Platelet-active drugs in the secondary prevention of cardiovascular events: an overview

Experimental and clinical evidence strongly suggests that platelets play an important role in a variety of acute cardiovascular events, as well as in the vascular disease with which they are frequently associated. Hence, there is considerable interest in the use of platelet-active drugs in the prevention of these events. A sound pharmacologic basis for the use of platelet-active agents will require further advances in our understanding of the pathophysiology of cardiovascular events and the pharmacology of these drugs.

Acquired storage pool deficiency with increased platelet-associated IgG. Report of five cases

Acquired abnormalities of platelet aggregation have been reported with increasing frequency. We studied five patients (including two with systemic lupus erythematosus and one with compensated chronic idiopathic thrombocytopenic purpura) in whom platelet aggregation responses to collagen, epinephrine and ADP are impaired; in all cases, we found that levels of platelet-associated immunoglobulin G (IgG) were increased. In all five patients substances stored in platelet-dense granules (ATP, ADP, serotonin and calcium) were diminished. The content of the alpha-granule substance, beta-thromboglobulin, was also decreased in most cases, whereas the levels of two secretable acid hydrolase enzymes (beta-glucuronidase and beta-N-acetyl glucosaminidase) were within normal limits. These findings are similar to those observed in subtypes of congenital storage pool deficiency. However, in contrast to the congenital disorder, a membrane-bound (nonsecretable) acid phosphatase was also decreased in the patients with acquired storage pool deficiency. These findings suggest that impaired platelet aggregation on an acquired basis may, in some patients, be due to immune platelet damage resulting in a distinctive type of platelet storage pool deficiency.

Release of human platelet factor V activity is induced by both collagen and ADP and is inhibited by aspirin

Factor V activity in suspensions of human platelets washed by albumin density gradient separation increased in response to stimulation by both collagen and adenosine diphosphate (ADP). The appearance of factor V activity extracellularly had the characteristics of platelet secretion and was partially inhibited by aspirin and by the antimetabolites 2-deoxyglucose and antimycin A. Some increase in factor V activity was also observed in platelet suspensions during the initial response to ADP; this activity was not detected extracellularly, but remained associated with the platelets. Patients with storage pool deficiency (SPD) whose platelets are deficient only in dense granule substances released normal amounts of factor V activity, whereas decreased amounts were released in a patient whose platelets have both dense and alpha granule deficiencies. These findings suggest that a portion of platelet factor V is associated with, and released from, alpha granules.

Biphasic aggregation responses to ADP and epinephrine in some storage pool deficient platelets: relationship to the role of endogenous ADP in platelet aggregation and secretion

Biphasic aggregation responses to ADP and epinephrine have been consistently observed in platelets of two patients with storage pool deficiency (SPD) despite marked reductions in platelet ADP contents. The nature of these aggregation responses was examined in relation to the role of secreted ADP as a mediator of secondary aggregation. Platelets from these patients secreted greater quantities of ADP after stimulation by epinephrine than did SPD platelets with comparable ADP deficiencies and absent second-phase aggregation, and were more sensitive than both other SPD platelets and normal platelets to the aggregating effects of low concentrations of ADP. Epinephrine-induced secondary aggregation was also associated with measurable, though less than normal, MDA formation in these patients' platelets, whereas no MDA formation occurred in SPD platelets with impaired epinephrine responses. CP/CPK did not inhibit epinephrine-induced responses in the SPD patients showing biphasic aggregation, whereas these responses in normal platelets were inhibited only with higher levels of CP/CPK than those required for inhibition of the responses to exogenous ADP. These findings suggest that secondary aggregation responses may be mediated by both prostaglandin endoperoxide production and secreted ADP, and are consistent with possibility that such mediation is primarily an intracellular process.

Optimal ordering policies for continuous review perishable inventory models

This paper extends the notions of perishable inventory models to the realm of continuous review inventory systems. The traditional perishable inventory costs of ordering, holding, shortage or penalty, disposal and revenue are incorporated into the continuous review framework. The type of policy that is optimal with respect to long run average expected cost is presented for both the backlogging and lost-sales models. In addition, for the lost-sales model the cost function is presented and analyzed.

Red blood cells: their dual role in thrombus formation

Red blood cells may have a physical and chemical effect on the interaction between platelets and blood vessel surfaces. Under flow conditions in which primarily physical effects prevail, platelet adhesion increases fivefold as hematocrit values increase from 10 to 40 percent but undergoes no further increase from 40 to 70 percent, implying a saturation of the transport-enhancing capabilities of red cells. For flow conditions in which platelet-surface reactivity is more dominant, platelet adhesion and thrombus formation increase monotonically as hematocrit values increase from 10 to 70 percent. Thus red cells may have a significant influence on hemostasis and thrombosis; the nature of the effect is apparently related to the flow conditions.

Effect of shear rate on platelet interaction with subendothelium in citrated and native blood. II. Relationships among platelet adhesion, thrombus dimensions, and fibrin formation

The adhesion of human platelets to subendothelium of rabbit aorta and the formation of microthrombi (aggregation) were investigated in an annular chamber after perfusion of native or citrated blood at wall shear rates ranging from 650 to 3300 sec-1. Previous studies demonstrated reduced adhesion and increased thrombi with native as compared to citrated blood and suggested that thrombi grow larger and higher in native blood. In the present study, thrombus dimensions were quantified by newly developed stereological techniques. The thrombus volume per surface area of subendothelium increased with increasing shear rate and was significantly greater in native blood. At 1300 sec-1 thrombi less than 5 and less than 10 micrometers in height were more frequent in citrated blood, and those greater than 20 micrometers were much more frequent in native blood. Maximum thrombus heights were 62 +/- 7 and 71 +/- 5 micrometers in native and 17 +/- 2 and 28 +/- 4 in citrated blood at shear rates of 650 and 1300 sec-1, respectively. Surface coverage with fibrin was 30%, 11%, and 0% at shear rates of 650, 1300, and 3300 sec-1, respectively. In addition, the effects of approximately 15 and approximately 45 mM citrate (final concentration in plasma) were studied at shear rates of 200, 650, and 2600 sec-1. At a citrate concentration of approximately 45 mM aggregation was abolished, and adhesion was strongly inhibited at the highest shear rate. We conclude that citrate (1) inhibits platelet attachment kinetics, thus acting to reduce adhesion, and (2) inhibits platelet aggregation and thrombus stability, thus reducing thrombus heights, which (3) may indirectly lead to increased adhesion.

Isolated deficiency of platelet procoagulant activity

This is a study of a 34 year old woman with a moderate to severe bleeding disorder in whom impaired platelet procoagulant activity (PPA) was found by several methods, including tests of factor 3 availability (PF-3a), prothrombin consumption and contact activation. No deficiencies of platelet adhesion, aggregation, secretion, metabolism or granule-bound substances were detectable. Under adequate platelet coverage, this woman underwent two surgical procedures without difficulty. These findings demonstrate the role of PPA in hemostasis and indicate that a defect in PPA can be an isolated occurrence. The abnormalities in PF-3a found in this patient could be due to the diminished number of factor V binding sites, resulting in impaired factor Xa binding, found in separate studies by Majerus et al.

Platelet membrane glycoproteins in thrombasthenia, Bernard-Soulier syndrome, and storage pool disease

Quantitative polyacrylamide gel electrophoresis has been carried out on patients with Bernard-Soulier syndrome, Glanzmann's thrombasthenia, and storage pool defect in order to clarify the abnormalities in their platelet membrane glycoproteins. Normal individuals had values (expressed as PAS staining units/mg of membrane protein) of 5.11 +/- 0.63 for glycoprotein 1 (Mr 150,000), 2.35 +/- 0.35 for glycoprotein II (Mr 120,000), 0.89 +/- 0.22 for glycoprotein III (Mr 100,000), and 1.34 +/- 0.64 for glycoprotein IV (Mr 85,000). Total PAS staining of these four major bands was 9.70 +/- 1.26 PAS units/mg of membane protein. Patients with Bernard-Soulier syndrome completely lacked glycocalicin and had about one half (1.90 PAS units/mg) of the glycoprotein I of normal controls. These was no significant reduction in glycoproteins II, III, and IV, but total PASstaining was reduced to 4.40 units/mg, reflecting the importance of the contribution of glycoprotein I to this parameter. Thrombasthenic platelets gave values for glycoprotein II of 0.66, which were about 25% of controls, and the values for glycoprotein III (0.34) were about 40% of controls. Patients with storage pool disease gave values within the normal range with the exception of one family which showed, in addition, small platelets and an associated lipid defect. In thic case of glycoprotein (2.71) was significantly elevated.

Prostacyclin (prostaglandin I2, PGI2) inhibits platelet adhesion and thrombus formation on subendothelium

Prostaglandin I2 (prostacyclin, PGI2), a substance synthesized in the wall of blood vessels, has been previously shown to inhibit the aggregation of platelets in stirred platelet-rich plasma. We used a method in which segments of deendothelialized rabbit aorta are perfused at arterial shear rates with human blood and found that both platelet adhesion and thrombus formation on subendothelium was inhibited in blood containing 10 nM PGI2. PGI2 appears to reduce adhesion by inhibiting platelet spreading. These findings suggest that PGI2 could regulate the deposition of platelets on vascular surfaces.

Effect of shear rate on platelet interaction with subendothelium in citrated and native blood. I. Shear rate--dependent decrease of adhesion in von Willebrand's disease and the Bernard-Soulier syndrome

Previous studies have demonstrated impaired adhesion of platelets to the subendothelium in von Willebrand's disease. These studies were performed by circulating (in a closed system) citrated whole blood through a chamber containing everted segments of rabbit aorta from which the endothelium had been removed by balloon catheter. The average wall shear rate was 800 sec-1, and the perfusion time was 10 min. In the present study we measured the interaction of platelets with subendothelium in native (nonanticoagulated) blood, using a recently described technique in which the vessel segments are perfused with directly sampled venous blood. The system was open; that is, the blood was not recirculated. We used blood flow rates of 20, 40, and 50 ml/min, which correspond to calculated shear rates of 1300, 2600, and 3300 sec-1 and perfusion times of 3, 2, and 2 min, respectively. For comparison, parallel studies at 1300 sec-1 were also obtained with citrated blood. In normal subjects, at a shear rate of 1300 sec-1, platelet adhesion was less in native blood than in citrated blood, but thrombus formation was greater. Platelet adhesion in five patients with von Willebrand's disease was decreased in both citrated and native blood. The magnitude of the adhesion defect was strongly dependent on the shear rate. Thus, in citrated blood studied at a shear rate of 1300 sec-1, adhesion was 75% less than in normal subjects, whereas in previous studies at 800 sec-1 the reduction in adhesion was 29%. In native blood, adhesion in von Willebrand's disease was normal at a shear rate of 1300 sec-1, whereas 53% and 77% reductions in adhesion were obtained at shear rates of 2600 and 3300 sec-1, respectively. The latter shear rates studied, adhesion of platelets in native blood was also decreased in the Bernard-Soulier syndrome but was normal in hemophilia and afibrinogenemia. Our findings with native blood provide further evidence that impaired adhesion of platelets to the vessel wall accounts for the hemostatic defect in von Willebrand's disease. In addition, this adhesion defect is shear rate-dependent.

Dissociation of factor VIII in the presence of proteolytic inhibitors

When gel filtration of factor VIII is performed with buffers of high ionic strength (1.0 M NaCl or 0.25 M CaCl2), the procoagulant activity elutes with proteins of relatively low molecular weight. It has been suggested that in the presence of proteolytic inhibitors, the procoagulant activity would appear at the void volume. To test this hypothesis, chromatography with buffers of high ionic strength was performed in the presence of benzamidine hydrochloride, soy bean trypsin inhibitor, heparin, DFP, and aprotinin. Under all of these conditions, the procoagulant activity continued to elute with proteins of low molecular weight. Similar findings were obtained after chromatographing cryoprecipitate prepared from the plasma of a normal subject who had received heparin. Thus, at present there is no direct evidence to suggest that proteolysis is involved in the dissociation of factor VIII by buffers of high ionic strength.

Stabilization of factor VIII in plasma by the von Willebrand factor. Studies on posttransfusion and dissociated factor VIII and in patients with von Willebrand's disease

In normal plasma, the ratio of the procoagulant activity of factor VIII (VIII(AHF)) to that of the von Willebrand factor activity (ristocetin cofactor, VIII(VWF)) or factor VIII antigen (VIII(AGN)) is approximately 1, but ratios > 1 (e.g., VIII(AHF) > VIII(VWF) or VIII(AGN)) may be observed in some patients with von Willebrand's disease and in the "late" posttransfusion plasmas of patients with this disorder. The lability of VIII(AHF) was studied by incubating plasma, diluted 1:10 in imidazole buffer pH 7.1, for 6 h at 37 degrees C. With normal plasmas, 77+/-12% (SD) of the original VIII(AHF) activity remained after incubation. VIII(AHF) was labile (e.g., 35-55% residual activity) in the "late" posttransfusion plasmas (VIII(AHF) >> VIII(VWF)) of a patient with von Willebrand's disease, but not in the "early" posttransfusion plasmas (VIII(AHF) approximately VIII(VWF)). VIII(AHF) was also labile in the (base-line) plasmas of three patients with von Willebrand's disease in whom the ratios of VIII(AHF) to VIII(VWF) were 4.4 to 8.1, but not in the plasmas of four other patients in whom the ratio was approximately 1. The electrophoretic mobility of factor VIII antigen was increased in two of the three patients with labile VIII(AHF). In both of these patients, and in the late posttransfusion plasmas, labile VIII(AHF) activity could be stabilized by the addition of purified von Willebrand factor (lacking VIII(AHF) activity) or by hemophilic plasma, but not by plasmas of patients with severe von Willebrand's disease. Thus, VIII(VWF) may serve to stabilize VIII(AHF) and this might explain the posttransfusion findings in von Willebrand's disease.

Platelet interaction with collagen fibrils in flowing blood. II. Impaired adhesion-aggregation in bleeding disorders. A comparison with subendothelium

Anticoagulated whole blood from patients and control subjects was circulated through an annular perfusion chamber in which the fibrillar collagen of alpha chymotrypsin-digested subendothelium and intact subendothelium were exposed. The blood flow conditions corresponded to those in arteries (830 sec-1 wall shear rate). Platelet surface interaction was measured morphometrically. Decreased adhesion to fibrillar collagen associated with normal spreading and normal adhesion-induced formation of platelet thrombi was found with blood of patients with von Willebrand's disease and the Bernard Soulier Syndrome, indicating a defect in the initial attachment reaction of platelets with collagen. Platelets of patients with thrombasthenia did normally adhere to the collagen fibrils and also lost their subcellular organelles during this reaction, but they totally failed to adhere to each other. In storage pool disease platelet thrombus formation was consistently inhibited whereas adhesion and spreading was inhibited in some patients and normal in others. In contrast adhesion was always normal after ingestion of aspirin which consistently caused a marked inhibition of platelet thrombi. These findings correspond -- in essence -- to those previously described on intact subendothelium. However, the observed defects are more pronounced on the fibrillar collagen than on intact subendothelium.