Changes in lung lipid during spontaneous labor in fetal sheep

Lung liquid clearance in preterm lambs assessed by magnetic resonance imaging

BACKGROUND

Postnatal adaptation requires liquid clearance and lung aeration. However, their relative contribution to the expansion of functional residual capacity (FRC) has not been fully investigated. We studied evolution of lung liquid removal and lung aeration after birth in preterm lambs.

METHODS

Lung liquid content and lung volume were assessed at birth and every 30 min over 2 h using magnetic resonance imaging (MRI) in three groups of lambs delivered by cesarean: preterm, late preterm, and late preterm with antenatal steroids. Lung function and mechanics of the respiratory system were also measured.

RESULTS

Lung liquid content increased by approximately 30% in the preterm group (P < 0.05), whereas it did not change significantly in the late preterm lambs. Antenatal steroids induced a 50% drop in the lung liquid content (P < 0.05). Total lung volume increased in all groups (P < 0.05) but was higher in the late preterm + steroids group relative to other groups (P < 0.05). Compliance and resistances of the respiratory system were significantly correlated with lung liquid content (P < 0.05).

CONCLUSION

FRC expansion results mainly from an increase in lung volume rather than a decrease in lung liquid in preterm and late preterm lambs. Antenatal steroids promote FRC expansion through increases in lung volume and liquid clearance.

Clinical implication of lung fluid balance in the perinatal period

At birth, lung fluid produced during fetal life must be cleared immediately and efficiently before the first breath takes place, in order for infants to achieve a normal and successful transition from prenatal to postnatal life. Postnatal lung fluid resorption is mediated through activation of airway epithelial sodium channels (ENaC). The observation that ENaC expression is a gestational age-dependent process contributes to our understanding of the development of respiratory distress in both term and preterm infants due to impaired clearing of fluid from their lungs. As fluid absorption, mediated by ENaC activity, in postnatal life has a significant biological role in preventing respiratory distress, any strategy that enhances ENaC activity can potentially help to decrease its incidence and associated morbidity.

Risk factors and management of transient tachypnea of the newborn

Transient tachypnea of the newborn (TTN) is the consequence of delayed clearance of fetal lung liquid in the newborn. With recognition of the increased risk in babies born by Cesarean sections, epidemiologic association with maternal asthma and increasing research on the possible role of genetic polymorphisms of ion-channel subunits, our understanding of the pathophysiology of this condition has vastly improved. We now know that the late-preterm infant, born at 34–36 weeks gestation, is at increased risk for both TTN and respiratory distress syndrome due to surfactant deficiency. As the incidence of Cesarean sections rises, there is likelihood of increased respiratory morbidity in newborns that will necessitate additional medical interventions and exposure to complications of intensive care. This review focuses on the risk factors that are associated with the development of TTN and the treatment strategies that are employed for the management of this condition.

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.

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.

Oxytocin-induced labor augments IL-1beta-stimulated lung fluid absorption in fetal guinea pig lungs

We tested the hypothesis that oxytocin-induced labor augmented IL-1beta-induced/-stimulated lung fluid absorption in preterm guinea pig fetuses. IL-1beta was administered subcutaneously daily to timed-pregnant guinea pigs for 3 days with and without simultaneous cortisol synthesis inhibition by metyrapone. At day 3, oxytocin was administered, and fetuses were delivered by abdominal hysterotomy at 61 and by oxytocin-induced birth at 68 days gestation. Delivered fetuses were instilled with isosmolar 5% albumin into the lungs, and lung fluid movement was measured over 1 h by mass balance. Lung fluid absorption was induced in 61-day and stimulated in 68-day gestation lungs by IL-1beta. Labor induction by oxytocin augmented IL-1beta-induced/-stimulated lung fluid absorption. Metyrapone pretreatment did not affect oxytocin-induced/-stimulated lung fluid absorption, while completely blocking IL-1beta-induced/-stimulated fluid absorption. Fetal lung fluid absorption, when present, was always propranolol and amiloride sensitive, suggesting that beta-adrenoceptor stimulation and amiloride-sensitive sodium channels were critical for fluid absorption. Epithelial sodium channel and Na-K-ATPase subunit expressions were both increased by IL-1beta, but not further by oxytocin. Our results indicate that IL-1beta release into the maternal blood circulation positively affects lung maturation due to the IL-1beta-induced release of cortisol and thus prepares the lungs for the epinephrine surge associated with labor.

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.

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.

Distal air space epithelial fluid clearance in near-term rat fetuses is fast and requires endogenous catecholamines

Knowledge about the conversion of the epithelium in the distal air spaces of the lung from secretion to absorption is imperative to the understanding of postnatal lung development; little such information is available in rats. Distal air space fluid clearance was therefore measured in 21- to 22-day gestation rat fetuses and newborn (40 h) rats. Distal air space fluid clearance was measured from the increase in (131)I-albumin concentration in an isosmolar, physiological solution instilled into the developing lungs. There was no net fluid movement across the distal air space epithelium in the lungs of 21-day gestation fetuses. Twenty-four hours later, distal air space fluid was cleared at a rapid rate in the 22-day gestation fetuses. Within the first 40 h after birth, the rate rapidly declined to adult levels. The high distal air space fluid clearance at 22 days gestation and at 40 h after birth was mediated by beta-adrenergic receptors as demonstrated by elevated plasma epinephrine levels and inhibition by propranolol. Interestingly, the elevated distal air space fluid clearance in the 22-day gestation fetuses was only minimally amiloride sensitive; however, amiloride sensitivity increased over the first 40 h after birth. In conclusion, these studies demonstrate that 1) rapid rates of net alveolar fluid clearance occur late in gestation in the rat and 2) this clearance is driven by elevations of endogenous epinephrine.

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.

Effects of glucagon on in vitro liquid production by lungs from fetal guinea pigs

BACKGROUND

Lung liquid reabsorption in newborns with respiratory distress syndrome can be deficient. Respiratory distress syndrome is often seen in infants of diabetic mothers, in whom the neonatal surge of glucagon is suppressed.

AIM

To investigate the possible effects of glucagon on lung liquid reabsorption.

METHODS

Lungs from near term fetal guinea pigs (62 (2) days gestation; term = 67 days) were supported in vitro for three hours; lung liquid production and reabsorption were monitored by a dye dilution method.

RESULTS

Untreated control preparations produced fluid at 1.75 (0.33) ml/h per kg body weight, and did not change significantly in three hours; those immersed in 10(-12) M glucagon during the middle hour showed no significant change, but those given higher concentrations all showed significant reductions in fluid production or even reabsorption (65.6 (10.3)% fall at 10(-11) M, 70.0 (6.3)% fall at 10(-10) M, and 90.6 (11.1)% fall at 10(-9) M; based on 54 preparations). At 10(-9) M glucagon, 12 out of 30 preparations reabsorbed fluid. The linear log dose-response curve (r(2) = 0.94) gave a theoretical threshold at 4 x 10(-15) M glucagon. Responses appeared to involve the amiloride sensitive Na(+) based reabsorptive system: responses to 10(-9) M glucagon appeared to be reduced by 10(-6) M amiloride, and were abolished by 10(-5) M amiloride (based on 72 preparations).

CONCLUSIONS

The results suggest that the surge of glucagon at birth may help to drain the lungs of fluid. As glucagon liberates cAMP, which also stimulates surfactant, glucagon is worth consideration for possible use in neonatal respiratory distress.

Alveolar epithelial fluid clearance is mediated by endogenous catecholamines at birth in guinea pigs

Transition from placental to pulmonary oxygenation at birth depends on a rapid removal of fetal lung fluid from the developing alveoli. Alveolar fluid clearance was examined in ventilated, anesthetized developing guinea pigs of the ages newborn, 2-d-old, 5-d-old, 30-d-old, and 60-d-old (adult). An isosmolar 5% albumin solution was instilled into the lungs of the guinea pigs; the guinea pigs were then studied for 1 h. Alveolar fluid clearance was measured from the increase in alveolar protein concentration as water was reabsorbed. Newborn guinea pigs had a very high alveolar fluid clearance rate that declined rapidly within the first 5 postnatal days towards adult levels. The high alveolar fluid clearance at birth was apparently mediated by the beta-adrenergic system as demonstrated by the elevated plasma epinephrine levels and the increased sensitivity to inhibition by the beta-adrenergic antagonist propranolol immediately after birth. Surprisingly, exogenous addition of epinephrine was not able to stimulate alveolar fluid clearance in the newborn lung, but exogenous epinephrine stimulation increased over time to adult levels. The elevated alveolar fluid clearance at birth was associated with a significantly greater amiloride sensitivity in the newborn guinea pig lung. Northern blot analysis of distal lung tissue as well as isolated alveolar epithelial type II cells showed and confirmed higher levels of the alpha-subunit of the epithelial sodium channel mRNA in the newborn lung that rapidly tapered off toward adult levels. In conclusion, these data demonstrate the importance of the beta-adrenergic system and amiloride-sensitive sodium transporting pathways for clearance of fetal lung fluid at birth.

Alpha-adrenoreceptor influences on liquid movements by in vitro lungs from fetal guinea pigs

Lungs from near-term fetal guinea pigs (60 +/- 2 days of gestation) were supported in vitro for 3 h; lung liquid production was monitored by a dye-dilution method. Studies of 30 fetuses showed that untreated preparations produced fluid at 1.34 +/- 0.21 ml.h-1.kg body wt-1, but epinephrine at concentrations known at delivery (10(-8) and 10(-7) M) produced significant reductions or fluid reabsorption (analysis of variance, regression analysis); at high levels (10(-6) and 10(-5) M, epinephrine had no effect. Maximal responses from 10(-7) M epinephrine involved alpha-adrenoreceptors, since they were abolished by 10(-6) M phentolamine (alpha-antagonist) but were unaffected by 10(-6) M propranolol (beta-antagonist; n = 36). Activation was through alpha2-adrenoreceptors, since responses were abolished by 10(-4) M yohimbine (alpha-antagonist; n = 24) but were resistant to 10(-5) M prazosin (alpha 1-antagonist; n = 24). At high levels of epinephrine (10(-5) M), where responses did not normally occur, reductions in lung liquid production were large if prazosin was also present (n = 24), and increases were significant if yohimbine was included (n = 24). In guinea pigs, epinephrine appears to activate lung fluid reabsorption through alpha 2-adrenoreceptors; at high concentrations only, it can also increase production through alpha 1-adrenoreceptors. Therefore, species differences appear to exist.

Massive decline in lung liquid before vaginal delivery at term in the fetal lamb

OBJECTIVE

Our aim was to determine the volume of liquid remaining in the lungs of the fetal lamb just before a normal vaginal delivery at term to assess the extent to which an excess of liquid in the airspaces might contribute to the respiratory morbidity that accompanies elective cesarean delivery.

STUDY DESIGN

The volume of liquid in the future airspace of the lungs was determined at the end of labor in eight fetal lambs at term from the dilution of an impermeable tracer (125I-labeled human serum albumin) mixed into the liquid. This volume was compared with that measured in a second group of 10 fetal lambs studied 7 days before the expected date of delivery (term = 147 days).

RESULTS

The volume of lung liquid present at the end of labor was 6.8 +/- 1.0 ml x kg(-1) (n = 8) compared with 28.2 +/- 1.8 ml x kg(-1) (n = 10) in the second group of lambs studied before the onset of labor at 140 days of gestation.

CONCLUSION

Our results indicate that the bulk (>75%) of the liquid that fills the lungs of the fetal lamb at 140 days of gestation is cleared at some time before normal term birth, suggesting that the adverse respiratory impact of elective cesarean delivery may be largely explained by denying the fetus this important adaptive mechanism.

Parturition and fetal adaptation

The fetus as "patient" during labor and birth has become an increasingly important concept during the past 20 years. However, what is understood about fetal status during labor and how the fetus prepares for its approaching separation from its mother? Current information indicates that not only is the term fetus well prepared for the adaptation to extrauterine life, but this transition is facilitated by normal labor.

Stimulation of lung epithelial liquid clearance by endogenous release of catecholamines in septic shock in anesthetized rats

Exogenous administration of beta-adrenergic agonists has previously been reported to increase lung liquid clearance by stimulation of active sodium transport across the alveolar epithelium. We hypothesized for this study that endogenous release of epinephrine in septic shock would stimulate liquid clearance from the airspaces in rats. Liquid clearance from the air spaces was measured by the concentration of protein over 4 h in a test solution of 5% albumin instilled into one lung. Bacteremic rats developed severe systemic hypotension and metabolic acidosis that was associated with a 100-fold rise in plasma epinephrine levels. There was a 100% increase in liquid clearance from the airspaces of the lung in the bacteremic compared with control rats. To determine the mechanisms responsible for this accelerated lung liquid clearance, amiloride (10(-3) M), a sodium transport inhibitor, was added to the air spaces. Amiloride prevented the increase in liquid clearance from the airspaces, indicating that this effect depended on increased uptake of sodium across the lung epithelium. The addition of propranolol (10(-4) or 10(-5) M) to the instillate also prevented the acceleration in alveolar liquid clearance in the bacteremic rats. We conclude that the release of endogenous catecholamines associated with septic shock markedly stimulates fluid clearance from the distal airspaces of the lung by a beta-adrenergic mediated stimulation of active sodium transport across the epithelial barrier. This data provides evidence for a previously unrecognized mechanism that can protect against or hasten the resolution of alveolar edema in pathological conditions, such as septic shock, that are associated with the endogenous release of catecholamines.