Hepatocytic transport of physiological concentrations of unconjugated bilirubin (UCB) has not been determined in isolated liver cells. Initial uptake of highly purified [3H]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 was much less evident at [UCB]=166 nM (supersaturated). At low, physiological UCB concentrations, specific UCB uptake showed saturative kinetics with an apparent Km of 41 nM, indicating carrier-mediated transport. With aqueous supersaturation, UCB entered hepatocytes mainly by passive diffusion.
