Alveolar epithelial cells (AEC) cover the alveolar surface of the lung, which is the major acid-excreting organ in mammals. Because these cells are subject to an inconstant extracellular milieu with respect to pCO2, the regulation of intracellular pH (pHi) in alveolar epithelial cells is of particular significance. In previous work it has been established that alveolar pneumocyte pHi is regulated by several ion transport mechanisms that are capable of effecting acid/base entry or extrusion from the cell. These ion transport mechanisms are plasma membrane proteins asymmetrically distributed across polarized epithelial cell surfaces, thereby making transepithelial transport of acid/base equivalents possible. Their distribution with respect to membrane polarity across the alveolar epithelium, whose apical lining fluid pH is acidic (6.9), has not previously been established. We will investigate the mechanisms whereby acid/base transport by AEC is regulated by studying three hypotheses: (1) Acid/base transport by AEC occurs via specific ion transport mechanisms having a characteristic distribution with respect to membrane polarity across the cells, and which represent the activity of specific transport protein isoforms. (2) These ion transport mechanisms are both acutely and chronically regulable in AEC with respect to activity, expression, and polarity by specific extracellular stimuli that can thereby modulate acid/base transport under normal conditions and in disease states. (3) Regulation and localization of acid/base transport mechanisms in AEC influence alveolar fluid pH. The objectives of the studies proposed herein are therefore to further characterize the acid/base transport mechanisms in AEC, and in particular, to study regulation of their activity, expression, and polarity by specific exogenous factors and lung injury. We will use rat alveolar epithelial cell monolayers cultured on tissue culture-treated polycarbonate filters for many of these studies. This model is especially well suited for studies on alveolar epithelial polarity and acid/base transport. We will validate our findings using in situ studies on rat alveolar epithelium. The long term goals of this project are to define the importance of acid/base transport in intracellular homeostasis of alveolar epithelial cells, and to demonstrate the role of acid/base transport by alveolar epithelial cells in modulation of the extracellular alveolar milieu.
