Lack of linear correlation between hepatic ligand uptake rate and unbound ligand concentration does not necessarily imply receptor-mediated uptake

Abstract

Previous studies of the hepatic uptake of several albumin-bound ligands, using constant and variable albumin concentrations, were interpreted as being inconsistent with the traditional mechanism of uptake, defined as an uptake rate directly proportional to unbound ligand concentration, and led to the formulation of the albumin receptor theory of hepatic uptake. Because other experimental designs have failed to confirm the albumin receptor theory, we reexamined, using the isolated perfused rat liver preparation, the traditional uptake mechanism under the conditions used in the original studies, of constant and variable albumin concentration. Livers (n=6) were perfused in randomized sequence with 10 different solutions containing 24-¹⁴C-taurocholate in a single-pass design. Five solutions contained fixed albumin (0.1 mM) and variable taurocholate (3–48 μM) concentrations, and five maintained the taurocholate-albumin ratio fixed at 0.06; absolute concentrations of taurocholate varied from 3–48 μM, and of albumin from 0.05–0.08 mM. At constant albumin concentration in hepatic inflow, elimination rate of taurocholate was linearly related to both total (C_in) and unbound (C_in,u) taurocholate concentration in hepatic inflow, indicating first-order elimination kinetics. When taurocholate and albumin were increased in hepatic inflow in a fixed molar ratio, taurocholate uptake rate was not linearly related to C_in,u but was still consistent with the traditional uptake mechanism. Moreover, the apparent saturation of taurocholate uptake by added albumin was consistent with the reduction in unbound fraction (f_u) in accordance with the traditional uptake mechanism. This study shows that although the traditional uptake mechanism dictates that ligand uptake rate is linearly related to unbound ligand concentration within the liver, uptake rate need not necessarily be linearly related to C_in,u.. Therefore, experiments in which lack of a linear relationship between uptake rate and C_in,u is found do not necessarily imply receptor-mediated uptake.