Glutamine transport in isolated epithelial intestinal cells. Identification of a Na+-dependent transport mechanism, highly specific for glutamine.

L-glutamine transport was evaluated in isolated cells from the guinea-pig small intestine by measuring [(3)H]- L-glutamine uptake. Villous and crypt cells expressed Na(+)-dependent and Na(+)-independent transport mechanisms. Glutamine transport systems were identified using various amino acids and analogues as inhibitors. In both villous and crypt cells, 2-(methylamino)-isobutyrate (MeAIB), a system A inhibitor, did not inhibit Na(+)-dependent glutamine influx. 2-Aminobicyclo(2,2,1)heptane-2-carboxylate (BCH), a system B(0) and B(0,+) substrate, had no effect on Na(+)-dependent influx. Serine, cysteine and threonine, system ASC inhibitors, reduced Na(+)-dependent influx by 50%. Asparagine, but not histidine, system N inhibitors, reduced Na(+)-dependent glutamine influx by 50%, however the effect of asparagine was not additive to that of threonine. The remaining Na(+)-dependent glutamine influx (50%) was only inhibited by glutamine itself, by Na(+) substitution ( N-methyl-glucamine, K(+), Li(+)) or by external pH reduction. Phenyl-acetyl-glutamine (PAG), a synthetic amino acid analogue, also inhibited this Na(+)-dependent, threonine-insensitive glutamine influx (IC(50) 2.45 mM). The Na(+)-independent uptake was partially inhibited by BCH, a system L inhibitor, and other neutral amino acids, but was not affect by PAG. Our results suggest that glutamine is transported in both villous and crypt cells by the Na(+)-independent system L, by the Na(+)-dependent system ASC and by an as yet undescribed Na(+)-dependent transport mechanism, highly specific for glutamine.