Pii: S0304-3940(01)02058-4

Human embryonic kidney 293 (HEK-293) cells stably transfected with the human serotonin (5-HT) or dopamine transporter (hSERT, hDAT), or the rat GABA transporter GAT-1 were incubated with saturating concentrations of transporter substrates (hSERT: [H]5-HT, [H]N-methyl-phenyl-pyridinium (MPP1); hDAT: [H]dopamine, [H]MPP; rGAT: [H]GABA). Uptake velocities decreased signi®cantly over time for [H]5-HT and [H]dopamine (already visible at 1 min), but not for [H]MPP or [H]GABA. In ef̄ ux experiments cells were preloaded and substrate diffusion into the medium was studied following the addition of appropriate uptake inhibitors. Fractional ef̄ uxes were (% min) 1.27, 0.72, 0.27 and 0.08 for [H]5-HT, [H]dopamine, [H]MPP and [H]GABA, respectively. The results suggest that in uptake experiments the more lipophilic substrates [H]5-HT and [H]dopamine leave the cells by diffusion already after a short time (1 min) of accumulation. q 2001 Elsevier Science Ireland Ltd. All rights reserved.