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

Abstract.

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-4-phenylpyridinium (MPP⁺) in Caco-2 cells.

Transepithelial flux as well as cellular accumulation of [³H]MPP⁺ were quantitatively similar when substrate was applied from the basolateral or apical cell membrane. Verapamil (100 µM) and rhodamine123 (10 µM) significantly reduced [³H]MPP⁺ transepithelial flux in the apical-to-basolateral direction. When cells were grown on plastic supports, [³H]MPP⁺ was rapidly accumulated in the cells, both by saturable and nonsaturable mechanisms. The kinetic parameters of the saturable component were: K _m: 449 µM and V _max: 2249 pmol per mg protein and 5 min. Uptake of [³H]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 [³H]MPP⁺ uptake into the cells. The P-glycoprotein antibody UIC2 (20 µg/ml) had no effect.

In conclusion, [³H]MPP⁺ is efficiently transported by Caco-2 cells in both basolateral-to-apical (secretion) and apical-to-basolateral (absorption) directions. Absorption of [³H]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.