Transport of glutamine across blood-facing membranes of perfused rat jejunum

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Abstract
Transport of glutamine and other neutral amino acids across the blood-facing membranes of isolated, dually perfused rat jejunum was measured using a paired-tracer isotope-dilution technique. Glutamine, asparagine, histidine, alanine, and leucine showed mutual inhibition of transport. The major component of physiological glutamine transport was saturable (Km = 0.88 .+-. 0.15 mM, Vmax = 454 .+-. 49 nmol .cntdot. g-1 .cntdot. min-1; mean .+-. SE), stereospecific and Na-independent and appeared to exhibit symmetry of glutamine transport; it most resembled system L. the minor Na-dependent component of glutamine transport resembled system A, i.e., it transported N-methylaminoisobutyric acid (Km .simeq. 10 .mu.M, Vmax .simeq. 1.2 nmol .cntdot. g-1 .cntdot. min-1). At 0.5 mM gluamine transport was insensitive to insulin and glucagon and was unaffected by perfusate pH (7.0-7.8). Glutamine extracted by the jejunum is rapidly utilized; at physiological blood glutamine concentrations the basolateral glutamine-transporter flux may thus not only restrict intestinal glutamine catabolism but also the consequent release of glutamine-derived ammonia (a substrate and stimulant of ureogenesis) into the portal circulation.