The internalization and release of heparins by cultured endothelial cells: the process is cell source, heparin...

Endothelial cells in vivo and in vitro take up heparin following administration. In in vitro systems, cellular and pericellular extracts of monolayers exposed to heparin, demonstrate internalization as well as cell surface attachment. To further investigate the characteristics of this exogenous heparin partitioning, we have exposed two types of endothelial cells to media containing cold and 125I labelled bovine lung, porcine mucosal or CY222 heparin. Heparin uptake, in pericellular and intracellular fractions of porcine cells, increased with concentration at 24 hrs exposure for all heparins. Only bovine heparin was preferentialy accumulated intracellularly. Cultured murine LE-II endothelial cells showed a greater accumulation in the intracellular fraction than porcine cells when exposed to porcine heparin. When bovine or CY222 heparin was administered for varying times, total heparin uptake increased with time. Heparin in the pericellular fraction was greater than intracellular at exposure durations from 5 mins to 6 hrs. At 16 hrs intracellular heparin markedly increased and far exceeded pericellular. The possibility of release of previously internalized heparin was studied by washing cultures previously exposed to heparin with heparin-free media. Heparin could be recovered up to 96 hours post exposure. A concurrent decrease in pericellular and cellular heparin was observed. These results show a differing distribution of heparin across endothelial cell membranes that is heparin source, concentration and time dependent. A previously unsuspected gradient-time mechanism is suggested. The degree of internalization differs for different endothelial cell sources. The protracted retention of heparin intracellularly by endothelial cells and subsequent release may be of therapeutic and physiological consequence.