Besides being excreted into bile, several anionic drugs and anionic drug conjugates are excreted from liver into plasma by a carrier-mediated process (sinusoidal efflux). In the study presented here, we investigated the influence of albumin on the net sinusoidal efflux of an organic anion from isolated perfused liver, with net sinusoidal efflux defined as the resultant of efflux and reuptake. We tested the hypothesis that albumin exerts its effect on net sinusoidal efflux through inhibition of repeat uptake rather than on the sinusoidal efflux process itself. We also studied possible acinar heterogeneity in net sinusoidal efflux rate of the organic anion. Isolated rat livers were preloaded with the nonmetabolizable anionic model compound dibromosulfophthalein in the absence (type I) or presence (type II) of bovine serum albumin. Fluorescence microscopy of dibromosulfophthalein in liver sections showed selective zone 1 loading after type I loading and homogeneous acinar distribution after type II loading. The rate of net sinusoidal efflux and biliary excretion was then studied in single-pass perfusions with various concentrations of bovine serum albumin in the medium. Net sinusoidal efflux of dibromosulfophthalein showed a marked dependence on albumin concentration in both type I and type II experiments. Net efflux rate reached a maximal value at a medium concentration of 300 mumol/L bovine serum albumin and was not increased further at 600 mumol/L. The initial uptake rate of dibromosulfophthalein was not significantly different in the 300 mumol/L and 600 mumol/L bovine serum albumin experiments.(ABSTRACT TRUNCATED AT 250 WORDS)
This study contains a pharmacokinetic analysis on the efflux of organic anions from the liver into the bloodstream (sinusoidal efflux) with specific reference to the influence of albumin. The net sinusoidal efflux rate of dibromosulfophthalein (DBSP) from preloaded livers, being the resultant of sinusoidal efflux and reuptake of ligand by hepatocytes downstream the sinusoid, can be strongly increased by the presence of bovine serum albumin (BSA), a protein having multiple binding sites for DBSP. We previously attributed this effect to a reduction of reuptake through extracellular binding of the organic anion to the protein, rather than to an intrinsic stimulatory effect on the actual membrane transport process from the cells. In the present study we tested this hypothesis using a pharmacokinetic multicompartment liver model. This model resembles the parallel tube model in that the liver is described by several compartments placed in series instead of a single well-stirred compartment and it takes into account rates of dissociation and association in binding to proteins in the sinusoidal space. The model parameters were fitted from the sinusoidal efflux and biliary excretion data from efflux experiments measuring the stimulatory effect of various concentrations of BSA. Equilibrium binding of DBSP to albumin as well as the dissociation rate constant (koff) were determined in vitro with rapid filtration techniques. The experimental data could not be fitted satisfactorily when using the experimentally obtained values of the protein association and dissociation rate constants (kon and koff). However, they could be simulated accurately assuming 16 times higher values for the association and dissociation rate constant compared to those determined in vitro. Time constants of the perfusate flow, liver (re)uptake, and protein association and dissociation indicate that binding equilibrium does not exist within the sinusoids and that, in particular at low protein concentrations, the net sinusoidal efflux rate is association rate-limited: A large fraction of the ligand effluxed from the cell into the median is taken up by the hepatocyte before binding to the proteins occurs. Higher kon and koff values predicted by the model might indicate altered DBSP-albumin binding characteristics upon passage through the liver but alternatively can be explained by an intrinsic effect of albumin on the carrier-mediated efflux process. Efflux experiments showed a marked stimulatory effect of the protein on sinusoidal efflux but only a moderate effect on biliary excretion, despite a strong decrease in liver content. These patterns indicate that sinusoidal efflux and biliary excretion occur from two different intracellular compartments that equilibrate slowly.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.