6533b861fe1ef96bd12c4566

RESEARCH PRODUCT

Voltage dependence of L-arginine transport by hCAT-2A and hCAT-2B expressed in oocytes from Xenopus laevis.

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Membrane potential and currents were investigated with the two-electrode voltage-clamp technique in Xenopus laevisoocytes expressing hCAT-2A or hCAT-2B, the splice variants of the human cationic amino acid transporter hCAT-2. Both hCAT-2A- and hCAT-2B-expressing oocytes exhibited a negative extracellularl-arginine concentration ([l-Arg]o)-sensitive membrane potential, additive to the K+diffusion potential, when cells were incubated in Leibovitz medium (containing 1.45 mM l-Arg and 0.25 mM l-lysine). The two carrier proteins produced inward and outward currents, which were dependent on the l-Arg gradient and membrane potential. Ion substitution experiments showed that the hCAT-induced currents were independent of external Na+, K+, Ca2+, or Mg2+. The apparent Michaelis-Menten constant values at −60 mV, obtained from plots of l-Arg-induced currents against [l-Arg]o, were 0.97 and 0.13 mM in oocytes expressing hCAT-2A and hCAT-2B, respectively; maximal currents amounted to −194 ± 8 and −84 ± 2 nA, respectively. At saturating [l-Arg]o, the current-voltage relationships of hCAT-2A-expressing oocytes became steeper, yielding an additional conductance up to 2 μS/oocyte, whereas those of hCAT-2B-expressing oocytes were simply shifted to the right, resulting in voltage-independent difference currents. The distinct electrochemical properties of the two isoforms of hCAT-2 are assumed to contribute differentially to the membrane transport and the maintenance of cationic amino acids in various tissues.