Figure 5. Mutual D-serine and MeAIB uptake by system A transporters in cells and synaptosomes. A, Uptake of [14C]MeAIB (50 μm) by Slc38a1 and Slc38a2 in HEK293 transfected cells incubated in Na+-containing HBSS medium and different D-serine concentrations. Blanks consisting of cells transfected with GFP were subtracted from the values. B, MeAIB inhibits the uptake of D-[3H]serine in HEK293 cells transfected with Slc38a1 and Slc38a2 but does not affect cells expressing the other D-serine transporters Slc1a4, Slc1a5, or Slc7a10/4f2hc. C, D-serine decreases the uptake of [14C]MeAIB (50 μm) in purified synaptosomes of the adult mouse neocortex monitored in Na+-containing HBSS medium. D, MeAIB inhibits the uptake of 5 μm D-[3H]serine in purified synaptosomes of adult mice. E, Inhibition of D-[3H]serine uptake in purified synaptosomes by MeAIB was additive to inhibitors of Slc1a4 (1 mm OHP) and Slc7a10 (10 μm LuAE). F, MeAIB slowed down the rate of D-[3H]serine uptake and decreased the achievable steady-state loading of synaptosomes. G, The inhibition of D-[3H]serine uptake by MeAIB is still observed when the inhibitor was added together with typical system A substrates (AA mix) at physiological-like concentrations, consisting of 14 μm L-alanine, 36 μm L-serine, and 230 μm L-glutamine. H, The uptake of L-[3H]glutamate (5 μm) in purified synaptosomes was insensitive to MeAIB. I, MeAIB reduces the uptake of D-[3H]serine in primary cortical neuronal cultures (DIV14). J, D-Serine (10 mm) or glutamine (2 mm) inhibits the transport of [14C]MeAIB (50 μm) by primary cortical neuronal cultures. All uptake experiments (A–J) were conducted in the presence of Na+. The data represent the average ± SEM of 2 (A, H), 3-4 (B), 4 (C, D, E, J), 3 (F), 5 (G), and 10 (I) independent experiments. Data were analyzed by repeated-measures ANOVA and Sidak's (E) or Dunnett's (J) multiple-comparison test. Two-tailed Student's paired t test (I). a, p = 0.001; b, p = 0.0015, ***p < 0.001.