Sequestration of organic cations by acidified hepatic endocytic vesicles and implications for biliary excretion

A number of cationic amine drugs that are taken up by liver and excreted into bile may accumulate in acidified intracellular organelles such as lysosomes and endosomes. These studies were undertaken to assess directly the uptake and accumulation of three types of model organic cationic amines by endocytic vesicles, and the role of vesicle acidification in this process. Uptake of tubocurarine (TC), vecuronium and tributylmethylammonium (TBuMA) by purified rat liver multivesicular bodies (MVB) (prelysosomal endocytic vesicles) was dependent upon MgATP, time and drug concentration. After 60 min, 52 to 81% of MVB cation content was dependent upon vesicle acidification (due to an electrogenic proton pump), but not upon an interior positive vesicle membrane potential. Nineteen to 42% of MVB cation content appeared due to binding to MVB membranes or to internal lipoproteins. Vesicle-to-medium ATP-dependent apparent concentration ratios for these three cations were 3.3 to 51. MVB uptake of these cations resembled uptake of methylamine, a tertiary amine known to distribute across organellar membranes according to pH gradients. By contrast, MVB uptake of the lipophilic quaternary amine methyldeptropine was not dependent upon MgATP or on development of MVB pH or membrane potential gradients. In further studies, TC, vecuronium and TBuMA were rapidly taken up by the isolated perfused rat liver and excreted in bile. Exposure to 250 mciroM primaquin (which partially alkalinized acidic endosomes and lysosomes) reduced accumulation of [3H]vecuronium in a lysosomal fraction by 23%, decreased perfusate disappearance of TC and TBuMA, but not of vecuronium, and decreased biliary appearance of all three cations. These studies suggest that acidified intracellular organelles sequester certain organic cationic drugs, possibly via a drug/proton antiporter, and/or diffusion followed by intravesicular protonation and trapping of tertiary amines. However, attempts at partial displacement of these drugs, accomplished through partial vesicle alkalization by primaquin, decreased excretion of TC, vecuronium and TBuMA, perhaps reflecting the small functional size of the displaceable organellar drug compartment and/or competition between primaquin and the organic cations for membrane transport processes.