TRANSPORT OF MONOCARBOXYLIC ACIDS AT THE BLOOD-BRAIN-BARRIER - STUDIES WITH MONOLAYERS OF PRIMARY CULTURED BOVINE BRAIN CAPILLARY ENDOTHELIAL-CELLS

The kinetics and mechanism of the transport of monocarboxylic acids (MCAs) were studied by using primary cultured bovine brain capillary endothelial cells. Concentration-dependent uptake of acetic acid was observed, and the kinetic parameters were estimated as follows: the Michaelis constant, K(t), was 3.41 +/- 1.87 mM, the maximum uptake rate, J(max), was 144.7 +/- 55.7 nmol/mg of protein/min and the nonsaturable first-order rate constant, K(d), was 6.66 +/- 1.98-mu-l/mg of protein/min. At medium pH below 7.0, the uptake rate of [H-3]acetic acid increased markedly with decreasing medium pH, whereas pH-independent uptake was observed in the presence of 10 mM acetic acid. An energy requirement for [H-3]acetic acid uptake was also demonstrated, because metabolic inhibitors (2,4-dinitrophenol and rotenone) reduced significantly the uptake rate (P < .05). Carbon-ylcyanide-p-trifluoro-methoxyphenylhydrazone, a protonophore, inhibited significantly the uptake of [H-3]acetic acid at medium pH of 5.0 and 6.0, whereas 4,4'-diisothiocyanostilben-2,2'-disulfonic acid did not. Several MCAs inhibited significantly the uptake rate of [H-3]acetic acid, whereas di- and tricarboxylic acids did not. The uptake of [H-3]acetic acid was competitively inhibited by salicylic acid, with an inhibition constant, K(i), of 3.60 mM, suggesting a common transport system between acetic acid and salicylic acid. Moreover, at the medium pH of 7.4, salicylic acid and valproic acid inhibited significantly the uptake of [H-3]acetic acid, demonstrating that the transport of MCA drugs could also be ascribed to the MCA transport system at the physiologic pH. Accordingly, it was suggested that a carrier-mediated transport system for MCA participates in the transport of not only endogenous short-chain MCAs but also exogenous aromatic MCA drugs at the blood-brain barrier.