OPPOSITE DIRECTED CURRENTS INDUCED BY THE TRANSPORT OF DIBASIC AND NEUTRAL AMINO-ACIDS IN XENOPUS OOCYTES EXPRESSING THE PROTEIN RBAT

Voltage- and current-clamp studies have been performed on a renal and intestinal protein (rBAT) which induces transport for neutral and diba sic amino acids when expressed in Xenopus oocytes. In current-clamp mo de, superfusion with L-leucine caused a hyperpolarization while superf usion with L-arginine depolarized the oocyte. Accordingly, in voltage- clamp experiments dibasic amino acids and neutral amino acids induced inward and outward currents, respectively. The relationship between cu rrents and substrate concentrations could be fitted by simple Michaeli s Menten kinetics. Currents induced by L-arginine and L-leucine were a lso voltage-dependent, pH changes from 6.25 to 8.75 did not affect the currents induced by saturating concentrations of L-arginine and L-leu cine, but reversed the direction of L-histidine-induced currents from inward to outward. The reversal potentials as well as the apparent K-m for L-histidine-induced currents were altered by the ambient pH. Curr ents induced by individual amino acids decreased during extended super fusion periods. However, extended superfusion with neutral amino acids increased dibasic amino acid induced currents, while prior superfusio n with dibasic amino acid resulted in an increase of currents induced by neutral amino acids. The reversal potentials for L-leucine- and L-a rginine-induced currents were depending on their intra- (after preload ing) and extracellular concentrations. In conclusion, rBAT mediated tr ansport of neutral and dibasic amino acids is associated with net outw ard or inward currents, respectively, which may be caused by an exchan ge of neutral with dibasic amino acids.