Conductive Na+ transport in an epithelial cell line (LLC-PK1) with characteristics of proximal tubular cells.

Na+ influx and efflux from confluent monolayers of an epithelial cell line with multiple differentiated characteristics of the straight segment of the renal proximal tubule were studied in the presence and absence of a pH gradient. The results show that Na+ influx in the absence of a pH gradient is inhibited by amiloride as well as by complete replacement of Cl- by an impermeable anion, such as isethionate. Dissipation of cell membrane potential by increasing the potassium concentration of the extracellular medium in the presence of valinomycin also inhibited Na+ influx, whereas sodium influx induced by an H+ gradient was not affected. Inhibition of Na+ influx by different maneuvers produced hyperpolarization of the plasma cell membrane, as would be expected if the sodium movement involved net displacement of charges. Calcium and other divalent and trivalent cations also inhibited Na+ influx measured in the absence of an H+ gradient. Na+ influx induced by a pH gradient, however, was not affected. Like the Na+-H+-exchange system, the conductive Na+ pathway is localized in the apical membrane of the epithelial cells. From these results, we conclude that at least a fraction of transepithelial Na+ transport in LLC-PK1 monolayers occurs through a simple rheogenic transport system.