IDENTIFICATION OF THE CATIONIC AMINO-ACID TRANSPORTER (SYSTEM Y(-BLOOD BRAIN BARRIER()) OF THE RAT)

Cationic amino acids are transported from blood into brain by a satura ble carrier at the blood-brain barrier (BBB). The transport properties of this carrier were examined in the rat using an in situ brain perfu sion technique. Influx into brain via this system was found to be sodi um independent and followed Michaelis-Menten kinetics with half-satura tion constants (K(m)) of 50-100 muM and maximal transport rates of 22- 26 nmol/min/g for L-lysine, L-arginine, and L-ornithine. The kinetic p roperties matched that of System y+, the sodium-independent cationic a mino acid transporter, the cDNA for which has been cloned from the mou se. To determine if the cloned receptor is expressed at the BBB, we as sayed RNA from rat cerebral microvessels and choroid plexus for the pr esence of the cloned transporter mRNA by RNase protection. The mRNA wa s present in both cerebral microvessels and choroid plexus and was enr iched in microvessels 38-fold as compared with whole brain. The result s indicate that System y+ is present at the BBB and that its mRNA is m ore densely expressed at cerebral microvessels than in whole brain.