Abstract

The hepatic transport mechanism for the fluorescent bivalent hydrophilic organic cation lucigenin (LU) was characterized employing kinetic and morphological methods. The extraction of LU by the perfused rat liver was 50% and uptake was saturable. LU did not inhibit the carrier-mediated hepatic uptake of the model organic cationic compounds tributylmethyl ammonium (type 1) and vecuronium (type 2) in isolated hepatocytes, whereas the uptake of LU in the perfused liver was not affected by either type of cation or by the cardiac glycoside cymarin, a potent type 2 inhibitor. The cytoskeleton-disrupting agents cytochalasin B and nocodazole, however, significantly lowered hepatic uptake of LU. In the intact liver, LU did not stimulate fluid phase endocytosis, as indicated by a lack of effect on the internalization of horseradish peroxidase. These kinetic data point to adsorptive endocytosis as the most probable uptake mechanism. This was confirmed by the inhibitory effect of neomycin and the polycation poly(L-lysine) on LU uptake. Fluorescence microscopy revealed that LU accumulated in the hepatocytes in discrete vesicular structures. Partial co-localization of rhodamine-dextran and acid phosphatase with LU indicated that part of the LU fluorescence was present in lysosomes, although not all lysosomes contained LU. Taken together, we conclude that we identified a novel vesicular pathway for uptake of organic cations by hepatocytes.