Heparin-releasable and platelet pools of tissue factor pathway inhibitor in rabbits.

Earlier studies from this laboratory have established that tissue factor pathway inhibitor (TFPI) functions as a natural anticoagulant protecting rabbits from intravascular coagulation triggered by the exposure of blood to small amounts of tissue factor. In addition to the TFPI circulating in plasma, humans have been shown to have heparin-releasable and platelet pools of TFPI. In order better to extrapolate from studies carried out in rabbits to an understanding of human hemostasis, we have examined the presence and extent of heparin-releasable and platelet pools of TFPI in rabbits. We find that in the rabbit the heparin-releasable pool of TFPI activity, as measured in a capacity assay, may be smaller relative to the plasma pool than in humans; that the platelet pool of TFPI activity is comparable to that of humans; and that rabbit TFPI, unlike human TFPI, has the same apparent molecular mass in all vascular pools. These studies extend our understanding of the properties of TFPI in rabbits and the appropriateness of using the rabbit for studies of TFPI relevant to human hemostasis.