Prior studies into the mechanism of hepatic uptake of organic anions have generally been interpreted in terms of transport across the plasma membrane. However, recent work by ourselves and others now suggests that other steps in the uptake process such as transfer from serum albumin to the plasma membrane, from the plasma membrane to cytoplasmic sites of sequestration, and from the cytosol back into the plasma may be equally important in determining the overall rate of removal of drugs, toxins and other lipophilic metabolites by the liver. These processes have not yet been systematically investigated. Two specific questions will be addressed. First, is dissociation from serum albumin spontaneous or is it catalyzed by contact with the liver cell? Recent kinetic observations support catalyzed dissociation; however, we have recently demonstrated that nonequilibrium binding can also account for these observations and have detailed an experimental approach which can resolve this uncertainty. Second, is the uptake rate determined by membrane or cytoplasmic transport processes? Recent analyses suggest that slow diffusion of organic anions through the cytoplasm can account for all of the kinetic features of carrier-mediated transport which are known to characterize uptake of this important class of compounds. These studies will be greatly facilitated by use of newly developed perfused rat liver and computer simulation models to generate and interpret kinetic data. The single-pass liver perfusion system permits rapid accumulation of highly precise uptake and efflux rates under both steady-state and transient conditions, making it feasible to study many previously uninvestigated factors which now appear critical to hepatic uptake. The computer simulation model includes many features not previously modeled including intra- and extracellular diffusion barriers and nonequilibrium binding to membranes and proteins. The information sought is essential for a full understanding of the mechanisms of hepatic detoxication of drugs and endogenous toxins and of the regulation of fatty acid metabolism, and it will permit a more complete understanding of the hepatic transport defects present in common conditions such as Gilbert's syndrome, viral hepatitis and cirrhosis.
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