IMPORTANCE OF THE LIVER IN PLASMA-CLEARANCE OF HEPATOCYTE GROWTH-FACTOR IN RATS

After intravenous administration of I-125-la led hepatocyte growth factor (HGF), trichloroacetic acid-precipitable radioactivity in the plasma disappeared rapidly with an early phase half-life of 4 min. The amounts of I-125-HGF distributed to the liver, adrenal, spleen, kidney, and lung tissues were much greater than those that could be accounted for by distribution to the extracellular space alone. The first-pass removal of I-125-HGF by the liver was approximately 26%; the liver accounted for approximately 70% of early-phase removal. The hepatic handling was also analyzed using a single-pass perfused liver system. The steady-state extraction ratio of tracer I-125-HGF was 0.48 but dropped to 0.23 in the presence of excess HGF (135 pM), demonstrating hepatic removal saturation of HGF. In the presence of excess HGF, the heparin-washable I-125-HGF, the heparin-resistant and acid-washable I-125-HGF, and the internalized I-125-HGF dropped to 54, 31, and 32% of the control values. The presence of at least two binding sites for HGF on the liver cell surfaces was made clear: the heparin-washable site and the heparin-resistant and acid-washable binding site, considered to have higher affinity for HGF. The internalization of I-125-HGF was observed to some extent even in the presence of excess HGF and phenylarsine oxide, known to be an inhibitor of polypeptides receptor-mediated endocytosis, suggesting the contribution of a relatively nonspecific internalization mechanism as well as receptor-mediated endocytosis.