Perinatal regulation of lung liquid absorption in pulmonary adaptation in newborn infants

Respiratory distress is a major contributor to morbidity in newborn infants. Insufficient clearance of lung liquid at birth causes maladaptation (transient tachypnea of the newborn, TTN) primarily in late preterm (delivery at 34 0/7 to 36 6/7 gestational weeks) and term infants. In small preterm infants excess lung liquid further complicates respiratory distress syndrome (RDS) caused by a lack of pulmonary surfactant. Importantly, the risk of respiratory morbidity remains elevated in early term infants (i.e. delivery at 37 0/7- 38 6/7 gestational weeks), especially if delivered by elective cesarean section (CS). Thus, major risk factors for respiratory morbidity include preterm delivery or delivery by elective CS. In both prematurity and CS, the hormonal milieu of the fetus, important in adaptation to air-breathing, differs from that of the term vaginally delivered infants. In particular, glucocorticoids (GCs) play an important role in the maturation of the fetal lung. GCs are often administered antenatally to mothers at risk of preterm delivery as an attempt to reduce respiratory morbidity in their preterm infants. Although relevant to preventive and treatment strategies for respiratory distress in newborn infants, data on airway ion and liquid transport and their hormonal regulation in human infants are limited. Therefore, the aim of this thesis was to acquire new data on lung liquid clearance and its molecular mechanisms in newborn infants. In particular, we studied their hormonal regulation during pulmonary adaptation. We used ultrasound to estimate the amount of lung liquid in term newborn infants, and compared infants delivered vaginally (VD) and by elective CS. The gene expression of the epithelial sodium channel (ENaC), Na-K-ATPase and serum- and glucocorticoid-inducible kinase 1 (SGK1) were determined in airway epithelial cells from nasal epithelium, used as a surrogate for the lower airway epithelium. Gene expression was quantified by measuring mRNA levels of these genes with a real-time reverse-transcription polymerase chain reaction. We determined the concentration of cortisol hormone in umbilical cord blood and saliva with liquid-chromatography tandem-mass spectrometry or enzyme-linked immunosorbent assay. The relationship of ENaC, Na-K-ATPase and SGK1 gene expression with cortisol was studied in cohorts of late preterm and term infants. In preterm infants, we studied the effect on airway ENaC expression of a repeated antenatal dose of the glucocorticoid betamethasone to the mother. The term infants and the majority of the preterm infants in the studies were born at the Women’s Hospital, Helsinki University Hospital. The lung liquid content of the term, healthy infants at three hours postnatally was significantly higher in infants delivered by elective CS than in VD infants. The umbilical cord cortisol concentration was lower in CS than in VD infants, and CS was associated with lower SGK1 expression at 1.5 hours after delivery. At this time point, the airway expression of ENaC, Na-K-ATPase, and SGK1 was lower in infants delivered late preterm and early term than in infants delivered ≥ 39 weeks. In addition, ENaC expression of late preterm and term infants correlated positively with umbilical cord blood and salivary cortisol concentrations. However, a repeat antenatal dose of betamethasone to the mother had no significant effect on ENaC expression in the preterm infants. This thesis work confirms that compared with infants delivered vaginally the amount of lung liquid remains more abundant after CS, and that ultrasound is a useful tool for evaluating lung liquid content in newborns. Low SGK1 expression after CS delivery and lack of labor could contribute to the insufficient activation of transmembrane sodium transport causing defective liquid absorption. In infants delivered <39 weeks of gestation airway epithelial amounts of ENaC, Na-K-ATPase, and SGK1 mRNAs were all lower than in those delivered ≥ 39 weeks. These physiological data are in line with the epidemiologic evidence of a higher risk for respiratory morbidity in infants delivered before 39 weeks of gestation, especially when delivered by CS. Thus, our data support the practice of postponing elective CS until 39 weeks. Our findings provide valuable information for planning strategies aimed at preventing or treating respiratory distress in preterm and term infants.