Effect of Heparin and Heparin Fractions on Platelet Aggregation

cofactor required for ristocetin. Fractionation of heparin yielded preparations that varied in molecular weight and, within a given molecular weight fraction, in affinity for antithrombin III. Fractions of high molecular weight (average 20,000) were more reactive with platelets than were fractions of low molecular weight (7,000). Anticoagulant activity did not parallel the platelet reactivity of heparin fractions. Among high molecular weight fractions, preparations of high or low antithrombin affinity were equally active in induction of platelet aggregation. In low molecular weight fractions, there was an inverse relationship between platelet reactivity and anticoagulant activity in normal platelet-rich plasma, but, in plateletrich plasma depleted of antithrombin, low molecular weight fractions of high and low antithrombin affinity reacted equally with platelets. These results suggest that formation of an antithrombin-heparin complex protected platelets from aggregation by heparin. Selection of heparin fractions of low molecular weight and high antithrombin affinity may improve anticoagulant therapy and development of thromboresistant heparin-coated artificial materials. This paper was presented in part at an Annual Meeting of the American Heart Association, Dallas, Tex., in November 1978, and published as an abstract (Circulation. 58[Suppl. II]: 206. Received for publication 27 April 1979 and in revisedform 30 July 1979.