Isolation and characterization of a digoxin transporter and its rat homologue expressed in the kidney

Digoxin, which is one of the most commonly prescribed drugs for the treatment of heart failure, is mainly eliminated from the circulation by the kidney. P-glycoprotein is well characterized as a digoxin pump at the apical membrane of the nephron. However, little is known about the transport mechanism at the basolateral membrane. We have isolated an organic anion transporter (OATP4C1) from human kidney. Human OATP4C1 is the first member of the organic anion transporting polypeptide (OATP) family expressed in human kidney. The isolated cDNA encodes a polypeptide of 724 aa with 12 transmembrane domains. The genomic organization consists of 13 exons located on chromosome 5q21. Its rat counterpart, Oatp4c1, is also isolated from rat kidney. Human OATP4C1 transports cardiac glycosides (digoxin, K-m = 7.8 muM and ouabain, K-m = 0.38 muM), thyroid hormone (triiodothyronine, K. = 5.9 muM and thyroxine), cAMP, and methotrexate in a sodium-independent manner. Rat Oatp4c1 also transports digoxin (Km = 8.0 muM) and triioclothyronine (Km = 1.9 muM). Immunohistochemical analysis reveals that rat Oatp4c1 protein is localized at the basolateral membrane of the proximal tubule cell in the kidney. These data suggest that human OATP40/rat Oatp4c1 might be a first step of the transport pathway of digoxin and various compounds into urine in the kidney.