Hepatic sequestration and modulation of the canalicular transport of the organic cation, daunorubicin, in the rat

In contrast to organic anions, substrates for the canalicular mdr1a and b a re usually organic cations and are often sequestered in high concentrations in intracellular acidic compartments. Because many of these compounds are therapeutic agents, we investigated if their sequestration could be regulat ed. We used isolated perfused rat liver (IPRL), isolated rat hepatocyte cou plets (IRHC), and WIF-B cells to study the cellular localization and biliar y excretion of the fluorescent cation, daunorubicin (DNR), Despite rapid (w ithin 15 minutes) and efficient (>90%) cellular uptake in the IPRL, only ap proximate to 10% of the dose administered (0.2-20 mu mol) was excreted in b ile after 85 minutes. Confocal microscopy revealed fluorescence predominant ly in vesicles in the pericanalicular region in IPRL, IRHC, and WIF-B cells , Treatment of these cells with chloroquine and bafilomycin A, agents that disrupt the pH gradient across the vesicular membrane, resulted in a loss o f vesicular fluorescence, reversible in the case of bafilomycin A. Taurocho late (TC) and dibutyryl cAMP (DBcAMP), stimulators Of transcytotic vesicula r transport, increased the biliary recovery of DNR significantly above cont rols, by 70% and 35%, respectively. The microtubule destabilizer, nocodazol e, decreased biliary excretion of DNR, No effect on secretion war noted in TR- mutant rats deficient in mrp2. Coadministration of verapamil, an inhibi tor of mdr1, also decreased DNR excretion. While TC and DBcAMP did not affe ct the fluorescent intensity or pattern of distribution in IRHC, nocodazole resulted in redistribution of DNR to peripheral punctuate structures. Thes e findings suggest that the organic cation, DNR, is largely sequestered in cells such as hepatocytes, yet its excretion can still be modulated.