(O2), isoprenaline failed to elicit a discernible change in I(SC), demonstrating that beta-adrenoceptor agonists do not stimulate Na(+) transport under these conditions. However, isoprenaline did elicit cAMP accumulation in these cells, indicating that functionally coupled beta-adrenoceptors are present. Further experiments showed that isoprenaline did increase I(SC) in cells treated (24 h) with a combination of tri-iodothyronine (T(3), 10 nM) and dexamethasone (200 nM). Studies of basolaterally permeabilised cells showed that these hormones are essential for the isoprenaline-evoked increase in the apical membrane's Na(+) conductance (G(Na)), whereas isoprenaline-evoked changes in apical Cl(-) conductance (G(Cl)) can occur in both control and hormone-treated cells. Irrespective of their hormonal status, FDLE cells thus express beta-adrenoceptors that are functionally coupled to adenylate cyclase, and allow beta-adrenoceptor agonists to modulate the apical membrane's anion conductance. However, T(3) and dexamethasone are needed if these receptors are to exert control over G(Na). These hormones may thus play an important role in the functional maturation of the lung by allowing beta-adrenoceptor-mediated control over epithelial Na(+) channels in the apical plasma membrane.

Beta-adrenoceptor-mediated control of apical membrane conductive properties in fetal distal lung epithelia

Distal lung epithelial cells isolated from fetal rats were cultured (48 h) on permeable supports so that transepithelial ion transport could be quantified electrometrically. Unstimulated cells generated a short-circuit current (I(sc)) that was inhibited (~80%) by apical amiloride. The current is thus due, predominantly, to the absorption of Na(+) from the apical solution. Isoprenaline increased the amiloride-sensitive I(sc) about twofold. Experiments in which apical membrane Na(+) currents were monitored in basolaterally permeabilized cells showed that this was accompanied by a rise in apical Na(+) conductance (G(Na(+))). Isoprenaline also increased apical Cl- conductance (G(Cl-)) by activating an anion channel species sensitive to glibenclamide but unaffected by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS). The isoprenaline-evoked changes in G(Na(+)) and G(Cl(minus sign)) could account for the changes in I(sc) observed in intact cells. Glibenclamide had no effect upon the isoprenaline-evoked stimulation of I(sc) or G(Na(+)) demonstrating that the rise in G(Cl-) is not essential to the stimulation of Na(+) transport.

Regulation of intracellular pH in Calu-3 human airway cells

The Calu-3 human cell line exhibits features of submucosal gland serous cells and secretes HCO(3)(-). The aim of this study was to identify the HCO(3)(-) transporters present in these cells by studying their role in the regulation of intracellular pH (pH(i)). Calu-3 cells were grown on coverslips, loaded with the pH-sensitive fluorescent dye BCECF, and their fluorescence intensity monitored as an indication of pH(i). Cells were acidified with NH(4)Cl (25 mM, 1 min) and pH(i) recovery recorded. In the absence of HCO(3)(-), initial recovery was 0.208 +/- 0.016 pH units min(-1) (n = 37). This was almost abolished by removal of extracellular Na(+) and by amiloride (1 mM), consistent with the activity of a Na(+)-H(+) exchanger (NHE). In the presence of HCO(3)(-) and CO(2), recovery (0.156 +/- 0.018 pH units min(-1)) was abolished (reduced by 91.8 +/- 6.7 %, n = 7) by removal of Na(+) but only attenuated (by 63.3 +/- 5.8 %, n = 9) by amiloride. 4,4-Dinitrostilbene-2,2-disulfonic acid (DNDS) inhibited recovery by 45.8 +/- 5.0 % (n = 7). The amiloride-insensitive recovery was insensitive to changes in membrane potential, as confirmed by direct microelectrode measurements, brought about by changing extracellular [K(+)] in the presence of either valinomycin or the K(+) channel opener 1-EBIO. In addition, forskolin (10 microM), which activates the cystic fibrosis transmembrane conductance regulator Cl(-) conductance in these cells and depolarises the cell membrane, had no effect on recovery. Removal of extracellular Cl(-) trebled pH(i) recovery rates, suggesting that an electroneutral, DNDS-sensitive, Cl(-)-HCO(3)(-) exchanger together with a NHE may be involved in pH(i) regulation and HCO(3)(-) secretion in these cells. RT-PCR detected the expression of the electrogenic Na(+)-HCO(3)(-) cotransporter NBC1 and the Cl(-)-HCO(3)(-) exchanger (AE2) but not the electroneutral Na(+)-HCO(3)(-) cotransporter NBCn1.

Hypoxic activation of an amiloride-sensitive cation conductance in alveolar epithelial cells

Imposing hypoxia (P(O(2)) = 23 mmHg) upon A549 cells elicited increased G(amil) although previous work had predicted a fall in this parameter. G(amil) appeared to be dependent upon glucocorticoid-driven gene expression, a process inhibited by ERK, an enzyme activated by oxidative stress. However, hypoxia transiently activated this enzyme and the response was blocked by glucocorticoids, showing that the rise in G(amil) occurs only if ERK activation is suppressed. Fluorimetric assays showed that lowering P(O(2)) elicited H(2)O(2) formation indicating that this maneuver actually imposes oxidative stress, thus explaining how hypoxia can elicit responses normally associated with a rise in P(O(2)).

Oxygen-evoked Na+ transport in rat fetal distal lung epithelial cells

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 -ENaC (epithelial Na+ channel -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 -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.

NF-kappaB blockade reduces the O2-evoked rise in Na+ conductance in fetal alveolar cells

Electrophoretic mobility shift assays revealed minimal levels of NF-kappaB activity in rat distal lung epithelial cells cultured at fetal (23 mmHg) or adult alveolar (100 mmHg) P(O2), but revealed significant activation of this transcription factor in cells exposed to a rise in P(O2) mimicking that experienced at birth. This response was entirely abolished by pretreating cells with 5 mM sulfasalazine (SSA). This shift in P(O2) also evoked a rise in apical Na+ conductance (G(Na+)) that may underlie the O2-evoked stimulation of Na+ transport seen in these cells. Pretreatment with SSA had no effect upon G(Na+) in cells cultured continually at adult or fetal P(O2) but did inhibit the increase in G(Na+) seen in cells that had experienced the rise in P(O2). O2-evoked activation of NF-kappaB may thus mediate the increased Na+ transport that occurs when the distal lung epithelial cells are exposed to a physiologically-relevant increase in P(O2).

Antioxidant/pro-oxidant equilibrium regulates HIF-1alpha and NF-kappa B redox sensitivity. Evidence for inhibition by glutathione oxidation in alveolar epithelial cells

The O(2) and redox-sensitive transcription factors hypoxia inducible factor-1alpha (HIF-1alpha) and nuclear factor-kappaB (NF-kappaB) are differentially regulated in the alveolar epithelium over fetal to neonatal oxygen tensions. We have used fetal alveolar type II epithelial cells to monitor their regulation in association with redox responsiveness to antioxidant pretreatment in vitro. N-Acetyl-l-cysteine, a glutathione (GSH) precursor and a potent scavenger of reactive oxygen species, induced HIF-1alpha and ameliorated NF-kappaB nuclear abundance and DNA binding activity, respectively, in a dose-dependent manner. Analysis of variations in glutathione homeostasis at ascending DeltapO(2) regimen with N-acetyl-(L)-cysteine reveals increased GSH at the expense of the oxidized form of glutathione (GSSG), thereby shifting GSH/GSSG into reduction equilibrium. Pyrrolidine dithiocarbamate (PDTC), which exerts both antioxidant and pro-oxidant effects, provoked a substantial increase in HIF-1alpha nuclear abundance, with no apparent effect on its activation. PDTC reduced NF-kappaB nuclear abundance and its inhibitory effects on binding activity are dose-dependent. Assessment of glutathione homeostasis with PDTC shows increasing levels of GSSG at the expense of GSH, lowering GSH/GSSG in favor of an oxidative equilibrium. Our results indicate the hypoxic activation of HIF-1alpha and the hyperoxic induction of NF-kappaB in the fetal epithelium is redox-sensitive and, thus, tightly regulated by the GSH/GSSG equilibrium. This highlights glutathione as a key regulatory component for determining genetic responsiveness to oxidant/antioxidant imbalance in normal lung development and pathophysiological conditions.

Differential effects of UTP and ATP on ion transport in porcine tracheal epithelium

Isolated segments of porcine tracheal epithelium were mounted in Ussing chambers, current required to maintain transepithelial potential difference at 0 mV (short circuit current, I(SC)) was monitored and effects of nucleotides upon I(SC) were studied. Mucosal UTP (100 microM) evoked a transient rise in I(SC) that was followed by a sustained fall below basal I(SC) maintained for 30 min. Mucosal ATP (100 microM) also stimulated a transient rise in I(SC) but in contrast to UTP did not inhibit basal I(SC). Submucosal UTP and ATP both transiently increased I(SC). UTP-prestimulated epithelia were refractory to ATP but prestimulation with ATP did not abolish the response to UTP. The epithelia thus appear to express two populations of apical receptors allowing nucleotides to modulate I(SC). The UTP-induced rise was reduced by pretreatment with either bumetanide (100 microM), diphenylamin-2-carboxylic acid (DPC, 1 mM), or Cl(-) and HCO(3)(-)-free solution whilst the fall was abolished by amiloride pretreatment. Thapsigargin (0.3 microM) abolished the UTP-induced increase in I(SC) but not the subsequent decrease. Staurosporine (0.1 microM) inhibited basal I(SC) and blocked UTP-induced inhibition of I(SC). Inhibitors of either protein kinase C (PKC) (D-erythro sphingosine) or PKA (H89) had no effect. This study suggests that UTP stimulates Cl(-) secretion and inhibits basal Na(+) absorption. ATP has a similar stimulatory effect, which may be mediated by activation of P2Y(2) receptors and an increase in [Ca(2+)](in), but no inhibitory effect, which is likely mediated by activation of a pyrimidine receptor and possible inhibition of a protein kinase other than PKC or PKA.

The effects of PO2 upon transepithelial ion transport in fetal rat distal lung epithelial cells

1. Isolated rat fetal distal lung epithelial (FDLE) cells were cultured (for 48 h) at PO2 levels between 23 and 142 mmHg. Higher PO2 levels between 23 and 142 mmHg. Higher PO2 was associated with increased short circuit current (ISC) and increased abundance of the Na+ channel protein alpha-ENaC. PO2 had no effect upon ISC remaining after apical application of amiloride (10 microM). 2. Studies of cells maintained (for 48 h) at PO2 levels of 23 mmHg or 100 mmHg, and subsequently nystatin permeabilized (50 microM), showed that high PO2 increased Na+ pump capacity. This response was apparent 24 h after PO2 was raised whilst it took 48 h for the rise in ISC seen in intact cells to become fully established. Both parameters were unaffected by raising PO2 for only 30 min. 3. Basolateral application of isoprenaline (10 microM) did not affect ISC in cells maintained at 23 mmHg but evoked progressively larger responses at higher PO2. The response seen at 142 mmHg was larger than at 100 mmHg, the normal physiological alveolar PO2. 4. Isoprenaline had no effect on Na+ pump capacity at PO2 levels of 23 mmHg or 100 mmHg, but stimulated Na+ extrusion at 142 mmHg. Increasing PO2 above normal physiological levels thus allows the Na+ pump to be controlled by isoprenaline. This may explain the enhanced sensitivity to isoprenaline seen under these slightly hyperoxic conditions. 5. Changes in PO2 mimicking those occurring at birth thus exert profound influence over Na+ transport in FDLE cells and the Na+ pump could be an important locus at which this control is exercised.

The influence of mode of delivery, hormonal status and postnatal O2 environment on epithelial sodium channel (ENaC) expression in perinatal guinea-pig lung

We have studied factors that potentially modulate the expression of mRNA coding for subunits of the amiloride-sensitive sodium channel, alphaENaC and betaENaC, in lungs of vaginally and Caesarean (CS)-delivered late gestation fetal guinea-pigs. Expression of alphaENaC and betaENaC mRNAs was developmentally regulated in the late gestation fetus, reaching peak levels at term (68 days post conception, PC) and postnatally, respectively. In animals delivered by CS at 65 days PC and term, alphaENaC mRNA expression was significantly increased by day 1 post partum, reaching levels greater than those normally achieved in vaginally delivered animals at term. In contrast, betaENaC mRNA levels remained significantly lower postnatally in animals delivered by CS at 65 days PC compared with those in vaginally and CS-delivered animals at term. Plasma cortisol and total triiodothyronine (T3) levels increased towards term, were higher 1 day after vaginal delivery but declined towards pre-term levels by day 3. Cortisol levels also increased rapidly in the CS-delivered animals, reaching levels similar to those in vaginally delivered animals at day 1. Plasma T3 levels at days 1 and 3 were significantly lower in animals delivered by CS at 65 days PC. The increase in alphaENaC mRNA paralleled the increase in plasma cortisol after delivery, but not T3, and inhibition of cortisol synthesis with 2-methyl-1,2-di-3-pyridyl-1-propanone (metyrapone) after CS delivery suppressed the increase in alphaENaC mRNA expression. Concomitant with the increase in alphaENaC mRNA expression after CS delivery at 65 days PC was an increase in the amiloride-blockable component of lung fluid clearance by day 3 postnatally. We conclude that in late gestation guinea-pigs delivered by CS there is a significant increase in lung alphaENaC expression postnatally, which is mediated, in part, by the postnatal rise in cortisol at delivery. This in turn leads to an increase in amiloride-sensitive lung fluid clearance, which is unrelated to labour.

Effect of luminal nucleotides on Cl- secretion and Na+ absorption in distal bronchi

P2Y receptor agonists stimulate Cl- secretion across both normal and cystic fibrosis (CF) airway epithelia, and therefore have potential for use in the treatment of CF. Although CF pathology is manifest primarily in the distal airways, most studies of P2Y-receptor-mediated airway epithelial ion transport have used cells cultured from proximal regions. Here we report the results of studies of P2Y-receptor-mediated ion transport in distal bronchi isolated from porcine lungs, cannulated and perfused. A luminal microelectrode was used to record transepithelial potential difference (PD) and cable analysis was applied to determine resistance (Rt) and equivalent short-circuit current (I(SC)). Luminal UTP (100 micromol/l) transiently hyperpolarized PD (from -5.8+/-0.3 to -6.5+/-0.4 mV) and increased I(SC) (from 47+/-6 to 55+/-8 microA cm(-2)) before inhibiting PD to below the pre-UTP level (-5.0+/-0.4 mV). The decline was attenuated by pretreatment with amiloride, and additional treatment with bumetanide inhibited the initial hyperpolarization, suggesting that UTP stimulates Cl- secretion and inhibits Na+ absorption across distal bronchi. Luminal addition of P2Y1 [ADP, 2-methylthio-ATP (2MeSATP)] and P2Y6 (UDP) receptor agonists had no effect on ion transport. Pretreatment with thapsigargin (0.3 micromol/l) did not prevent the UTP-induced increase in PD and I(SC) but attenuated the secondary fall in PD. Pretreatment with BAPTA/AM (50 micromol/l), however, had no effect on the response to UTP. Additional studies of isolated bronchial epithelial cells using Fura-2 showed that UTP increases [Ca2+]in, and this increase is blocked by pretreatment with thapsigargin. These results suggest that in intact distal bronchi luminal UTP stimulates Cl- secretion by a Ca2+-independent mechanism and inhibits Na+ absorption by a Ca2+-dependent mechanism. Both effects are likely to favour increased hydration of the airway surface, and may therefore be beneficial in CF.

P2Y2 receptor-mediated inhibition of ion transport in distal lung epithelial cells

1 Rat foetal distal lung epithelial cells were plated onto permeable supports where they became integrated into epithelial sheets that spontaneously generated short circuit current (ISC). 2 Apical ATP (100 microM) evoked a transient fall in ISC that was followed by a rise to a clear peak which, in turn, was succeeded by a slowly developing decline to a value below control. Apical UTP evoked an essentially identical response. 3 UDP and ADP were ineffective whilst ATP had no effect when added to the basolateral solution. These effects thus appear to be mediated by apical P2Y2 receptors. 4 The rising phase of the responses to ATP/UTP was selectively inhibited by anion transport inhibitors but persisted in the presence of amiloride, which abolished the inhibitory effects of both nucleotides. Thus, apical nucleotides appear to evoke a transient stimulation of anion secretion and sustained inhibition of Na+ absorption. 5 Basolateral isoprenaline (10 microM) elicited a rise in ISC but subsequent addition of apical ATP reversed this effect. Conversely, isoprenaline restored ISC to its basal level following stimulation with ATP. Apical P2Y2 receptors and basolateral beta-adrenoceptors thus allow their respective agonists to exert mutually opposing effects on ISC.

Inwardly rectifying K+ currents in fetal alveolar type II cells: regulation by protein kinase A and protein phosphatases

Fetal guinea-pig lung alveolar type II (ATII) cells have inwardly rectifying (IR) K+ currents that display Mg2+- and G-protein-dependent run-down. We have used the whole-cell patch-clamp technique to investigate further the regulation of these currents. Under control conditions [KCl-rich pipette solution (1 mM free Mg2+, 10 nM free Ca2+) and KCl-rich bath solution], we found that IR K+ currents diminished with a t1/2 of 7.6 min and were absent by 30 min. Experimental manoeuvres designed to inhibit phosphorylation increased the rate of current run-down. Thus, intracellular addition of 100 microM H-7, a general kinase inhibitor, reduced the t1/2 to 4.7 min and the currents were absent by 16 min. Similarly, protein kinase A (PKA) inhibitor peptide (50 nM) also accelerated run-down. Agents known to increase phosphorylation, such as db-cAMP (0.5 mM) and forskolin (10 microM), resulted in a significant slowing of run-down (t1/2>16 min) as did intracellular addition of the catalytic subunit of PKA (100 nM). Similarly, inhibition of dephosphorylation by either 1 microM okadaic acid [protein phosphatase 1/2A (PP-1/2A) inhibitor] or anti-human protein phosphatase 2Calpha (PP2C) antiserum decreased the rate of run-down. These results indicate that the phosphorylation-dependent activation state of the fetal ATII cell IR K+ channel is regulated by a complex interplay of kinases and phosphatases involving PKA (activation), and PP2C and PP-1/2A (inactivation).

Perinatal PTX-sensitive G-protein expression and regulation of conductive 22Na+ transport in lung apical membrane vesicles

Using apical membrane vesicles (AMV) prepared from mature foetal and early neonatal guinea pig lung we show that pertussis toxin (PTX)-sensitive G-protein regulation of conductive 22Na+ uptake undergoes rapid changes following birth. Thus, G-protein activation by intravesicular incorporation of 100 microM GTPgammaS into vesicles resuspended in NaCl, which in late gestation stimulated uptake, consistently induced inhibition of conductive Na+ uptake into AMV prepared from neonatal lung at 4 days of age (N4) (52+/-9%, n=8, P<0.05). This response was not significantly different in the presence of the relatively impermeant anion isethionate (Ise-) (69+/-9%, n=7, P<0.05). Changes in the regulation of uptake were already detectable on the day of birth (N0) in AMV resuspended in NaCl, with GTPgammaS inducing both stimulatory and inhibitory responses. These data indicate that the processes by which 22Na+ uptake into AMV is regulated by G-proteins undergoes a change at birth and by 4 days of age, G-protein regulation of uptake occurs predominantly via modulation of co-localised Na+ channels. Intravesicular incorporation of GDPbetaS or pre-treatment with PTX did not significantly alter conductive 22Na+ uptake in the presence of NaCl or NaIse suggesting that constitutively active G-proteins are not involved in this process. Pre-treatment of AMV with PTX prevented the inhibition of conductive 22Na+ uptake by GTPgammaS (105+/-16% n=7) indicating that a PTX-sensitive G-protein mediates the inhibition of channels in neonatal AMV. Western blotting demonstrated enrichment of Gialpha1, Gialpha2, Gialpha3 and Goalpha in the apical membrane preparations. We also show that there is a significant rise in the levels of Gialpha3 during the early neonatal period providing a potential candidate for the G-protein mediated changes in regulation of conductive 22Na+ uptake in neonatal AMV.

Systematic review of antistaphylococcal antibiotic therapy in cystic fibrosis

BACKGROUND

The respiratory tract in patients with cystic fibrosis is frequently colonised with Staphylococcus aureus. There is great diversity of clinical practice in this area of cystic fibrosis. A systematic review was conducted to study the evidence relating antistaphylococcal therapy to clinical outcome in patients with cystic fibrosis.

METHODS

A search strategy already evaluated for the study of the epidemiology of cystic fibrosis clinical trials was used. This yielded 3188 references from which 13 clinical trials of antistaphylococcal therapy were identified.

RESULTS

Substantial heterogeneity was observed between trials. In the 13 clinical trials a total of 19 antibiotics were used to assess a wide variety of outcome measures (11 clinical, six laboratory). Both intermittent and continuous treatment strategies were used. Sputum clearance of S aureus was more frequently achieved than any other beneficial outcome. A beneficial effect on pulmonary function was rarely measured or observed. Although five randomised clinical trials were identified, the extent of heterogeneity precluded the use of meta-analysis for further synthesis of information.

CONCLUSIONS

Antistaphylococcal treatment achieves sputum clearance of S aureus in patients with cystic fibrosis. Prophylactic antistaphylococcal treatment in young children with cystic fibrosis is likely to be of clinical benefit. It remains to be determined whether the use of "prophylactic" versus "intermittent" antistaphylococcal therapy in cystic fibrosis is associated with improved lung function and/or chest radiographic scores, an increase in bacterial resistance, or earlier acquisition of Pseudomonas aeruginosa. A large randomised clinical trial lasting approximately two years is urgently required to address this problem.

Differential regulation of Na+ and Cl- conductances by PTX-sensitive G proteins in fetal lung apical membrane vesicles

In apical membrane vesicles (AMV) prepared from late gestation fetal guinea pig lung we show that conductive 22Na+ uptake is modulated by at least two pathways involving pertussis toxin (PTX)-sensitive G proteins. Intravesicular incorporation of 100 microM GTPgammaS into vesicles resuspended in NaCl caused a significant stimulation (P<0. 05) of conductive Na+ uptake in AMV to 150+/-10% (n=10) of control, whereas GDPbetaS reduced uptake to 65+/-9% (n=4) of control. This contrasting response to GTPgammaS and GDPbetaS is characteristic of a G protein mediated pathway. GTPgammaS induced a significantly smaller stimulation, 125+/-8% (n=5) of control, in the presence of the relatively impermeant anion isethionate (Ise-). Taken together, these data indicate modulation of both Na+ and Cl- channels in the apical membrane by co-localised G protein(s). Treatment with PTX stimulated conductive 22Na+ uptake to 171+/-20% (n=13) of control in AMV resuspended in NaCl, but did not have a significant effect, 94+/-19% of control, in the presence of NaIse indicating the existence of tonic activation of Cl- channels in these AMV under resting conditions. As the combined effects of PTX and GTPgammaS diminished uptake, we propose that the G protein(s) responsible for Na+ channel activation in response to GTPgammaS is PTX-sensitive and that additional PTX-insensitive G proteins might also modulate 22Na+ uptake in these AMV. The presence of Gialpha1, Gialpha2, Gialpha3 and Goalpha in this apical membrane preparation was confirmed by PTX catalysed [32P]ADP-dependent ribosylation and Western blotting. Incubation of AMV with 200 microM DTT caused an inhibition of conductive Na+ uptake in AMV resuspended in NaCl or NaIse to 66+/-8% (n=11) and 64+/-8% (n=6) of control respectively. Pre-treatment with DTT did not affect the ability of GTPgammaS to stimulate conductive Na+ uptake suggesting that the regulation of 22Na+ uptake in late gestation guinea pig fetal lung AMV is unlikely to involve an associated regulatory protein.

Culture substrate-specific expression of P2Y2 receptors in distal lung epithelial cells isolated from foetal rats

ATP and UTP did not evoke [Ca2+]i signals in rat foetal lung epithelial cells grown on glass but elicited clear responses in cells grown into functionally polarised epithelia on permeable supports. Moreover, P2Y2 receptor mRNA could not be detected in cells on glass by the polymerase chain reaction but this mRNA species was clearly expressed by polarised cells. P2Y2 receptor expression thus appears to be a feature of the polarised phenotype.

G protein-coupled prostaglandin receptor modulates conductive Na+ uptake in lung apical membrane vesicles

Because G protein-regulated cation channels in type II pneumocytes constitute the most likely pathway for alveolar Na+ entry, we explored the hypothesis that a G protein-coupled prostaglandin (PG) E2 receptor controls perinatal lung alveolar Na+ transport. [3H]PGE2 binding to the alveolar apical membrane was trypsin sensitive and showed a rank order of competitive inhibition: PGE2 = PGE1 > PGD2 > PGF2 alpha. Kinetic analysis demonstrated both high-affinity [dissociation constant (KD) = 2.1 +/- 0.7 nM; maximal binding (Bmax) = 27 +/- 7 fmol/mg protein] and low-affinity (KD = 28 +/- 2 nM; Bmax = 265 +/- 29 fmol/mg protein) binding sites. Modulation of high-affinity GTPase activity identified a similar potency order (IC50 = 11 mM for PGF2 alpha vs. 10-50 microM for other PGs), suggesting that the receptors are G protein coupled. Finally, 1 microM PGE2 (approximately IC25) increased conductive 22Na+ uptake into membrane vesicles only in the presence of 100 microM intravesicular GTP. The KD value for the high-affinity binding site together with the rank order of PG effect on ligand binding and G protein function places this PG receptor in the EP3 subtype, whereas Na+ uptake studies suggest that it helps maintain perinatal lung Na+ homeostasis.

Direct modulation of G-proteins by polyunsaturated fatty acids: a novel eicosanoid-independent regulatory mechanism in the developing lung

Basal and fatty-acid-modulated G-protein function was studied in 1-3-day-pre-term, fetal guinea-pig, type II (fATII) pneumocyte apical membrane. Unstimulated (tonic) high-affinity GTPase activity (measured as [gamma-32P]GTP hydrolysis rate) was high and 77% pertussis toxin-insensitive. Alteration of this activity was used as a marker of G-protein regulation. Arachidonic acid (AA) showed a dose-dependent (IC50 = 48+/-8 microM) inhibition of activity at concentrations significantly below critical micellar concentrations; this effect was mimicked by other polyunsaturated fatty acids (IC50 for linoleic acid = 47 +/- 2 microM; IC50 for oleic acid = 106 +/- 11 microM). Saturated fatty acids showed no effect. The effect of AA on ouabain-insensitive ATPases in the same preparation was significantly lower, suggesting a specificity of the GTPase modulation effect. AA modulation of GTPase activity was not attenuated by blocking eicosanoid metabolism with inhibitors of 5'-lipoxygenase, cyclo-oxygenase and P-450. In order to explore further the mechanism of AA-G-protein interaction, the effect of AA on the time course and equilibrium binding of [35S]GTP[S] to apical membrane was studied. Consistent with our GTPase assay data, AA inhibited binding with an IC50 value of 71+/-1 microM; stearic acid did not mimic this effect. This is the first report of unsaturated-fatty-acid-specific modulation of lung G-protein function: since AA also up-regulates perinatal lung alveolar Na+ transport, we suggest this lipid/G-protein switch helps maintain pulmonary fluid homoeostasis around birth.

What is core? Guidelines for the core curriculum in paediatrics

A key point that the UK General Medical Council addressed in its recommendations on the undergraduate medical education was the concept of 'core curriculum' (General Medical Council 1993). Although enthusiastic for the idea of reducing factual overload, many medical teachers found themselves facing the task of how to define what a core curriculum is, what should be included and why. Predictably, our initial response is to include common and important topics, but how common is common, and how does one determine the relative importance of topics? We do not claim to have unravelled all the ambiguities surrounding the subject nor to have resolved all the controversies that are inevitably encountered. We hope, however, to describe some of the principles that governed our approach and put forward some guidelines, that may contribute to the debate.

Conductive Na+ transport in fetal lung alveolar apical membrane vesicles is regulated by fatty acids and G proteins

We have characterised G protein and fatty acid regulation of the Na+ conductance in purified apical membrane vesicles prepared from late gestation fetal guinea-pig lung. Addition of 100 microM GTP gamma S or beta gamma-methylene-GTP, irreversible G protein activators, stimulated conductive 22Na+ uptake (ratio of experimental to control 1.35 +/- 0.02 and 1.34 +/- 0.05, respectively). Conversely, the addition of GDP beta S, an irreversible G protein inhibitor, reduced conductive 22Na+ uptake from 1.00 (control) to 0.79 +/- 0.04. A range of saturated (myristic, palmitic, stearic), monounsaturated (elaidic, oleic) and polyunsaturated (linoleic, arachidonic) fatty acids all stimulated conductive 22Na+ uptake, by between 1.18 +/- 0.05 to 1.56 +/- 0.13 over the control. Both arachidonic acid and GTP gamma S-dependent stimulation were abolished in the presence of 10 microM amiloride. The non-metabolisable analogue of arachidonic acid, eicosa-5,8,11,14-tetraynoic acid also stimulated conductive 22Na+ uptake. Furthermore, addition of indomethacin and nordihydroguairetic acid, inhibitors of cyclooxygenase and lipoxygenase pathways of arachidonate metabolism respectively, did not affect the arachidonic acid stimulation suggesting a direct effect of fatty acid upon the Na+ channel Since mepacrine (50 microM), a phospholipase A2 inhibitor, did not affect the GTP gamma S-stimulated conductive 22Na+ uptake, and inhibition of G protein turnover by GDP beta S did not attenuate the arachidonic acid response we conclude that these two regulatory pathways modulate alveolar Na+ transport directly and independently of each other.

Inwardly rectifying K+ currents of alveolar type II cells isolated from fetal guinea-pig lung: regulation by G protein- and Mg2+-dependent pathways

K+ currents in alveolar type II cells, isolated from fetal guinea-pig lung, were studied using the whole-cell patch-clamp technique. Inwardly rectifying (IR) K+ currents were observed when cells were bathed in symmetrical KCl-rich solutions. When extracellular K+ was replaced by Na+, inward currents were greatly decreased and the zero-current potential moved from 0 mV to -69 mV, indicating high K+ selectivity. In recordings with an intracellular KCl-rich solution, containing 1.12 mM Mg2+ and 10(-8 )M free Ca2+, IR K+ currents slowly diminished with time. Addition of the irreversible G protein activator, guanosine 5'-O-(3-thiotriphosphate) (GTP [gamma-S]), to the intracellular solution accelerated the rate of current run-down. In experiments where the intracellular solution was Mg2+ free, current run-down was abolished. The rate of current run-down was found to increase with increasing free intracellular [Mg2+]. Raising the intracellular free [Ca2+] to 10(-6 )M under Mg2+-free conditions had no effect on the K+ currents. Extracellular Ba2+ blocked the IR K+ currents in a concentration- and voltage-dependent manner. Tolbutamide, a blocker of ATP-sensitive K+ (KATP) channels, had no effect on the currents. The single channel underlying the whole-cell IR K+ currents displayed inward rectification and had a conductance of 31 pS in symmetrical KCl-rich solutions. We demonstate that mRNA coding for IRK1 is expressed in this cell preparation. Possible functions for this channel are discussed.

G protein regulation of alveolar ion channels: implications for lung fluid transport

Vectorial ion flow across the alveolar epithelium provides the driving force for lung fluid secretion in the prenatal lung and for fluid reabsorption at birth and thereafter into adult life. Fluid secretion is dependent upon 'active' accumulation of Cl- into the lumen of the developing lung and any factor which interupts the production of this liquid template results in life-threatening abnormalities in lung growth. The direction of fluid flow is reversed at birth to bring about the reabsorption of the lung fluid so that gaseous exchange can be initated successfully in the neonate. This functional switch at birth involves active Na+ reabsorption; failure of this mechanism to activate adequately contributes to respiratory distress in the newborn. Although beta 2-adrenoceptor-cAMP-protein kinase activation provides a basic model for the switch, it is clear that local regulation of many of the components in the switching cascade are important to the overall efficiency with which it is achieved. Co-localized with apical Cl- and Na+ channels are pertussis toxin-sensitive G proteins, which exert their regulatory effects principally by direct protein-channel interactions. The modulation of channels by local G proteins is finely tuned by negative feedback mechanisms, which may include a novel double-bond specific fatty acid regulation of G protein turnover and the paracrine effects of locally produced eicosanoids. An understanding of how these overlapping pathways integrate to produce the smooth and ordered transition from Cl- secretion to Na+ absorption will allow the design of rational treatments for conditions that are characterized by disfunctional lung fluid homeostasis.

Nebulised antipseudomonal antibiotic therapy in cystic fibrosis: a meta-analysis of benefits and risks

BACKGROUND

To establish the benefits and risks of nebulised antipseudomonal therapy in cystic fibrosis the results of relevant randomised controlled trials were combined.

METHODS

The therapeutic end points compared were (a) number of pulmonary exacerbations requiring treatment with systemic antibiotics, (b) measurable alteration in respiratory tract pseudomonal load, (c) alteration in lung function on spirometric assessment, (d) development of resistance in respiratory tract Pseudomonas strains to the nebulised antipseudomonal used in each randomised controlled trial, and (e) renal and auditory impairment.

RESULTS

Five studies were suitable for meta-analysis, eight others could not be included because of inadequate outcome description or the lack of appropriate randomisation. Meta-analysis shows benefit for nebulised antipseudomonal antibiotic therapy with no demonstrable adverse effect other than a possible increase in in vitro antibiotic resistance of Pseudomonas aeruginosa of the respiratory tract.

CONCLUSIONS

Although inferences drawn from individual randomised controlled trials concerning the benefits and risks of this form of therapy are conflicting, pooled effect size establishes benefit with nebulised antipseudomonal antibiotic therapy and emphasises its relevance to the integration of information in other areas of controversy relating to the treatment of this disease.

Conductive cation transport in apical membrane vesicles prepared from fetal lung

In order to characterise the apically-located conductive cation pathway of the type II pneumocyte, apical plasma membranes were prepared from mature fetal guinea pig lung. The protocol yielded purified apical membranes that enriched 19-fold with the brush border enzyme marker alkaline phosphatase; there was no significant contamination with other cellular membranes. A technique for imposing an outwardly-directed electrochemical Na+ gradient was used to amplify conductive 22Na+ uptake into vesicles. Uptake of 22Na+ was time-dependent, proportional to the magnitude of the Na+ gradient, specific and sensitive to the amiloride analogues phenamil and EIPA (apparent minimum Ki values of 50 nM and 10 microM, respectively, with maximum uptake inhibition of 42% and 39% at 100 microM). Uptake experiments in which the outwardly-directed Na+ gradient was replaced by outwardly-directed gradients of small monovalent cations and molecular cations were performed. The Na+/K+ permeability ratio was 1.2:1, and over the extended range of small monovalent cations, a permeability sequence of Na+ > K+ > Li+ > Rb+ > Cs+ was observed, indicating the presence of fixed negative charge in or spatially close to the pore. The molecular cation permeability sequence of NH4+ > methylamine+ > dimethylamine+ > choline+ > N-methyl-D-glucamine+ > tetraethylammonium+ > tetramethylammonium+, after transformation, gives an estimate of 8 A for the conducting pore diameter. These data are consistent with the presence in the apical membrane of fetal type II pneumocytes of a cation specific channel with low Na+ selectivity and amiloride sensitivity.

Amiloride-sensitive Na+ channels in fetal type II pneumocytes are regulated by G proteins

We have used the patch-clamp technique to record single channels in excised membrane patches from type II pneumocytes freshly isolated from fetal guinea pig lung by elastase digestion and differential filtration. The 10/56 patches exhibited spontaneous channel activity with a mean open-state probability (NPo) of 0.5 +/- 0.1. In symmetrical Na(+)-rich solutions, the channels had a unitary conductance of 11.1 +/- 0.5 pS and showed current reversal at approximately 0 mV. Superfusing the inner membrane leaflet of the patch with a K(+)-rich solution resulted in single-channel current activity with a conductance of 5.6 +/- 0.2 pS being resolved. Current reversed at +22.1 +/- 1.9 mV, which is compatible with a PNa+/PK+ of 1.8 +/- 0.1. The addition of 0.1 mM guanosine 5'-O-(3-thiotriphosphate) to the cytoplasmic face of the patch elicited channel activity in 12/31 previously quiescent patches, whereas, in spontaneously active patches, channel NPo was increased. Amiloride, in the concentration range 0.4-4 microM, reduced the frequency of observed spontaneous (or activatable) channel activity, reduced NPo, and induced flickery channel behavior. No activity was seen in the presence of 10 microM amiloride in the pipette. This is the first direct observation of a G protein regulated Na(+)-conductive pathway in alveolar epithelium, and it may represent one route by which the alveolar epithelium of the fetus can regulate the Na(+)-driven fluid reabsorption necessary for the adaptation of the newborn lung to air breathing at birth.

G protein-regulated large-conductance chloride channels in freshly isolated fetal type II alveolar epithelial cells

Using the patch-clamp technique, we have recorded single channels in cell-attached and inside-out excised patches from the plasma membrane of type II alveolar epithelial cells freshly isolated from fetal guinea pig lung by elastase digestion and differential filtration. In cell-free patches the channels were highly selective for Cl- (PCl:Pcat = 9:1), had a large unitary conductance (375 pS +/- 23 pS), and current reversal of 0 mV in either symmetrical Na(+)-rich solutions or when the inner membrane leaflet was bathed in a K(+)-rich solution. The large-conductance Cl- channel exhibited little or no voltage inactivation at positive potentials, remained open for a significant amount of time at potentials negative to -40 mV, and was blocked at all potentials by 0.1 mM 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid. Channel activity was independent of intracellular calcium concentration. Bath addition of the nonmetabolizable analogue of GTP, GTP gamma S (0.1 mM), caused a voltage-dependent inhibition of channel activity [open probability (Po) plot was shifted by at least +25 mV]. Smaller channels (25 +/- 3 pS) were recorded in the cell-attached configuration with a current-voltage (I-V) relationship which was compatible with a Cl- conductance. On excision, the patches previously containing small-conductance channels exhibited only large-conductance Cl- channel behavior. These large-conductance, G protein-regulatable Cl- channels may provide a route for alveolar cell Cl- exit and as such may be an integral part of the mechanism responsible for secretion of fetal lung fluid.

Effect of benzamil on sheep tracheal epithelium

Apical membrane potential (Va), voltage divider ratio (f), transepithelial potential (Vt) and transepithelial resistance (Rt) were measured in isolated sheep tracheal epithelium. Individual membrane resistances and electromotive forces were calculated by a method described in the text. Whereas Va hyperpolarized and fa increased in tissues acutely exposed to amiloride or benzamil, the values for these parameters in tissues bathed for 1 h with mucosal benzamil (0.38 microM) were not significantly different from those found in control tissues. Circuit analysis revealed a higher value for both apical and basolateral membrane resistances in the benzamil-treated tissues. These results suggest a decrease in the basolateral potassium conductance secondary to the initial decrease in the apical sodium conductance, thereby ensuring that the electrical driving force for other ions across the apical membrane is restored to normal in the face of a hyperpolarizing perturbation to the apical membrane.

Sodium-proton exchange across the apical membrane of the alveolar type II cell of the fetal sheep

In order to detect and characterise Na(+)-H+ countertransport in the fetal lung epithelium we have studied under a variety of conditions the effect of an outward facing H+ gradient on Na+ uptake into purified apical membrane vesicles prepared from alveolar type II cells. Kinetic analysis of the data reveals both a diffusional and a saturable component of total Na+ uptake. Evidence for the presence of a Na(+)-H+ exchanger is demonstrated by (1) stimulation of Na+ uptake by proton loading of vesicles both in the presence and absence of chemical voltage clamping; (2) saturation kinetics with respect to external Na+ with a Km of 16 mM and a Vmax of 2.1 nmol/mg protein per min; (3) amiloride inhibition of Na+ uptake driven by pH gradient. We conclude that although diffusion may be the major component of total Na+ uptake at physiological external Na+ concentration, Na(+)-H+ countertransport provides a possible mechanism for the acidification of fetal lung liquid in-vivo in addition to its established role in intracellular pH and volume regulation.

Dibutyryl cAMP induces a gestation-dependent absorption of fetal lung liquid

The maturation of the adenosine 3',5'-cyclic monophosphate-(cAMP) dependent pathway controlling fetal lung liquid secretion was examined in experiments in which the lungs of chronically catheterized fetal lambs (123-141 days gestational age) were exposed to dibutyryl cAMP (DBcAMP, 10(-4) M). The effect of DBcAMP was markedly gestation dependent, with the greatest effect observed in the most mature fetuses. In immature fetuses (less than 130 days, mean age 125 days) DBcAMP caused slowing of secretion, with maximal effect at 5 h. With increasing maturity the effect of DBcAMP was more pronounced and occurred earlier so that in mature fetuses (mean age 140 days) lung liquid absorption took place, with maximal effect at 2 h. Changes in lung liquid volume flow induced by DBcAMP could be blocked by addition of 10(-4) M amiloride to lung liquid. It is concluded that 1) DBcAMP induces a change in lung liquid secretion that, like epinephrine, is mediated via an increase in Na+ permeability of the apical membrane of the lung epithelium and 2) the rate-limiting step in the maturation of this process must lie beyond the generation of intracellular cAMP.

Ionic permeabilities of the cell membranes of sheep tracheal epithelium

1. The ionic permeabilities of the apical and basolateral membranes of sheep tracheal epithelial cells have been evaluated by studying the effect of ion replacements on the apical (Va) and basolateral (Vb) membrane potentials and apical voltage divider ratio (fa). 2. The results provide evidence for the existence of conductances for Cl-, Na+ and K+ at the apical membrane, and of an important K+ conductance at the basolateral membrane. These characteristics agree with the model for Cl- secretion and Na+ absorption proposed for tracheal epithelium of other species. 3. The short-circuit current (Isc) across this tissue was found to increase significantly when HCO3- and CO2 were added to the bathing solutions. The addition of 0.5 mM-SITS (4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid) to the serosal bathing solution decreased Isc only if HCO3- and CO2 were present in the bathing solutions. These results suggest the existence of a HCO3- secretion in sheep tracheal epithelium under short-circuit conditions. 4. The results obtained in HCO3- replacement studies are compatible with an electrically silent HCO3- entry across the basolateral membrane and an electrogenic exit at the apical membrane of the cells.

Transport of sodium into apical membrane vesicles prepared from fetal sheep alveolar type II cells

A method is described for isolating apical plasma membrane vesicles from fetal alveolar type II cells. The procedure yields purified apical membranes which are enriched 24-fold with the brush-border enzyme marker, alkaline phosphatase. Contamination of this fraction by basolateral membranes and organelles is minimal. Evidence for transport of Na+ into an intravesicular space is demonstrated by: (1) time-dependent uptake of Na+ with release of accumulated Na+ by treatment with detergent; (2) a linear inverse correlation between Na+ uptake and medium osmolarity. In addition, Na+ uptake is shown to be anion dependent (SCN- greater than Cl- greater than gluconate-) and sensitive to amiloride inhibition at a concentration of 1 mM.

The role of amiloride-blockable sodium transport in adrenaline-induced lung liquid reabsorption in the fetal lamb

Adrenaline was infused intravenously at rates of 0.1-1.0 microgram/min into chronically catheterized fetal lambs (125-141 days gestation) to induce slowing of secretion or reabsorption of lung liquid. 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. 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 difference between measured flux ratios and those predicted from the forces determining passive flux provided evidence for active transport of Cl- from plasma to lung lumen, as previously demonstrated by Olver & Strang (1974). When adrenaline was infused, there was evidence of active Na+ transport in the direction lung lumen to plasma and an associated decrease in active Cl- transport in the opposite direction. These changes were greatest when the response to adrenaline was reabsorption. Amiloride, when mixed into the lung liquid to give a calculated concentration of 10(-4) M, abolished the changes in p.d. and ion flux induced by adrenaline. In experiments using amiloride concentrations between 10(-8) and 10(-4) M it was shown that 50% inhibition of the reabsorptive response to adrenaline (KI) was induced by 4 X 10(-6) M-amiloride in the lung lumen. Thus adrenaline-induced slowing of secretion or reabsorption of lung liquid is mediated by active Na+ transport from lung lumen to plasma and depends on amiloride-inhibitable Na+ channels on the luminal surface of the pulmonary epithelium.

Sodium and chloride transport by the tracheal epithelium of fetal, new-born and adult sheep

In vitro measurements were made of Na+ and Cl- isotopic fluxes across the tracheal epithelium of mature fetal lambs (130-143 days gestation), new-born lambs (up to 41 days of age) and adult sheep under conditions of continuous short circuiting. The effects of a variety of drugs were examined, but only in the case of amiloride and isoprenaline were observations made in all three groups. Experiments designed to elucidate the mechanism of basal Cl- secretion were performed in adult trachea only. Under resting conditions the net flux of Na+ from lumen to submucosa exceeds that of Cl- in the reverse direction in fetal and adult trachea. In the new-born the two fluxes are more or less equivalent in magnitude. In none of the three groups is the sum of ion fluxes significantly different from the short-circuit current (Isc). Removal of Na+ from, or addition of furosemide (10(-3) M) to, the solution bathing the submucosal surface of adult trachea has the effect of reducing Isc by an amount which approximates to the Cl- current (29%). At a concentration of 10(-4) M on the submucosal side of adult trachea, ouabain causes potential difference and Isc to fall to zero within 70 min of addition to the bathing solution. Nevertheless, there remains a significant net Na+ flux from submucosa to lumen. The addition of isoprenaline (10(-4) M) to the medium bathing the submucosal surface of both fetal and adult trachea causes an increase in the one-way flux of Cl- from submucosa to lumen with consequent increase in net Cl- flux towards the lumen. (The Na+ fluxes are unchanged.) However, in the adult the Cl- secretory response to isoprenaline is very much less and is not accompanied by an increase in electrical conductance. As judged by the change in Isc, all the post-natal fall in beta-agonist responsiveness takes place within the 3 week period following birth. Whereas, in the fetus, the effect of luminally applied amiloride on the Na+ fluxes is negligible, in the adult the one-way flux of Na+ from lumen to submucosa is reduced by 35% with a consequent 60% fall in net Na+ flux towards the submucosa.(ABSTRACT TRUNCATED AT 400 WORDS)

Domestic metallic mercury poisoning

In a family exposed to metallic mercury vapour two patients had acrodynia, one had the nephrotic syndrome, and one person remained well. Recognition of the variable manifestations of the disease and prevention of further exposure were the most important aspects of management. Recovery appeared to be complete as blood mercury levels fell to normal. Urinary mercury levels were too variable to be reliable as indications of progress.

Effects of adrenaline and of spontaneous labour on the secretion and absorption of lung liquid in the fetal lamb

In the chronically catheterized fetal lamb, intravenous infusion of adrenaline at 0.5 microgram/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. 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 microgram/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. 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. (0.48 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. 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.

Epithelial solute permeability, ion transport and tight junction morphology in the developing lung of the fetal lamb

1. Experiments were performed on exteriorized fetal lambs of between 69 days' gestation and term (147 days) in order to observe changes in lung volume and lung liquid secretion rate, and to delineate any alterations in solute permeability, ion transport and tight junction morphology in the maturing lung epithelium. Whilst it was technically possible to measure solute permeability as early as 69 days it was not feasible to apply the Ussing flux ratio technique before 84 days.2. Fetal lung liquid volume and secretion rate, when normalized for body weight, increase linearly with gestation, whereas tracheal volume expressed in the same manner remains constant.3. When expressed in terms of pore theory, epithelial permeability to small polar non-electrolytes does not change between 69 days and term (equivalent pore radius 0.66 nm and 0.64 nm respectively).4. In the immature fetus of 69-76 days, mean epithelial tight junction strand number is 8.3, whereas by the end of gestation it has fallen to 4.6.5. The transfer constants (min(-1)) for sodium and chloride movement in the direction lung liquid to plasma are, respectively, some 6 and 4 times greater at 84-87 days than at term.6. As in the mature fetus, the lung epithelium at 84-87 days actively transports chloride from plasma to lung lumen, albeit with a slightly reduced transport e.m.f. Sodium movement does not, at any gestational age, differ from the predictions for passive transfer.7. In lung liquid the concentrations of chloride and potassium increase and that of bicarbonate decreases during gestation, whilst that of sodium does not change. The rises in lung liquid chloride and potassium concentrations follow those in plasma, maintaining plasma/lung liquid ratios of 0.7 and 0.95 respectively. However, plasma bicarbonate remains constant and the plasma/lung liquid ratio for bicarbonate rises from 3 at 69-76 days to 20 near term as the lung liquid bicarbonate falls from 9.8 to under 2 m-mole kg(-1) H(2)O.8. Whereas lung liquid protein concentration remains constant and low at about 0.35 g l.(-1), plasma protein concentration rises from 23 g l.(-1) at 69-76 days to 43 g l.(-1) near term. During the same period arterial blood pressure doubles.

Development of intercellular junctions in the pulmonary epithelium of the foetal lamb

The integrity of epithelial tight junctions in foetal mammalian lungs is essential to maintain the unique ionic composition of lung liquid, and to prevent leakage of serum proteins into peripheral air spaces. In the present study the development of intercellular junctions of the lining epithelium of foetal lamb lungs during gestation was examined by light and electron microscopy. Both thin sections and freeze-fracture replicas were examined by electron microscopy. By 39 days of gestation, epithelial tight junctions consist of a minimum of 3.1 +/- 1.6 (s.D.) and a maximum of 5.8 +/- 2.0 discontinuous rows of particles and short segments of strands on P face ridges and in complementary E face grooves, while from 58 to 76 days they are composed of a network of 4.3 +/- 1.6 to 7.7 +/- 1.9 focally interrupted P face strands. Complementary replicas show that many of the discontinuities on the P face are due to separation of junctional particles on to the E face during fracturing, and not to an absence of junctional particles. From 76 days to term, epithelial tight junctions (exclusive of upper airway epithelium which was not examined) resemble those of adult lungs, and consist of a continuous network of 4.5 +/- 2.0 to 7.5 +/- 2.5 P face strands and complementary particle-free grooves. Permeability measurements, published elsewhere, indicate that the epithelium is functionally 'tight' from 69 days onwards. Tight junctions in peripheral air-space epithelium, therefore, are structurally continuous and functionally 'tight' early in foetal lung development, and form seals at one end of long, narrow intercellular spaces; these features may be important for coupled ion and water transport. When the bounding epithelial cells become flattened, these narrow intercellular spaces remain intact as a result of complex interdigitations of adjacent cell membranes. Desmosomes were present throughout gestation near the abluminal side of the tight junctions and occasionally near the base of the intercellular space. These junctions may serve to connect cells to each other at a time when tight junctions may be mechanically weak. In addition, gap junctions are associated with tight junctions from the glandular through the canalicular stages of lung development. They disappear by 120 days when the epithelial cells are differentiated.

The role of catecholamines in lung liquid absorption at birth

We have examined the effect on lung liquid secretion of catecholamines infused 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 be blocked by propranolol indicate a mode of action involving beta-adrenergic receptors.