Further insights into sinusoidal organic anion uptake



Previous studies in cultured rat hepatocytes revealed that initial uptake of sulfobromophthalein (BSP) was markedly reduced upon removal of Cl from the medium. In the present study, unidirectional Cl gradients were established in short-term cultured rat hepatocytes and their effect on BSP uptake was determined. These investigations revealed that BSP uptake requires external Cl and is not stimulated by unidirectional Cl gradients, suggesting that BSP transport is not coupled to Cl transport. In contrast, BSP transport is stimulated by an inside-to-outside OH gradient, consistent with OH exchange or H+ cotransport. As the presence of Cl is essential for but not directly coupled to BSP transport, binding of 35S-BSP to hepatocytes was determined at 4° C. This revealed an ∼ 10-fold higher affinity of cells for BSP in the presence as compared to the absence of Cl (Ka + 3.2 ± 0.8 vs. 0.42 ± 0.09 μM−1; P < 0.02). Affinity of BSP for albumin was Cl-independent, and was ∼ 10% of its affinity for cells in the presence of Cl. These results indicate that extracellular Cl modulates the affinity of BSP for its hepatocyte transporter.