Expression of the renal Na+/dicarboxylate cotransporter, NaDC-1, in COS-7 cells.

The cloned rabbit renal Na+/dicarboxylate cotransporter, NaDC-1, was transiently expressed in the mammalian monkey kidney cell line, COS-7. Cells transfected with the plasmid pSV-201, containing the cDNA for the Na+/dicarboxylate cotransporter, expressed sodium-dependent succinate and citrate transport after 48 h. Cells transfected with control plasmid, pSV-SPORT, did not express sodium-dependent transport of succinate or citrate. The transport of succinate in cells expressing NaDC-1 was inhibited by di-and tricarboxylic acids, but not by monocarboxylic acids. Sodium-dependent transport of succinate was insensitive to changes in pH, while sodium-dependent citrate transport was stimulated by acidic pH. Succinate transport by NaDC-1 was saturable with an apparent Michaelis constant, Km, around 0.5 mM. The kinetics of sodium activation of succinate transport by NaDC-1 were sigmoidal, with an apparent Hill coefficient of 2.9, indicating that three sodium ions are involved in the transport of each succinate. Succinate transport by NaDC-1 was inhibited by lithium. The functional characteristics of NaDC-1 expressed in COS-7 cells correspond to those of the Na+/dicarboxylate cotransporter of the apical membrane of the renal proximal tubule.