Characterization of sodium‐dependent monocarboxylate transporter‐1 (SMCT1)‐mediated GHB transport in FRTL‐5 cells

Abstract

PurposeTo characterize γ‐hydroxybutyrate (GHB) uptake kinetics in Rat Thyroid Follicular (FRTL‐5) cells, which express the Na+‐dependent monocarboxylate (SMCT1) transporter.MethodsSMCT1 expression and GHB uptake kinetics were analyzed in FRTL‐5 cells. Transport was evaluated with respect to Na+, pH, concentration and inhibitors.ResultsAt pH 7.4, Na+‐dependent GHB uptake could be described with a simple Michaelis‐Menten equation (Km, 0.77 ± 0.41 mM; Vmax, 3.59 ± 2.0 nmol/mg/min) suggesting a single transporter‐mediated uptake process. GHB uptake was not altered at different pH values (5.5 ‐ 7.4) in the presence of Na+, but was significantly increased at low pH in the absence of Na+, suggesting an additional uptake pathway. Na+‐dependent GHB uptake was inhibited by MCT substrates (D‐lactate, L‐lactate, pyruvate and butyrate), NSAIDs (ibuprofen, ketoprofen and naproxen) and probenecid. IC50 values for L‐lactate, ibuprofen, ketoprofen and probenecid were 106.7, 31.6, 64.4 and 379 uM, respectively. All four inhibitors also inhibited GHB uptake in ratMCT1 transfected MDA/MB231 cells, suggesting they also inhibit proton‐dependent MCT1.ConclusionsThe present study characterized Na+‐dependent GHB transport in FRTL‐5 cells and furthers our understanding of the roles of Na+‐ and proton‐dependent MCTs in GHB disposition.SupportNIH grant DA023223 and a fellowship for MAF from Pfizer Inc.