1.
Adenosine kinase (AK) inhibitors can enhance
adenosine levels and potentiate
adenosine receptor activation. As the AK inhibitors 5' iodotubercidin (ITU) and 5-amino-5'-deoxyadenosine (NH(2)dAdo) are
nucleoside analogues, we hypothesized that
nucleoside transporter subtype expression can affect the potency of these inhibitors in intact cells. 3. Three
nucleoside transporter subtypes that mediate
adenosine permeation of rat cells have been characterized and cloned: equilibrative transporters rENT1 and rENT2 and concentrative transporter
rCNT2. We stably transfected rat C6
glioma cells, which express rENT2
nucleoside transporters, with rENT1 (rENT1-C6 cells) or
rCNT2 (rCNT2-C6 cells)
nucleoside transporters. 3. We tested the effects of ITU and NH(2)dAdo on [(3)H]-
adenosine uptake and conversion to [(3)H]-
adenine nucleotides in the three cell types. NH(2)dAdo did not show any cell type selectivity. In contrast, ITU showed significant inhibition of [(3)H]-
adenosine uptake and [(3)H]-
adenine nucleotide formation at concentrations < or =100 nM in rENT1-C6 cells, while concentrations > or =3 microM were required for C6 or rCNT2-C6 cells. 4. Nitrobenzylthioinosine (
NBMPR; 100 nM), a selective inhibitor of rENT1, abolished the effects of nanomolar concentrations of ITU in rENT1-C6 cells. 5. This study demonstrates that the effects of ITU, but not NH(2)dAdo, in whole cell assays are dependent upon
nucleoside transporter subtype expression. Thus, cellular and tissue differences in expression of
nucleoside transporter subtypes may affect the pharmacological actions of some AK inhibitors.