ROLE OF ALTERED MEMBRANE FUNCTION IN XENOBIOTIC TOXICITY

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In recent years, we have examined the mechanism and energetics of renal organic anion (OA) and cation (OC) transport. These are the primary systems which govern the elimination of foreign chemicals. OA transport was shown to be indirectly coupled to metabolic energy through Na/alpha- ketoglutarate (alphaKG) co-transport and OA/alphaKG exhange. We are currently examinng both plasma membrane and intracellular events associated with secretory transport and initiating studies of the molecular biology of these systems. This work indicates that the rate of OA transport may be modulated by manipulations which alter either the internal alphaKG concentration or its in-to-out gradient. In addition, imaging studies indicate that sequestration within intracellular vesicles reduces the cytoplasmic OA concentrations, apparently protecting intracellular sites during secretion. Sequestration of OA is both carrier mediated and energy dependent. OC are also sequestered, suggesting that vesicular packaging may be a general means of protecting transporting epithelial cells during secretion. The mechanism of OC sequestration was a secondary active proton/OC exchange, driven by a proton-ATPase present in these endosomal vesicles. Initial evidence indicates that these vesicles move in a basolateral to apical direction, raising the possibility that they may also play a direct role in transcellular movement of secreted xenobiotics. Expression cloning techniques are currently in use to identify and clone specific membrane proteins responsible for individual transport steps in secretion of foreign anions and cations. Finally, cell culture techniques have been applied to develop intact epithelial sheet preparations for analysis of these transport systems, their control mechanisms, and their sensitivity to xenobiotic toxicity. These studies have shown for the first time that a renal line (the OK cell) demonstrates OA secretion and in comparison with primary cultures of rat proximal tubule, they provide a means to analyze control of transepithelial OA and OC transport in intact epithelial.