Voltage dependence of L-arginine transport by hCAT-2A and hCAT-2B expressed in oocytes from Xenopus laevis |
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Authors: | Nawrath H Wegener J W Rupp J Habermeier A Closs E I |
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Affiliation: | Department of Pharmacology, Johannes Gutenberg University, 55101 Mainz, Germany. nawrath@mail.uni-mainz.de |
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Abstract: | Membrane potential and currents were investigated with thetwo-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- andhCAT-2B-expressing oocytes exhibited a negative extracellularL-arginine concentration ([L-Arg]o)-sensitive membrane potential,additive to the K+ diffusion potential, when cells wereincubated in Leibovitz medium (containing 1.45 mM L-Arg and0.25 mM L-lysine). The two carrier proteins produced inwardand outward currents, which were dependent on the L-Arggradient and membrane potential. Ion substitution experiments showedthat the hCAT-induced currents were independent of externalNa+, K+, Ca2+, or Mg2+.The apparent Michaelis-Menten constant values at 60 mV, obtained fromplots of L-Arg-induced currents against[L-Arg]o, were 0.97 and 0.13 mM in oocytesexpressing hCAT-2A and hCAT-2B, respectively; maximal currentsamounted to 194 ± 8 and 84 ± 2 nA, respectively. Atsaturating [L-Arg]o, the current-voltagerelationships of hCAT-2A-expressing oocytes became steeper, yielding anadditional conductance up to 2 µS/oocyte, whereas those ofhCAT-2B-expressing oocytes were simply shifted to the right, resultingin voltage-independent difference currents. The distinctelectrochemical properties of the two isoforms of hCAT-2 are assumed tocontribute differentially to the membrane transport and the maintenanceof cationic amino acids in various tissues. |
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