A COMPARATIVE-STUDY ON THE UPTAKE OF ALPHA-AMINOISOBUTYRIC-ACID BY NORMAL AND IMMORTALIZED HUMAN EMBRYONIC KIDNEY-CELLS FROM PROXIMAL TUBULE

We investigated whether an immortalized human kidney epithelial cell l ine (IHKE), compared with normal embryonic cells (NHKE), can be used a s a representative system with which to characterize the transport of neutral amino acids in the proximal tubule of the human kidney. The IH KE cell line, immortalized by treatment with NiSO4, exhibited microvil li and enzyme markers specific for highly specialized tubule cells. Th e Na+-dependent uptake of alpha-aminoisobutyric acid (AIB) by IHKE and NHKE cells occurred by means of a single transport system with identi cal half-saturation constants, but the capacity for uptake was higher in the immortalized cells. Proton-dependent influx of AIB was also med iated by a single transport component with similar uptake characterist ics in both types of cells. Imposition of an H+-gradient to a Na+-grad ient reduced the sodium dependent uptake of AIB with the exception of short incubation time (1 min), where addition of a proton gradient pro duced a marked increase in the Na+-dependent influx of AIB in NHKE but not in IHKE cells. Competition experiments revealed that the Na+-depe ndent uptake at 50 muM AIB was reduced by neutral alpha-amino acids in the two cell lines. L-Glutamate, L-aspartate, L-arginine and the beta -amino acid taurine had no effect. Only in the IHKE cell line did addi tion of 5 mM L-lysine produce a slight inhibition. Except for L-prolin e all of the neutral and acidic amino acids tested reduced the H+-depe ndent uptake of AIB in the IHKE cell line. By contrast, addition of L- aspartate did not influence the transport of AIB in NHKE cells. L-Argi nine, but not L-lysine decreased the influx in both cell lines. We con clude that the IHKE cell line has retained the capability to accumulat e AIB by transport protein(s) similar to those present for neutral alp ha-amino acids in NHKE cells.