UPTAKE OF NEUTRAL ALPHA-AMINO AND BETA-AMINO ACIDS BY HUMAN PROXIMAL TUBULAR CELLS

The transport characteristics of amino acids in primary cell cultures from the proximal tubule of human adults (AHKE cells) were examined, u sing alpha-aminoisobutyric acid (AIB) and beta-alanine as representati ves of alpha- and beta-amino acids, respectively, The Na+-gradient dep endent influx of ALE occurred by a single, saturable transport system, whereas the Na+-gradient dependent uptake data for beta-alanine could be described in terms of two-independent transport components as well as one-transport one-leak model with identical kinetic constants for the high-affinity system. Competition experiments revealed that all th e neutral amino acids tested reduced the uptake of AIB, whereas there was no effect of taurine, L-aspartic acid, and L-arginine. By contrast , the influx of beta-alanine was only drastically reduced by beta-amin o acids, whereas the inhibition by neutral alpha-amino acids was relat ively low. Nor did L-arginine and L-aspartic acid affect the uptake of beta-alanine into AHKE cells. Comparison with the results obtained fo r normal (NHKE) and immortalized (IHKE) embryonic cells suggested an u naltered expression of the types of transport carriers for neutral alp ha- and beta-amino acids in the embryonic and AHKE cells. However, the uptake capacity of the above-mentioned transport proteins was relativ ely smaller in the embryonic kidney compared with the adult human kidn ey, which may explain, at least partly, the phenomenon of physiologic amino aciduria in neonates.