CHARACTERIZATION OF L-GLUTAMINE TRANSPORT BY TYPE-II ALVEOLAR CELLS

This study characterized the sodium-dependent transport of L-glutamine (L-Gln) by rat type II alveolar cells. Uptake of 50-mu-M glutamine (Gln) was determined and found to be linear for at least 15 min. The sodium-dependent velocity represented > 80% of the total uptake at all time points. Kinetic studies of sodium-dependent Gln uptake at concentrations between 0.005 and 8 mM showed uptake to occur via two saturable transport systems, a high-affinity carrier [Michaelis constant (K(m)) = 259 +/- 19-mu-M, maximum velocity (V(max)) = 0.942 +/- 0.08 nmol.mg protein-1.30 s-1] and a low-affinity transporter (K(m) = 4.96 +/- 1.2 mM, V(max) = 2.98 +/- 0.19 nmol.mg protein-1.30 s-1). Uptake of Gln via the low-affinity system was nearly completely blocked by 10 mM 2-methylaminoisobutyric acid (MeAIB), and increasing concentrations of Gln almost totally inhibited transport of MeAIB, indicating the presence of system A. Further inhibition studies of the high-affinity transporter showed marked inhibition by serine, cysteine, and nonradioactive Gln. Lithium did not substitute for sodium, strongly suggesting that L-Gln was not transported by system N. Furthermore, transport of Gln by the high-affinity carrier was not affected by hormones or by changes in external pH. We conclude that sodium-dependent L-Gln transport by rat type II alveolar cells occurs predominantly via system A and system ASC.