Sodium and lithium interactions with the Na+/dicarboxylate cotransporter

The two-electrode voltage clamp was used to study the currents associated with transport of succinate by the cloned Na+/dicarboxylate cotransporter, NaDC-1, expressed in Xenopus oocytes, The presence of succinate induced inward currents which were dependent on the concentrations of succinate and sodium,;md on the membrane potential. At -50 mV, the K-0.5(succinate) was 180 mu M and the K-0.5(Na+) was 19 mM. The Hill coefficient was 2.3, which is consistent with a transport stoichiometry of 3 Na+:1 divalent anion substrate. Currents wt:re induced in NaDC-1 by a range of di- and tricarboxylates, including citrate, methylsuccinate, fumarate, and tricarbally-late. Although Na+ is the preferred cation, Li+ was also able to support transport. The K-0.5(succinate) was approximately 10-fold higher in Li+ compared with Na+. In the presence of Na+, however, Li+ was a potent inhibitor of transport. Millimolar concentrations of Li+ resulted in decreases in apparent succinate affinity and in the I-max(succinate). Furthermore, lithium inhibition under saturating sodium concentrations showed hyperbolic kinetics, suggesting that one of the three cation binding sites in NaDC-1 has a higher affinity for Li+ than Na+. We conclude that NaDC-1 is an electrogenic anion transporter that accepts either Na+ or Li+ as coupling cations. However, NaDC-1 contains a single high affinity binding site for Li+ that, when occupied,. results in transport inhibition, which may account for its potent inhibitory effects on renal dicarboxylate transport.