ADAPTIVE REGULATION OF TAURINE TRANSPORT IN 2 CONTINUOUS RENAL EPITHELIAL-CELL LINES

Taurine is actively transported by a β-amino acid transporter located on the proximal tubule apical surface. We have characterized taurine transport into confluent monolayers of two continuous renal epithelial cell lines: LLC-PK1, a cell of porcine proximal tubular origin, and the Madin-Darby canine kidney cell line (MDCK) of distal origin. Taurine uptake is linear up to 90 minutes in LLC-PK1 cells and 180 minutes in MDCK cells. This process is highly dependent upon Na+ as the cation and either Cl- or Br- as the anion. Taurine uptake is inhibited by another β-amino acid, β-alanine, to a greater extent than the α-analog, L-alanine or other α-amino acids. Incubation of cell monolayers with taurine-free medium (0 μM taurine) induces an increase in Na+-dependent taurine uptake when compared to cells exposed to standard medium (50 μM taurine). When cells were incubated in medium containing high taurine (500 μM), uptake was decreased as compared to control cells. This adaptive response is evident by 12 hours in both cell lines and is the result of changes in the apparent transport maximum (J(max)) rather than the apparent Km for taurine. The changes in transport observed after manipulation of medium taurine concentration were not associated with differences in taurine efflux. In summary, taurine is transported by a β-specific, Na-Cl dependent process in both renal epithelial cell lines. Although the factors which regulate taurine transport are not known, an increased transport maximum is observed in cells which have been taurine-starved, and a decreased J(max) is seen in cells supplied with excess taurine. This adaptive response mimics the actions observed for the brush border transporter.