Uptake of neutral α- and β-amino acids by human proximal tubular cells

Abstract

The transport characteristics of amino acids in primary cell cultures from the proximal tubule of human adults (AHKE cells) were examined, using α-aminoisobutyric acid (AIB) and β-alanine as representatives of α- and β-amino acids, respectively. The Na +-gradient dependent influx of AIB occurred by a single, saturable transport system, whereas the Na +-gradient dependent uptake data for β-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 the 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 β-alanine was only drastically reduced by β-amino acids, whereas the inhibition by neutral α-amino acids was relatively low. Nor did l-arginine and l-aspartic acid affect the uptake of β-alanine into AHKE cells. Comparison with the results obtained for normal (NHKE) and immortalized (IHKE) embryonic cells suggested an unaltered expression of the types of transport carriers for neutral α- and β-amino acids in the embryonic and AHKE cells. However, the uptake capacity of the above-mentioned transport proteins was relatively smaller in the embryonic kidney compared with the adult human kidney, which may explain, at least partly, the phenomenon of physiologic amino aciduria in neonates.