Monocarboxylate transporter-mediated transport of γ-hydroxybutyric acid in human intestinal Caco-2 cells

The objectives of this study were to determine mRNA expression of monocarboxylate transporters (MCT) and to evaluate intestinal transport of the MCT substrates γ-hydroxybutyrate (GHB) and Dlactate in human intestinal Caco-2 cells. The presence of mRNA for MCT1, 2, 3, and 4 was observed in Caco-2 cells. The uptake of both GHB and D-lactate in Caco-2 cells was demonstrated to be pH- and concentration-dependent and sodium-independent. The uptake of GHB and D-lactate was best described by a Michaelis-Menten equation with passive diffusion (GHB: K_m = 17.6 ± 10.5 mM, V_max = 17.3 ± 11.7 nmol/min/mg, and P = 0.38 ± 0.15 μl/min/mg; and D-lactate: K_m = 6.0 ± 2.9 mM, V_max = 35.0 ± 18.4 nmol/min/mg, and P = 1.3 ± 0.6 μl/min/mg). The uptake of GHB and D-lactate was significantly decreased by the known MCT inhibitor α-cyano-4-hydroxycinnamate and the MCT substrates GHB and D-lactate but not by the organic cation tetraethylammonium chloride. Directional flux studies with both GHB and D-lactate suggested the involvement of carrier-mediated transport with the permeability in the apical to basolateral direction higher than that in the basolateral to apical direction. These findings confirm the presence of MCT1-4 in Caco-2 cells and demonstrate GHB and D-lactate transport characteristics consistent with proton-dependent MCT-mediated transport.