Alveolar fluid clearance in late-gestational guinea pigs after labor induction: mechanisms and regulation

The intertwining of oxytocin's effects on social affiliation and inflammation

•Oxytocin is an ancient adaptive hormone that promotes social affiliation to maximize fitness and longevity.•Oxytocin is a multifaceted hormone that regulates stress responses at all levels of cellular organization within individuals.•Oxytocin's dual actions on sociability and inflammation highlight its powerful capacity as a modulator of human health.

Birth signalling hormones and the developmental consequences of caesarean delivery

Rates of delivery by caesarean section (CS) are increasing around the globe and, although several epidemiological associations have already been observed between CS and health outcomes in later life, more are sure to be discovered as this practice continues to gain popularity. The components of vaginal delivery that protect offspring from the negative consequences of CS delivery in later life are currently unknown, although much attention to date has focused on differences in microbial colonisation. Here, we present the case that differing hormonal experiences at birth may also contribute to the neurodevelopmental consequences of CS delivery. Levels of each of the 'birth signalling hormones' (oxytocin, arginine vasopressin, epinephrine, norepinephrine and the glucocorticoids) are lower following CS compared to vaginal delivery, and there is substantial evidence for each that manipulations in early life results in long-term neurodevelopmental consequences. We draw from the research traditions of neuroendocrinology and developmental psychobiology to suggest that the perinatal period is a sensitive period, during which hormones achieve organisational effects. Furthermore, there is much to be learned from research on developmental programming by early-life stress that may inform research on CS, as a result of shared neuroendocrine mechanisms at work. We compare and contrast the effects of early-life stress with those of CS delivery and propose new avenues of research based on the links between the two bodies of literature. The research conducted to date suggests that the differences in hormone signalling seen in CS neonates may produce long-term neurodevelopmental consequences.

The inflammatory event of birth: How oxytocin signaling may guide the development of the brain and gastrointestinal system

The role of oxytocin (OT) as a neuropeptide that modulates social behavior has been extensively studied and reviewed, but beyond these functions, OT's adaptive functions at birth are quite numerous, as OT coordinates many physiological processes in the mother and fetus to ensure a successful delivery. In this review we explore in detail the potential adaptive roles of oxytocin as an anti-inflammatory, protective molecule at birth for the developing fetal brain and gastrointestinal system based on evidence that birth is a potent inflammatory/immune event. We discuss data with relevance for a number of neurodevelopmental disorders, as well as the emerging role of the gut-brain axis for health and disease. Finally, we discuss the potential relevance of sex differences in OT signaling present at birth in the increased male vulnerability to neurodevelopmental disabilities.

Persistent pulmonary hypertension of the newborn following elective cesarean delivery at term

OBJECTIVE

To evaluate the frequency of persistent pulmonary hypertension of the newborn (PPHN) following elective cesarean at greater than 34 weeks' gestation in an academically affiliated community hospital.

METHODS

Retrospective cohort study involving chart review of 300 newborns with PPHN between 1999 and 2006. Infants less than 34 weeks' or with congenital anomalies were excluded. Subjects were divided into two groups: (1) intended vaginal delivery and (2) elective cesarean.

RESULTS

A total of 125 neonates were included. In all, 46 were delivered vaginally, 53 by cesarean after a trial of labor, and 26 by elective cesarean. No statistically significant differences were noted between groups in birth weight, gestational age, or length of stay. The crude relative risk (RR) of PPHN in cesareans prior to labor (elective cesareans) when compared to intended vaginal deliveries was 2.0 (95% CI 1.3-3.1). The RR of PPHN in elective cesareans when compared to spontaneous labor resulting in vaginal deliveries was 3.4 (95% CI 2.1-5.5). The adjusted RRs for these outcomes comparing the same delivery groups when considering gestational age at birth (less vs. equal to or more than 37 weeks') were 2.2 (95% CI 1.4-3.4) and 3.7 (95% CI 2.3-6.1), and birth weight (less vs. equal to or more than 2500 g) were 1.9 (95% 1.3-3.0) and 3.4 (95% CI 2.1-5.5), respectively. The incidence of PPHN in the elective cesarean group was 6.9 per 1000 deliveries. The number of cesareans to be avoided to prevent one case of PPHN in this cohort was 387 (number needed to harm, 95% CI 206.8-3003.1).

CONCLUSIONS

Our findings include a high rate of PPHN following elective cesarean delivery, and suggest that physicians should consider this added morbidity when performing elective cesareans.

Inflammation in fetal sheep from intra-amniotic injection of Ureaplasma parvum

Bronchopulmonary dysplasia is associated with chorioamnionitis and fetal lung inflammation. Ureaplasma species are the bacteria most frequently isolated from chorioamnionitis. Very chronic ureaplasma colonization of amniotic fluid causes low-grade lung inflammation and functional lung maturation in fetal sheep. Less is known about shorter exposures of the fetal lung. Therefore, we hypothesized that ureaplasmas would cause an acute inflammatory response that would alter lung development. Singleton ovine fetuses received intra-amniotic Ureaplasma parvum serovar 3 or control media at 110, 117, or 121 days and were delivered at 124 days gestational age (term = 150 days). Inflammation was assessed by 1) cell counts in bronchoalveolar lavage fluid (BALF), and 2) cytokine mRNA measurements, immunohistochemistry, and flow cytometry for inflammatory cells and elastin and α-smooth muscle actin (α-SMA) staining in lung tissue. Neutrophils were increased in BALF 3 days after exposure to ureaplasmas (P = 0.01). Myeloperoxidase-positive cells increased after 3 days (P = 0.03), and major histocompatibility complex (MHC) class II-positive cells increased after 14 days of ureaplasma exposure (P = 0.001). PU.1 (macrophage marker)- or CD3 (T lymphocyte marker)-positive cells were not induced by ureaplasmas. CD3-positive cells in the posterior mediastinal lymph node increased in ureaplasma-exposed animals at 3, 7, and 14 days (P = 0.002). Focal elastin depositions decreased in alveolar septa at 14 days (P = 0.002), whereas α-SMA increased in arteries and bronchioli. U. parvum induced a mild acute inflammatory response and changed elastin and α-SMA deposition in the lung, which may affect lung structure and subsequent development.

IL-1beta-induced cortisol stimulates lung fluid absorption in fetal guinea pigs via SGK-mediated Nedd4-2 inhibition

We tested the hypothesis that interleukin (IL)-1beta-induced cortisol synthesis stimulates distal lung fluid absorption in fetal guinea pigs via induction of serum- and glucocorticoid-regulated kinase (SGK) and inhibition of neural precursor cell expressed, developmentally downregulated protein 4-2 (Nedd4-2). IL-1beta was subcutaneously administered daily to timed-pregnant guinea pigs over 3 days. Fetuses were obtained by abdominal hysterotomy at gestation day (GD)61 and GD68 and instilled with an isosmolar 5% albumin solution into the lungs. Distal lung fluid movement was measured over 1 h from the change in distal air space protein concentration. Fetal lungs were secreting lung fluid at GD61 while absorbing lung fluid at GD68. Distal lung fluid absorption was induced at GD61 by IL-1beta but unaffected at GD68. Plasma cortisol concentrations were increased by IL-1beta at GD61 and endogenously at GD68. Distal lung fluid absorption was measured and correlated to SGK and Nedd4-2 expression and to alpha-epithelial Na channel (ENaC) expression. SGK was increased by IL-1beta and late during gestation (GD68), while Nedd4-2 was decreased by IL-1beta and late during gestation. alpha-ENaC was induced by IL-1beta at GD61 and increased late during gestation. Thus our study suggests that cortisol-stimulated fetal lung fluid absorption is mediated by increased ENaC expression and may be governed by the SGK/Nedd4-2 pathway. These observations may explain why babies delivered preterm after intrauterine inflammation have a reduced risk of developing severe respiratory distress.

NKCC-1 and ENaC are down-regulated in nitrofen-induced hypoplastic lungs with congenital diaphragmatic hernia

Congenital diaphragmatic hernia (CDH) is accompanied by pulmonary hypoplasia and pulmonary hypertension. Fetal lung growth is dependent on the secretion of lung liquid, which normally is absorbed at partus. The ion channel NKCC-1 is involved in this secretory process, but has recently also been reported to be implicated in absorption. CDH patients show a disturbed transition from secretion to absorption. alpha- and beta-ENaC are essential for lung liquid absorption. Common for all transcellular ion transport is the need for Na/K-ATPase as a primary driving force. The aim of the study was first to map the normal pulmonary expression of the above proteins during late gestation and secondly to see if the expression was affected in a CDH rat model. Pregnant Sprague-Dawley rat dams were given nitrofen on gestational day 9.5 to induce CDH. The fetuses were removed on gestational days E18 and E21. In addition, newborn rats were harvested postpartum on day P2. The fetuses were put into one of two groups: hypoplastic lungs without CDH (N-CDH) and hypoplastic lungs with CDH (N+CDH). The pulmonary expression of NKCC-1, alpha-/beta-ENaC and Na/K-ATPase was then analyzed using Western blot. We found that the protein levels of NKCC-1 on gestational days E18 and E21 were significantly lower among fetuses with N+CDH as well as N-CDH compared to controls. The expression of beta-ENaC was also significantly down-regulated in both the groups on E18 and E21. The protein levels of alpha-ENaC and Na/K-ATPase were not found to be significantly decreased, but both showed a tendency towards down-regulation. The marked down-regulation of NKCC-1 in fetal hypoplastic lungs with CDH indicates a possibly decreased lung liquid production. This may be one of the mechanisms behind the disturbed pulmonary development in CDH. We also show that beta-ENaC is down-regulated. Down-regulation of beta-ENaC may result in abnormal lung liquid absorption, which could be one of the mechanisms behind the respiratory distress seen in CDH patients postpartum.

Stimulation of MAP kinase pathways after maternal IL-1beta exposure induces fetal lung fluid absorption in guinea pigs

BACKGROUND

We tested the hypothesis that maternal interleukin-1beta (IL-1beta) pretreatment and induction of fetal cortisol synthesis activates MAP kinases and thereby affects lung fluid absorption in preterm guinea pigs.

METHODS

IL-1beta was administered subcutaneously daily to timed-pregnant guinea pigs for three days. Fetuses were obtained by abdominal hysterotomy and instilled with isosmolar 5% albumin into the lungs and lung fluid movement was measured over 1 h by mass balance. MAP kinase expression was measured by western blot.

RESULTS

Lung fluid absorption was induced at 61 days (D) gestation and stimulated at 68D gestation by IL-1beta. Maternal IL-1beta pretreatment upregulated ERK and upstream MEK expression at both 61 and 68D gestation, albeit being much more pronounced at 61D gestation. U0126 instillation completely blocked IL-1beta-induced lung fluid absorption as well as IL-1beta-induced/stimulated ERK expression. Cortisol synthesis inhibition by metyrapone attenuated ERK expression and lung fluid absorption in IL-1beta-pretreated fetal lungs. JNK expression after maternal IL-1beta pretreatment remained unaffected at either gestation age.

CONCLUSION

These data implicate the ERK MAP kinase pathway as being important for IL-1beta induction/stimulation of lung fluid absorption in fetal guinea pigs.

Congenital diaphragmatic hernia prevents absorption of distal air space fluid in late-gestation rat fetuses

We hypothesized that congenital diaphragmatic hernia (CDH) may decrease distal air space fluid absorption due to immaturity of alveolar epithelial cells from a loss of the normal epithelial Na+ transport, as assessed by amiloride and epithelial Na+ channel (ENaC) and Na-K-ATPase expression, as well as failure to respond to endogenous epinephrine as assessed by propranolol. Timed-pregnant dams were gavage fed 100 mg of nitrofen at 9.5-day gestation to induce CDH in the fetuses, and distal air space fluid absorption experiments were carried out on 22-day gestation (term) fetuses. Controls were nitrofen-exposed fetuses without CDH. Absorption of distal air space fluid was measured from the increase in 131I-albumin concentration in an isosmolar, physiological solution instilled into the developing lungs. In controls, distal air space fluid absorption was rapid and mediated by β-adrenoceptors as demonstrated by reversal to fluid secretion after propranolol. Normal lung fluid absorption was also partially inhibited by amiloride. In contrast, CDH fetuses continued to show lung fluid secretion, and this secretion was not affected by either propranolol or amiloride. CDH lungs showed a 67% reduction in α-ENaC and β-ENaC expression, but no change in α1-Na-K-ATPase expression. These studies demonstrate: 1) CDH delays lung maturation with impaired distal air space fluid absorption secondary to inadequate Na+ uptake by the distal lung epithelium that results in fluid-filled lungs at birth with reduced capacity to establish postnatal breathing, and 2) the main stimulus to lung fluid absorption in near-term control fetuses, elevated endogenous epinephrine levels, is not functional in CDH fetuses.

Mechanisms of pulmonary edema clearance

The mechanisms of pulmonary edema resolution are different from those regulating edema formation. Absorption of excess alveolar fluid is an active process that involves vectorial transport of Na+out of alveolar air spaces with water following the Na+osmotic gradient. Active Na+transport across the alveolar epithelium is regulated via apical Na+and chloride channels and basolateral Na-K-ATPase in normal and injured lungs. During lung injury, mechanisms regulating alveolar fluid reabsorption are inhibited by yet unclear pathways and can be upregulated by pharmacological means. Better understanding of the mechanisms that regulate edema clearance may lead to therapeutic interventions to improve the ability of lungs to clear fluid, which is of clinical significance.

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.

Vasotocin and vasopressin stimulation of the chloride secretion in the human bronchial epithelial cell line, 16HBE14o-

1. Effects of neuropeptides of the vasopressin family on Cl(-) secretion have not yet been reported in lung. Using the 16HBE14o- bronchial epithelial cell line, we investigated their action on Cl(-) secretion. 2. In symmetrical Cl(-) solutions, basolateral application of arginine vasotocin (AVT), oxytocin or isotocin induced a transient I(sc) stimulation (I(peak)), whereas arginine vasopressin (AVP) did not. The effects of different Cl(-) channel blockers and of a protein kinase C (PKC) inhibitor suggest that CFTR is involved in I(peak). The calcium-activated K(+) channel (SK4) and the Cl(-)/HCO(-)(3) exchanger favor the driving force for AVT-mediated Cl(-) secretion. The antagonists of V1a (SR49059)- and V1b (SSR149415)-receptors blocked I(peak), while SR121463B, a V2 receptor antagonist, did not. These results point to the stimulation of a V1-like receptor mediating I(peak) and presenting an efficacy order, AVT>oxytocin>isotocin>>AVP. 3. When a serosal to mucosal Cl(-) gradient was applied, AVT and AVP both stimulated I(sc) according to a biphasic profile, I(peak) being followed by a plateau phase (I(plateau)). The pharmacology of I(plateau) suggests that CFTR channels are involved and that Na(+)/K(+)/2Cl(-) is the only transporter associated with I(plateau). dDAVP, a V2 receptor agonist-induced I(plateau) with the same potency as AVP, suggesting the involvement of V2 receptors in the AVP-induced I(plateau). V2 receptors are present on both opposite membranes, while V1-like receptors are mainly expressed on the basolateral membranes. RT-PCR experiments show the expression of V1a, V1b, V2 and vasopressin-activated calcium-mobilizing (VACM) receptors mRNAs.

Stimulation of distal airspace fluid clearance in guinea pigs involves bumetanide-sensitive ion transport

OBJECTIVE

This study was undertaken to test the hypothesis that beta-adrenoceptor stimulation of fetal lung fluid absorption in near-term guinea pig fetuses involves bumetanide-sensitive ion transport.

STUDY DESIGN

Fetuses were obtained from timed-pregnant guinea pigs at 61 to 69 days' gestation with and without oxytocin-induced preterm labor. The fetuses were placed on continuous positive airway pressure oxygenation, and an isosmolar 5% albumin solution was instilled into the lungs. Distal airspace fluid clearance was measured over 1 hour from the increase in distal airspace protein concentration as fluid was reabsorbed with and without the Cl(-) transport inhibitor bumetanide.

RESULTS

Fetal lungs began to absorb distal airspace fluid at 64 to 66 days' gestation, and at birth, distal airspace fluid clearance rapidly quadrupled. Labor induction by oxytocin stimulated distal airspace fluid clearance. Distal airspace fluid clearance, when present, was sensitive to propranolol-inhibition and depended on beta-adrenoceptor stimulation. Fluid secretion at 61 days' gestation was reduced by bumetanide instillation. Bumetanide addition was only inhibitory when distal airspace fluid clearance was propranolol-sensitive.

CONCLUSION

Beta-adrenoceptor stimulation from endogenous fetal epinephrine increased fetal distal airspace fluid clearance and involved bumetanide-sensitive ion transport.

IL-1β stimulates alveolar fluid absorption in fetal guinea pig lungs via the hypothalamus-pituitary-adrenal gland axis

We tested the hypothesis that interleukin (IL)-1β-induced cortisol synthesis stimulates alveolar fluid clearance in preterm fetuses. IL-1β was administered subcutaneously daily to timed-pregnant guinea pigs for 3 days with and without simultaneous cortisol synthesis inhibition by metyrapone. Fetuses were obtained by abdominal hysterotomy at 61 and 68 days gestation and instilled with isosmolar 5% albumin in the lungs, and alveolar fluid movement was measured over 1 h from the change in alveolar protein concentration. Alveolar fluid clearance was induced at 61 days gestation and stimulated at 68 days gestation by IL-1β, which both were attenuated by cortisol synthesis inhibition. Plasma ACTH and cortisol concentrations were increased by IL-1β at both gestational ages, and metyrapone reduced cortisol concentrations. IL-1β was mostly low or undetectable in both fetal and maternal blood. Prenatal alveolar fluid clearance, when present as well as IL-1β induced, was always propranolol and amiloride sensitive, suggesting that β-adrenoceptor stimulation and amiloride-sensitive Na+channels were critical for fluid absorption. IL-1β increased lung β-adrenoceptor density at gestation day 61, and cortisol synthesis inhibition attenuated the increased β-adrenoceptor density. Epithelial Na+channel and Na+-K+-ATPase subunit expressions were both increased by IL-1β and attenuated by cortisol synthesis inhibition. These results may explain why babies delivered preterm after intrauterine inflammation have a reduced risk of developing severe respiratory distress.

Maternal intravaginal prostaglandin E2 gel before elective caesarean section at term to induce catecholamine surge in cord blood: randomised, placebo controlled study

OBJECTIVE

To test the hypothesis that the application of intravaginal prostaglandin E(2) gel before elective caesarean section (ECS) will induce a catecholamine surge in umbilical arterial blood.

DESIGN

Randomised, double blind, placebo controlled trial.

SETTING

A regional perinatal referral centre.

PATIENTS

Mothers booked for ECS at or above 38 weeks gestation.

INTERVENTIONS

Thirty six consenting mothers were randomly allocated to receive either 2 mg intravaginal prostaglandin E(2) gel (study group; n = 18) or an equal volume of K-Y jelly as a placebo (control group; n = 18) 60 minutes before the ECS. Computer generated random numbers contained in coded, sealed envelopes were used for allocation. The obstetric and neonatal teams were blinded to the randomisation status of enrolled mothers.

MAIN OUTCOME MEASURES

Catecholamine concentrations in the umbilical arterial blood samples collected at delivery.

RESULTS

The median (interquartile range) neonatal gestation and birth weight were 271 (269-274) days and 3605 (3072-3970) g for the study group and 271 (270-273) days and 3340 (3000-3622) g for the control group. Median (interquartile range) noradrenaline (norepinephrine) concentrations in the umbilical arterial blood were significantly higher in the study group than the control group (15.9 (9.8-28.92) v 4.6 (1.65-14.4) ng/l, p = 0.03). Adrenaline (epinephrine) concentrations did not differ significantly between the two groups (1.6 (< 0.5-3.1) v 1.4 (< 0.5-2.75) ng/l, p = 0.6). No treatment related complications occurred.

CONCLUSION

A labour related catecholamine surge could be simulated by intravaginal prostaglandin E(2) gel.

Chronic bronchial allergic inflammation increases alveolar liquid clearance by TNF-alpha -dependent mechanism

Bronchial inflammation in allergic asthma is associated with active exudation from the bronchial tree into the interstitial space of both mucosa and submucosa. The aim of this study was to evaluate epithelial and endothelial permeability as well as alveolar fluid movement in a model of chronic allergic inflammation in Brown-Norway rats sensitized and challenged with ovalbumin (OA). Control groups were challenged with saline solution (C), and rats were immunized by OA but not challenged (Se). Lung sections showed a marked inflammatory infiltrate associated with perivascular and peribronchiolar edema in OA. To measure alveolar liquid clearance, a 5% bovine albumin solution with 1 microCi of (125)I-labeled human albumin was instilled into the air spaces. Alveolar-capillary barrier permeability was evaluated by intravascular injection of 1 microCi of (131)I-labeled albumin. Endothelial permeability was significantly increased in OA, from 0.08 +/- 0.01 in the C group to 0.19 +/- 0.03 in OA group (P < 0.05). Final-to-initial protein ratio was also statistically higher in OA (1.6 +/- 0.05) compared with C (1.38 +/- 0.03, P = 0.01) and Se groups (1.42 +/- 0.03, P = 0.04). Administration of anti-tumor necrosis factor-alpha antibodies within the instillate significantly decreased this ratio (1.32 +/- 0.08, P = 0.003 vs. OA). To conclude, we demonstrated a tumor necrosis factor-alpha-dependent increase in alveolar fluid movement in a model of severe bronchial allergic inflammation associated with endothelial and epithelial leakage.

Lung epithelial fluid transport and the resolution of pulmonary edema

The discovery of mechanisms that regulate salt and water transport by the alveolar and distal airway epithelium of the lung has generated new insights into the regulation of lung fluid balance under both normal and pathological conditions. There is convincing evidence that active sodium and chloride transporters are expressed in the distal lung epithelium and are responsible for the ability of the lung to remove alveolar fluid at the time of birth as well as in the mature lung when pathological conditions lead to the development of pulmonary edema. Currently, the best described molecular transporters are the epithelial sodium channel, the cystic fibrosis transmembrane conductance regulator, Na+-K+-ATPase, and several aquaporin water channels. Both catecholamine-dependent and -independent mechanisms can upregulate isosmolar fluid transport across the distal lung epithelium. Experimental and clinical studies have made it possible to examine the role of these transporters in the resolution of pulmonary edema.

Ca(2+)-dependent stimulation of alveolar fluid clearance in near-term fetal guinea pigs

We investigated the importance of changes in intracellular Ca(2+) concentration ([Ca(2+)](i)) for amiloride-sensitive alveolar fluid clearance (AFC) in late-gestational guinea pigs. Fetal guinea pigs of 61, 68, and 69 days (term) gestation were investigated under normal conditions and after oxytocin-induced preterm labor. AFC or alveolar fluid secretion was measured using an impermeable tracer technique. At 61 days gestation there was net secretion of fluid into the lungs, and at birth the lungs cleared 49 +/- 7% of the instilled fluid volume over 1 h. Induction of preterm labor with oxytocin induced AFC at 61 days gestation. When present, AFC was inhibited or reversed to net fluid secretion by amiloride (10(-3) M). Inhibition of membrane Ca(2+) channels by verapamil (10(-4) M) or depletion of intracellular Ca(2+) by thapsigargin (10(-5) M) reduced AFC when net AFC was evident. Amiloride lacked an inhibitory effect on AFC when instilled with verapamil or thapsigargin. The results indicate that AFC via amiloride-sensitive pathways develops during late gestation, and that inducing preterm labor precociously may activate such pathways. Our results suggest that Ca(2+) may act as a second messenger in mediating catecholamine-stimulated AFC.

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.