Heparin, platelets and blood coagulation: implications for low-dose heparin prophylactic regimens in venous thrombosis

Normal and low molecular weight heparins: interaction with human platelets

Porcine mucosal heparin was chemically depolymerized. The depolymerization was stopped at different steps to obtain two low molecular weight (LMW) heparins with a molecular weight of 10 000 and 6000, respectively. The LMW heparins were tested in vitro for anti-clotting activities and for platelet serotonin release in different systems in comparison with normal heparins, dermatan and heparan sulphate. After addition of various amounts of heparin preparations to washed platelets, no significant release was observed for all tested heparins. On the contrary, different heparins showed an inhibition of serotonin-release induced by collagen in platelet rich plasma, whereas the ADP-induced release was increased. The effect on the platelet release appears related to the molecular weight. In fact, it is significant only for normal heparins whereas it is not for LMW heparins. A good relation was observed, also, between anti-activated factor X activity/antiglobal clotting activity (Xa/APTT) ratio of different heparins and the effect on platelet release.

Two platelet aggregation inhibitors in tsetse (Glossina) saliva with studies of roles of thrombin and citrate in in vitro platelet aggregation

Two inhibitors of platelet aggregation have been identified in saline extracts of Glossina morsitans (tsetse) salivary glands. A protein fraction (MW greater than 30 000) inhibited primary and secondary aggregation to ADP, secondary aggregation to adrenalin, and aggregation to collagen. It also caused disaggregation of platelets stimulated by ADP and adrenalin. These properties could be explained by ADP hydrolytic activity. A previously identified antithrombin fraction (MW 11 000-13 000) abolished thrombin-induced aggregation. It did not affect platelet aggregation to ADP, adrenalin or collagen, as compared to aggregation in citrate, when used as sole anticoagulant for platelet rich plasma or when added to citrated platelet rich plasma. These results fail to support hypotheses (i) that thrombin plays an important role in platelet aggregation by other agonists and (ii) that secondary platelet aggregation is an artefact induced by citrate. It is proposed that these inhibitors may be important in maintaining mouthpart and crop patency during feeding. Their discovery suggests that other arthropods may also have antiplatelet agents in their saliva which might be entomologically important, provide useful tools for platelet studies, and be of potential therapeutic interest.

Preventing hemorrhage in high-risk hemodialysis: regional versus low-dose heparin

Hemodialysis in patients with increased risk for hemorrhage can be accomplished with either a regional or a low, total dose of heparin. In a prospective study of 69 series of dialyses performed on an alternating schedule of heparinization for each patient, bleeding complications during and immediately following dialysis occurred in 23 of 122 dialyses (19%) with regional heparin compared to 13 of 133 dialyses (10%) with low-dose heparin (P less than 0.05). The incidence of hemorrhage correlated with the estimated degree of bleeding risk both at expected and at occult bleeding sites, and was the same or higher with regional heparin in all categories. Hemorrhage was not correlated with preexisting coagulation abnormalities, concurrent anticoagulant drugs, level of azotemia, or ability to successfully limit systemic heparinization during dialysis. The incidence of partial clotting of the dialyzer was 3 to 5% with both heparin protocols. We conclude that regional heparinization has no clinical or practical advantage over low-total-dose heparin in preventing bleeding associated with hemodialysis.

[Influence of five commercial heparin preparations on the collagen induced platelet aggregation (author's transl)]

The ADP- and collagen-induced aggregation of platelets in PRP of healthy probands was tested in vitro in the presence of 4 heparin preparations (Novo, Vitrum, Liquemin, Haemoderivate) and of a heparinoid (Eleparon). None of the preparations caused a significant direct aggregation. The collagen-induced aggregation was inhibited by all mentioned preparations; thereby the heparins proved to be effective approx. to the same degree whereas the heparinoid proved to be significantly less effective. The ADP-induced aggregation was potentiated by all preparations. A considerably different sensitivity to heparin could be observed in the single PRP's. A special method was found out to exclude the influence of the heparin-sensitivity of the PRP's to compare the special effects of the heparins.

Adenosine diphosphate induced platelet aggregation and release reaction in heparinized platelet rich plasma and the influence of added citrate

Platelets in heparinized platelet-rich plasma (PRP) from 54 volunteers were examined for their ability to aggregate and to release tritiated 5-hydroxy tryptamine (3H-5HT) in response to adenosine diphosphate (ADP). Release of 3H-5HT was considerable in 26 of the preparations. When release was high, lower concentrations of ADP were required to induce irreversible aggregation. Second phase aggregation reflected extensive release reaction. Citrate always augmented the release induced by ADP.

Impairment by heparin of primary haemostasis and platelet [14C]5-hydroxytryptamine release

Heparin was administered to 34 normal subjects by intravenous injection (100 mu/kg) and the template bleeding time was significantly increased both 10 min and 120 min following injection. Before heparin the bleeding time was 5-3 +/- 1.0 min (mean +/- 1 SD); 10 min after injection it was 9.8 +/- 5.6 min (P less than 0.001); and 120 min after injection it was 7.2 +/- 3.9 min (P less than 0.001). Increases in the bleeding time were unrelated to changes in platelet count, and independent of heparin's effect on plasma coagulation. In blood drawn 10 min and 120 min following heparin injection, there was significantly less [14C]5-HT released from platelet-rich plasma (PRP) in response to collagen, 0.41 mM epinephrine and 8 micron ADP, although in vitro addition of heparin (0.1 mu/ml, 0.5 mu/ml and 2.5 mu/ml) to baseline PRP of three subjects did not depress [14C]5-HT release. Our experiments suggest that intravenous administration of a therapeutic dose of heparin can cause a significant reversible inpairment of platelet haemostatic properties, possibly by an indirect mechanism.

Heparin-induced immune thrombocytopenia

We studied five patients in whom severe thrombocytopenia developed during intermittent intravenous heparin treatment of arterial and venous thrombosis. Platelet aggregation was demonstrated when heparin (0.5 U per milliliter) was incubated with the patients' citrated platelet-rich plasma or with normal platelet-rich plasma in the presence of the patients' serum. Antiplatelet antibody was not detected in the patient globulin fractions prepared from serum collected within one week after heparin withdrawal by use of the platelet factor 3 availability technic. When the studies were repeated with modifications to detect heparin-dependent antiplatelet antibodies, positive results were obtained in four of five patients. The data suggest that a casual relation, mediated by an immune mechanism, existed between heparin therapy and thrombocytopenia, and that this syndrome may occur more often than has previously.

The effect of heparin on platelet aggregation by common inductors and by ristocetin in congenital bleeding disorders due to factor VIII or fibrinogen defects

Platelet aggregation by various inductors was studied in citrated and heparinized plasma of the following groups of subjects: Normal, hemophilia A, combined factor V and factor VIII deficiency, v. Willeprand's disease and congenital afibrinognemia. The results may be summarized as follows: A-platelet aggregation in citrated plasm 1) platelet aggregation by common inductors ADP, adrenalin and collagen was normal in all groups of subjects but for the patients with congential afibrinogenemia in whom adrenalin induced aggregation was absent or markedly refuced whereas ADP and collagen gave slightly reduced or near normal aggregation curves. 2) platelet aggregation by ristocetin was normal in all groups of subjects but for v. Willebrand's disease in which it was absent. B-platelet aggregation in heparized plasma. 1) platelet aggregation by common inductors resulted to be normal in all groups of subjects except in congenital afibrinogenemia. In this latter case the pattern was still mildly defective but here was an increased aggregation as compared to citrated plasma. These findings have been interpretemmon inductors. 2) platelet aggregation by ristocetin resulted to be absent in all groups of subjects investigated. The possible mechanism of action of the inhibitory effect exercised py heparin with regard to restocetin is discussed.

The role of thrombin in ADP-induced platelet aggregation and release: a critical evaluation

The role of thrombin in ADP-induced aggregation and release in vitro was critically examined. The addition of heparin or hirudin to citrated platelet rich plasma did not prevent aggregation or release. The addition of citrate to heparinized plasma restored secondary aggregation and release. Hirudin did not prevent irreversible aggregation. These results are incompatible with the hypothesis that thrombin is a primary and necessary mediator of platelet aggregation (Ardlie & Han, 1974; Han & Ardlie, 1974a, b, c). This hypothesis is based in part on the assumption that EDTA enhances the elution of clotting factors from platelets; we found no enhanced elution of factors II, V, X, VIII, IX or XI when platelets were washed in EDTA.