Na+/amino acid coupling stoichiometry of rheogenic system B0,+ transport in Xenopus oocytes is variable.
Using electrophysiological and radiotracer studies in parallel, we have investigated the characteristics of the endogenous Na(+)-dependent amino acid transporter (system B0,+) in Xenopus oocytes with regard to ion dependence, voltage dependence and transport stoichiometry. In voltage-clamped oocytes (-60 mV) superfusion with saturating concentrations of amino acids (1 mM) in 100 mM NaCl resulted in reversible, inward currents (mean +/- SEM): alanine, 1.83 +/- 0.09 nA (n = 21); arginine, 2.54 +/- 0.18 nA (n = 17); glutamine, 1.73 +/- 0.10 nA (n = 19). Only arginine evoked a current in choline medium (0.50 +/- 0.13 nA, n = 10), whereas Cl- replacement had no effect on evoked currents. The glutamine-evoked current was saturable (Imax = 1.73 nA, glutamine Km = 0.12 mM) and linearly dependent upon voltage between -90 and -30 mV. Using direct and indirect (activation) methods, we found that transport can proceed with Na+/amino acid coupling stoichiometry of either 1:1 or 2:1, but coupling was the same for each amino acid tested (alanine, arginine and glutamine) within a batch of oocytes (i.e. from a single toad). Despite the net single positive charge on arginine, the magnitude of the net transmembrane charge movement during Na(+)-coupled arginine transport was identical to that for the zwitterionic neutral amino acids glutamine and alanine; this may be explained by a concomitant stimulation of K+ efflux during arginine transport with a putative coupling of 1 K+:1 arginine.
Mackenzie
B
B
Department of Anatomy and Physiology, University of Dundee, UK.
Harper
A A
AA
Taylor
P M
PM
Rennie
M J
MJ
D000409:Alanine / Q000378:metabolism; D026905:Amino Acid Transport Systems; D000596:Amino Acids / Q000378:metabolism*; D000818:Animals; D001693:Biological Transport, Active; D002352:Carrier Proteins / Q000378:metabolism; D005071:Evoked Potentials; D005260:Female; D066298:In Vitro Techniques; D009865:Oocytes / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism; D012964:Sodium / Q000378:metabolism*; D014982:Xenopus laevis
D026905:Amino Acid Transport Systems; D000596:Amino Acids; D002352:Carrier Proteins; D012964:Sodium; D000409:Alanine; D011188:Potassium