Characterization of 3-[I-123]iodo-L-alpha-methyl tyrosine transport in astrocytes of neonatal rats

3-[I-123]Iodo-L-alpha -methyl tyrosine (I-123-IMT) is used for diagnosis an d monitoring of brain tumours by means of single-photon emission tomography . As recently shown, I-123-IMT is predominantly mediated into rat C6 glioma cells by sodium-independent system L for large neutral amino acids. Until now, I-123-IMT transport in non-neoplastic glial cells has not been examine d. Therefore, the aim of this study was to examine the cellular pathways an d precise transport kinetics of I-123-IMT uptake into astrocytes of neonata l rats. In particular sodium-independent I-123-IMT transport into neonatal astrocytes was compared with sodium-independent I-123-IMT uptake into neopl astic rat C6 glioma cells. Competitive inhibition experiments showed that I-123-IMT is exclusively tra nsported via sodium-independent system L into the neonatal astrocytes (92%) . Kinetic analysis of sodium-independent I-123-IMT uptake into neonatal ast rocytes and into C6 glioma cells revealed apparent Michaelis constants K-M = 13.9 +/- 0.5 muM and K-M = 33.9 +/- 4.1 muM, respectively, which are in t he same range of K-M values as those recently determined for amino acid tra nsport into neoplastic and non-neoplastic glial cells. Indeed, the K-M valu es in the micromolar range correspond to the expression of the LAT-1 subuni t of system L both in the neonatal astrocytes and in C6 glioma cells. Howev er, sodium-independent maximum transport velocities (V-max) differed signif icantly between neonatal astrocytes and C6 glioma cells (11.1 +/- 0.3 and 3 9.9 +/- 3.3 nmol/mg protein/10 min, respectively).