Function of taurine transporter (Slc6a6/TauT) as a GABA transporting protein and its relevance to GABA transport in rat retinal capillary endothelial cells.

The purpose of this study was to identify the uptake mechanism of gamma-aminobutyric acid (GABA) via taurine transporter (Slc6a6/TauT) and its relationship with GABA transport at the inner BRB. Rat Slc6a6/TauT-transfected HeLa cells exhibited Na(+)-, Cl(-)-, and concentration-dependent [3H]GABA uptake with a Km of 1.5 mM. Taurine, beta-alanine, and GABA markedly inhibited Slc6a6/TauT-mediated uptake of [3H]GABA. The uptake of [3H]GABA by a conditionally immortalized rat retinal capillary endothelial cell line (TR-iBRB2) was Na(+)-, Cl(-)-, and concentration-dependent with a Km of 2.0 mM. This process was more potently inhibited by substrates of Slc6a6/TauT, taurine and beta-alanine, than those of GABA transporters, GABA and betaine. In the presence of taurine, there was competitive inhibition with a Ki of 74 microM. [3H]Taurine also exhibited competitive inhibition with a Ki of 1.8 mM in the presence of GABA. In conclusion, rat Slc6a6/TauT has the ability to use GABA as a substrate and Slc6a6/TauT-mediated GABA transport appears to be present at the inner BRB.