Vectorial sodium transport across the mammalian alveolar epithelium: it occurs but through which cells?

The fluid that fills the alveolar spaces in the fetal lung is cleared shortly after birth, mainly as a consequence of active transport of sodium ions (Na ) across the alveolar epithelium. This transport establishes an osmotic gradient that favors reabsorption of intra-alveolar fluid.1 Studies that demonstrate both the reabsorption of intratracheally instilled isotonic fluid or plasma from the alveolar spaces of adult anesthetized animals and resected human lungs, and the partial inhibition of this process by amiloride and ouabain, indicate that adult alveolar epithelial cells are also capable of actively transporting Na ions (see reviews2,3). Although it remains unclear whether active Na transport plays an important role in keeping alveolar spaces free of fluid in the normal lung, a variety of studies have clearly established that active Na transport limits the degree of alveolar edema under pathological conditions in which the alveolar epithelium has been damaged. For example, intratracheal instillation of a Na channel blocker in rats exposed to hyperoxia increased the amount of extravascular lung water.4 Conversely, intratracheal instillation of adenoviral vectors expressing the Na ,K -ATPase genes increased survival of rats exposed to hyperoxia.5 Moreover, patients with acute lung injury who are still able to concentrate alveolar protein (as a result of active Na reabsorption) have a better prognosis than those who cannot.6,7 Insight into the nature and regulation of transport pathways has come from electrophysiological studies of freshly isolated and cultured alveolar type II (ATII) cells. These cells, which make up 67% of the alveolar epithelial cell population, but which constitute only 3% of the alveolar surface area in the adult lung, can be isolated at high purity and cultured to form confluent monolayers.8 The results of a variety of electrophysiological studies, of both confluent monolayers of ATII cells mounted in Ussing chambers, and individual cells by patchclamp analysis, indicate that Na ions diffuse passively into ATII cells through apically located amiloride-sensitive cation and sodium selective channels9–11 and are extruded across the basolateral cell membranes by the ouabain-sensitive Na ,K ATPase.12 The cation channels on the apical surface usually constitute the rate-limiting step in this process, offering more than 90% of the resistance to transcellular Na transport. Over the last 20 years, there have been only a handful of publications on the transport properties of alveolar type I (ATI) cells. There are two major reasons for this relative dearth of studies: first, in their original immunocytochemical study of the distribution of Na ,K -ATPase subunit proteins in lungs, Schneeberger and McCarthy13 reported that the ATPase was present in the basolateral membranes of ATII but not of ATI cells; similar findings were reported by Ingbar et al14 in rat fetal lungs. Second, isolation of a pure population of ATI cells has been difficult and accomplished only in a couple of laboratories. Thus, until recently, the prevailing wisdom has been that ATII cells are metabolically active and responsible for surfactant secretion and ion transport, whereas the squamous ATI cells are merely a “passive” component of the alveolar epithelial barrier. However, in a recent study, Johnson et al15 clearly demonstrated that ATI cells isolated from rat lungs exhibit amilorideand ouabain-sensitive Na and K transport, at considerable higher levels than in ATII cells. Furthermore, these cells immunostained for the amiloride-sensitive sodium channel (ENaC) and Na ,K -ATPase proteins. In a study published in this issue of Circulation Research, Ridge et al16 used immunofluorescent and electron microscopy techniques to determine the distribution of two subunits of the Na ,K -ATPase ( 1 and 2) in alveolar epithelial cells in vivo. Interestingly, they found that while both subunits were present in ATI cells, only the 1 subunit of Na ,K -ATPase was detected in ATII cells. Most importantly, the authors ingeniously exploited the differential sensitivity to ouabain of the two Na ,K -ATPase subunits ( 1 and 2) to demonstrate the importance of vectorial sodium transport across ATI cells to normal lung fluid balance: specifically perfusion of isolated lungs with solutions containing 100 nmol/L ouabain resulted in 60% inhibition of basal alveolar fluid clearance (AFC) (secondary to an osmotic gradient generated by the active transport of sodium) and totally abolished the increase in AFC that follows intratracheal instillation of isoproterenol, a -agonist. The authors argued that because ouabain at these low concentrations blocks the 2 subunit (which is expressed only in ATI cells) but not the 1 (which is expressed in both ATI and ATII cells), active vectorial transport of sodium across ATI cells must play a major role in both basaland -adrenergic– stimulated fluid clearance across the alveolar epithelium. Needless to say, the scientific community is reluctant to accept a shift in paradigm without solid, incontrovertible evidence. Thus, it behooves us to carefully examine potential limitations of this study, which by no means detract from its The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association. From the Department of Anesthesiology, University of Alabama at Birmingham, Birmingham, Ala. Correspondence to Sadis Matalon, PhD, Alice McNeal Professor of Anesthesiology, University of Alabama at Birmingham, UAB Dept of Anesthesiology, 1530 3rd Ave South, Birmingham, AL 35294-2172. E-mail [email protected] (Circ Res. 2003;92:348-349.) © 2003 American Heart Association, Inc.