Function and regulation of taurine transport at the inner blood-retinal barrier

In the retina, taurine exerts a number of neuroprotective functions as an osmolyte and antioxidant. The purpose of the present study was to elucidate the taurine transport system(s) at the inner blood-retinal barrier (BRB). [(3)H]Taurine transport at the inner BRB was characterized using in vivo integration plot analysis and a conditionally immortalized rat retinal capillary endothelial cell line (TR-iBRB2 cells). The expression of the taurine transporter (TauT) was demonstrated by RT-PCR and immunoblot analyses. The apparent influx permeability clearance of [(3)H]taurine in the rat retina was found to be 259 muL/(ming retina), supporting carrier-mediated influx transport of taurine at the BRB. [(3)H]Taurine uptake by TR-iBRB2 cells was Na(+)-, Cl(-)- and concentration-dependent with a K(m) of 22.2 muM and inhibited by TauT inhibitors, such as beta-alanine and hypotaurine. RT-PCR and immunoblot analyses demonstrated that TauT is expressed in TR-iBRB2 and primary cultured human retinal endothelial cells. The uptake of [(3)H]taurine and the expression of TauT mRNA in TR-iBRB2 cells increased under hypertonic conditions but decreased following pretreatment with excess taurine. In conclusion, TauT most likely mediates taurine transport and regulate taurine transport at the inner BRB.