CHARACTERIZATION OF A SODIUM-DEPENDENT TAURINE TRANSPORTER IN RABBIT CHOROID-PLEXUS

Taurine, a beta-amino acid, plays an important role as a neuromodulato r and is necessary for the normal development of the brain. Since de n ovo synthesis of taurine in the brain is minimal and in vivo studies s uggest that taurine does not cross the blood-brain barrier, we examine d whether the choroid plexus, the blood-cerebrospinal fluid barrier, p lays a role in taurine transport in the central nervous system. The up take of [H-3]taurine into ATP-depleted choroid plexus from rabbit was substantially greater in the presence of an inwardly directed Na+ grad ient, whereas in the absence of a Na+ gradient taurine accumulation wa s negligible. A transient inside-negative potential gradient enhanced the Na+-driven uptake of taurine into the tissue slices, suggesting th at the transport process is electrogenic. Na+-driven taurine uptake wa s saturable with an estimated V-max, of 111+/-20.2 nmol/g per 15 min a nd a K-m of 99.8+/-29.9 mu M. The estimated coupling ratio of Na+ and taurine was 1.80+/-0.122. Na+-dependent taurine uptake was significant ly inhibited by beta-amino acids, but not by alpha-amino acids, indica ting that the transporter is selective for beta-amino acids. Na+-depen dent taurine uptake showed some selectivity for anions: the accumulati on was comparable in the presence of Cl-, Br- and thiocynate whereas I -, SO42- and gluconate did not stimulate the uptake significantly. Col lectively, our results demonstrate that taurine is transported in the choroid plexus via a Na+-dependent, saturable and apparently beta-amin o acid selective mechanism. This process may be functionally relevant to taurine homeostasis in the brain.