title: Tolbutamide uptake via pH- and membrane-potential-dependent transport mechanism in mouse brain capillary endothelial cell line.
abstract: The purpose of this study was to investigate the transport mechanism of tolbutamide across the blood-brain barrier (BBB) using MBEC4 cells as an in vitro BBB model.
  
  
  METHODS
  The BBB transport of tolbutamide was studied by using a mouse brain capillary endothelial cell line, MBEC4, cultured on dishes with their luminal membrane facing the culture medium.
  
  
  RESULTS
  The uptake of [14C]tolbutamide by MBEC4 cells was dependent on temperature and energy. The uptake coefficient of [14C]tolbutamide increased markedly with decreasing pH of the external medium from neutral to acidic. Valinomycin and replacement of chloride with sulfate or gluconate significantly increased the initial uptake of [14C]tolbutamide, while replacement with nitrate significantly decreased it. The uptake was significantly reduced by a proton ionophore, FCCP, and an anion-exchange inhibitor, DIDS. The initial uptake of [14C]tolbutamide was saturable with Kt of 0.61+/-0.03 mM (pH 7.4) and 1.76+/-0.19 mM (pH 6.5). At pH 6.5, the initial uptake of [14C]tolbutamide was significantly reduced by several sulfa drugs, salicylic acid, valproic acid and probenecid, and was competitively inhibited by sulfaphenazole (Ki=3.47+/-0.50 mM) and valproic acid (Ki=2.29+/-0.43 mM).
  
  
  CONCLUSIONS
  These observations indicate the existence of a pH- and membrane-potential-dependent anion exchange and/or proton-cotransport system(s) for concentrative uptake of tolbutamide and sulfa drugs in MBEC4 cells.
authors.affiliation: Department of Biopharmaceutics, Graduate School of Pharmaceutical Sciences, Kyushu University, Fukuoka, Japan.
authors.forename: Noriko
authors.initials: N
authors.lastname: Koyabu
authors.forename: Hitomi
authors.initials: H
authors.lastname: Takanaga
authors.forename: Hirotami
authors.initials: H
authors.lastname: Matsuo
authors.forename: Mikihiko
authors.initials: M
authors.lastname: Naito
authors.forename: Takashi
authors.initials: T
authors.lastname: Tsuruo
authors.forename: Hisakazu
authors.initials: H
authors.lastname: Ohtani
authors.forename: Yasufumi
authors.initials: Y
authors.lastname: Sawada
mesh_terms: D000818:Animals; D001692:Biological Transport / Q000502:physiology; D001921:Brain / Q000378:metabolism*; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D042783:Endothelial Cells / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D008564:Membrane Potentials / Q000502:physiology; D051379:Mice; D014044:Tolbutamide / Q000378:metabolism*
chemical_list: D014044:Tolbutamide