Increases in transepithelial vectorial Na + transport facilitates Na +-dependent l-DOPA transport in renal OK cells

Abstract

The present study evaluated the hypothesis of whether increases in vectorial Na + transport translate into facilitation of Na +-dependent l-DOPA uptake in cultured renal epithelial tubular cells. Increases in vectorial Na + transport were obtained in opossum kidney (OK) cells engineered to overexpress Na +–K +-ATPase after transfection of wild type OK cells with the rodent Na +–K +-ATPase α 1 subunit. The most impressive differences between wild type and transfected OK cells are that the latter overexpressed Na +–K +-ATPase accompanied by an increased activity of the transporter. Non-linear analysis of the saturation curve for l-DOPA uptake revealed a Vmax value (in nmol mg protein/6 min) of 62 and 80 in wild type and transfected cells, respectively. The uptake of a non-saturating concentration (0.25 μM) of [ 14C]- l-DOPA in OK-WT cells was not affected by Na + removal, whereas in OK-α 1 cells accumulation of [ 14C]- l-DOPA was clearly dependent on the presence of extracellular Na +. When Na + was replaced by choline, the inhibitory profile of neutral l-amino acids, but not of basic and acidic amino acids, upon [ 14C]- l-DOPA uptake in both cell types, was significantly greater than that observed in the presence of extracellular Na +. It is concluded that enhanced ability of OK cells overexpressing Na +–K +-ATPase to translocate Na + from the apical to the basal cell side correlates positively with their ability to accumulate l-DOPA, which is in agreement with the role of Na + in taking up the precursor of renal dopamine.