Uptake mechanism of fractionated [3H]heparin in isolated rat Kupffer cells: Involvement of scavenger receptors

Abstract

The uptake of fractionated [3H]heparin was examined to elucidate the uptake mechanism in isolated rat Kupffer cells. The equilibrium binding of fractionated [3H]heparin to Kupffer cells was concentration-dependent with the dissociation constant of 5.7 nM and the maximum binding capacity of 1.5 pmol/106 cells. Several ligands of scavenger receptors inhibited the binding of fractionated [3H]heparin to Kupffer cells competitively and also the internalization of heparin, suggesting the involvement of scavenger receptors in the uptake of fractionated [3H]heparin. Fractionated [3H]heparin was also suggested to be internalized according to first order kinetics with the apparent internalization rate constant of 0.010 min-1. Lowering temperature from 37 to 4°C reduced the fraction internalized from 33% to 6% without affecting the total association, while the fraction internalized at 25°C was comparable with that at 37°C. Metabolic inhibitors (2,4-dinitrophenol and rotenone), an inhibitor of receptor- mediated and adsorptive endocytosis of polypeptides (phenylarsine oxide) and phagocytosis inhibitors (cytochalasine B and colchicine) did not inhibit the internalization of fractionated [3H]heparin. As known inhibitors of receptor-mediated and adsorptive endocytosis of polypeptides and phagocytosis did not affect the uptake of fractionated heparin, the scavenger receptor-mediated uptake is suggested to be ATP-independent and different from receptor- mediated and adsorptive endocytosis of polypeptides and phagocytosis, although for temperature dependency it showed the typical characteristics of receptor-mediated endocytosis.