Analysis of Regulatory Mechanisms for Tubular Secretion of Organic Cations by Cultured Renal Cells

To clarify the regulatory mechanisms for the tubular secretion of organic cations, we examined transcellular transport of 14C-tetraethylammonium (TEA) by monolayers of a pig kidney epithelial cell line LLC-PK1 grown on microporous membrane filters. TEA was taken up progressively by the monolayers from the basolateral side, and was transported unidirectionally to the apical side. The basolateral-to-apical transport of TEA was stimulated by lowering pH of the apical side, resulting in a decrease in the TEA accumulation. The TEA efflux from the monolayers to the apical side caused a decrease in the intracellular pH monitored by a pH-sensitive fluorescent dye BCECF, indicating H+/TEA antiport. Transport studies for quinidine revealed that there may be distinct organic cation transport systems in both the basolateral and apical membranes, which can be distinguished from the systems mediating TEA transport. Basolateral uptake of TEA was decreased by lowering pH of the basolateral medium. Cell acidification by a prepulse with NH4Cl also decreased the basolateral TEA uptake. Kinetical analysis suggested that pH sensitivity of the basolateral TEA transport was caused by changes in Km value. These results suggest that the activity of the basolateral organic cation transporter and the apical H+/organic cation antiporter is regulated by an environmental pH and a pH-gradient across the apical membranes, respectively.