Development and regulation of alveolar fluid clearance in the guinea pig

Abstract: Alveolar fluid clearance is driven by active absorption of Na+ across the alveolar epithelium. Development and hormonal regulation of alveolar fluid clearance over 1 hour was studied by instillation of a protein-containing, isosmolar fluid into lungs of fetal, newborn, and adult guinea pigs. After 1 hour, a sample of remaining alveolar fluid was collected and the change in protein concentration was used to calculate secretion or absorption of fluid. The technique is based on the principle that large molecules cross the alveolar epithelium at a much slower rate than water, and as such, a change in concentration reflects water movement across the cell layer. At 61 days gestation (term = 69 days), the guinea pig lung secreted fluid into the future air spaces of the lungs. Between 64 and 66 days gestation, alveolar fluid clearance slowly begun to develop and then rapidly increased to a maximal level at birth (62 ? 5% of the instilled fluid was absorbed over one hour). Alveolar fluid clearance then decreased to adult levels (37 ? 5% fluid absorption) within a few days after birth. Preterm and neonatal, but not adult alveolar fluid clearance depended on endogenous epinephrine stimulation. Alveolar fluid clearance could be stimulated with exogenous ?-adrenergic agonists in the lung by stimulation of ?1-adrenergic receptors, when there was no endogenous ??adrenergic stimulation. The adult basal alveolar fluid clearance was maintained at a relatively high rate by endogenous cortisol, which functioned by continuously stimulating ?ENaC (epithelial Na+ channel subunit) mRNA transcription and de novo ENaC protein synthesis. Oxytocin-induced preterm labor induced alveolar fluid clearance at 61 days gestation, when there normally was fluid secretion. Since propranolol inhibited this effect and the fetal plasma epinephrine levels were increased, oxytocin acted through release of endogenous epinephrine.Basal and stimulated alveolar fluid clearance was always sensitive to amiloride-instillation (ENaC inhibitor) at all ages when there was fluid absorption. Preterm alveolar fluid clearance was almost completely inhibited by amiloride; the sensitivity decreased with age, and alveolar fluid clearance in adult guinea pigs was inhibited by around 40%. Development of ?ENaC mRNA expression and amiloride-sensitivity suggested that ENaC represented a major pathway for alveolar fluid clearance before birth and that alternative pathways for alveolar fluid clearance developed in the postnatal lungs. Since, the amiloride-sensitive fraction of alveolar fluid clearance was not different from that in control guinea pigs after ?-adrenergic stimulation of alveolar fluid clearance, both amiloride-sensitive and amiloride-insensitive pathways were simultaneously stimulated. ?-adrenergic stimulation of alveolar fluid clearance was mediated by intracellular cAMP as a second messenger. Also, intracellular Ca2+ concentration changes inhibited both stimulated and basal alveolar fluid clearance, at all ages, suggesting that disturbance of normal intracellular Ca2+ concentrations may affect Na+ transport pathways. IN CONCLUSION: The major findings in this study were that an amiloride-sensitive alveolar fluid clearance can be induced in otherwise fluid-secreting lungs by oxytocin-induced preterm labor. These results may be important for the survival and health of prematurely born infants and infants delivered by emergency cesarean section, especially if cesarean-delivered before the start of labor, since these infants frequently display a respiratory distress that may be related to immature fluid absorption.