Rat liver parenchymal cells express Na(+)-dependent and Na(+)- independent
nucleoside transport activity. The Na(+)-dependent component shows kinetic properties and substrate specificity similar to those reported for plasma membrane vesicles [Ruiz-Montasell, Casado, Felipe and Pastor-Anglada (1992) J. Membr. Biol. 128, 227-233]. This transport activity shows apparent K(m) values for
uridine in the range 8-13 microM and a Vmax of 246 pmol of
uridine per 3 min per 10(5) cells. Most
nucleosides, including the analogue
formycin B, cis-inhibit Na(+)-dependent
uridine transport, although
thymidine and
cytidine are poor inhibitors.
Inosine and
adenosine inhibit Na(+)-dependent
uridine uptake in a dose-dependent manner, reaching total inhibition.
Guanosine also inhibits Na(+)-dependent
uridine uptake, although there is some residual transport activity (35% of the control values) that is resistant to high concentrations of
guanosine but may be inhibited by low concentrations of
adenosine. The transport activity that is inhibited by high concentrations of
thymidine is similar to the
guanosine-resistant fraction. These observations are consistent with the presence of at least two Na(+)-dependent transport systems. Na(+)-dependent
uridine uptake is sensitive to
N-ethylmaleimide treatment, but Na(+)-independent transport is not. Nitrobenzylthioinosine (
NBTI) stimulates Na(+)-dependent
uridine uptake. The
NBTI effect involves a change in Vmax, it is rapid, dose-dependent, does not need preincubation and can be abolished by depleting the Na+ transmembrane electrochemical gradient. Na(+)-independent
uridine transport seems to be insensitive to
NBTI. Under the same experimental conditions,
NBTI effectively blocks most of the Na(+)-independent
uridine uptake in
hepatoma cells. Thus the stimulatory effect of
NBTI on the concentrative
nucleoside transporter of liver parenchymal cells cannot be explained by inhibition of
nucleoside efflux.