Amino acid transport in kidney. Heterogeneity of alpha-aminoisobutyric uptake.

1. More than one mode of transport can be observed for concentrative uptake of a-aminoisobutyric acid (AIB) by rat kidney cortex slices when the initial concentration of AIB in the medium is varied between 0.2 and 16 mM. 2. The relative contribution from both systems to the total observed uptake at various concentrations of AIB was calculated. The K, values for the two systems were 3.87 rnM and 24.8 m&r. 3. Sodium-dependent inhibitory reactions influence the uptake of 0.8 rnM AIB more than of 8.0 mM AIB; inhibitors affecting -SH groups have the opposite effect. 4. L-Alanine is an effective competitive inhibitor of AIB transport at high substrate concentrations; more than one Ki value for L-alanine can be derived depending on the concentration of AIB. External L-proline enhances the affinity of AIB for uptake at low substrate concentrations; an inhibitory effect is observed at high substrate concentrations. Although omission of sodium from the medium abolishes concentrative uptake of AIB at all concentrations, it is restored at low substrate concentration by the addition of 4 mra L-proline to the external sodium-free medium. L-Proline placed internally stimulates AIB accumulation at low substrate concentrations only in the presence of sodium. 5. A site at which a ternary complex between proline or sodium and the AIB-carrier complex may be formed is proposed for AH3 transport at low concentrations; another system with different properties is used predominantly for AIB transport at high concentrations.