Tolbutamide uptake via pH- and membrane-potential-dependent transport mechanism in mouse brain capillary endothelial cell line.
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.
Koyabu
Noriko
N
Department of Biopharmaceutics, Graduate School of Pharmaceutical Sciences, Kyushu University, Fukuoka, Japan.
Takanaga
Hitomi
H
Matsuo
Hirotami
H
Naito
Mikihiko
M
Tsuruo
Takashi
T
Ohtani
Hisakazu
H
Sawada
Yasufumi
Y
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*
D014044:Tolbutamide