Sodium‐Dependent Uptake of Nucleosides by Dissociated Brain Cells from the Rat

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Abstract
Sodium-dependent 3H-labeled nucleoside transport was studied using a mixed population of dissociated brain cells from adult rats. The accumulation of [3H]adenosine during brief (15-s) incubation periods was significantly greater in the presence of 110 μM Na+ than in its absence. This occurred at substrate concentrations that ranged from 0.25 to 100 μM. Similar findings were observed for the rapid accumulation of [3H]uridine. Kinetically, the rapid accumulation of [3H]adenosine in both the absence and the presence of Na+ was best described by a two-component system. In the presence of Na+, the KT and Vmax values for the high-affinity component were 0.9 μM and 8.9 pmol/mg of protein/15 s, and those for the low-affinity component were 313 μM and 3,428 pmol/mg of protein/15 s, respectively. In the absence of Na+, the KT value for the high-affinity component was significantly higher (1.8 μM). [3H]Uridine accumulation was best described kinetically by a one-component system that in the presence of Na+ had KT and Vmax values of 1.0 mM and 2.6 nmol/mg of protein/15 s, respectively. As was found for [3H]adenosine, in the absence of Na+, the KT value was significantly higher (1.8 mM). The sodium-dependent transport of [3H]adenosine was inhibitable by ouabain and 2,4-dinitrophenol. Of the three nucleoside transport inhibitors tested, only nitrobenzylthioinosine demonstrated high affinity and selectivity in blocking the sodium component. Thus, high-affinity sodium-dependent nucleoside transport systems, in addition to facilitated diffusion systems, exist on brain cells from adult rats.