Monovalent cation conductance in Xenopus laevis oocytes expressing hCAT-3.
hCAT-3 (human cationic amino acid transporter type three) was investigated with both the two-electrode voltage clamp method and tracer experiments. Oocytes expressing hCAT-3 displayed less negative membrane potentials and larger voltage-dependent currents than native or water-injected oocytes did. Ion substitution experiments in hCAT-3-expressing oocytes revealed a large conductance for Na+ and K+. In the presence of L-Arg, voltage-dependent inward and outward currents were observed. At symmetrical (inside/outside) concentrations of L-Arg, the conductance of the transporter increased monoexponentially with the L-Arg concentrations; the calculated Vmax and KM values amounted to 8.3 microS and 0.36 mM, respectively. The time constants of influx and efflux of [3H]L-Arg, at symmetrically inside/outside L-Arg concentrations (1 mM), amounted to 79 and 77 min, respectively. The flux data and electrophysiological experiments suggest that the transport of L-Arg through hCAT-3 is symmetric, when the steady state of L-Arg flux has been reached. It is concluded that hCAT-3 is a passive transport system that conducts monovalent cations including L-Arg. The particular role of hCAT-3 in the diverse tissues remains to be elucidated.
Gilles
Wolfgang
W
Department of Pharmacology, Johannes-Gutenberg University, Obere Zahlbacher Str. 67, 55101 Mainz, Germany.
Vulcu
Sebastian D
SD
Liewald
Jana F
JF
Habermeier
Alice
A
Vékony
Nicole
N
Closs
Ellen I
EI
Rupp
Johanna
J
Nawrath
Hermann
H
D000818:Animals; D001120:Arginine / Q000494:pharmacology; D027201:Cationic Amino Acid Transporter 1 / Q000378:metabolism*; D002414:Cations, Monovalent; D004553:Electric Conductivity; D006801:Humans; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology*; D009865:Oocytes / Q000187:drug effects / Q000502:physiology*; D011188:Potassium / Q000378:metabolism*; D011994:Recombinant Proteins / Q000378:metabolism; D012964:Sodium / Q000378:metabolism*; D014982:Xenopus laevis
D001120:Arginine; D027201:Cationic Amino Acid Transporter 1; D002414:Cations, Monovalent; D011188:Potassium; D011994:Recombinant Proteins; D012964:Sodium; C499482:SLC7A3 protein, human