Enhancement of unbound clearance of ICG by plasma proteins, demonstrated in human subjects and interpreted without assumption of facilitating structures

The kinetics of hepatic removal of protein-bound substances were studied in nine human subjects with various liver diseases by the use of indocyanine green as a model substance. Intrinsic hepatic clearance of indocyanine green was measured by means of a constant infusion of indocyanine green and concentration measurements of indocyanine green in arterial and hepatic venous plasma samples. During the indocyanine green infusion, 1-1.51 of dextran-70 was given whereby a stable dilution of the plasma protein concentration by a factor of 0.6-0.8 was obtained. In each of the subjects, the intrinsic clearance of indocyanine green increased after the protein dilution (range 11-64%). Elimination of ethanol (not protein bound), similarly assessed, was not significantly changed. The traditional hypothesis that unbound clearance (intrinsic clearance divided by the free fraction of the ligand) is independent of protein concentration was refuted since in each of the subjects the protein dilution was followed by a reduction of the unbound clearance of ICG (P < 0.005, n = 9). We examined whether these observations imply some special mechanism (e.g. a hepatocyte protein receptor) by which the unbound clearance is enhanced by the binding protein(s). The previously developed pseudofacilitation model - describing the effects of ligand-protein diffusion and dissociation in an unstirred plasma layer near the hepatocyte - was extended to the case of a mixture of binding proteins, and parameter-free bounds were derived to predict the response of intrinsic clearance to protein dilution. The observed changes of the intrinsic clearance values did not violate these bounds (P < 0.002, n = 9). Thus no facilitating mechanisms are necessary to account for the observed deviations from the traditional hypothesis