Molecular cloning and functional expression of the murine noradrenaline transporter

The cDNA of the murine noradrenaline transporter (mNAT) was cloned from the RNA of the placenta of a C57BL/6 mouse. The cloned mNAT differs from a previously published sequence in two amino acids within the C-terminal region. A cDNA obtained from an inbred mouse strain showed a further amino acid exchange (Ile(505)Val) within the fifth intracellular loop. The pharmacological properties of both, the wild-type mNAT and the variant (mNAT-I(505)V), were studied in human embryonic kidney HEK293 cells transfected with the corresponding cDNA. The kinetic constants for transport (K (m), V (max)) of [(3)H]noradrenaline ([(3)H]-NA) and binding (K (D), B (max)) of the selective NAT inhibitor [(3)H]nisoxetine were not different between the two isoforms; the mean kinetic constants amounted to about 4 microM and 120pmol/mg protein for K (m) and V (max) and 6nM and 18pmol/mg protein for K (D) and B (max), respectively. [(3)H]-NA transport by both isoforms showed the typical properties of an NAT because it was dependent on sodium and chloride and inhibited with almost identical K (i) values by various NAT substrates and inhibitors. The only significant pharmacological difference identified between the two mNAT isoforms was an about threefold higher affinity for cocaine of the very rare mNAT-I(505)V variant.