NEUTRAL AMINO-ACID-TRANSPORT BY ISOLATED SMALL INTESTINAL-CELLS FROM GUINEA-PIGS

Neutral amino acid transport was examined by using isolated enterocytes. Cells transport L-alanine by at least three different mechanisms: two Na+-dependent systems (A and ASC) and one Na+-independent mechanism (system L), in addition to passive entry. System A was characterized by measuring the Na+-dependent MeAIB uptake was concentrative and saturable. V(max) was obtained at uptake was concentrative and saturable. V(max) was obtained at 80 mM Na+ in the incubation medium and K(t app) for Na+ was 21.5 mM. K(t app) for MeAIB was 6.75 +/- 0.37 mM and the V(max) was 14.2 +/- 0.3 nmol . mg-1 . min-1. System ASC was studied by evaluating the Na+-dependent L-alanine uptake, insensitive to MeAIB and inhibitable by L-serine and L-cysteine. Uptake by this mechanism was also concentrative and saturable. Maximal uptake was obtained with 80 mM Na+ in the incubation medium and K(t app) for Na+ was 29.7 mM. K(t app) for L-alanine was 7.02 +/- 0.61 mM and V(max) was 5.44 +/- 0.19 nmol.mg-1.min-1. The Na+-independent system L was studied by measuring cycloleucine uptake in Na+-free medium. It had a saturable and a nonsaturable component. Only the saturable component was concentrative; it was inhibited by 2-amino-2-norbornanecarboxylic acid and was capable of mediating exchange diffusion. K(t app) for cycloleucine was 4.05 +/- 0.72 mM and the V(max) was 31.9 +/- 1.3 nmol.mg-1.min-1. These results confirm the existence of Na+-dependent systems A and ASC and Na+-independent system L in isolated enterocytes.