Pulmonary vasodilator drugs decrease lung liquid production in fetal sheep

Enhanced ventricular pump function and decreased reservoir backflow sustain rise in pulmonary blood flow after reduction of lung liquid volume in fetal lambs

Although a reduction in lung liquid volume increases fetal pulmonary blood flow, the changes in central flow patterns that sustain this increased pulmonary perfusion are unknown. To address this issue, eight anesthetized late-gestation fetal sheep were instrumented with pulmonary trunk (PT), ductus arteriosus (DA), and left pulmonary artery (PA) micromanometer catheters and transit-time flow probes, with blood flow profile and wave intensity analyses performed at baseline and after withdrawal of lung liquid via an endotracheal tube. Reducing lung liquid volume by 19 ± 6 ml/kg (mean ± SD) augmented right ventricular power by 34% (P < 0.001), with distribution of an accompanying increase in mean PT blood flow (245 ± 63 ml/min, P < 0.001) to the lungs (169 ± 91 ml/min, P = 0.001) and across the DA (77 ± 92 ml/min, P = 0.04). However, although PT and DA flow increments were confined to systole and were related to an increased magnitude of flow-increasing, forward-running compression waves, the rise in PA flow spanned both systole (108 ± 66 ml/min) and diastole (61 ± 32 ml/min). Flow profile analysis showed that the step-up in PA diastolic flow was associated with diminished PA diastolic backflow and accompanied by a lesser degree of diastolic right-to-left DA shunting. These data suggest that an increased pulmonary blood flow after reduction of lung liquid volume is associated with substantial changes in PT-DA-PA interactions and underpinned by two main factors: 1) enhanced right ventricular pump function that increases PA systolic inflow and 2) decreased PA diastolic backflow that arises from a fundamental change in PA reservoir function, thereby resulting in greater passage of systolic inflow through the lungs.

Selective type 5 phosphodiesterase inhibition alters pulmonary hemodynamics and lung liquid production in near-term fetal lambs

Nitric oxide causes dilation of the pulmonary circulation and reduction in net lung liquid production in the fetal lamb, two critical perinatal events. Phosphodiesterase inhibition alone causes similar changes and also enhances the effects of nitric oxide. To better define the cyclic guanosine 5'-monophosphate (GMP) pathway in these events, we studied the effects of a specific phosphodiesterase inhibitor, E4021, on pulmonary arteries and veins isolated from near-term fetal lambs, as well as in intact, chronically instrumented late-gestation fetal lambs. In the in vitro experiments, both pulmonary arteries and veins relaxed to E4021 in a dose-dependent manner, although pulmonary veins were significantly more sensitive to E4021. Pretreatment with N(G)-nitro-l-arginine (L-NNA) abolished this response in arteries but not in veins. In both arteries and veins, pretreatment with beta-phenyl-1,N2-etheno-8-bromoguanosine-3',5'-cyclic monophosphorothionate blunted relaxations to E4021. In the in vivo experiments, E4021 infusion into either the pulmonary artery or central venous circulation increased pulmonary blood flow and decreased pulmonary vascular resistance, and these responses were blunted by pretreatment with L-NNA. Net lung liquid production, measured by a dye-dilution technique using blue dextran, decreased when E4021 was infused directly into the pulmonary artery and this effect was not altered by L-NNA. There was no effect on lung liquid production when E4021 was infused into the central venous circulation. Taken together, these results suggest that the pulmonary hemodynamic effects of E4021 involve the cyclic GMP pathway and are primarily nitric oxide synthase dependent. In contrast, the effects on E4021 on net lung liquid production appear to be independent of nitric oxide synthase, suggesting that these two critical perinatal events might be modulated independently.

Pulmonary and systemic nitric oxide metabolites in a baboon model of neonatal chronic lung disease

We report on developmental changes of pulmonary and systemic nitric oxide (NO) metabolites in a baboon model of chronic lung disease with or without exposure to inhaled NO. The plasma levels of nitrite and nitrate, staining for S-nitrosothiols and 3-nitrotyrosine in the large airways, increased between 125 d and 140 d of gestation (term 185 d) in animals developing in utero. The developmental increase in NO-mediated protein modifications was not interrupted by delivery at 125 d of gestation and mechanical ventilation for 14 d, whereas plasma nitrite and nitrate levels increased in this model. Exposure to inhaled NO resulted in a further increase in plasma nitrite and nitrate and an increase in plasma S-nitrosothiol without altering lung NO synthase expression. These data demonstrate a developmental progression in levels of pulmonary NO metabolites that parallel known maturational increases in total NO synthase activity in the lung. Despite known suppression of total pulmonary NO synthase activity in the chronic lung disease model, pulmonary and systemic NO metabolite levels are higher than in the developmental control animals. Thus, a deficiency in NO production and biological function in the premature baboon was not apparent by the detection and quantification of these surrogate markers of NO production.

Effect of nitric oxide on the development of nitrofen-induced fetal hypoplastic lung explants

BACKGROUND/PURPOSE

Nitric oxide (NO) is an important cell-signaling molecule, and its generators, nitric oxide synthases, are expressed temporospatially in fetal rat lung. Recently, NO has been reported to modulate branching of the fetal rat lung lobe in vitro. We designed this study to evaluate the effect of NO on the morphogenesis of hypoplastic lung using nitrofen-induced rat lung explant model.

METHODS

A hypoplastic fetal lung model and a normal control lung model were induced by feeding a pregnant rat with nitrofen (100 mg) or olive oil on day 9.5 of gestation, respectively. Fetal lungs were harvested on day 13.5 and placed in organ culture containing serum-free medium Dulbecco modified Eagle medium. An NO donor, DETA NONOate (DETA/NO), was added daily in the culture medium. The lung cultures were divided into 4 groups: group 1 (n = 8), normal controls without DETA/NO; group 2 (n = 22), normal controls with DETA/NO; group 3 (n = 13), hypoplastic lungs without DETA/NO; group 4 (n = 22), hypoplastic lungs with DETA/NO. The fetal lungs were incubated for 48 hours at 37 degrees C with 5% CO2. Lung bud count and area of the specimens were measured under computer-assisted digital tracings. The rate of increase in bud count and lung area was calculated as the ratio of each value at 48 hours minus each value at 0 hour, divided by the value at 0 hour.

RESULTS

The lung bud count was significantly increased in group 2 compared with group 1 at a concentration of 50 micromol/L DETA/NO (P < .05). In the nitrofen group, the lung bud count was significantly increased in group 4 compared with group 3 at 100 micromol/L DETA/NO added (P < .05). There was no significant difference in the rate of increase in whole lung area among the 4 groups. The peak increase rates of lung area and bud count were significantly lower in group 4 compared with group 2.

CONCLUSIONS

This study demonstrates that the NO donor, DETA/NO, promotes branching of the nitrofen-induced hypoplastic fetal lung explant. These data suggest that NO may modulate the development of the nitrofen-induced hypoplastic lung.

Developmental changes in nitric oxide synthase isoform expression and nitric oxide production in fetal baboon lung

Nitric oxide (NO), produced by NO synthase (NOS), plays a critical role in multiple processes in the lung during the perinatal period. To better understand the regulation of pulmonary NO production in the developing primate, we determined the cell specificity and developmental changes in NOS isoform expression and action in the lungs of third-trimester fetal baboons. Immunohistochemistry in lungs obtained at 175 days (d) of gestation (term = 185 d) revealed that all three NOS isoforms, neuronal NOS (nNOS), endothelial NOS (eNOS), and inducible NOS (iNOS), are primarily expressed in proximal airway epithelium. In proximal lung, there was a marked increase in total NOS enzymatic activity from 125 to 140 d gestation due to elevations in nNOS and eNOS, whereas iNOS expression and activity were minimal. Total NOS activity was constant from 140 to 175 d gestation, and during the latter stage (160-175 d gestation), a dramatic fall in nNOS and eNOS was replaced by a rise in iNOS. Studies done within 1 h of delivery at 125 or 140 d gestation revealed that the principal increase in NOS during the third trimester is associated with an elevation in exhaled NO levels, a decline in expiratory resistance, and greater pulmonary compliance. Thus, there are developmental increases in pulmonary NOS expression and NO production during the early third trimester in the primate that may enhance airway and parenchymal function in the immediate postnatal period.

Regulation of endothelial nitric oxide synthase: location, location, location

Endothelial nitric oxide synthase (eNOS) is expressed in vascular endothelium, airway epithelium, and certain other cell types where it generates the key signaling molecule nitric oxide (NO). Diminished NO availability contributes to systemic and pulmonary hypertension, atherosclerosis, and airway dysfunction. Complex mechanisms underly the cell specificity of eNOS expression, and co- and post-translational processing leads to trafficking of the enzyme to plasma membrane caveolae. Within caveolae, eNOS is the downstream target member of a signaling complex in which it is functionally linked to both typical G protein-coupled receptors and less typical receptors such as estrogen receptor (ER) alpha and the high-density lipoprotein receptor SR-BI displaying novel actions. This compartmentalization facilitates dynamic protein-protein interactions and calcium- and phosphorylation-dependent signal transduction events that modify eNOS activity. Further understanding of these mechanisms will enable us to take preventive and therapeutic advantage of the powerful actions of NO in multiple cell types.

Nitric oxide decreases lung liquid production via guanosine 3',5'-cyclic monophosphate

We studied the role of cGMP in nitric oxide (NO)-induced changes in lung liquid production (J(v)) in chronically instrumented fetal sheep. Forty-five studies were done in which J(v) was measured by a tracer dilution technique. Left pulmonary arterial flow (Q(lpa)) was measured by a Doppler flow probe. There were two series of experiments. In the first, we gave 8-bromo-cGMP, a cGMP analog, by either the pulmonary vascular or intraluminal route; in the second, we used agents to inhibit or enhance endogenous cGMP activity. When infused directly into the pulmonary circulation, 8-bromo-cGMP significantly increased Q(lpa) but had no effect on J(v). Conversely, when instilled into the lung liquid, 8-bromo-cGMP had no effect on Q(lpa) but significantly reduced J(v). Inhibition of guanylate cyclase activity with methylene blue totally blocked, whereas phosphodiesterase inhibition with Zaprinast significantly enhanced, the effect of instilled NO on J(v). Thus the reduction in lung liquid caused by NO appears to be mediated by cGMP, perhaps through a direct effect on the pulmonary epithelium.

Molecular basis of cell-specific endothelial nitric-oxide synthase expression in airway epithelium

Nitric oxide (NO) plays an important role in airway function, and endothelial NO synthase (eNOS) is expressed in airway epithelium. To determine the basis of cell-specific eNOS expression in airway epithelium, studies were performed in NCI-H441 human bronchiolar epithelial cells transfected with the human eNOS promoter fused to luciferase. Transfection with 1624 base pairs of sequence 5' to the initiation ATG (position -1624) yielded a 19-fold increase in promoter activity versus vector alone. No activity was found in lung fibroblasts, which do not express eNOS. 5' deletions from -1624 to -279 had modest effects on promoter activity in H441 cells. Further deletion to -248 reduced activity by 65%, and activity was lost with deletion to -79. Point mutations revealed that the GATA binding motif at -254 is mandatory for promoter activity and that the positive regulatory element between -248 and -79 is the Sp1 binding motif at -125. Electrophoretic mobility shift assays yielded two complexes with the GATA site and three with the Sp1 site. Immunodepletion with antiserum to GATA-2 prevented formation of the slowest migrating GATA complex, and antiserum to Sp1 supershifted the slowest migrating Sp1 complex. An electrophoretic mobility shift assay with H441 versus fibroblast nuclei revealed that the slowest migrating GATA complex is unique to airway epithelium. Thus, cell-specific eNOS expression in airway epithelium is dependent on the interaction of GATA-2 with the core eNOS promoter, and the proximal Sp1 binding site is also an important positive regulatory element.

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.

Nitric oxide synthase isoform expression in the developing lung epithelium

Nitric oxide (NO), generated by NO synthase (NOS), is an important mediator of physiological processes in the airway and lung parenchyma, and there is evidence that the pulmonary expression of the endothelial isoform of NOS (eNOS) is developmentally regulated. The purpose of the present study was to delineate the cellular distribution of expression of eNOS in the developing respiratory epithelium and to compare it with inducible (iNOS) and neuronal (nNOS) NOS. Immunohistochemistry was performed on fetal (125-135 days gestation, term 144 days), newborn (2-4 wk), and maternal sheep lungs. In fetal lung, eNOS expression was evident in bronchial and proximal bronchiolar epithelia but was absent in terminal and respiratory bronchioles and alveolar epithelium. Similar to eNOS, iNOS was detected in bronchial and proximal bronchiolar epithelia but not in alveolar epithelium. However, iNOS was also detected in terminal and respiratory bronchioles. nNOS was found in epithelium at all levels including the alveolar wall. iNOS and nNOS were also detected in airway and vascular smooth muscle. The cellular distribution of all three isoforms was similar in fetal, newborn, and adult lungs. Findings in the epithelium were confirmed by isoform-specific reverse transcription-polymerase chain reaction assays and NADPH diaphorase histochemistry. Thus the three NOS isoforms are commonly expressed in proximal lung epithelium and are differentially expressed in distal lung epithelium. All three isoforms may be important sources of epithelium-derived NO throughout lung development.

The effects of cGMP on fetal sheep pulmonary blood flow and lung liquid production

This study was designed to determine the effects of a membrane permeant phosphodiesterase-resistant analog of cGMP on lung liquid production and pulmonary blood flow at the time of birth. Experiments were performed on seven fetal sheep prepared for chronic measurements of lung liquid production (Jv), pulmonary blood flow (Qp) and pressure, as well as systemic pressure. Injection of either 8-bromo-cGMP or saline were made via a catheter inserted in the left pulmonary artery. Experiments consisted of 1 h of control, 1 h of infusion, and 2 h of recovery. Data were analyzed by ANOVA and Newman-Keuls test. After infusion of 8-bromo-cGMP, Jv was decreased by 70 and 44% from control in h 3 and 4, respectively. Qp was elevated by 100 mL/min in h 2 and 3 and continued to be elevated by 50 mL/min in h 4. Saline infused animals showed no significant changes in Qp and Jv. This study demonstrates that 8-bromo-cGMP decreases lung liquid production and increases pulmonary blood flow in near term fetal sheep. Although blood flow increased in h 2, lung liquid production did not decrease at this time, suggesting a time dissociation between changes in pulmonary blood flow and lung liquid production. Thus, it is possible that a common transduction pathway involving cGMP may be responsible for lung liquid reduction and elevation of pulmonary blood flow at birth. However, Qp and Jv may not be causally related.

Nitric oxide decreases lung liquid production in fetal lambs

To examine the effect of nitric oxide on fetal lung liquid production, I measured lung liquid production in fetal sheep at 130 +/- 5 days gestation (range 122-137 days) before and after intrapulmonary instillation of nitric oxide. Thirty-one studies were done in which net lung luminal liquid production (JV) was measured by plotting the change in lung luminal liquid concentration of radiolabeled albumin, an impermeant tracer that was mixed into the lung liquid at the start of each study. To see whether changes in JV might be associated with changes in pulmonary hemodynamics, pulmonary and systemic pressures were measured and left pulmonary arterial flow was measured by an ultrasonic Doppler flow probe. Variables were measured during a 1- to 2-h control period and for 4 h after a small bolus of isotonic saline saturated with nitric oxide gas (10 or 100%) was instilled into the lung liquid. Control (saline) instillations (n = 6) caused no change in any variable over 6 h. Nitric oxide instillation significantly decreased JV and increased pulmonary blood flow; these effects were sustained for 1-2 h. There was also a significant but transient decrease in pulmonary arterial pressure. Thus intrapulmonary nitric oxide causes a significant decrease in lung liquid and is associated with a decrease in pulmonary vascular resistance. In a separate series of experiments either amiloride or benzamil, which blocks Na+ transport, was mixed into the lung liquid before nitric oxide instillation; still, there was a similar reduction in lung liquid production. Thus the reduction in lung liquid secretion caused by nitric oxide does not appear to depend on apical Na+ efflux.