title: Polarized distribution of Na(+)-H+ antiport activity in rat alveolar epithelial cells.
abstract: In this study, we investigated the polarized distribution of Na(+)-H+ antiport activity in alveolar epithelial cell monolayers. Rat alveolar type II pneumocytes were grown on detachable tissue culture-treated Nuclepore filters. The membrane filters, with their adherent intact alveolar epithelial cell monolayers, were mounted in a cuvette designed to contain two fluid compartments separated by the monolayer. Cells were loaded with the pH-sensitive dye 2',7'-biscarboxyethyl-5,6-carboxylfluorescein and intracellular pH (pHi) measured spectrofluorometrically. Monolayers were studied at ambient temperature on days 3-4 in culture, coincident with the development of high tissue resistance (RT > or = 2000 omega.cm2). Cells were incubated in HCO(3-)-free Na+ buffer [(in mM) 140 NaCl, 6 HEPES, pH 7.4] and acidified by NH3 prepulse. Rates of realkalinization (JH+) were calculated as the product of the initial rate of recovery (dpHi/dt) and the intracellular buffer capacity (beta i). Under control conditions, recovery occurred with an initial JH+ of 28.4 mM/min. When 100 microM dimethylamiloride (DMA), an amiloride analogue with enhanced specificity for inhibiting the Na(+)-H+ antiporter, was present in the basolateral fluid, recovery was inhibited by > 90%. Conversely, when the monolayers were acidified in Na+ buffer containing DMA (100 microM) in the apical fluid, acidification and recovery were identical to control. Recovery from acidification was inhibited by basolateral DMA with a one-half maximal inhibitory concentration (IC50) of 100 nm and by basolateral amiloride with an IC50 of 10 microns. Recovery was completely inhibited by omission of Na+ from the basolateral fluid, but omission of Na+ from apical fluid had no effect. We conclude that Na(+)-H+ antiport activity is located exclusively on the basolateral surface of these alveolar epithelial cell monolayers, where it most likely represents the high-amiloride affinity isoform of the Na(+)-H+ antiporter, NHE-1. The Na(+)-H+ antiporter, asymmetrically distributed to the basolateral surface of the polarized alveolar epithelium, contributes to intracellular homeostasis in alveolar pneumocytes and may also play a role in signal transduction in these cells.
authors.affiliation: Division of Pulmonary and Critical Care Medicine, University of Southern California, Los Angeles 90033.
authors.forename: R L
authors.initials: RL
authors.lastname: Lubman
authors.forename: E D
authors.initials: ED
authors.lastname: Crandall
mesh_terms: D000584:Amiloride / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D002021:Buffers; D016764:Cell Polarity; D004847:Epithelial Cells; D004848:Epithelium / Q000378:metabolism; D006706:Homeostasis; D006863:Hydrogen-Ion Concentration; D007425:Intracellular Membranes / Q000378:metabolism; D008297:Male; D011650:Pulmonary Alveoli / Q000166:cytology / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012964:Sodium / Q000378:metabolism / Q000494:pharmacology; D017923:Sodium-Hydrogen Exchangers / Q000378:metabolism*; D014018:Tissue Distribution
chemical_list: D002021:Buffers; D017923:Sodium-Hydrogen Exchangers; C039465:5-dimethylamiloride; D000584:Amiloride; D012964:Sodium