Tetraethylammonium transport by OK cells

Mechanisms exist in renal proximal tubules for the mediated transepithelial secretion or reabsorption of endogenous and exogenous organic cations. In the studies presented here, the uptake of the organic cation tetraethylammonium (TEA) into confluent monolayers of opossum kidney cells was evaluated to determine if these cells might serve as an in vitro model of this transport pathway. 3H-TEA entered opossum kidney cells in a time-dependent manner. Uptake at early time points was saturable with an apparent Km of 59.1 +/- 11.2 microM and a Vmax of 1,292 +/- 210 fmol/micrograms of DNA. TEA uptake was inhibited in a dose-dependent manner by several other organic cations including amiloride, cimetidine, verapamil, procainamide, quinidine and N1-methylnicotinamide. With 1 mM concentrations of these compounds, uptake was virtually eliminated. However, another organic cation, N'-methylnicotinamide caused only minimal inhibition. TEA uptake was significantly reduced by sodium azide, suggesting dependence on oxidative phosphorylation. An alkaline medium pH enhanced TEA uptake, but, at the same pH, uptake was similar in the presence or absence of bicarbonate. When cellular pH was altered by ammonium chloride addition or removal, TEA uptake was not affected. Thus, organic cation/proton exchange, as has been demonstrated previously in apical membrane vesicles prepared from proximal tubules, is evidently not responsible for TEA uptake. Similarly, uptake does not appear to result from organic cation/organic cation exchange. These results indicate that the plasma membrane of opossum kidney cells contains a transport system(s) for the mediated uptake of organic cations and that these cells may be a useful mode for further study of renal epithelial transport of these solutes.