UNBOUND LIGAND DRIVES HEPATOCYTE TAUROCHOLATE AND BSP UPTAKE AT PHYSIOLOGICAL ALBUMIN CONCENTRATION

Unbound ligand drives hepatocyte taurocholate and BSP uptake at physio logical albumin concentration. Am. J. Physiol. 266 (Gastrointest. Live r Physiol. 29): G425-G432, 1994. - We have recently shown (D. Sorrenti no, R. B. Robinson, C.-L. Kiang, and P. D. Berk. J. Clin. Invest. 84: 1325-1333, 1989) that, in a variety of isolated cell types, the uptake of oleate at physiological albumin concentrations is consistent with traditional pharmacokinetic theory (i.e., driven by unbound ligand). L ower albumin concentrations were associated with a deviant uptake patt ern for which alternative theories have been proposed. Whether other c lasses of organic anions exhibit similar behavior is unknown. Therefor e, we examined the effect of albumin on uptake of two widely studied o rganic anions, sulfobromophthalein (BSP) and taurocholate. Initial upt ake velocity of [S-35]BSP and [H-3]taurocholate by isolated hepatocyte s was studied employing a fixed albumin concentration and ligand-to-al bumin molar ratios from 0.01:1 to 2:1 for taurocholate and 0.031:1 to 0.75:1 for BSP. In other experiments, albumin and ligand were altered in parallel, keeping;their molar ratio constant. Unbound taurocholate concentrations were measured directly by equilibrium dialysis; unbound BSP concentrations were calculated from published data (K. J. Baker a nd S. E. Bradley. J. Clin. Invest. 45: 281-287, 1966). At 600 mu M alb umin, uptake of both ligands was a function of the unbound ligand conc entration. At low ligand-to-albumin molar ratios and consequent unboun d ligand concentrations this relationship was linear; over the entire range of unbound ligand concentrations studied, both ligands exhibited Michaelis-Menten kinetics, with definable maximal velocity and Michae lis constant values. At low albumin concentrations, the relationships between uptake and unbound ligand were unchanged for taurocholate; how ever, BSP exhibited altered kinetics similar to those observed with ol eate. Nontraditional uptake kinetics at low albumin concentrations app ear to correlate with very high affinity for albumin.