Transepithelial transport by pulmonary alveolar type II cells in primary culture.

Abstract

Fluid and electrolyte transport by epithelial cells in vitro can be recognized by the ability of cultured cells to form domes and by the electrical properties of monolayer cultures. Pulmonary alveolar epithelial cells are thought to be partially responsible for fluid movement in the fetal lung, but their role in electrolyte transport in the adult lung is not known. We isolated alveolar type II cells from adult rat lung and maintained them on plastic culture dishes alone, on plastic culture dishes coated with an extracellular matrix, and on collagen-coated Millipore filters. Numerous large domes were formed on culture dishes coated with the extracellular matrix; smaller domes were formed on uncoated plastic culture dishes. Sodium butyrate (3 mM) stimulated dome formation. Transmission electron microscopy showed that the epithelial cells had flattened but still retained lamellar inclusions and that the cells were polarized with microvilli on the apical surface facing the culture medium. The electrical properties of the monolayers maintained on collagen-coated Millipore filters were tested in two laboratories. The transepithelial potential differences were 0.7 ± 0.1 mV (24 filters, seven experiments) and 1.3 ± 0.1 mV (13 filters, two experiments) apical side negative, and the corresponding resistances were 217 + 11 ohm.cm2 and 233 ± 12 ohm.cm2. Terbutaline (10 FM) produced a biphasic response with a transient decrease and then a sustained increase in potential difference. Amiloride (0.1 mM) completely abolished the potential difference when it was added to the apical side but not when it was added to the basal side, whereas 1 mM ouabain inhibited the potential difference more effectively from the basal side. Thus, type II cells form a polarized epithelium in culture, and these cells actively transport electrolytes in vitro.The alveolar space of adult mammalian lungs is lined by only a thin film of fluid. Although the amount offluid is not known precisely, one estimate is that 20 ml of alveolar fluid is distributed over a surface area ofabout 70 m2 (1). When lungs are fixed for electron microscopy by vascular perfusion, the only areas that appear to contain fluid are the corners ofthe alveoli, where the radius ofcurvature is short (2). At the corners, the net force of surface tension, which is a function of the radius ofcurvature and the actual surface tension, is directed to draw fluid into the alveolus (3-7). How this force is counterbalanced is not known. Although most investigators have tended to discount the possibility of active transport across alveolar epithelium (7, 8), recent observations in vivo suggest that there may be active transport of fluid from the airspace into the interstitium (9, 10).Cell culture techniques have been used to study transepithelial fluid movement. Epithelial cells that transport fluid form domes or hemicysts in culture (11,12

Innate Immune Response of Human Alveolar Type II Cells Infected with Severe Acute Respiratory Syndrome–Coronavirus

“…Although the air-liquid interface culture conditions greatly increased the susceptibility of type II cells to SARS-CoV, the mechanism for this increased susceptibility to infection is not yet established. In the lung, very little fluid is present on the apical surface of the alveolar epithelium (37,38). Hence, the A/L conditions approximate conditions in vivo.…”

Beta-adrenergic agonists increase lung liquid clearance in anesthetized sheep.

“…Recently, it has been suggested that alveolar and lung liquid clearance may also depend on active metabolic processes for ion transport (3)(4)(5). This hypothesis is supported by work in different experimental models in which the presence of active sodium transport has been identified (6)(7)(8)(9)(10).…”

“…For example, beta-adrenergic agonists increase sodium transport across the alveolar barrier in fetal lambs (1 1), isolated rat lungs (12), and cultured alveolar type II cells (7,13). However, whether increased active transport of sodium induced by beta-adrenergic agents increases lung liquid clearance in the intact adult animal has never been studied.…”

Cystic fibrosis decreases the apical membrane chloride permeability of monolayers cultured from cells of tracheal epithelium.

“…It is now well established that epithelial cells will grow in culture to establish confluent monolayers with tightjunctions separating discrete apical and basolateral cell membranes (4)(5)(6)(7)(8). Furthermore, the electrical properties of cultured cell sheets resemble those of the original tissue (9)(10)(11)(12).…”