Transfer of glutamine between astrocytes and neurons is an essential part o
f the glutamate-glutamine cycle in the brain. Here we have investigated how
the neural glutamine transporter (rATA1/GlnT) works. Rat ATA1 was expresse
d in Xenopus laevis oocytes and examined using two-electrode voltage-clamp
recordings, ion-sensitive microelectrodes and tracer flux experiments. Glut
amine transport via rATA1 was electrogenic and caused inward currents that
did not reverse at positive holding potentials. Currents were induced by a
variety of neutral amino acids in the following relative order Ala > Ser/Gl
n/Asn/His/Cys/Met > MeAIB/Gly > Thr/Pro/Tyr/Val, where MeAIB is the amino a
cid analogue N-methylaminoisobutyric acid. The uptake of glutamine and the
corresponding currents depended on Na+ and PH. Hill-coefficient and flux st
udies with (NaCl)-Na-22 indicated a cotransport stoichiometry 1 Na+ per tra
nsport cycle. The transporter also showed uncoupled Na+ transport, particul
arly when alanine was used as the substrate. Although substrate uptake incr
eased strongly with increasing PH, no change of intracellular PH was observ
ed during transport. A decrease of the intracellular PH similarly inhibited
glutamine transport via ATA1, suggesting that the PH dependence was an all
osteric effect on the transporter.