Fetal Lung Liquid Production Research Articles

Cloning and cellular expression of aquaporin 9 in ovine fetal membranes

Amniotic fluid (AF) absorption across fetal membranes is essential for AF volume homeostasis, balancing fetal swallowing, urine flow, and lung liquid production. In sheep, AF is absorbed primarily across the amniotic membrane into fetal vasculature situated between the amnion and chorion. Aquaporins (AQPs) are cell membrane proteins that serve as water channels. Recent studies have demonstrated the expression of AQP 1, 3, 8, and 9 in human chorioamniotic membranes and placenta. As AF dynamics continued to be explored primarily in the ovine model, we sought to clone and characterize the expression of ovine AQP9 in fetal membranes. Ovine AQP9 gene was cloned with the use of homology reverse transcriptase-polymerase chain reaction (RT-PCR). RT-PCR and Northern analysis were used to determine AQP9 gene expression, and immunohistochemistry (IHC) used to localize AQP9 protein expression in ovine fetal membranes. A 2085-base pair (bp) full-length complementary DNA (cDNA) sequence of ovine AQP9 was cloned. The ovine AQP9 cDNA is 86%, 82%, and 82%, and the predicted amino acid sequence (295 amino acids) is 77%, 71%, and 69% identical to human, rat, and mouse AQP9, respectively. RT-PCR and Northern analysis detected AQP9 messenger RNA expression in ovine amnion and allantois, but not in placenta, chorion, or umbilical cord. Immunohistochemistry localized AQP9 protein in epithelia of amnion and allantois. The presence of significant AQP9 messenger RNA and protein expression in ovine fetal membranes suggests that AQP9 may be a major water channel for intramembranous AF resorption in sheep. The cloning of ovine AQP9 and the demonstration of AQP9 expression in amnion and allantois significantly enhances our understanding of ovine AF regulation and offers the potential for therapeutic approaches for the treatment of oligohydramnios and polyhydramnios.

Alterations in maternal corticosteroid levels influence fetal urine and lung liquid production.

This study was designed to test the hypotheses that disruption of maternal adrenal secretion in late pregnancy requires fetal adaptations in order to maintain fetal blood volume and fetal viability. Pregnant ewes were adrenalectomized at approximately 112 days, and cortisol and aldosterone were replaced to either normal pregnant levels (with 1 mg/kg per day of cortisol and 3 microg/kg per day of aldosterone) or normal nonpregnant levels of aldosterone or cortisol (0.5 mg/kg per day of cortisol or 1.5 microg/kg per day of aldosterone). Fetal blood volume, blood pressure, lung liquid production, urine production, free water clearance, and glomerular filtration rate were measured at 130 days. In a separate group, fetal organ blood flow was measured. Fetal blood volume was not significantly decreased by disruption of maternal corticosteroid secretion. However fetal urine production and free water clearance were reduced in fetuses of low cortisol or low aldosterone ewes. Fetal lung liquid secretion was also significantly reduced in the low aldosterone group. The glomerular filtration rate was reduced in fetuses of all adrenalectomized ewes, regardless of replacement dose. Fetal blood pressure was significantly reduced in the fetuses of low aldosterone ewes; blood flow to several fetal organs was increased in this group, indicating that decreased vascular resistance may contribute to the relative hypotension. Alterations in maternal adrenal corticosteroid levels resulted in fetal adaptation to maintain fetal blood volume despite relative maternal hypovolemia. These adaptations occurred at the expense of fetal urine and lung liquid production.

Fetal lung liquid secretion: insights using the tools of inhibitors and genetic knock-out experiments.

At birth, the newborn infant’s organ for gas exchange switches from the placenta to the lungs. Despite major advances in our understanding of the fundamental mechanisms involved in fetal lung development, respiratory disorders continue to be the major cause of perinatal morbidity and mortality. It is therefore not surprising that many investigators continue their research into the mechanisms responsible for the normal development of the fetal lung. The initial key observation that led to our present understanding of fetal lung liquid production occurred in the 1940s when Jost and Policard (1) showed that the fluid within the fetal lung arises from the lung and did not, as it had been believed, represent aspirated amniotic fluid. At this time investigators speculated that the fluid was a transudate arising from the vasculature. However, experiments in the 1960s demonstrated that the fetal lung liquid

Effects of intrauterine growth restriction on lung liquid dynamics and lung development in fetal sheep

Objective: The aim of this study was to determine the effects of intrauterine growth restriction on fetal lung liquid and lung development. Study Design: Intrauterine growth restriction was induced in 7 fetal sheep from 120 to 140 days' gestation (term, approximately 147 days' gestation) by umbilicoplacental embolization. We used 6 control fetuses. Volumes and production rates of fetal lung liquid were measured, and lungs were removed post mortem (140 days' gestation) for analysis of concentrations of deoxyribonucleic acid, protein, and messenger ribonucleic acid for surfactant proteins A, B, and C. Results: Umbilicoplacental embolization induced fetal hypoxemia, hypoglycemia, and intrauterine growth restriction. At 140 days' gestation lung weight was reduced by 34%, and the fetal lung liquid production rate (15.9 ± 1.8 mL/h for intrauterine growth restriction vs 24.8 ± 3.9 mL/h for control) and volume (110.9 ± 16.3 mL for intrauterine growth restriction vs 178.1 ± 11.9 mL for control) were reduced in the intrauterine growth restriction group. After adjustment for body weight, however, values were not different from those in the control group. Pulmonary deoxyribonucleic acid and plasma cortisol concentrations were elevated by intrauterine growth restriction, but levels of messenger ribonucleic acid for surfactant proteins were unchanged. Conclusion: In intrauterine growth restriction, lung liquid and lung growth were proportionate to body weight, and surfactant protein expression was unaffected. Alterations in lung deoxyribonucleic acid concentrations suggest that the lungs may be structurally immature. (Am J Obstet Gynecol 2001;184:209-16.)

The association between meconium and the production and reabsorption of lung liquid and lactate loss by in vitro lungs from fetal guinea pigs

Objective: We sought to use an in vitro approach to determine relationships between meconium and fetal lung liquid production and to relate meconium to possible problems in lung liquid removal at birth. Study Design: Near-term fetal guinea pigs were divided according to the level of spontaneously occurring meconium (no meconium, light meconium, or heavy meconium). Their lungs were maintained in vitro in Krebs-Henseleit saline solution for 3 hours. Lung liquid production or reabsorption was measured by a dye-dilution method, and loss of lactate to the lung liquid and outer Krebs-Henseleit solution was monitored. Reabsorptions were investigated by activating powerful responses with dinitrophenol during the middle hour. Results: During the first hour, lungs from meconium-free fetuses produced liquid at 1.25 ± 0.12 mL/kg–1 body weight · h–1; rates from light or heavy meconium groups were not significantly different (n = 18). Similarly, total lactate loss was not significantly different between the meconium-free and light-meconium groups but was twice as high in heavy-meconium preparations (73.68 ± 10.60 μmol/L · g–1 dry lung tissue · h–1; P <.025, analysis of variance). The meconium-free and heavy-meconium groups continued to produce fluid, with no significant change throughout the 3 hours of incubation; lactate losses fell slightly. Therefore there were no problems with fluid production with meconium present, but the high-lactate losses with heavy meconium suggested long-term intrauterine hypoxia. Dinitrophenol produced powerful reabsorptions in lungs from meconium-free fetuses (–0.85 ± 0.35 mL · kg–1 body weight · h–1; P <.005, analysis of variance; P <.0005, regression analysis) but failed to do so in heavy-meconium fetuses (n = 36). Lactate losses rose 2-fold in both groups (P <.005 to P < 0.0005, analysis of variance and regression analysis), despite already elevated losses with heavy meconium (n = 12). Therefore, in heavy-meconium fetuses, dinitrophenol affected metabolic pathways but did not activate fluid reabsorption, suggesting damage to reabsorptive mechanisms. Conclusion: Unless major airways are blocked, meconium is not associated with reduced fetal lung liquid production, which can cause poor lung development, but there may well be poor fluid removal after birth because of compromised reabsorptive mechanisms, which are unlikely to be helped by possible hormonal intervention. (Am J Obstet Gynecol 2000;183:235-44.)

The association between meconium and the production and reabsorption of lung liquid and lactate loss by in vitro lungs from fetal guinea pigs*1

Objective: We sought to use an in vitro approach to determine relationships between meconium and fetal lung liquid production and to relate meconium to possible problems in lung liquid removal at birth. Study Design: Near-term fetal guinea pigs were divided according to the level of spontaneously occurring meconium (no meconium, light meconium, or heavy meconium). Their lungs were maintained in vitro in Krebs-Henseleit saline solution for 3 hours. Lung liquid production or reabsorption was measured by a dye-dilution method, and loss of lactate to the lung liquid and outer Krebs-Henseleit solution was monitored. Reabsorptions were investigated by activating powerful responses with dinitrophenol during the middle hour. Results: During the first hour, lungs from meconium-free fetuses produced liquid at 1.25 ± 0.12 mL/kg–1 body weight · h–1; rates from light or heavy meconium groups were not significantly different (n = 18). Similarly, total lactate loss was not significantly different between the meconium-free and light-meconium groups but was twice as high in heavy-meconium preparations (73.68 ± 10.60 μmol/L · g–1 dry lung tissue · h–1; P <.025, analysis of variance). The meconium-free and heavy-meconium groups continued to produce fluid, with no significant change throughout the 3 hours of incubation; lactate losses fell slightly. Therefore there were no problems with fluid production with meconium present, but the high-lactate losses with heavy meconium suggested long-term intrauterine hypoxia. Dinitrophenol produced powerful reabsorptions in lungs from meconium-free fetuses (–0.85 ± 0.35 mL · kg–1 body weight · h–1; P <.005, analysis of variance; P <.0005, regression analysis) but failed to do so in heavy-meconium fetuses (n = 36). Lactate losses rose 2-fold in both groups (P <.005 to P < 0.0005, analysis of variance and regression analysis), despite already elevated losses with heavy meconium (n = 12). Therefore, in heavy-meconium fetuses, dinitrophenol affected metabolic pathways but did not activate fluid reabsorption, suggesting damage to reabsorptive mechanisms. Conclusion: Unless major airways are blocked, meconium is not associated with reduced fetal lung liquid production, which can cause poor lung development, but there may well be poor fluid removal after birth because of compromised reabsorptive mechanisms, which are unlikely to be helped by possible hormonal intervention. (Am J Obstet Gynecol 2000;183:235-44.)

The effect of dopamine on lung liquid production by in vitro lungs from fetal guinea-pigs.

1. The neuroendocrine system of the lungs has no clear function. However, previous studies of one of its products, somatostatin, have implicated it in lung liquid removal at birth. The present study extends this concept by investigating the effects of dopamine, a major product of this system, on lung liquid reabsorption. 2. The effects of dopamine on fetal lung liquid production and reabsorption were tested on in vitro lungs from fetal guinea-pigs of 60 +/- 2 days of gestation (term = 67 days). Dopamine was placed in the outer bathing saline during the middle hour of 3 h incubations. Fluid movements across the pulmonary epithelium were monitored by a dye dilution method, and changes in rates over 1 h intervals were tested for significance by analysis of variance and regression analysis. 3. Dopamine was able to reduce fluid production or cause reabsorption (based on 42 preparations). Control preparations and those given 10-8 M dopamine showed no significant changes; those given higher concentrations showed significant reductions in production or reabsorption (P < 0.025 to P < 0.0005), according to dose (42.6 +/- 10.8% reduction at 10-7 M; 75.4 +/- 5.9% reduction at 10-6 M; 92.1 +/- 7.0% reduction at 10-5 M and 121.4 +/- 12.8% (reabsorption) at 10-4 M dopamine). The linear log dose-response curve (r = 0.99) showed a theoretical threshold at 1.7 x 10-9 M dopamine. 4. Effects were mediated through specific dopamine receptors (based on 78 preparations). Dopamine at 10-6 M was tested together with each of three dopamine receptor antagonists at 10-5 M. The general dopamine receptor antagonist haloperidol and the more specific D2 receptor blocker domperidone both abolished responses, but the D1 receptor antagonist SCH 23390 was without effect. This suggested that D2 dopamine receptors mediated the responses, and that responses were not due to conversion of dopamine to adrenaline or noradrenaline. 5. There was no evidence that responses involved amiloride-sensitive Na+ transport (based on 54 preparations). Apical amiloride at 10-6, 10-5 or 10-4 M, and the more specific Na+ channel blocker benzamil (10-5 M), had no effect on responses to dopamine, in contrast to their effects on responses to adrenaline in sheep. 6. It is suggested that internal release of dopamine by the neuroendocrine system of the lungs may influence lung liquid reabsorption at birth. This system, which also produces somatostatin, another agent active on lung liquid production, is maximally developed and activated at birth; it is also deficient in hyaline membrane disease.

Mechanism of arginine vasopressin suppression of ovine fetal lung fluid secretion: lack of V2-receptor effect.

Fetal lung liquid production is essential for in utero pulmonary development, and the resorption of lung liquid at birth facilitates neonatal transition. Lung liquid also contributes importantly to amniotic fluid volume. Factors that influence lung liquid production/resorption may, therefore, impact fetal pulmonary growth and development as well as amniotic fluid homeostasis. Arginine vasopressin (AVP) inhibits fetal lung liquid production and facilitates lung liquid resorption. In view of studies administering fetal AVP or the V2 receptor agonist, 1 desamino 8-D AVP (dDAVP) for the regulation of amniotic fluid volume, we sought to determine the impact of AVP V2 receptor stimulation on fetal lung liquid production. Eight near-term ovine fetuses (130 +/- 2 d; term = 145 d) were prepared with hind limb vascular catheters, a bladder catheter, and a tracheal catheter. Each ewe received vascular and amniotic catheters. After 5 days of recovery, the animals were studied for a 2-hr control period and for 2 hr following intravenous dDAVP injection (1 ng/kg). Lung liquid and urine flow rates and plasma electrolyte and osmolality composition were determined at 30-min intervals. To confirm fetal lung liquid suppression in response to AVP, two animals received an intravenous injection of AVP (2 ng/kg) 24 hr after the first experiment. dDAVP administration had no effect on lung liquid production (3.4 +/- 0.4 ml/kg/h), electrolyte concentrations, or osmolality. Fetal urine electrolyte concentrations and osmolality (154 +/- 24-295 +/- 22 mOsm/kg H2O) increased in response to dDAVP, whereas urine flow decreased (9.4 +/- 2.5-4.2 +/- 1.5 ml/kg/h). In the fetuses exposed to AVP, lung liquid production decreased (3.3-1.6 ml/kg/h). As AVP, although not dDAVP, inhibited fetal lung liquid production, these results indicate that AVP effects on lung liquid are likely mediated via the AVP V1 receptor, not the V2 receptor. The use of fetal administration of dDAVP for the regulation of urine production and amniotic fluid volume will not adversely impact lung liquid production.

Mechanism of arginine vasopressin suppression of ovine fetal lung fluid secretion: Lack of V2‐receptor effect

Fetal lung liquid production is essential for in utero pulmonary development, and the resorption of lung liquid at birth facilitates neonatal transition. Lung liquid also contributes importantly to amniotic fluid volume. Factors that influence lung liquid production/resorption may, therefore, impact fetal pulmonary growth and development as well as amniotic fluid homeostasis. Arginine vasopressin (AVP) inhibits fetal lung liquid production and facilitates lung liquid resorption. In view of studies administering fetal AVP or the V2 receptor agonist, 1 desamino 8-D AVP (dDAVP) for the regulation of amniotic fluid volume, we sought to determine the impact of AVP V2 receptor stimulation on fetal lung liquid production. Eight near-term ovine fetuses (130 ± 2 d; term = 145 d) were prepared with hind limb vascular catheters, a bladder catheter, and a tracheal catheter. Each ewe received vascular and amniotic catheters. After 5 days of recovery, the animals were studied for a 2-hr control period and for 2 hr following intravenous dDAVP injection (1 ng/kg). Lung liquid and urine flow rates and plasma electrolyte and osmolality composition were determined at 30-min intervals. To confirm fetal lung liquid suppression in response to AVP, two animals received an intravenous injection of AVP (2 ng/kg) 24 hr after the first experiment. dDAVP administration had no effect on lung liquid production (3.4 ± 0.4 ml/kg/h), electrolyte concentrations, or osmolality. Fetal urine electrolyte concentrations and osmolality (154 ± 24–295 ± 22 mOsm/kg H2O) increased in response to dDAVP, whereas urine flow decreased (9.4 ± 2.5–4.2 ± 1.5 ml/kg/h). In the fetuses exposed to AVP, lung liquid production decreased (3.3–1.6 ml/kg/h). As AVP, although not dDAVP, inhibited fetal lung liquid production, these results indicate that AVP effects on lung liquid are likely mediated via the AVP V1 receptor, not the V2 receptor. The use of fetal administration of dDAVP for the regulation of urine production and amniotic fluid volume will not adversely impact lung liquid production. J. Matern.–Fetal Med. 7:177–182, 1998. © 1998 Wiley-Liss, Inc.

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.

Mechanism of arginine vasopressin suppression of ovine fetal lung fluid secretion: Lack of V2-Receptor effect

Fetal lung liquid production is essential for in utero pulmonary development, and the resorption of lung liquid at birth facilitates neonatal transition. Lung liquid also contributes importantly to amniotic fluid volume. Factors that influence lung liquid production/resorption may, therefore, impact fetal pulmonary growth and development as well as amniotic fluid homeostasis. Arginine vasopressin (AVP) inhibits fetal lung liquid production and facilitates lung liquid resorption. In view of studies administering fetal AVP or the V2 receptor agonist, 1 desamino 8-D AVP (dDAVP) for the regulation of amniotic fluid volume, we sought to determine the impact of AVP V2 receptor stimulation on fetal lung liquid production. Eight near-term ovine fetuses (130 ± 2 d; term = 145 d) were prepared with hind limb vascular catheters, a bladder catheter, and a tracheal catheter. Each ewe received vascular and amniotic catheters. After 5 days of recovery, the animals were studied for a 2-hr control period and for 2 hr following intravenous dDAVP injection (1 ng/kg). Lung liquid and urine flow rates and plasma electrolyte and osmolality composition were determined at 30-min intervals. To confirm fetal lung liquid suppression in response to AVP, two animals received an intravenous injection of AVP (2 ng/kg) 24 hr after the first experiment. dDAVP administration had no effect on lung liquid production (3.4 ± 0.4 ml/kg/h), electrolyte concentrations, or osmolality. Fetal urine electrolyte concentrations and osmolality (154 ± 24–295 ± 22 mOsm/kg H2O) increased in response to dDAVP, whereas urine flow decreased (9.4 ± 2.5–4.2 ± 1.5 ml/kg/h). In the fetuses exposed to AVP, lung liquid production decreased (3.3–1.6 ml/kg/h). As AVP, although not dDAVP, inhibited fetal lung liquid production, these results indicate that AVP effects on lung liquid are likely mediated via the AVP V1 receptor, not the V2 receptor. The use of fetal administration of dDAVP for the regulation of urine production and amniotic fluid volume will not adversely impact lung liquid production.

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.

The Effects of cGMP on Fetal Pulmonary Blood Flow and Lung Liquid Production. ♦ 268

The Effects of cGMP on Fetal Pulmonary Blood Flow and Lung Liquid Production. ♦ 268

Cyclic Guanosine Monophosphate (cGMP) Reduces Fetal Lung Liquid Production Independent of Changes in Pulmonary Blood Flow. † 1483

Cyclic Guanosine Monophosphate (cGMP) Reduces Fetal Lung Liquid Production Independent of Changes in Pulmonary Blood Flow. † 1483

REDUCTION IN FETAL LUNG LIQUID PRODUCTION WITH CYCLIC ADENOSINE MONOPHOSPHATE(cAMP) IS ASSOCIATED WITH A REDUCTION IN PULMONARY VASCULAR RESISTANCE.† 1958

At birth, both fetal lung liquid production and pulmonary vascular resistance are rapidly reduced. We have previously shown that mediators of pulmonary vasodilation, including prostaglandins, also cause significant reductions in fetal lung liquid production (J Appl Physiol 1995; 1212). Since cAMP is a known mediator of both lung liquid resorption and of prostaglandin-induced vasorelaxation, we asked whether it might be an important mediator of transitional events at birth. To see if cAMP, at a concentration sufficient to cause significant reduction in lung liquid production, would lower pulmonary resistance, we studied its effects on pulmonary arterial pressure and flow. Eight chronically prepared fetal sheep were studied at 131 ± 5 d gestation. Net lung luminal liquid production(Jv) was measured by plotting the change in lung luminal liquid concentration of radiolabelled albumin, an impermeant tracer that was mixed into the lung liquid at the start of each study. Variables were measured during a 1-2 h baseline, then for 2-3 h following an intratracheal instillation of dibutyryl cAMP (initial concentration ≅10-4 M). Instillation of cAMP reduced Jv from 25 ± 8 ml/h to 4 ± 8 ml/h (P<.001) and pulmonary artery pressure from 46 ± 3 mm Hg to 43± 4 mm Hg (P<.05). There was a modest increase in left pulmonary blood flow (P=.10) such that the calculated pulmonary vascular resistance fell from 0.89 ± 0.24 to 0.66 ± 0.17 mm Hg/ml/min (P<.05). Systemic arterial and left atrial pressures and heart rate did not change significantly following cAMP instillation. Control (saline) instillations(n=6) caused no significant change in any variable.

Pulmonary vasodilator drugs decrease lung liquid production in fetal sheep.

To examine a potential relationship between pulmonary vasodilatation and fetal lung liquid production, I measured lung liquid production in 20 fetal sheep at 130 +/- 4 days gestation while using several agents known to increase pulmonary blood flow. Thirty-two studies were done in which left pulmonary arterial flow (Qlpa) was measured by an ultrasonic Doppler flow probe and 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. Qlpa and Jv were measured during a 1- to 2-h baseline period and then during a 1- to 2-h infusion period in which the fetuses received either an intravenous infusion of acetylcholine (n = 8), prostaglandin D2 (n = 10), or the leukotriene blocker FPL-55712 (n = 7). These vasodilators work by different mechanisms, each mechanism having been implicated in the decrease in pulmonary vascular resistance seen at birth. Control (saline) infusions (n = 7) caused no change in either Qlpa or Jv over 4 h. All vasodilator agents significantly increased pulmonary blood flow and decreased Jv. Pulmonary arterial pressure did not change significantly in either the control, acetylcholine, prostaglandin, or leukotriene-blocker studies, indicating that pulmonary vascular resistance decreased. Thus agents that increase pulmonary blood flow by mechanisms that occur at birth also decrease lung liquid production in fetal lambs.

Ovine fetal lung fluid response to atrial natriuretic factor

The fetal lung is an important site of fluid production and is postulated to serve a regulatory role in fetal fluid balance. To assess the role of atrial natriuretic factor on fetal lung liquid production, we studied the effect of intravenous atrial natriuretic factor infusion on tracheal fluid production in fetal sheep with chronic vascular and tracheal catheters. Ovine fetuses (mean gestation = 130 days +/- 1 day) received successive 40-minute intravenous infusions of increasing doses of synthetic fragment 1-28 atrial natriuretic factor (5, 25, and 100 ng/min.kg-1). In response to the 25 ng/min.kg-1 infusion, fetal tracheal fluid production decreased from 1.2 +/- 0.3 ml/10 min to 0.6 +/- 0.2 ml/10 min (p less than 0.05), and remained suppressed during the 100 ng/min.kg-1 infusion (0.5 +/- 0.2 ml/10 min). There was a significant inverse correlation between tracheal fluid production and fetal plasma atrial natriuretic factor levels (r = -0.61, p less than 0.001). Basal tracheal fluid sodium and potassium concentrations (147 +/- 1 mEq/L and 5 +/- 1 mEq/L) and osmolality (291 +/- 3 mOsm) did not change during the atrial natriuretic factor infusion periods. The observation that atrial natriuretic factor acts to decrease fetal lung fluid production suggests that atrial natriuretic factor may be important in the fetal adaptive response to extrauterine life.

Effect of beta-adrenergic blockade on lung liquid secretion during fetal asphyxia

The hypothesis tested in this study was that beta-adrenergic stimulation is responsible for the inhibition of fetal lung liquid production during moderate fetal asphyxia. In chronically catheterized fetal sheep, net lung liquid production rates were measured over three consecutive periods: a control period, a period of reduced uterine blood flow (RUBF) or epinephrine infusion, and periods of RUBF or epinephrine infusion in the presence of the beta-adrenergic receptor antagonist propranolol. The net production rate of fetal lung liquid was decreased from a mean control value of 7.7 +/- 1.0 to 1.5 +/- 0.4 ml/h (P less than 0.001) by RUBF; the administration of propranolol had no further effect on these liquid production rates (1.1 +/- 0.5 ml/h). In other experiments epinephrine infusion reduced the net production rate of fetal lung liquid from a mean control value of 7.2 +/- 1.4 to 1.7 +/- 1.8 ml/h (P less than 0.025); the addition of propranolol reversed this inhibition (secretion rate 6.1 +/- 1.4 ml/h, P less than 0.005). We conclude that the inhibition of fetal lung liquid production induced by moderate fetal asphyxia does not solely result from catecholamine stimulation of pulmonary beta-receptors.

Bioelectric properties and ion transport across excised canine fetal and neonatal airways.

Knowledge of liquid secretion by fetal lung stems from studies of sheep. We extended these studies to dogs and examined the persistence of the fetal pattern of airway epithelial permeability and ion transport in the neonatal animal. Plasma and lung liquid from fetal dogs were analyzed for Na+, K+, Cl-, and HCO3-. Only the Cl- concentration of fetal lung liquid (129 meq/l) was significantly different from that of fetal plasma (111 meq/l). Segments of trachea from fetal and neonatal (less than 1, 7-10, and 21-46 days after birth) dogs were excised and mounted in flux chambers. The transepithelial potential difference (PD) of all tissues was oriented lumen negative (9.8-14.8 mV). Under short-circuit conditions, unidirectional Na+ flows were symmetrical. Cl- was secreted, and the secretion was equivalent to short-circuit current (Isc). Cl- secretion persisted under open-circuit conditions. Lobar bronchi from 21- to 46-day neonates absorbed Na+ (1.9 mueq.cm-2.h-1), but unidirectional flows of Cl- were symmetrical. Amiloride (10(-4) M) reduced Isc of neonatal bronchi by 47% but did not affect fetal bronchi. Isoproterenol increased Isc of both fetal (33%) and neonatal (40%) bronchi. These responses suggest that fetal bronchi do not absorb Na+ but can be stimulated to secrete Cl-. We conclude that Cl- secretion by epithelium of large airways may contribute to fetal lung liquid production, but it is unlikely that the tracheal epithelium is involved in fluid absorption at birth. Whereas fetal bronchi appear to secrete Cl-, neonatal bronchi absorb Na+.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of maternal hyperglycemia on acid base balance and lung liquid production in the fetal sheep.

In 10 chronically catheterized fetal sheep, the effects of 100 g of glucose infused (intravenous) to the pregnant ewe on lung liquid production and acid-base status were investigated. Maternal and fetal hyperglycemia occurred within 15 min of the infusion of glucose. There was a significant increase in fetal PCO2 from 41.67 +/- 1.00 to 46.89 +/- 1.83 mm Hg and a decrease in fetal arterial pH from 7.42 +/- 0.016 to 7.33 +/- 0.026 (n = 7). This acidosis was probably the result of fetal lactacidemia. There was also a decrease in fetal PO2 (p less than 0.001) following the infusion of glucose to the ewe. This, along with the acidosis, might account for the increased risk of unexplained fetal death in the diabetic pregnancy especially near term. Following the infusion of glucose to the ewe there was an increase in maternal plasma osmolality by 16 +/- 3.35 mosmol/kg (n = 9) and in fetal plasma osmolality by 14 +/- 2.64 mosmol/kg (n = 9; p less than 0.001). Fetal lung liquid production fell from 0.195 +/- 0.04 to 0.093 +/- 0.02 ml/min (n = 6; p less than 0.001). There was no change in the excretion of osmol by the lungs. The decreased lung liquid production was probably due to a decrease in the net movement of fluid across the pulmonary epithelium.