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Articles in PresS, published online ahead of print November 30, 2001
Am J Physiol Lung Cell Mol Physiol, 10.1152/ajplung.00330.2001
Submitted on August 15, 2001
Accepted on November 1, 2001
1 Division of Pulmonary & Critical Care Medicine, University of Southern California, Los Angeles, CA, USA; Will Rogers Institute Pulmonary Research Center, University of Southern California, Los Angeles, CA, USA
* To whom correspondence should be addressed. E-mail: rlubman{at}hsc.usc.edu.
We investigated the acid/base permeability properties of electrically resistive monolayers of alveolar epithelial cells (AEC) grown in primary culture. AEC monolayers were grown on tissue culture-treated polycarbonate filters. Filters were mounted in a partitioned cuvette containing two fluid compartments (apical and basolateral) separated by the adherent monolayer, cells loaded with the pH-sensitive dye BCECF, and intracellular pH (pHi) determined. Monolayers in HCO3--free Na+-buffer (Na 140 mM, HEPES 6 mM, pH 7.4) maintained a transepithelial pH gradient between the two fluid compartments over 30 min. Replacement of apical fluid by either acidic (6.4) or basic (8.0) buffer resulted in minimal changes in pHi. In contrast, replacement of basolateral fluid by either acidic or basic buffer resulted in transmembrane proton fluxes and intracellular acidification or alkalinization. Intracellular alkalinization was blocked 
80% by dimethylamiloride (DMA, 100 µM), an inhibitor of Na+/H+ exchange, whereas acidification was not affected by a series of acid/base transport inhibitors. Additional experiments in which AEC monolayers were grown in the presence of acidic (6.4) or basic (8.0) medium revealed differential effects on bioelectric properties depending on whether extracellular pH was altered in the apical or basolateral fluid compartments bathing the cells. Acid exposure reduced (and base exposure increased) ISC (short-circuit current) from the basolateral side, whereas apical exposure did not affect ISC in either case. We conclude that AEC monolayers are relatively impermeable to transepithelial acid/base fluxes, primarily due to the impermeability of intercellular junctions and of the apical, rather than basolateral, cell membrane. The principal basolateral acid exit pathway observed under these experimental conditions is Na+-H+ exchange, whereas proton uptake into the cells occurs across the basolateral cell membrane by a different, currently undetermined, mechanism. These results are consistent with the ability of the alveolar epithelium to maintain an apical to basolateral (airspace to blood) pH gradient in situ.
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