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

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 K_T and V_max 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 K_T 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 K_T and V_max 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 K_T 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.