Heparin, monocytes, and procoagulant activity

Monocytes are able to specifically bind heparin rapidly, reversibly, and satura- bly with 5.5 ± 2.6 X 10⁶ binding sites per monocyte and a dissociation constant of 330 ± 221 nmol/1. Moreover, at least 40-50% of the cell-bound heparin can be internalized. Heparin binding by monocytes is affected by cell stimulation with an increase of the availability of binding sites after challenge with calcium ionophore, lipopolysaccharide, and interleukin 2. The interaction of heparin with monocytes reversibly decreases the expression of tissue factor on the cellular surface, so hampering the cellular procoagulant potential.