Uptake of [H-3]bilirubin in freshly isolated rat hepatocytes: role of freebilirubin concentration

Hepatocytic transport of physiological concentrations of unconjugated bilir ubin (UCB) has not been determined in isolated liver cells. Initial uptake of highly purified [H-3]UCB was measured in rat hepatocytes in the presence of human serum albumin at various free, unbound UCB concentrations, [UCB]. At [UCB] = 42 nM (below aqueous solubility of 70 nM), uptake was strictly temperature dependent; this mas much less evident at [UCB] = 166 nM (supers aturated), At loa, physiological UCB concentrations, specific UCB uptake sh owed saturative kinetics with an apparent K-m of 41 nM, indicating carrier- mediated transport. With aqueous supersaturation, UCB entered hepatocytes m ainly by passive diffusion. (C) 1999 Federation of European Biochemical Soc ieties.