We investigated whether an immortalized human kidney epithelial cell line (IHKE), compared with normal embryonic cells (NHKE), can be used as a representative system with which to characterize the transport of neutral amino acids in the proximal tubule of the human kidney. The IHKE cell line, immortalized by treatment with NiSO4, exhibited microvilli and enzyme markers specific for highly specialized tubule cells. The Na(+)-dependent uptake of alpha-aminoisobutyric acid (AIB) by IHKE and NHKE cells occurred by means of a single transport system with identical half-saturation constants, but the capacity for uptake was higher in the immortalized cells. Proton-dependent influx of AIB was also mediated by a single transport component with similar uptake characteristics in both types of cells. Imposition of an H(+)-gradient to a Na(+)-gradient reduced the sodium dependent uptake of AIB with the exception of short incubation time (1 min), where addition of a proton gradient produced a marked increase in the Na(+)-dependent influx of AIB in NHKE but not in IHKE cells. Competition experiments revealed that the Na(+)-dependent uptake at 50 microM 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 addition of 5 mM L-lysine produce a slight inhibition. Except for L-proline all of the neutral and acidic amino acids tested reduced the H(+)-dependent 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-Arginine, but not L-lysine decreased the influx in both cell lines. We conclude that the IHKE cell line has retained the capability to accumulate AIB by transport protein(s) similar to those present for neutral alpha-amino acids in NHKE cells.