Liquid flow across the epithelium of the artificially perfused lung of fetal and postnatal sheep

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

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

The role of lung inflation and sodium transport in airway liquid clearance during lung aeration in newborn rabbits

BACKGROUND

Recent phase-contrast X-ray imaging studies suggest that inspiration primarily drives lung aeration and airway liquid clearance at birth, which questions the role of adrenaline-induced activation of epithelial sodium channels (ENaCs). We hypothesized that pressures generated by inspiration have a greater role in airway liquid clearance than do ENaCs after birth.

METHODS

Rabbit pups (30 d of gestation) were delivered and sedated, and 0.1 ml of saline (S) or amiloride (Am; an ENaC inhibitor) was instilled into the lungs before mechanical ventilation. Two other groups (30 d of gestation) were treated similarly but were also given adrenaline (S/Ad and Am/Ad) before mechanical ventilation.

RESULTS

Amiloride and adrenaline did not affect functional residual capacity (FRC) recruitment (P > 0.05). Amiloride increased the rate of FRC loss between inflations (Am: -5.2 ± 0.6 ml/kg/s), whereas adrenaline reduced the rate of FRC loss (S/Ad: -1.9 ± 0.3 ml/kg/s) as compared with saline-treated controls (S: -3.5 ± -0.6 ml/kg/s; P < 0.05).

CONCLUSION

These data indicate that inspiration is a major determinant of airway liquid clearance and FRC development during positive pressure ventilation. Although ENaC inhibition and adrenaline administration had no detectable effect on FRC development, ENaC may help to prevent liquid from re-entering the airways during expiration.

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

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

Alveolar epithelial CNGA1 channels mediate cGMP-stimulated, amiloride-insensitive, lung liquid absorption

Impairment of lung liquid absorption can lead to severe respiratory symptoms, such as those observed in pulmonary oedema. In the adult lung, liquid absorption is driven by cation transport through two pathways: a well-established amiloride-sensitive Na(+) channel (ENaC) and, more controversially, an amiloride-insensitive channel that may belong to the cyclic nucleotide-gated (CNG) channel family. Here, we show robust CNGA1 (but not CNGA2 or CNGA3) channel expression principally in rat alveolar type I cells; CNGA3 was expressed in ciliated airway epithelial cells. Using a rat in situ lung liquid clearance assay, CNG channel activation with 1 mM 8Br-cGMP resulted in an approximate 1.8-fold stimulation of lung liquid absorption. There was no stimulation by 8Br-cGMP when applied in the presence of either 100 μM L: -cis-diltiazem or 100 nM pseudechetoxin (PsTx), a specific inhibitor of CNGA1 channels. Channel specificity of PsTx and amiloride was confirmed by patch clamp experiments showing that CNGA1 channels in HEK 293 cells were not inhibited by 100 μM amiloride and that recombinant αβγ-ENaC were not inhibited by 100 nM PsTx. Importantly, 8Br-cGMP stimulated lung liquid absorption in situ, even in the presence of 50 μM amiloride. Furthermore, neither L: -cis-diltiazem nor PsTx affected the β(2)-adrenoceptor agonist-stimulated lung liquid absorption, but, as expected, amiloride completely ablated it. Thus, transport through alveolar CNGA1 channels, located in type I cells, underlies the amiloride-insensitive component of lung liquid reabsorption. Furthermore, our in situ data highlight the potential of CNGA1 as a novel therapeutic target for the treatment of diseases characterised by lung liquid overload.

Amiloride-insensitive Na+and fluid absorption in the mammalian distal lung

The ability of the distal lung epithelia to actively transport Na+, with Cl−and water following, from the alveolar spaces inversely correlates with morbidity and mortality of infants, children, and adults with alveolar pulmonary edema. It is now recognized, in contrast to many other Na+transporting epithelia, that at least half of this active transport is not sensitive to amiloride, which inhibits the epithelial Na+channel. This paper reviews amiloride-insensitive Na+and fluid transport in the mammalian distal lung unit under basal conditions and speculates on potential explanations for this amiloride-insensitive transport. It also provides new information, using primary cultures of rat fetal distal lung epithelia and alveolar type II cells grown under submersion and air-liquid interface culture conditions, regarding putative blockers of this transport.

Developmental regulation of lumenal lung fluid and electrolyte transport

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

Expulsion of liquid from the fetal lung during labour in sheep

Effective gas exchange after birth requires clearance of most of the liquid filling the lung during gestation. To date the focus has been on active Na(+) transport from lung lumen to interstitium, but Na(+) transport begins only close to delivery, making it an unlikely mechanism for clearing the bulk of fetal lung liquid. We hypothesised that fetal trunk muscle contractions, known to occur in labour, are involved in lung liquid clearance. We measured maternal uterine contractions, fetal tracheal flow directly and fetal electromyograms in thoracic and abdominal muscles. During labour in five fetal sheep, brief flow pulses were observed in the trachea, most of which expelled a small volume of lung liquid. Tracheal flow pulses were associated with fetal muscle contractions 89% of the time, which were associated on 91% of occasions with uterine contractions. Our results suggest that liquid contained in the fetal lung is cleared before and during labour as a result of fetal muscular effort, perhaps stimulated by uterine contractions.

Developmental Regulation of Lung Liquid Transport

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.

Fetoscopic temporary tracheal occlusion for congenital diaphragmatic hernia: prelude to a randomized, controlled trial

OBJECTIVE

As previously reported, high postnatal mortality seen in fetuses with congenital diaphragmatic hernia (CDH) with liver herniation and low lung-to-head ratio (LHR) appears to be improved in fetuses who undergo fetoscopic temporary tracheal occlusion (TO). To test whether further evolution of this technique produces results that justify a randomized controlled trial comparing prenatal intervention to postnatal care, the authors analyzed 11 additional cases and the cumulative experience with 19 cases.

METHODS

The authors analyzed retrospectively the outcome of 11 new and 8 previously reported cases of fetoscopic temporary tracheal occlusion. Various factors were studied including maternal morbidity, antenatal outcome, physiologic lung response, and neonatal course.

RESULTS

Temporary TO can be accomplished using 3 5-mm radially expanding uterine ports without hysterotomy. Obstetric morbidity included mild pulmonary edema in 6 cases, chorioamniotic separation and premature rupture of membranes in 12 patients, and preterm labor and delivery in all patients. Thirteen of 19 (68%) neonates survived for 90 days after delivery; one died in utero, and 5 died after birth. Late mortality included one death caused by sepsis and 2 by complications associated with tracheostomies. Morbidity from gastroesophageal reflux requiring Nissen fundoplication, tracheal injury requiring repair or tracheostomy, and recurrent hernias after diaphragmatic repair were characteristic in longterm survivors.

CONCLUSIONS

Fetoscopic temporary TO may improve outcome in poor-prognosis fetuses with CDH. However, complications related to tracheal dissection, premature delivery and late morbidity are significant. This experience has led to simpler techniques for fetoscopic tracheal occlusion and to an National Institutes of Health-sponsored randomized controlled trial comparing fetoscopic tracheal occlusion with optimal postnatal care.

Invited review: Clearance of lung liquid during the perinatal period

At birth, the distal lung epithelium undergoes a profound phenotypic switch from secretion to absorption in the course of adaptation to air breathing. In this review, we describe the developmental regulation of key membrane transport proteins and the way in which epinephrine, oxygen, glucocorticoids, and thyroid hormones interact to bring about this crucial change in function. Evidence from molecular, transgenic, cell culture, and whole lung studies is presented, and the clinical consequences of the failure of the physiological mechanisms that underlie perinatal lung liquid absorption are discussed.

Re-evaluating the Na(+) conductance of adult rat alveolar type II pneumocytes: evidence for the involvement of cGMP-activated cation channels

1. Alveolar epithelial type II pneumocytes were isolated and purified from adult rat lung by elastase digestion and differential adhesion, and cultured in serum-free medium for approximately 2 days on glass coverslips for subsequent patch-clamp studies employing symmetrical sodium isethionate solutions. 2. Whole-cell Na(+) currents exhibited essentially linear current-voltage relationships which were mildly inhibited (by approximately 25 %) by 10 microM amiloride. In contrast, 1 mM Zn(2+) inhibited the currents by approximately 55 % with an IC(50) of approximately 134 microM and maximal blockade achieved between 5 and 10 mM. The effects of Zn(2+) and amiloride were additive, and independent of the order of blocker addition. 3. Gd(2+), Zn(2+) and La(3+) at 10 mM were all effective at rapidly, reversibly and significantly blocking the amiloride-insensitive currents by approximately 60%. in contrast, Ni(2+) was a very weak inhibitor (30 % inhibition at 10 mM). 4. Pimozide (10 microM) caused inhibition of whole-cell cation conductance by approximately 55 %. The inhibitory effect of pimozide was concentration dependent with an IC(50) of approximately 1 microM and was maximally effective between 10 and 30 microM. Sequential addition of Zn(2+) and pimozide, in either order, revealed no overlapping inhibitory effect on the amiloride-insensitive conductance, and supported the notion that the Zn(2+)- and pimozide-sensitive currents are identical. 5. The amiloride-insensitive, Zn(2+)-blockable conductance was characterised by a Na(+)/K(+) permeability ratio (P(Na)/P(K)) of 0.73 +/- 0.02. 6. 8Br-cGMP (100 microM), a membrane-permeable analogue of cGMP, evoked a robust activation of whole-cell cation conductance to 220 % of control. This activation was apparent in either the absence or the presence of 10 microM amiloride, but was completely abolished in the presence of Zn(2+). 7. These data support the in vivo and in situ observations of a substantial amiloride-resistant Na(+) conductance, demonstrate directly that cyclic nucleotide-gated non-selective cation channels are functionally expressed in alveolar epithelial type II cells, and suggest that these channels may contribute to the fluid-reabsorptive driving force in adult lung.

Volume and secretion rate of lung liquid in the final days of gestation and labour in the fetal sheep

1. Most of the liquid that fills the lung of the fetal sheep in late gestation is cleared by the end of labour. Clearance of this liquid has a beneficial effect on postnatal gas exchange and therefore represents an important adaptation for postnatal life. Despite its importance, there is disagreement about whether clearance begins prior to labour, or occurs entirely within labour. 2. To address this issue, we made serial determinations of lung liquid volume by indicator dilution during late gestation and labour in the fetal sheep. 3. Regression analysis demonstrated that lung liquid volume exhibited a plateau level in the near-term fetus before it began to decline. Two models provided a fit to the decline in volume. In one, lung liquid clearance occurred in two linear phases, the first beginning 70 h before the study was terminated when the ewe was in advanced labour, the second occupying the last 8 h of the study period. In the initial phase, average lung liquid volume fell from 38.3 to 26.4 ml x kg(-1) before a rapid decline in the second phase reduced the volume to 13.8 ml x kg(-1). An exponential decay model was also found to fit the data; this showed a gradual decline in lung liquid volume in the 2 days preceding onset of labour, followed by a much more rapid decline within labour. 4. The rate of lung liquid secretion also declined in two linear phases, both of which commenced earlier than the changes in lung liquid volume. An exponential decay model also gave a significant fit to the data, but the fit was significantly weaker than that achieved with the two-slope model. 5. We conclude that clearance of lung liquid begins well before commencement of labour in the full term fetal sheep, and then accelerates once labour is established. In our study, lung liquid volume fell even in the absence of reabsorption of liquid across the pulmonary epithelium, indicating that outflow of liquid through the trachea must have occurred at a rate in excess of the secretion rate.

Fetoscopic temporary tracheal occlusion by means of detachable balloon for congenital diaphragmatic hernia

Occlusion of the fetal trachea blocks the egress of fetal lung fluid and stimulates the growth of hypoplastic lungs in fetuses with diaphragmatic hernia. Accomplishing temporary and reversible occlusion of the fetal trachea has proven difficult without invasive fetal surgery. Using simultaneous real-time ultrasonography and fetal bronchoscopy through a single uterine port, we placed a detachable balloon in the trachea of 2 fetuses with severe diaphragmatic hernia. In both fetuses the fetal lung subsequently enlarged, allowing survival after birth.

Effect of lung water content, manipulated by intratracheal furosemide, surfactant, or a mixture of both, on compliance and viscoelastic tissue forces in lung-lavaged newborn piglets

OBJECTIVE

To study the impact of lung water content and its reduction by a topically applied diuretic on respiratory and lung tissue mechanics in comparison with surfactant administration in surfactant-deficient newborn piglets with lavage-induced lung injury.

DESIGN

Controlled, randomized study.

SETTING

Animal research facility.

SUBJECTS

Newborn piglets. TREATMENT Piglets were surfactant depleted by lung lavage and, after a pretreatment period, randomly treated with intratracheal furosemide, furosemide and surfactant, or with surfactant alone.

MEASUREMENTS AND MAIN RESULTS

Dynamic compliance (C(DYN)), static compliance (C(ST)), stress-adaptation pressures (P(DIFF)) and post mortem lung water content were determined. Static compliance in the furosemide-surfactant group was not significantly higher than in the surfactant group. At the end of the study, C(ST) did not differ between the three groups because C(ST) in the furosemide group had increased to values similar to those of the surfactant-containing treatment groups: C(ST) F+S: 0.73 +/- 0.2 mL/cm H2O/kg body weight (BW); C(ST) S: 0.61 +/- 0.11 mL/cm H2O/kg BW; and C(ST) F: 0.60 +/- 0.19 mL/cm H2O/kg BW). Compliance was inversely and P(DIFF) was directly correlated to lung water (LW) content (C(ST) vs. LW: r2 = .59, p = .001; C(DYN) vs. LW: r2 = .49, p = .006; P(DIFF) vs. LW: r2 = .37, p = .059), independent of the type of treatment. Changes in C(ST) and C(DYN) were inversely related to changes in P(DIFF). Intrapulmonary furosemide was more rapidly absorbed when administered to the surfactant-depleted lung alone compared with the mixture with surfactant, and intrapulmonary furosemide had a rapid systemic effect.

CONCLUSION

Although the combination of surfactant with a diuretic failed to increase respiratory compliance to a significantly larger extent than surfactant alone, furosemide at the end of the study increased respiratory compliance to a level similar to surfactant-containing treatments. Lung water content and, to a lesser extent, the absence or presence of surfactant appeared to determine lung mechanics, and its impact on lung mechanics was similar to surfactant administration.

Lack of a role for cyclic nucleotide gated cation channels in lung liquid absorption in fetal sheep

1. Late gestation fetal sheep were chronically catheterised in utero to allow measurement of the rate of production of lung liquid (Jv) from 132-143 days gestation (term, 147 days), and to test the hypothesis that cyclic nucleotide gated cation channels mediate a component of fetal lung liquid absorption. 2. In eight experiments, 0.5 microg min-1 adrenaline caused a significant (P < 0.005) reduction in Jv from +18. 12 +/- 3.52 to -10.27 +/- 5.26 ml h-1. Dichlorobenzamil (a blocker of cyclic nucleotide gated cation channels) at 1.5 x 10-5 M did not significantly inhibit the adrenaline-induced lung liquid absorption (Jv dichlorobenzamil, -5.77 +/- 2.78 ml h-1; P > 0.1) when the data were grouped, but did exert a significant gestational effect (r = 0. 90, P < 0.01). Subsequent addition of 10-4 M amiloride (a blocker of epithelial sodium channels) abolished the adrenaline-induced absorption of lung liquid (mean Jv amiloride, +6.45 +/- 1.59 ml h-1; P < 0.01 relative to Jv adrenaline and P < 0.005 relative to Jv dichlorobenzamil). 3. In seven experiments, 0.5 microg min-1 adrenaline caused a significant (P < 0.0005) reduction in Jv from +18.95 +/- 2. 98 to -10.08 +/- 3.75 ml h-1. Amiloride (10-4 M) inhibited the adrenaline response (Jv amiloride, +5.46 +/- 1.09 ml h-1; P < 0.005). However, subsequent addition of 1.5 x 10-5 M dichlorobenzamil had no additive effect to that of amiloride (Jv dichlorobenzamil, +4.58 +/- 0.93 ml h-1; P > 0.1). 4. In six experiments, the cGMP analogue 8-Br-cGMP at 10-4 M caused a significant (P < 0.05) reduction in Jv from +15.20 +/- 2.81 to +11.63 +/- 1.71 ml h-1. Amiloride (10-4 M) did not block the effect of 8-Br-cGMP (Jv amiloride, +14.00 +/- 2.49 ml h-1; not significantly different from 8-Br-cGMP). Subsequent addition of 1.5 x 10-5 M dichlorobenzamil also did not block the effect of 8-Br-cGMP (Jv dichlorobenzamil, +11.37 +/- 1.22 ml h-1; not significantly different from either Jv amiloride or Jv 8-Br-cGMP). 5. We conclude that, in fetal sheep, neither adrenaline nor cGMP stimulate lung liquid absorption by actions on cyclic nucleotide gated cation channels, and that the effect of cGMP on fetal lung liquid secretion is minor and does not involve epithelial sodium channels. The effect of dichlorobenzamil, when given before amiloride, was probably due to an action on amiloride sensitive epithelial sodium channels.

A novel role for cyclic nucleotide-gated cation channels in lung liquid homeostasis in sheep

1. Sheep lungs were artificially perfused in situ with warmed whole oxygenated sheep blood. The airspaces of the lungs were filled with liquid containing an impermeant tracer, to allow measurement of the rate of net transepithelial liquid movement under various conditions. 2. Dichlorobenzamil (1.5 x 10-5 M), a blocker of cyclic nucleotide-gated cation channels, inhibited the resting absorption of lung liquid in sheep aged 6 months (n = 5) (from -36.47 +/- 4.62 to -4.36 +/- 5.27 ml h-1, means +/- s.e.m.; P < 0.005, paired t test). Amiloride (10-4 M), a blocker of epithelial sodium channels, had no additive effect to that of dichlorobenzamil. 3. In the lungs of sheep aged 6 months (n = 4), amiloride (10-4 M) partially inhibited the resting absorption of liquid (from -35.21 +/- 8.57 to -11.05 +/- 4.91 ml h-1; P < 0.05, one-tailed paired t test), and dichlorobenzamil (1.5 x 10-5 M) exerted an additive effect to that of amiloride resulting in secretion at +6.29 +/- 3.05 ml h-1 (P < 0. 01, paired t test). 4. In the lungs of sheep aged 6 weeks (n = 3), amiloride (10-4 M) also inhibited the resting absorption of liquid (from -26.36 +/- 14.05 to -5.17 +/- 8.27 ml h-1; P < 0.05, one-tailed paired t test); however, dichlorobenzamil (1.5 x 10-5 M) did not exert an additive effect to that of amiloride. 5. In the lungs of sheep aged 6 months (n = 4), amiloride (10-4 M) partially inhibited the resting absorption of liquid (from -35.70 +/- 8.58 to -6.79 +/- 4.28 ml h-1; P < 0.05, paired t test), and pimozide (1.5 x 10-4 M), another blocker of cyclic nucleotide-gated cation channels, also exerted an additive effect to that of amiloride, resulting in secretion of lung liquid at +15.36 +/- 9.14 ml h-1 (P < 0.05, paired t test). 6. We conclude that cyclic nucleotide-gated cation channels mediate a component of lung liquid absorption in sheep aged 6 months (but not in sheep aged 6 weeks), and that a mechanism for lung liquid secretion (present in fetuses) is retained at 6 months of age.

The regulation of lung liquid absorption by endogenous cAMP in postnatal sheep lungs perfused in situ

1. The lungs of two groups of lambs aged 0-2 weeks and 6-12 weeks were artificially perfused in situ with warmed and oxygenated sheep blood. The airspaces of the lungs were filled with liquid containing an impermeant tracer to allow estimation of net liquid movement across the pulmonary epithelium at rest and after administration of certain drugs. 2. Dibutyryl cAMP (dB-cAMP, 10-4 M) stimulated the rate of lung liquid (LL) absorption in the lungs of four neonatal sheep aged 9-12 days, from -1.43 +/- 0.2 to -2.75 +/- 0.3 ml h-1 (kg body wt)-1 (P < 0.05, comparison of regression lines by Student's t test), but had no effect in four juvenile sheep aged 6-12 weeks (P > 0.10). 3. Theophylline, a non-selective phosphodiesterase (PDE) inhibitor (5 x 10-4 M), increased LL absorption from a resting rate of -1.55 +/- 0.3 to -3.62 +/- 0.5 ml h-1 kg-1 in the lungs of four sheep aged 1-12 days and from -1.47 +/- 0.3 to -3.73 +/- 0.4 ml h-1 kg-1 in four sheep aged 6-12 weeks (P < 0.05, Student's paired t test). 4. The beta-adrenergic antagonist sotalol (10-4 M) reduced LL absorption rate from -1.47 +/- 0.1 to -1.22 +/- 0.1 ml h-1 kg-1 (P < 0.01) in the lungs of four sheep aged 4-13 days, while theophylline given after sotalol had no effect. In four sheep aged 6-12 weeks, sotalol had no effect on LL absorption rate, whereas theophylline given after sotalol increased LL absorption rate from -1.06 +/- 0.1 to -1.92 +/- 0.2 ml h-1 kg-1 (P < 0.05). 5. The A1/A2 purinergic receptor blocker 7-(beta-chloroethyl) theophylline (CET; given at 5 x 10-6 M and 10-4 M) had no effect on LL absorption rate in the lungs of four sheep aged 6-12 weeks, confirming that theophylline produced its effect of increasing LL absorption by inhibiting PDE hydrolytic activity. 6. The selective PDE IV (cAMP-specific) PDE inhibitor rolipram was given in the perfused lungs of seven sheep aged 6-12 weeks at doses between 10-8 and 10-4 M, increasing LL absorption rate at concentrations of 10-6 M and above; the half-maximal effective concentration was estimated to be 5.9 x 10-7 M. 7. Rolipram (10-5 M) increased LL absorption rate from -1.99 +/- 0.2 to -3.18 +/- 0.5 ml h-1 kg-1 in the perfused lungs of four sheep aged 6-11 days, and from -1.21 +/- 0.4 to -3.45 +/- 0.3 ml h-1 kg-1 in the perfused lungs of four sheep aged 6-12 weeks (P < 0.05). Sotalol (10-4 M) reduced LL absorption rate from -3.39 +/- 0.8 to -2. 18 +/- 0.4 ml h-1 kg-1 (P < 0.05) in four sheep aged 10-14 days, while rolipram given after sotalol had no effect. In four sheep aged 6-12 weeks, sotalol had no effect on resting LL absorption rate, whereas rolipram given after sotalol increased absorption rate from -1.27 +/- 0.1 to -2.02 +/- 0.6 ml h-1 kg-1 (P < 0.05). 8. We conclude that cAMP mediates a component of LL absorption postnatally, and that while beta-adrenergic stimulation was the sole source of endogenous cAMP in neonates, this was not the case in juveniles, in whom cAMP originated, at least in part, from other sources.

Airway and alveolar permeability and surface liquid thickness: theory

The thickness of airway surface liquid (ASL) can be calculated as the ratio of the permeability coefficient of an absorbed inert tracer to the percentage rate in which it decreases in content in the airway lumen. The percentage clearance of radiolabeled diethylenetriaminepentaacetic acid (DTPA) from human airways or lungs has been measured many times, with a mean value of 1.04 +/- 0.25 (SD) %/min. Rates of clearance from animal lungs of most species give values of the same order, although they are lower in the sheep and higher in the dog. Permeability coefficients have not been measured simultaneously with percentage clearances and not at all for human tissues. Values for mannitol and sucrose, of which the former gives a permeability coefficient approximately 25% greater than that for sucrose and DTPA in airway tubes and isolated mucosal sheets from experimental animals, give a mean approximately 7.1 x 10(-7) cm/s. This corresponds to thickness of ASl of approximately 20-150 microns for various species. The assumptions underlying this estimate are discussed. It is concluded that ASL thickness in vivo may be considerably greater than in vitro measurements involving rapid freezing of the airway wall. Estimates of alveolar permeability suggest that either it is very considerably lower than that of the airway epithelium, that methods to measure alveolar permeability mainly reflect airway permeability, or both.

Pulmonary interstitial pressure in premature rabbits

By micropuncture technique we measured pulmonary interstitial pressure (Pip) from birth up to 6 h postnatal age in anesthetized and paralyzed cesarian delivered term (31 days gestation) and premature (27 to 30 days gestation) rabbits. In term cesarian delivered rabbits Pip followed the time course of vaginally delivered rabbits, namely, it increased from about zero at birth up to about 5 cmH2O at 2 h, as a result of alveolar fluid reabsorption, subsequently it decreased becoming subatmospheric due to progressive interstitial fluid drainage. In ventilated lung regions of premature rabbits, Pip also peaked to about 5 cmH2O at 2 h but its subsequent decrease was markedly slowed down while in atelectatic regions of premature rabbits Pip remained slightly subatmospheric. Up to 6 h, the wet/dry weight ratio of the lung was higher in premature relative to vaginally and cesarian delivered term rabbits (at birth 8.4 +/- 0.9 vs. 7.5 +/- 0.8). In 29-31 days rabbits, plasma protein concentration at birth was 3.6 +/- 0.5 g/dl (within 95% confidence limits for vaginally delivered rabbits, considered as control) while in 27-28 days rabbit it was 3.1 +/- 0.4 g/dl (at the lower edge of control confidence limits). In the first postnatal hours, the increase in Pip favoured fluid reabsorption into pulmonary microcirculation in term cesarian delivered rabbits and in ventilated regions of premature rabbits. Conversely, in the atelectatic regions of premature rabbits the unchanged Pip value in the postnatal hours favours fluid filtration from microcirculation into lung interstitium.

Active transport in the alveolar epithelium of the adult lung: vestigial or vital?

Active secretion by mammalian fetal pulmonary alveolar epithelium is well recognized, as is the role of the adult epithelium in the secretion of surfactant. Recent studies have demonstrated active absorption by adult epithelium involving two sodium-dependent pathways. This finding has focused attention on how poorly we understand both the disposition of alveolar liquid and the physiological role of surfactant. In this paper we review the evidence that the adult mammalian alveolar epithelium absorbs solutes by active transport, and we assess the physiological importance of the resulting liquid movements.

Anion exchange in type II pneumocytes freshly isolated from adult guinea-pig lung

We have studied chloride influx and efflux in a highly purified preparation of type II cells freshly isolated from adult guinea-pig lung using 36Cl-. Chloride uptake was time-dependent, saturable (Km < 10 mM) and was inhibited by 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS; Ki approximately 80 microM). In the absence of external chloride (substituted by gluconate), 36Cl- uptake exhibited an overshoot above equilibrium. The rate of 36Cl- entry was strongly inhibited by addition of external nitrate; sulphate was a weaker inhibitor. 36Cl- efflux was stimulated by external bromide > bicarbonate > or = chloride > or = citrate; and was inhibited by propionate > acetate > oxalate. Although the "chloride channel blocker" 4-nitro-2-(3-phenylpropylamino)benzoate (0.14 mM) caused an inhibition, 36Cl- influx did not appear to be electrogenic. These data are compatible with the existence of a substantial electroneutral anion-exchange pathway for chloride transport in freshly isolated adult type II pneumocytes.