Platelet antiheparin activity. Assay based on factor-Xa inactivation by heparin and antifactor Xa

Mechanisms for the anticoagulant effects of synthetic antithrombins

The important roles of thrombin in the development and propagation of thrombosis are well recognized. In addition to being the enzyme for clotting fibrinogen (the major protein component of blood clots), thrombin accelerates its own generation by activating factor V, factor VIII, factor XI and platelets. It accelerates the stabilization of clots by activating factor XIII to factor XIIIa, the enzyme which crosslinks fibrin. There are probably two major pathways for regulating the availability of thrombin in vivo: inactivation of thrombin (by antithrombin III/vessel wall heparan sulfate and perhaps by other endogenous antithrombins) and the inactivation of factor Va and factor VIIIa by activated protein C. Factor Va and factor VIIIa accelerate the production of thrombin. However, when thrombin becomes bound to fibrin (in clots or possibly on cell surfaces), the ability of antithrombin III/heparin to inactivate thrombin is then reduced significantly. Impairment by fibrin of thrombin inhibition by antithrombin III may account in part for the inability of unfractionated heparin to prevent post-operative deep vein thrombosis in up to 20% of patients who undergo major elective orthopaedic surgery, and may also explain the need for oral anticoagulants after unfractionated and low molecular weight heparins are used to initiate the treatment of established deep vein thrombi. The ineffectiveness of the antithrombin III/heparin pathway for inhibiting thrombin under some circumstances has been a contributory factor for the development, evaluation and identification of other inhibitors of thrombin which are more able than antithrombin III/heparin to inactivate thrombin when the enzyme is bound to fibrin. The focus of this review is to detail how these synthetic agents, by directly or indirectly inactivating thrombin, can also effectively inhibit prothrombin activation in vitro.

Increased sulphation improves the anticoagulant activities of heparan sulphate and dermatan sulphate

Heparan sulphate and dermatan sulphate have both antithrombotic and anticoagulant properties. These are, however, significantly weaker than those of a comparable amount of standard pig mucosal heparin. Antithrombotic and anticoagulant effects of glycosaminoglycans depend on their ability to catalyse the inhibition of thrombin and/or to inhibit the activation of prothrombin. Since heparan sulphate and dermatan sulphate are less sulphated than unfractionated heparin, we investigated whether the decreased sulphation contributes to the lower antithrombotic and anticoagulant activities compared with standard heparin. To do this, we compared the anticoagulant activities of heparan sulphate and dermatan sulphate with those of their derivatives resulphated in vitro. The ratio of sulphate to carboxylate in these resulphated heparan sulphate and dermatan sulphate derivatives was approximately twice that of the parent compounds and similar to that of standard heparin. Anticoagulant effects were assessed by determining (a) the catalytic effects of each glycosaminoglycan on the inhibition of thrombin added to plasma, and (b) the ability of each glycosaminoglycan to inhibit the activation of 125I-prothrombin in plasma. The least sulphated glycosaminoglycans were least able to catalyse the inhibition of thrombin added to plasma and to inhibit the activation of prothrombin. Furthermore, increasing the degree of sulphation improved the catalytic effects of glycosaminoglycans on the inhibition of thrombin by heparin cofactor II in plasma. The degree of sulphation therefore appears to be an important functional property that contributes significantly to the anticoagulant effects of the two glycosaminoglycans.

Defective platelet aggregation to the calcium ionophore A23187 in a patient with a lifelong bleeding disorder

A patient with a lifelong bleeding disorder is presented with a prolonged bleeding time and abnormal aggregation and secretion responses to arachidonic acid, thromboxane A2, PAF-acether and the divalent calcium ionophore A23187. Platelet alpha and dense granule contents and morphology appear normal. The proposed defect is due to an abnormality of a platelet intracellular calcium dependent process.

Familial bleeding tendency with partial platelet thromboxane synthetase deficiency: reorientation of cyclic endoperoxide metabolism

Three family members from three successive generations presented with a moderate bleeding tendency and a functional platelet defect. They had absent aggregation with arachidonic acid (0.6--3 microM), reversible aggregation with ADP (4 microgram) and cyclic endoperoxide analogues, single wave aggregation only with adrenaline (5.4 microgram) and a prolonged template bleeding time (> min). Malondialdehyde formation was reduced after N-ethylmaleimide stimulation (2--6 nmol/10(9) platelets; control values 8--12 nmol) and serum thromboxane B2 values were reduced (33--101 ng/ml; control values 200--700 ng/ml). When the platelets were incubated with [3H]arachidonic acid the final metabolite of the lipoxygenase pathway (HETE) was produced in normal amounts but the production of thromboxane B2 and HHT was decreased whereas prostaglandin F2a, and E2 and probably D2 were increased. Evidence for enhanced production of prostaglandin D2 was also provided by the rise in the patient's platelet cyclic AMP levels following stimulation with arachidonic acid. The patient's washed platelets stimulated the production of 6-keto PGF 1a by aspirin-pretreated cultured bovine endothelial cells. The plasma levels of 6-keto PGF1a (439--703 pg/ml; normal 181 +/- 46 pg/ml) were raised. The decreased production of thromboxane B2, HHT and malondialdehyde and increased formation of prostaglandin F2a, E2, D2 and of 6-keto PGF1a are compatible with a partial platelet thromboxane synthetase deficiency and reorientation of cyclic endoperoxide metabolism. The markedly prolonged bleeding time would result not only from reduced formation of thromboxane A2 but also from increased production of the aggregation inhibiting prostaglandins PGI2 and PGD2.

Early increase of a new platelet coagulant activity in rats fed a thrombogenic diet

A recently described platelet coagulant activity (factor X activating activity), whose pathophysiological significance is as yet unknown, was studied in rats fed a fat-rich (thrombogenic) diet for 1, 2 and 7 weeks and compared to rats fed normal laboratory chow. Whatever the duration of the special feeding period, a highly significant shortening of the clotting time, used for measuring this activity, was observed. When the platelet coagulant activity of individual "fat-fed"rats was quantitated by reference to that of individual control animals, we found a mean increase of 350% (n = 9) after one week and 267% (n = 3) after two weeks of dietary treatment. Partial thromboplastin time, thrombin time and soluble fibrin monomer complexes did not differ in control and treated animals. It seems that platelet coagulant activity, as measured in our test system, is one of the first laboratory parameters to be modified by fat-rich diets. These findings may be relevant to an understanding of the role of platelet coagulant activities other than platelet factor 3 in thrombotic phenomena.

Radioimmunoassay of platelet factor 4 and beta-thromboglobulin: development and application to studies of platelet release in relation to fibrinopeptide A generation

Platelet and fibrinogen survival and turnover studies have shown that platelet activation and fibrin formation may occur to different degrees in different thrombotic disorders. More direct evidence of differential involvement of platelet activation and fibrin formation should be provided by specifically measuring the products of these reactions, i.e. released platelet proteins and fibrinopeptide A. Two platelet proteins, platelet factor 4 (PF4) and beta-thromboglobulin (betaTG), were isolated and characterized, and sensitive and specific radioimmunoassays were developed to measure them. These assays were employed, along with the radioimmunoassay for fibrinopeptide A (FPA), to study the release of PF4 and betaTG in relation to FPA cleavage. PF4 and betaTG were released by ADP and collagen with time course and concentration dependence similar to that of [14C]serotonin release. FPA was not cleaved from fibrinogen during ADP or collagen-induced platelet release. Thrombin caused release of PF4 and betaTG as well as cleavage of FPA. Cleavage of FPA occurred with concentrations of thrombin about 100 times less than did release of PF4 and betaTG, and release of [14C]serotinin required still higher thrombin concentrations. Release of [14C]serotonin and platelet proteins was similar as a function of time. Sodium citrate was found to inhibit platelet release induced by thrombin.

Comparison of heparin and a semi-synthetic heparin analogue, A73025. II. Some effects on platelet function

A comparison has been made of some effects of a semi-synthetic heparin analogue, A73025, and heparin upon platelet function. In several of the in vitro tests performed, such as their potentiating effects on ADP and adrenaline induced aggregation and their effects on the aggregation of washed platelets by activated factor X, heparin proved to be more potent than A73025. Following intravenous injection of twice the quantity of A73025, an equivalent anti-factor Xa activity was obtained, in the agreement with our previous studies. However, it was found that PRP containing heparin and A73025 with comparable anti-Factor Xa acitvity responded differently to the addition of thrombin, as A73025 barely inhibited thrombin induced aggregation. Similarly, A73025 had little effect on the dilute thrombin clotting time of plasma, following intravenous injection. Heparin and A73025 were neutralized to approximately the same degree by a crude PF4 preparation.

Metabolism and function of human platelets washed by albumin density gradient separation

A method for washing platelets by albumin density gradient separation, originally designed for the study of platelet coagulant activities, has been modified for platelet aggregation and metabolic studies. Platelets are sedimented into a continuous density gradient of isosmolar albumin containing apyrase to protect them from clumping and physical injury and are resuspended in calcium-free Tyrode's solution. The mean recovery of platelets after two separations relative to platelet-rich plasma (PRP) was 90.3%. When small amounts of plasma were added to washed platelet suspensions, aggregation and release of [14C]5-hydroxytryptamine (5HT) in response to adenosine diphosphate (adp) or 5HT were similar to results obtained with PRP. When fibrinogen was substituted for plasma, ADP-induced aggregation occurred but was feeble. Without added plasma or fibrinogen, platelets were refractory to ADP and insensitive to the cyclic endoperoxide analogue U44619. When both ADP and U44619 were added simultaneously, in low concentrations, to washed platelets without added plasma or fibrinogen, aggregation occurred immediately. Washed platelets were not aggregated by adrenaline, which potentiated ADP-induced aggregation. Several biochemical measurements which are sensitive indicators of cellular damage were normal in washed platelets, including [14C]adenine uptake, adenylate energy charge, hypoxanthine formation and the response of adenylate cyclase to stimulation by PGE1 or PGD2. Platelet coagulant activities were not made available and heparin-neutralizing activity (HNA) was not spontaneously released by the washing procedure, but the washed platelets responded normally to appropriate agents by developing coagulant activities and releasing HNA. The ultrastructure of washed platelets was similar to those in control PRP. Inclusion of apyrase in the first albumin gradient had a beneficial effect on platelet morphology, aggregation and metabolism, but washing at 37degreesC compared with 25degreesC did not. Albumin density gradient separation is a useful method for isolating platelets for aggregation and metabolic studies.

Platelet consumption and plasma concentration of platelet factor 4 (PF-J)

In 10 cancer patients platelet concentration, platelet consumption, platelet half-life time and plasms concentrations of platelet factor 4 were determined. A high freguency of thrombocytosis and increased platelet consumption were observed, and the plasms concentrations of platelet factor 4 was increased. However, no significant correlation was found between platelet consumption and the result of a single determination of platelet factor 4 concentrations in plasma.

Comparison of the coagulant activities of platelets and phospholipids

The coagulant activities of various phospholipid preparations were compared with those of platelets. Folch phospholipid with maximal platelet factor 3 (PF3) activity produced long recalcified clotting times of relatively undiluted plasma in plastic tubes whereas untreated or ADP-treated platelets with minimal PF3 activity produced short clotting times in the same test system which is sensitive to activators of the contact system of intrinsic coagulation. Bell and Alton phospholipids with maximal PF3 activity produced recalcified clotting times similar to those in the presence of platelets. Bell and Alton phospholipids had tissue factor activity, but Folch phospholipid and platelets did not. Bell and Alton phospholipids and gum acacia (used as a vehicle in one of the preparations) activated factor XII as did platelets, but Folch phospholipid did not. The multiple coagulant activities of Bell and Alton phospholipids (i.e. PF3, tissue factor and contact activating) may account for the absence of coagulant superiority of platelets in the undiluted system in plastic tubes. The coagulant activities of platelets are also complex but different from Bell and Alton phospholipids whereas Folch phospholipid would appear to possess only PF3 activity.

The relationship of platelet coagulant activities to venous thrombosis following hip surgery

Platelets have recently been shown to trigger intrinsic coagulation by two alternative pathways, protect active clotting factors from inactivation by plasma inhibitors and catalyse intrinsic coagulation reactions on the platelet surface to form fibrin. To determine whether these platelet coagulant activities (PCA) might have a role in the pathogenesis of DVT, 29 patients have been studied before and after arthroplasty or other surgery for fractured hip or degenerative hip disease. The occurrence of DVT was detected by [125I]fibrinogen uptake in the legs and confirmed by venography. In patients who developed DVT, all PCA increased progressively and significantly on day 1 (mean rise, 146% of baseline), day 3 (228%) and day 5 (298%) after surgery before isotopic evidence of DVT appeared (mean 3.27 days postoperatively). In patients without DVT no changes in PCA were observed. Plasma coagulation factor assays were no different in patients with a without DVT. Platelet counts and total platelet antiheparin activity increased during the early postoperative period in DVT patients but not in patients without DVT. It is suggested that progressive increases in PCA concerned with triggering and catalysing intrinsic coagulation reactions may play a pathogenetic role in DVT after hip surgery.

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.

Hereditary giant platelet syndrome. Absence of collagen-induced coagulant activity and deficiency of factor-XI binding to platelets

The platelets from two related patients with the hereditary giant platelet syndrome were examined. They were larger than normal but otherwise ultrastructurally normal; they contained increased storage pools of adenine nucleotides and heparin-neutralizing activity and took up serotonin at an increased rate. They aggregated normally with ADP and collagen but failed to aggregate with bovine factor VIII and Ristocetin. Some change in shape occurred with ADP, and the reduction in adenylate energy change after addition of ADP to platelet-rich plasma was smaller than normal. Platelet coagulant activities including contact product forming activity, intrinsic factor-Xa forming activity and platelet factor 3 activity were normal or increased, but collagen-induced coagulant activity was absent whether tested in washed platelet suspensions or platelet-rich plasma. Platelet washing experiments showed decreased binding of factors V and VIII to hereditary giant platelets and no detectable factor XI in washed platelet suspensions. It is concluded that (1) the hereditary giant platelets studied lacked a binding mechanism for factors, V, VIII and XI; (2) the normal development of collagen-induced coagulant activity apparently depends upon the binding of factor XI to the platelet membrane; and (3) the defective prothrombin consumption observed in these patients may have resulted from the failure of their platelets to bind factor XI.