ENDOGENOUS ANTITHROMBIN ASSOCIATED WITH MICROVASCULAR ENDOTHELIUM - QUANTITATIVE-ANALYSIS IN PERFUSED RAT HEARTS

A recirculating Langendorff heart preparation is used to characterize the endogenous antithrombin associated reversibly with murine vascular endothelium. Rat hearts are perfused clear of blood and then recircul ated with a physiological salt solution. Addition of heparin educes an tithrombin activity continuously into the perfusate during 6 min of re circulation. This process contrasts with a more rapid equilibration of the system as assessed by displacement of [I-125]thrombin with hirudi n or with a heparin-antithrombin mixture. Perfusion of washed hearts w ith [I-125]factor Xa, which evidences no significant binding to the co ronary endothelium identifies a minor fraction of the endogenous antit hrombin that reacts immediately with factor Xa, i.e., at a rate indica tive of heparin enhancement. This rapid-reacting antithrombin is not r eproducibly detected with [I-125]thrombin, which binds preferentially to thrombomodulin in this system. The amount of antithrombin reacting rapidly with factor Xa is too low to detect as a burst of antithrombin activity eluted into the perfusate when the hearts are perfused with heparin. It is concluded that the murine myocardial microvasculature h arbors at least two pools of antithrombin, the minor of which is in an activated configuration characteristic of association with heparin. T he major pool is in a more slowly accessible compartment or configurat ion.