Characterization of the transport of the organic cation [3H]MPP+ in human intestinal epithelial (Caco-2) cells

The aim of this study was to characterize the transport of organic cations at the intestinal level, by studying the characteristics of the transport of 1-methyl-1-phenylpyridinium (MPP+) in Caco-2 cells. Transepithelial flux as well as cellular accumulation of [H-3]MPP+ were quantitatively similar when substrate was applied from the basolateral or apical cell membrane. Verapamil (100 mu M) and rhodamine123 (10 mu M) significantly reduced [H-3]MPP+ transepithelial flux in the apical-to-basolateral direction. When cells were grown on plastic supports, [H-3]MPP+ was rapidly accumulated in the cells, both by saturable and nonsaturable mechanisms. The kinetic parameters of the saturable component were: K-m: 449 mu M and V-max: 2249 pmol per mg protein and 5 min. Uptake of [H-3]MPP+ was metabolic energy-dependent and Na+-, pH- and potential-independent. It was inhibited by several organic cations (verapamil, rhodamine123, daunomycin, vinblastine, tetrabutylammonium and vecuronium) but not by others (tetraethylammonium and N-methylnicotinamide). Decynium22 and corticosterone inhibited [H-3]MPP+ uptake into the cells. The P-glycoprotein antibody UIC2 (20 mu g/ml) had no effect. In conclusion, [H-3]MPP+ is efficiently transported by Caco-2 cells in both basolateral-to-apical (secretion) and apical-to-basolateral (absorption) directions. Absorption of [H-3]MPP+ at the apical membrane seems to occur through a carrier-mediated mechanism belonging to the Amphiphilic Solute Facilitator (ASF) family of transporters, but distinct from the known members of this family.