AbstractSeveral organic cations are actively transported by proximal renal tubules by mediated processes across both the apical and basolateral cell membranes. In order to evaluate this transport system in a cultured renal epithelium, uptake of3H‐tetraethylammonium (TEA) across the apical membrane was measured in LLCPK1cells, a cell line with several characteristics of proximal tubules.3H‐TEA progressively entered these cells and reached a near‐steady state by 30 min. Three‐minute uptake was saturable with an apparent Vmax of 1,669 ± 129 fmoles/μg DNA and apparent Km of 34.0 ± 3.4 μM.3H‐TEA uptake was inhibited by an excess of nonradioactive TEA, other organic cations, sodium azide, and hypothermia. An alkaline external pH was associated with greater3H‐TEA uptake than an acid pH. However, efflux of3H‐TEA from cells was not appreciably affected by changes in external pH. Preincubation of cells in acid or alkaline media did not affect uptake. Alteration of cell pH by ammonium chloride addition or removal had little effect on3H‐TEA uptake. Finally, uptake of3H‐TEA was not accelerated by preloading cells with an excess of nonradioactive TEA. These results indicate that intact LLCPK1cells possess a mechanism(s) in their apical membranes for the mediated transport of a prototypic organic cation. The mechanism(s) involved in this transport is uncertain. However, neither organic cation/proton nor organic cation/organic cation exchange appears to be th
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