Developmental differences in rabbit lung epithelial cell Na(+)-K(+)-ATPase

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.

Alveolar epithelial type I cells contain transport proteins and transport sodium, supporting an active role for type I cells in regulation of lung liquid homeostasis

Transport of lung liquid is essential for both normal pulmonary physiologic processes and for resolution of pathologic processes. The large internal surface area of the lung is lined by alveolar epithelial type I (TI) and type II (TII) cells; TI cells line >95% of this surface, TII cells <5%. Fluid transport is regulated by ion transport, with water movement following passively. Current concepts are that TII cells are the main sites of ion transport in the lung. TI cells have been thought to provide only passive barrier, rather than active, functions. Because TI cells line most of the internal surface area of the lung, we hypothesized that TI cells could be important in the regulation of lung liquid homeostasis. We measured both Na(+) and K(+) (Rb(+)) transport in TI cells isolated from adult rat lungs and compared the results to those of concomitant experiments with isolated TII cells. TI cells take up Na(+) in an amiloride-inhibitable fashion, suggesting the presence of Na(+) channels; TI cell Na(+) uptake, per microgram of protein, is approximately 2.5 times that of TII cells. Rb(+) uptake in TI cells was approximately 3 times that in TII cells and was inhibited by 10(-4) M ouabain, the latter observation suggesting that TI cells exhibit Na(+)-, K(+)-ATPase activity. By immunocytochemical methods, TI cells contain all three subunits (alpha, beta, and gamma) of the epithelial sodium channel ENaC and two subunits of Na(+)-, K(+)-ATPase. By Western blot analysis, TI cells contain approximately 3 times the amount of alphaENaC/microg protein of TII cells. Taken together, these studies demonstrate that TI cells not only contain molecular machinery necessary for active ion transport, but also transport ions. These results modify some basic concepts about lung liquid transport, suggesting that TI cells may contribute significantly in maintaining alveolar fluid balance and in resolving airspace edema.

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

We tested the hypothesis that labor-induced epinephrine release would stimulate alveolar fluid clearance in preterm fetuses. Preterm fetuses were obtained by cesarean section from timed-pregnant guinea pigs at 61-69 days postconception. Fetal guinea pigs were euthanized and placed on continuous positive airway pressure oxygenation, and an isosmolar 5% albumin solution was instilled. Alveolar fluid clearance was measured over 1 h. The fetal lung began to absorb fluid at 64-66 days postconception, and at birth, alveolar fluid clearance quadrupled. Baseline alveolar fluid clearance when present was sensitive to propranolol inhibition and depended on beta-adrenergic stimulation. Measurements of plasma epinephrine in fetal animals confirmed high epinephrine levels in 66- to 69-day postconception fetuses. Prenatal alveolar fluid clearance when present was highly amiloride sensitive, suggesting that amiloride-sensitive Na+ channels were critical. Oxytocin-induced labor initiated an amiloride- and propranolol-sensitive net alveolar fluid clearance in 61-day-gestation animals. Moreover, oxytocin induced significant epinephrine release in all fetuses. These results have clinical implications for infants delivered by cesarean section before the onset of labor. Use of pharmacological agents to induce labor may reduce the occurrence and severity of perinatal respiratory distress.

Embryonic epithelial membrane transporters

Embryonic epithelial membrane transporters are organized into transporter families that are functional in several epithelial organs, namely, in kidney, lung, pancreas, intestine, and salivary gland. Family members (subtypes) are developmentally expressed in plasma membranes in temporospatial patterns that are 1) similar for one subtype within different organs, like aquaporin-1 (AQP1) in lung and kidney; 2) different between subtypes within the same organ, like the amiloride-sensitive epithelial sodium channel (ENaC) in lung; and 3) apparently matched among members of different transporter families, as alpha-ENaC with AQP1 and -4 in lung and with AQP2 in kidney. Finally, comparison of temporal expression patterns in early embryonic development of transporters from different families [e.g., cystic fibrosis transmembrane conductance regulator (CFTR), ENaC, and outer medullary potassium channel] suggests regulatory activating or inactivating interactions in defined morphogenic periods. This review focuses on embryonic patterns, at the mRNA and immunoprotein level, of the following transporter entities expressed in epithelial cell plasma membranes: ENaC; the chloride transporters CFTR, ClC-2, bumetanide-sensitive Na-K-Cl cotransporter, Cl/OH, and Cl/HCO(3); the sodium glucose transporter-glucose transporter; the sodium/hydrogen exchanger; the sodium-phosphate cotransporter; the ATPases; and AQP. The purpose of this article is to relate temporal and spatial expression patterns in embryonic and in early postnatal epithelia to developmental changes in organ structure and function.

Developmental changes in water permeability across the alveolar barrier in perinatal rabbit lung

Lung fluid is reabsorbed rapidly at birth to permit alveolar respiration. We reported previously that expression of aquaporins (AQP) 1, 4, and 5 in rat lung increased just after birth. The hypothesis was tested that the increased AQP expression is associated with increased osmotic water permeability (Pf) between the airspace and capillary compartments. Pf was measured in isolated perfused fetal and newborn rabbit lungs using a pleural surface fluorescence method (Carter, E.P., M.A. Matthay, J. Farinas, and A.S. Verkman. 1996. J. Gen. Physiol. 108:133-142). In response to perfusate osmolality increase from 300 to 600 mosM, initial rates of osmotic equilibration were 1.13+/-0.13 mosM/s at 0-12 h after birth, increasing to 1.52+/-0.19 mosM/s at 12-24 h, and 1.83+/-0.10 mosM/s at 24-84 h. Corresponding Pf values (in cm/s x 10(-2)), computed from d[mosM]/dt and alveolar surface-to-volume ratios, were 1.03+/-0.11 (0-12 h), 1.51+/-0.16 (12-24 h), and 1.88+/-0.09 (24-84 h). Pf was relatively low in prenatal (1.22-1.27, fetal days 29 and 31) and adolescent (1.25+/-0.08, 21-d) rabbit lungs. To test for involvement of molecular water channels, measurements were made of Arrhenius activation energy (Ea), mercurial inhibition, diffusional water permeability (Pd), and AQP expression. Temperature-dependence measurements showed a 25% decrease in Ea for Pf in lungs < 1 d vs. 4 d. Pf was decreased 30% by 0.5 mM HgCl2 in < 1-d lungs and 44% in 4-d lungs. Pd was 1.0 x 10(-)5 cm/s and did not change when Pf was increased by 75%. RNase protection assay showed increased transcript expression in the first 24 h after birth for rabbit isoforms of AQP1 and AQP4. These results provide the first functional data on water permeability in perinatal lung. The increased water permeability after birth may facilitate the maintenance of dry alveoli.

Activity and expression of Na(+)-K(+)-ATPase in human placental cytotrophoblast cells in culture

1. To determine whether there is a change during differentiation, the activity and expression of Na(+)-K(+)-ATPase were studied in mononucleate cytotrophoblast cells (18 h culture) and syncytiotrophoblast-like cells (66 h culture). A choriocarcinoma-derived cell line (JAr) which, unlike the cytotrophoblast cells, divides in culture, was also studied for comparison. 2. Na(+)-K(+)-ATPase activity was assessed by measurement of ouabain-sensitive 86Rb+ uptake. Na(+)-K(+)-ATPase expression was determined by (i) measurement of [3H]ouabain binding and (ii) Northern hybridization to measure expression of alpha-1 and beta 1-subunit mRNA. 3. There was no significant difference in either activity or expression of Na(+)-K(+)-ATPase during differentiation of cytotrophoblast cells. However, expression of alpha 1- and beta 1-subunit mRNA was significantly lower in 66 vs. 18 h cultured cytotrophoblast cells. 4. Both Na(+)-K(+)-ATPase activity and [3H]ouabain binding was significantly greater in JAr cells than either cytotrophoblast cell groups, although expression of alpha 1- and beta 1-subunit mRNA was the same as cytotrophoblast cells cultured for 18 h. 5. It is concluded that N(+)-K(+)-ATPase activity and protein expression does not change during differentiation of cytotrophoblast cells but that there are changes in expression at the transcriptional or post-transcriptional level.