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

Nitric oxide regulation of fetal and newborn lung development and function

In the developing lung, nitric oxide (NO) and cyclic guanosine monophosphate (cGMP) signaling are essential in regulating lung formation and vascular tone. Animal studies have linked many anatomical and pathophysiological features of newborn lung disease to abnormalities in the NO/cGMP signaling system. They have demonstrated that driving this system with agonists and antagonists alleviates many of them. This research has spurred the rapid clinical development, testing, and application of several NO/cGMP-targeting therapies with the hope of treating and potentially preventing significant pediatric lung diseases. However, there are instances when the therapeutic effectiveness of these agents is limited. Studies indicate that injury-induced disruption of several critical components within the signaling system may hinder the promise of some of these therapies. Recent research has identified basic mechanisms that suppress NO/cGMP signaling in the injured newborn lung. They have also pinpointed biomarkers that offer insight into the activation of these pathogenic mechanisms and their influence on the NO/cGMP signaling system's integrity in vivo. Together, these will guide the development of new therapies to protect NO/cGMP signaling and safeguard newborn lung development and function. This review summarizes the important role of the NO/cGMP signaling system in regulating pulmonary development and function and our evolving understanding of how it is disrupted by newborn lung injury.

Cardiac emergencies in neonates and young infants

Cardiac emergencies in children are not infrequent. Early recognition and management are essential to save life and prevent any comorbidity. The presentation of cardiac emergencies and etiologies is variable depending on the age of child at the time of presentation and type of cardiac lesion. Cyanotic and noncyanotic congenital heart diseases are the main causes in neonates and infants. Acquired heart diseases and dysrhythmia are more common causes for cardiac emergencies in toddler and childhood. In this review, we discuss the most common causes for cardiac emergencies in neonates and young infants highlighting important points in the presentation and management that are essential for early recognition and timely management of infants presenting with these conditions.

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.

Oxygen alters caveolin-1 and nitric oxide synthase-3 functions in ovine fetal and neonatal lung microvascular endothelial cells

We determined the effect of oxygen [∼100 Torr (normoxia) and ∼30–40 Torr (hypoxia)] on functions of endothelial nitric oxide (NO) synthase (NOS-3) and its negative regulator caveolin-1 in ovine fetal and neonatal lung microvascular endothelial cells (MVECs). Fetal NOS-3 activity, measured as NO production with 0.5–0.9 μM 4-amino-5-methylamino-2,7-difluorofluorescein, was decreased in hypoxia by 14.4% ( P < 0.01), inhibitable by the NOS inhibitor N-nitro-l-arginine, and dependent on extracellular arginine. Caveolar function, assessed as FITC-BSA (160 μg/ml) endocytosis, was decreased in hypoxia by 13.5% in fetal and 22.8% in neonatal MVECs ( P < 0.01). NOS-3 and caveolin-1 were physically associated, as demonstrated by coimmunoprecipitation and colocalization, and functionally associated, as shown by cross-activation of endocytosis, by their specific antibodies and activation of NOS by albumin. Caveolin peptide, containing the sequence for the PKC phosphorylation site of caveolin, and caveolin antiserum against the site increased NO production and endocytosis by 12.3% ( P < 0.05) and 16% ( P < 0.05), respectively, in normoxia and increased endocytosis by 25% ( P < 0.001) in hypoxia. PMA decreased NO production in normoxia and hypoxia by 19.32% ( P < 0.001) and 11.8% ( P < 0.001) and decreased endocytosis in normoxia by 20.35% ( P < 0.001). PKC kinase activity was oxygen sensitive, and threonine phosphorylation was enhanced in hypoxia. Pertussis toxin increased caveolar and NOS functions. These data support our hypothesis that increased Po2at birth promotes dissociation of caveolin-1 and NOS-3, with an increase in their activities, and that PKC and an oxygen-sensitive cell surface G protein-coupled receptor regulate caveolin-1 and NOS-3 interactions in fetal and neonatal lung MVECs.

Effects of Phosphodiesterase 5 Inhibitor on Pulmonary Vascular Reactivity in the Fetal Lamb

BACKGROUND

Nitric oxide released by pulmonary vascular endothelium is a potent vasodilator related to increased cyclic guanosine monophosphate (cGMP) content. Hydrolysis of cGMP is achieved predominately by cGMP-specific phosphodiesterases. Sildenafil is a selective phosphodiesterase-5 (PDE5) inhibitor. The purpose of the study is to assess the effects of sildenafil on pulmonary vascular circulation during the perinatal period.

METHODS

Thirty-two pregnant ewes were operated on at the end of gestation, and fetal lambs were prepared with catheters placed into the aorta, vena cava, pulmonary artery, and left atrium. An ultrasonic flow transducer and an inflatable vascular occluder were placed respectively around the left pulmonary artery and the ductus arteriosus. Fetal lambs were randomly divided into two groups: (1) sildenafil group, infused continuously with sildenafil for 24 hours at a rate of 1 mg/h; or (2) control group, infused with saline for 24 hours. After 24 hours of infusion, we compared basal pulmonary vascular resistance and the pulmonary vascular responses to increase in fetal PaO2 and to acute ductus arteriosus compression causing "shear stress."

RESULTS

Sildenafil infusion did not change mean aortic and pulmonary artery pressures, increased mean left pulmonary blood flow by 160%, and decreased pulmonary vascular resistance by 60% (p < 0.05). However, both mean flow (Q) and pulmonary vascular resistance returned to baseline values after 2 hours of sildenafil infusion. Despite similar baseline values, pulmonary vascular resistance during maternal O2 inhalation was lower in the sildenafil group than in the control group (0.21 +/- 0.03 versus 0.33 +/- 0.03 mm Hg.mL(-1).min(-1), respectively; p < 0.01). Furthermore, drop in pulmonary vascular resistance during acute ductus arteriosus compression was greater in the sildenafil group (from 0.56 +/- 0.06 to 0.26 +/- 0.04 mm Hg.mL(-1).min(-1)) than in the control group (from 0.55 +/- 0.05 to 0.39 +/- 0.03 mm Hg.mL(-1).min(-1); p < 0.01).

CONCLUSIONS

Although sildenafil induces a transient pulmonary vasodilation, it mediates a sustained change in vascular reactivity, especially to birth-related stimuli in the ovine fetal lung. These data suggest that PDE5 is involved in the regulation of pulmonary vascular reactivity during the perinatal period and may potentiate birth-related pulmonary vasodilator stimuli.

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.

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.

Effect of antenatal dexamethasone treatment on Ca2+-dependent nitric oxide synthase activity in rat lung

We investigated the effects of dexamethasone on nitric oxide synthase activity, nitrate/nitrite concentration, and cGMP concentration in the lungs of premature and full-term neonate rats. Dexamethasone or vehicle alone was administered to the mother (1 mg/kg/d, s.c., 2 d), and the neonate was killed 24 h after birth. Ca2+-dependent nitric oxide synthase activity and nitrate/nitrite and cGMP concentrations in lungs of dexamethasone-treated neonates, both premature and full-term, were significantly higher than those in the lungs of the control rats. Ca2+-dependent nitric oxide synthase activity, nitrate/nitrite concentration, and cGMP concentration in the lungs of control rats showed developmentally associated increases during late gestation and in the early postnatal period. The activation of the nitric oxide synthasenitric oxide-cGMP system by antenatal dexamethasone treatment may be related to the improvement of pulmonary function by antenatal glucocorticoid therapy to minimize respiratory distress syndrome.

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.