Submitted on October 25, 2007
Accepted on December 4, 2007
Liver Sinusoidal Endothelial Cells are the Principal Site for Elimination of Unfractionated Heparin from the Circulation
Cristina Ionica Oie1*, Randi Olsen2, Bard Smedsrod3, and John-Bjarne Hansen1
1 Department of Medicine, University of Tromso, Tromso, Norway
2 Electron Microscopy, University of Tromso, Tromso, Norway
3 Cell Biology and Histology, University of Tromso, Tromso, Norway
* To whom correspondence should be addressed. E-mail: cristina{at}fagmed.uit.no.
The mechanism of elimination of blood borne heparin was studied. Unfractionated heparin (UFH) was tagged with FITC, which served as both a visual marker and a site of labelling with 125I. The labelling procedure did not alter the anticoagulant activity or binding specificity of the glycosaminoglycan. Labelled heparin administered i.v. to rats (0.1 IU/kg) had a circulatory t1/2 of 1.7 min, which was increased to 16 min upon coinjection with unlabelled UFH (100 IU/kg). At 15 min after injection, 71% of recovered radioactivity was found in liver. Liver cell separation revealed the following relative uptake capacity, expressed per cell: LSEC:parenchymal cell:Kupffer cell = 15:3.6:1. Fluorescence microscopy on liver sections showed accumulation of FITC-UFH only in cells lining the liver sinusoids. No fluorescence was detected in parenchymal cells or endothelial cells lining the central vein. Fluorescence microscopy of cultured LSECs following binding of FITC-UFH at 4°C and chasing at 37°C, showed accumulation of the probe in vesicles located at the periphery of the cells after 10 min, with transfer to large, evenly stained vesicles in the perinuclear region after 2 h. Immunogold electron microscopy of LSECs to probe the presence of FITC following injection with FITC-UFH along with BSA-gold to mark lysosomes demonstrated colocalization of the probe with the gold particles in the lysosomal compartment. Receptor-ligand competition experiments in primary cultures of LSECs indicated the presence of a specific heparin receptor, functionally distinct from the hyaluronan/scavenger receptor (Stabilin2). The results suggest a major role for LSECs in heparin elimination.
