Assembly of the platelet prothrombinase complex is linked to vesiculation of the platelet plasma membrane. Studies in Scott syndrome: an isolated defect in platelet procoagulant activity

Activation of human platelets by complement proteins C5b-9 is accompanied by the release of small plasma membrane vesicles (microparticles) that are highly enriched in binding sites for coagulation factor Va and exhibit prothrombinase activity. We have now examined whether assembly of the prothrombinase enzyme complex (factors VaXa) is directly linked to the process of microparticle formation. Gel-filtered platelets were incubated without stirring with various agonists at 37 degrees C, and the functional expression of cell surface receptors on platelets and on shed microparticles was analyzed using specific monoclonal antibodies and fluorescence-gated flow cytometry. In addition to the C5b-9 proteins, thrombin, collagen, and the calcium ionophore A23187 were each found to induce formation of platelet microparticles that incorporated plasma membrane glycoproteins GP Ib, IIb, and IIIa. These microparticles were enriched in binding sites for factor Va, and their formation paralleled the expression of catalytic surface for the prothrombinase enzyme complex. Little or no microparticle release or prothrombinase activity were observed when platelets were stimulated with epinephrine and ADP, despite exposure of platelet fibrinogen receptors by these agonists. When platelets were exposed to thrombin plus collagen, the shed microparticles contained activated GP IIb-IIIa complexes that bound fibrinogen. By contrast, GP IIb-IIIa incorporated into C5b-9 induced microparticles did not express fibrinogen receptor function. Platelets from a patient with an isolated defect in inducible procoagulant activity (Scott syndrome) were found to be markedly impaired in their capacity to generate microparticles in response to all platelet activators, and this was accompanied by a comparable decrease in the number and function of inducible factor Va receptors. Taken together, these data indicate that the exposure of the platelet factor Va receptor is directly coupled to plasma membrane vesiculation and that this event can be dissociated from other activation-dependent platelet responses. Since a catalytic membrane surface is required for optimal thrombin generation, platelet microparticle formation may play a role in the normal hemostatic response to vascular injury.

Inhibition of human platelet function in vitro and ex vivo by acetaminophen

The effects of acetaminophen (APAP) in vitro, or ex vivo following APAP ingestion, on human platelet aggregation, 14C-5HT secretion, and thromboxane B2 (TxB2) formation were assessed. APAP added in vitro to citrated platelet-rich plasma (PRP) inhibited aggregation, secretion, and TxB2 formation induced by collagen, epinephrine, arachidonate, and the ionophore A23187, but had no effect on the responses induced by the endoperoxide analog U44069. Arachidonate-induced responses were inhibited by lower concentrations of APAP than were the responses to the other agonists. In PRP obtained 1 hour after ingestion of 650 mg or 1000 mg APAP, arachidonate-induced TxB2 formation was inhibited by 40-99% in five subjects tested, whereas inhibition of collagen- or epinephrine-induced TxB2 formation was less consistent. Aggregation and secretion responses were not altered by APAP ingestion in 4 of the 5 subjects, but were inhibited in the remaining subject, who had the highest plasma APAP levels. In contrast to aspirin and indomethacin, APAP-induced inhibition of collagen-stimulated TxB2 formation could be partially overcome with increasing collagen concentrations. No such partial correction occurred with epinephrine, however. In washed platelet suspensions labeled with 3H-arachidonate, both APAP and aspirin inhibited the formation of labeled PGD2 and PGE2, as well as TxB2. These results suggest that APAP acts in human platelets as a reversible inhibitor of cyclo-oxygenase, as found previously in other tissues, and that recent APAP ingestion can, on occasion, produce inhibition of platelet functional responses measured in vitro.

Evidence for the presence of tissue factor activity on subendothelium

By a variety of methods, tissue factor activity was demonstrated in the subendothelium of rabbit aorta and human umbilical artery. In one method, everted segments of de-endothelialized vessels were mounted in an annular perfusion chamber and the subendothelial surface was exposed to nonanticoagulated human blood under controlled flow. Procoagulant activity was assessed by measuring fibrin deposition on subendothelium and fibrinopeptide A (FPA) levels in post chamber blood. Both fibrin deposition and FPA were decreased with rabbit vessel segments exposed (at a shear rate of 650 seconds-1) to blood from patients with factor VII deficiency and with umbilical artery segments (at shear rates of 90 to 180 seconds-1) that had been pretreated with a monoclonal antibody to human tissue factor. In a second method, everted umbilical artery segments were mounted on a stir bar and the subendothelial surface was exposed, with stirring, to plasma or purified coagulation factors. The capacity of the surface to clot plasma on addition of calcium was inhibited by the antibody to tissue factor. The surface also activated purified 3H-factor X in the presence of factor VIIa, but not in its absence, and this surface property was almost entirely eliminated by pretreating the vessel segments with antitissue factor. Tissue factor activity in subendothelium could play a role in both the arrest of bleeding and in promoting the formation of thrombi at sites of vascular injury.

Fibrinogen-independent platelet adhesion and thrombus formation on subendothelium mediated by glycoprotein IIb-IIIa complex at high shear rate

Platelet adhesion and thrombus formation on subendothelium, studied at a shear rate of 2,600 s-1, were inhibited by two synthetic peptides known to interact with GPIIb-IIIa. One peptide (HHLGGAKQAGDV) corresponds to the carboxyl terminal segment of the fibrinogen gamma-chain (gamma 400-411) and the other (RGDS) contains the amino acid sequence Arg-Gly-Asp (RGD) common to fibronectin, von Willebrand factor, vitronectin and the alpha-chain of fibrinogen. Neither platelet adhesion nor thrombus formation were decreased in a patient with severe congenital fibrinogen deficiency and this was equally true when his blood was further depleted of the small amounts of fibrinogen present utilizing an anti-fibrinogen antibody. In normal subjects, adhesion and thrombus formation were inhibited by the Fab' fragments of a monoclonal anti-GPIIb-IIIa antibody (LJ-CP8), which interferes with the interaction of platelets with all four adhesive proteins in both the fluid and solid phase. However, another anti-GPIIb-IIIa antibody (LJ-P5) that had minimal effects on the interaction of platelets with fibrinogen, but inhibited to varying degrees platelet interaction with other adhesive proteins, was equally effective. The findings demonstrate that, at a shear rate of 2,600 s-1, adhesive proteins other than fibrinogen are involved in GPIIb-IIIa-mediated platelet adhesion and thrombus formation on subendothelium. In addition, since LJ-P5 inhibited the binding of soluble von Willebrand factor and vitronectin, these adhesive proteins may be involved in platelet thrombus formation. In contrast to the results obtained at a shear rate of 2,600 s-1, fibrinogen could play a role in mediating platelet-platelet interactions with weak agonists or lower shear rates.

The response of platelets to epinephrine in storage pool deficiency--evidence pertaining to the role of adenosine diphosphate in mediating primary and secondary aggregation

Aggregation responses and thromboxane (Tx) formation in ten patients with storage pool deficiency (SPD) specific to the dense granules (delta-SPD) were studied to assess further the role of dense granule adenosine diphosphate (ADP) in mediating platelet aggregation by epinephrine. The ability of epinephrine to elicit secondary aggregation (SA) responses was highly variable in delta-SPD when tested at 5 mumol/L epinephrine, but was consistently abnormal when tested over a range of concentrations. The occurrence of SA in both delta-SPD patients and normal subjects was correlated with the magnitude of the rate of primary aggregation (PA). This PA rate was normal, on average, for the entire patient group but was greater in patients with more consistent SA responses. The PA findings were related to the Kd value obtained in binding studies with 3H-yohimbine, but not with the number of alpha 2-receptor sites. Studies on Tx production (assessed by radioimmunoassay of TxB2) showed that the ability to synthesize Tx from arachidonate was not impaired in delta-SPD, and that there was an absolute positive correlation between epinephrine-induced SA and Tx production. Aggregation in delta-SPD platelets in response to the Tx receptor agonist U44069 was consistently decreased, but could be corrected by addition of ADP. The results of the study suggest that dense granule-derived ADP is not required for PA by epinephrine, but mediates SA as a synergistic agonist with TxA2. This role of ADP in SA may be elucidated more precisely by further studies on platelet activation processes in delta-SPD.

Specific correction of impaired acid hydrolase secretion in storage pool-deficient platelets by adenosine diphosphate

Storage pool-deficient (SPD) platelets, which have decreased amounts of dense-granule and/or alpha-granule constituents, contain normal amounts of lysosomal acid hydrolases, but in some cases exhibit impaired secretion of these enzymes. We examined this impaired secretion response in SPD patients with varying extents of granule deficiencies, and determined the effects of added dense-granule constituents. Acid hydrolase secretion was impaired in patients with severe dense-granule deficiencies, but not in patients with lesser dense-granule deficiencies, including those with alpha-granule deficiencies as well. When dense-granule constituents (ADP, ATP, serotonin, Ca+2, pyrophosphate) were added to gel-filtered platelets, ADP, but none of the other constituents, completely corrected the impairment of thrombin and A23187-induced secretion in SPD platelets. The concentration of ADP required to normalize thrombin-induced secretion varied markedly, from 0.01 to 10 microM, among the individual patients. Fixation of platelets with formaldehyde before centrifugation did not prevent the enhancement of secretion by ADP. Excess ATP, which acts as a specific antagonist of ADP-mediated responses, completely blocked this enhancement of secretion in SPD platelets by ADP, and partially inhibited acid hydrolase secretion induced by low, but not high, concentrations of thrombin in normal platelets as well. Treatment of normal platelets with acetylsalicylic acid in vivo, but not in vitro, produced an impairment of acid hydrolase secretion similar in extent to that in SPD platelets, but which could not be completely corrected by added ADP. One possible explanation of these results is that the impairment of acid hydrolase secretion may be secondary to the dense-granule deficiency in SPD platelets, and that secreted ADP may potentiate the lysosomal secretion response in normal platelets as well.

Impairment of phosphatidylinositol metabolism in a patient with a bleeding disorder associated with defects of initial platelet responses

Phosphoinositide/polyphosphoinositide (PI/PPI) metabolism, measured by the increase of 3H-phosphatidic acid (PA) and the decrease of 3H-phosphatidylinositol (PI) in 3H-arachidonate-labeled platelet suspensions, was assessed in five patients whose platelet functional defects included impaired initial rates of ADP, epinephrine and U44069 aggregation in platelet-rich plasma (PRP). In one patient, 3H-PA formation induced by collagen and thrombin was reduced or absent on two of three occasions, and the decrease in 3H-PI was reduced on one of these two occasions in response to collagen and A23187, and on all 3 occasions in response to thrombin. The variations in the formation of 3H-PA in this patient on different occasions broadly paralleled the variations in the initial rates of ADP and U44069 aggregation and in epinephrine aggregation seen in PRP. No such abnormalities of PI metabolism were found in four other patients with similar, but not identical, functional defects. These results suggest an impairment affecting metabolism of PI/PPI via the PI/PPI cycle in this patient's platelets. The association of abnormalities of PI metabolism with defects of initial platelet responses provides further support for a physiological role of phosphoinositide metabolism in the early activation mechanism of platelets.

Evidence for tissue factor-dependent activation of the classic extrinsic coagulation mechanism in blood obtained from bleeding time wounds

The activation of platelets and the coagulation mechanism was studied by collecting blood from a standard bleeding time incision at 30-second intervals and measuring the plasma concentrations of fibrinopeptide A (FPA), platelet factor 4 (PF4), and thromboxane B2 (TxB2). FPA was observed in the first samples (30 to 60 seconds) obtained, increased progressively until cessation of bleeding, and was markedly diminished after heparin administration, thus indicating that thrombin formation occurs early in incisional blood. PF4 increased monotonically throughout blood sampling, whereas the major increase in TxB2 appeared near the cessation of bleeding. The initial increase in FPA content occurred normally in patients with deficiencies of either factor IX or VIII, was markedly diminished in patients with factor X or V deficiency, and was delayed in patients with factor VII deficiency. These studies suggest that tissue factor activation of the classic (activation of factor X) extrinsic coagulation mechanism occurs as an early event during the arrest of bleeding from bleeding time incisions. The relation of the aforementioned to platelet activation is less clear because there was no consistent correlation between decreased FPA formation and impaired PF4 secretion or TxB2 production. In fact, the latter were normal in some subjects with the most impaired FPA formation, which suggests that both collagen and thrombin, perhaps synergistically, may contribute to platelet activation during the primary arrest of bleeding.

Heterogeneous defects of platelet secretion and responses to weak agonists in patients with bleeding disorders

Eleven patients with mild bleeding disorders had as a common abnormality, impaired platelet aggregation and secretion with low concentrations (0.5-1.0 micrograms/ml) of collagen and, in most cases, an absence of second phase aggregation with epinephrine. Platelet granule contents were normal, ruling out storage pool deficiency. To characterize further the platelet abnormalities, we measured aggregation, 14C-5HT secretion, and TxB2 formation induced by a variety of platelet agonists. In eight of the 11 patients we observed decreased initial rates as well as extents of aggregation with one or more weak agonists (ADP, epinephrine, thromboxane A2 and the endoperoxide analogue U44069), i.e. agonists which induced secretion only as a result of aggregation, but normal responses to strong agonists such as arachidonate and high (10 micrograms/ml) concentrations of collagen, which can induce secretion in the presence or absence of aggregation. In all of these patients, TxB2 formation with arachidonate and all concentrations of collagen was normal. The platelet defects in these eight patients have been designated as weak agonist response defects (WARDs). In contrast, the initial aggregation responses to all weak agonists were normal in the three other patients, while secretion and TxB2 formation induced by strong agonists were impaired. Thus, in contrast to the eight patients above, the platelet defects in these three patients were characteristic of defects in the secretion response per se. The results obtained in the 11 patients studied indicate that these types of platelet disorders, previously referred to as primary secretion defects, include defects in the initial platelet responses which precede secretion (WARD) as well as defects in the secretory mechanism per se. Both groups of defects appear to be heterogeneous in nature.

Role of shear rate and platelets in promoting fibrin formation on rabbit subendothelium. Studies utilizing patients with quantitative and qualitative platelet defects

The deposition of platelets on subendothelium of rabbit aortic segments exposed to non-anticoagulated human blood increased progressively with increasing wall shear rates (50-2,600 s-1), whereas fibrin deposition decreased. Studies in normal subjects and patients with platelet disorders suggested that, under the conditions used, platelets were essential for fibrin deposition at intermediate (650 s-1) but not low (50 s-1) shear rates. Fibrin deposition was markedly diminished in a patient with Scott syndrome whose platelets have a diminished capacity to bind Factor Xa and activate Factors IX and II. In glycoprotein IIb-IIIa deficiency, fibrin deposition was normal (or somewhat increased), whereas in glycoprotein Ib deficiency the association of fibrin with platelets, but not subendothelium, was decreased. The findings indicate that platelets, perhaps through surface localization of coagulation proteins, promote fibrin deposition on subendothelium at arterial shear rates and suggest that agents directed against platelet coagulant properties could be antithrombotic.

A new von Willebrand variant (type I, New York): increased ristocetin-induced platelet aggregation and plasma von Willebrand factor containing the full range of multimers

We report three members of a family who had reduced levels of plasma von Willebrand factor (vWF) and increased ristocetin-induced platelet aggregation (RIPA) (aggregation of platelet-rich plasma with ristocetin at a concentration of 0.45 mg/mL), as previously reported in type IIB and pseudo-von Willebrand's disease (vWD). However, in contrast to the latter two disorders in which the larger vWF multimers are absent in plasma, the entire range of vWF multimers was observed in the patients' plasma after sodium dodecyl sulfate-agarose gel electrophoresis, and all vWF multimers (including the largest) were present in the same proportion as in normal plasma and type I vWD. Thus, despite increased RIPA, the levels and multimeric pattern of vWF in this family's plasma were indistinguishable from those in type I vWD in which RIPA is usually decreased. Addition of ristocetin to the patients' platelet-rich plasma resulted in the removal of vWF (and, more selectively, of the large multimers) at lower concentrations of ristocetin than normal, as in type IIB and pseudo-vWD. The defect in the patients was localized to their vWF, which had an enhanced capacity for aggregating washed normal platelets in the presence of low concentrations of ristocetin and for aggregating pseudo-vWD platelets (in the absence of ristocetin). Both glycoproteins (GP) Ib and IIb-IIIa were involved in the enhanced aggregation response. RIPA (at low ristocetin concentrations) in the patients' platelet-rich plasma was abolished by a monoclonal antibody (AP1) to GPIb and was markedly reduced by monoclonal antibodies (10E5 and LJP9) that block adenosine diphosphate and thrombin-induced binding of vWF and fibrinogen to GPIIb-IIIa but was unaffected by an antibody (LJP5) that only blocks vWF binding. Partial inhibition of the initial aggregation slope (and complete inhibition of second phase aggregation) was achieved with creatine phosphate/creatine phosphokinase. EDTA blocked second-phase aggregation but was without effect on the initial slope. The findings in this family combine some features of both type I vWD (normal pattern of vWF multimers in plasma) and type IIB vWD (increased RIPA) and further demonstrate the increasing complexity of the structure-function relationships in vWD.

Platelet adhesion and thrombus formation on subendothelium in platelets deficient in glycoproteins IIb-IIIa, Ib, and storage granules

Patients whose platelets are deficient in glycoprotein (GP) Ib, IIb-IIIa (thrombasthenia), or granule substances (storage pool deficiency, SPD) were studied to define further the properties of platelets that mediate platelet adhesion and thrombus formation on subendothelium. Both nonanticoagulated and citrated blood were exposed to everted, de-endothelialized rabbit vessel segments under controlled flow conditions and shear rates varying from 650 to 3,300 sec-1. Morphometry was used to measure platelet thrombus dimensions and the percentage of the subendothelial surface covered with contact (C) or spread (S) platelets. Adhesion was defined as C + S. The results in SPD demonstrated (1) reduced thrombus dimensions in delta-SPD (pure dense granule deficiency) in proportion to the magnitude of the dense granule defect; (2) an even greater reduction in thrombus dimensions in patients with combined deficiencies of alpha and dense granules (alpha delta-SPD); and (3) impaired platelet adhesion at several conditions in alpha delta-SPD and, in delta-SPD, a hematocrit-dependent impairment of adhesion in citrated blood at 2,600 sec-1. In thrombasthenia, platelets were present as a monolayer on the subendothelial surface in both nonanticoagulated and citrated blood, indicating an absolute requirement for GPIIb-IIIa in promoting platelet-platelet interaction at all shear rates and perfusion times. Two types of abnormalities in platelet-vessel wall interactions were observed. In nonanticoagulated blood, the percentage of platelets in the C phase was consistently increased at all shear rates, but C + S values were normal. These observations indicate that platelets deficient in GPIIb-IIIa do not spread normally on the subendothelial surface exposed to nonanticoagulated blood. With citrated blood, the C + S value in thrombasthenia was reduced at both 800 and 2,600 sec-1, as in von Willebrand's disease, and a similar degree of reduction (about 50%) was observed in normal blood treated with a monoclonal antibody to GPIIb-IIIa. The findings, together with theoretical considerations, are consistent with an hypothesis that GPIIb-IIIa mediates the spreading of platelets on subendothelium following the initial attachment through GPIb and that GPIIb-IIIa may be considered an adhesion site on the platelet membrane. Abnormalities of GPIIb-IIIa may, depending on the conditions of study, result in either increased values of C platelets or decreased values of C + S. The results of the study further suggest that a complex interaction of platelet granule factors and membrane GP mediate platelet adhesion and thrombus formation.

Impaired factor X and prothrombin activation associated with decreased phospholipid exposure in platelets from a patient with a bleeding disorder

Platelets from a platelet factor 3-deficient patient, which was first described by Weiss et al (Am J Med 67:206, 1979), were found to be equally impaired in their ability to promote factor X and prothrombin activation. Compared to normal platelets, the patient's platelets showed upon stimulation with thrombin plus collagen a much slower generation and a considerably lower level of platelet prothrombin- and factor X-converting activities. Treatment of stimulated platelets with phospholipases revealed a decreased exposure of negatively charged phospholipid at the outer surface of the patient's platelets, relative to control's. We suggest that the combined impairment of prothrombin- and factor X-converting activities in this patient is due to a defect in the mechanism by which phosphatidylserine becomes exposed at the outer surface of stimulated platelets.

Factor VIII/von Willebrand factor in subendothelium mediates platelet adhesion

The present studies were undertaken to determine whether factor VIII/von Willebrand factor (vWF) present in the vessel wall (in addition to that in plasma) may mediate the attachment of platelets to subendothelium. Subendothelium from everted rabbit aorta was exposed to human citrated blood flowing through an annular perfusion chamber at 40 mL/min (wall shear rate of 2,600 s-1 for five minutes). The vessel segments were incubated at 37 degrees C for one hour with various dilutions of either goat-anti-rabbit factor VIII/vWF serum or an IgG fraction prepared from the serum. Control segments were incubated with serum or IgG from a nonimmunized goat. Values of platelet contact (C), platelet adhesion (C + S), and thrombus formation (T) on the subendothelium were evaluated by a morphometric technique. Compared with vessels incubated with fractions prepared from a normal goat, a significant decrease in platelet adhesion (C + S), ranging from 45% to 65%, was observed on vessels incubated with various dilutions (1:5 to 1:50) of either serum or IgG fractions of goat-anti-rabbit factor VIII/vWF. A similar decrease in platelet adhesion was observed with vessels incubated with an F(ab')2 fragment against rabbit factor VIII/vWF prepared in the goat. When goat-anti-rabbit factor VIII/vWF IgG was added to rabbit blood (1:75 dilution), platelet adhesion was reduced to the same extent (65%) on normal rabbit vessels and on vessels pre-incubated with goat-anti-rabbit factor VIII/vWF. Immunofluorescence studies revealed the presence of rabbit factor VIII/vWF in the subendothelium of rabbit aorta and the continued binding of the goat-anti-factor VIII/vWF antibodies on subendothelium during the perfusion studies. No uptake of human factor VIII/vWF on the rabbit subendothelium was observed by this immunologic technique; human factor VIII/vWF was found to be entirely associated with the attached human platelets. Thus, factor VIII/vWF in the vessel wall may mediate platelet attachment to subendothelium in a manner similar to that of plasma factor VIII/vWF.

Platelet interaction with rabbit subendothelium in von Willebrand's disease: altered thrombus formation distinct from defective platelet adhesion

Blood interaction with the subendothelium of rabbit aorta was investigated in an annular perfusion chamber using patients with von Willebrand's disease, hemophilia, and afibrinogenemia. The vessels were exposed to nonanticoagulated blood for a range of flow conditions (wall shear rates of 650-3,300 s-1) and exposure times (1.5-10 min). The resultant platelet and fibrin interaction was quantified by the use of several morphometric techniques, one of which was developed to measure more precisely the dimensions (height and volume) of platelet thrombi attached to the subendothelium. A major finding was that under flow conditions in which little or no defect in platelet adhesion was observed in von Willebrand's disease, platelet thrombus height and volume in this disorder were significantly reduced as compared with normal controls or patients with hemophilia. Thus, Factor VIII/von Willebrand factor (VIII/VWF) may mediate not only the adhesion of platelets to subendothelium but also platelet-platelet attachments necessary for normal thrombus development. The level of Factor VIII:coagulant activity (VIII: C) was also observed to influence the resultant thrombus height and volume deposited on subendothelium, presumably through the generation of thrombin or some other procoagulant factor preceding fibrin formation, since normal values of thrombus dimensions were always observed in a patient with a fibrinogen deficiency. The influence of VIII:C became greater as shear rate was reduced, whereas as shear rate was increased, VIII/VWF was more dominant in determining the resultant platelet deposition on subendothelium. Thus, the deficiencies of VIII:C and VIII/VWF in hemophilia and von Willebrand's disease can lead to various abnormalities in platelet and fibrin association with subendothelium. The importance of a particular deficiency will depend strongly on the local blood flow conditions.

Fibrin formation, fibrinopeptide A release, and platelet thrombus dimensions on subendothelium exposed to flowing native blood: greater in factor XII and XI than in factor VIII and IX deficiency

Fibrin deposition and platelet thrombus dimensions on subendothelium were studied in four groups of patients with coagulation factor deficiencies. Five patients with factor VIII deficiency (APTT 120 +/- 8 sec) and three patients with factor IX deficiency (APTT 125 +/- 11 sec) were severe bleeders, whereas four patients with factor XII deficiency and seven with factor XI deficiency were either asymptomatic or only mild bleeders despite APTT values of 439 +/- 49 and 153 +/- 13 sec, respectively. Everted segments of deendothelialized rabbit aorta were exposed at a shear rate of 650 sec(-1) for 5 and 10 min to directly sampled venous blood in an annular chamber. Blood coagulation was evaluated by measuring fibrin deposition (percent surface coverage) on the subendothelium and post-chamber fibrinopeptide A levels; platelet thrombus dimensions on the subendothelium were evaluated by determining the total thrombus volume per surface area (using an optical scanning technique) and the average height of the three tallest thrombi. Consistent differences were observed among the patient groups for both the 5-min and 10-min exposure times. The larger of the 5- and 10-min exposure-time values was used to calculate group averages. Fibrin deposition in normal subjects was 81% +/- 5% surface coverage, and post-chamber fibrinopeptide A values were 712 +/- 64 ng/ml. Markedly decreased fibrin deposition and fibrinopeptide A levels were observed in factor VIII deficiency (2% +/- 1% and 102 +/- 19 ng/ml) and factor IX deficiency (11% +/- 7% and 69 +/- 11 ng/ml). In contrast, significantly higher values were obtained in patients deficient in factor XI (33% +/- 5% and 201 +/- 57 ng/ml) and factor XII (66% +/- 12% and 306 +/- 72 ng/ml). Differences in thrombus dimensions were also observed. In normal subjects, the value for thrombus volume and average height of the tallest thrombi were 8.3 +/- 1.3 cu micron/sq micron and 145 +/- 11 micron, respectively, and in patients were as follows: FVIII, 2.7 +/- 0.6 and 71 +/- 7; FIX, 4.5 +/- 1.8 and 88 +/- 14; FXI, 11.8 +/- 1.9 and 125 +/- 10; and FXII, 7.9 +/- 3.1 and 130 +/- 25. Platelet thrombus dimensions were normal in a patient with fibrinogen deficiency, indicating that the smaller thrombi in factor VIII and factor IX deficiencies were probably due to impaired evolution of thrombin rather than diminished fibrin formation.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence that platelet alpha-granules are a major determinant of platelet density: studies in storage pool deficiency

Density-related subpopulations of platelets were obtained by centrifuging platelets obtained from venous blood on a discontinuous gradient of arabinogalactan (Stractan). Four subpopulations were obtained in fractions of the gradient with densities less than or equal to 1.058, 1.065, 1.070, and 1.092 g/ml. The percentage of the total platelet population recovered in these various fractions in 5 patients with congenital storage pool deficiency who are deficient only in dense granules (delta-SPD) was similar to that observed in 8 normal subjects. In contrast, a shift in the density distribution towards a higher percentage of less dense platelets was observed in 3 patients (alpha delta-SPD) who are deficient in both alpha and dense granules. The results of these studies strongly suggest that alpha-granules are a major determinant of platelet density.

Decreased platelet adhesion on vessel segments in von Willebrand's disease: a defect in initial platelet attachment

The adhesion of platelets to subendothelium exposed to flowing blood involves two distinct morphological stages: (1) platelet contact (C), the initial attachment of unspread, discoid platelets to the subendothelium, and (2) spread platelets (S), the attachment that results as contact platelets spread on the surface and become more firmly bound to it. A defect in either initial platelet attachment or platelet spreading can result in reduced levels of platelet adhesion (C + S). The combined observation of decreased platelet adhesion (C + S) and increased platelet contact (C) has been previously utilized to conclude that a defect exists in the ability of platelets to spread on subendothelium in von Willebrand's disease. In this present investigation, we demonstrate, by modeling the contact and spreading stages of platelet adhesion as a classic set of reactions in series, that the combination of reduced adhesion (C + S) and increased contact (C) is inconclusive with regard to the nature of the adhesion defect in von Willebrand's disease. Decreased adhesion (C + S) coupled with increased platelet contact (C) can result from either decreased rates of initial attachment or decreased rates of spreading. In fact, given the complexity of the temporal behavior of platelet contact (C) and platelet spreading (S), and the relatively small fraction (less than 10%) of the platelets that are in contact (C) at any time, we conclude that a determination of the nature of the adhesion (C + S) defect in von Willebrand's disease is not statistically feasible under conditions in which both contact and spreading occur simultaneously. Experiments were conducted in which blood anticoagulated with EDTA was exposed to subendothelium digested with alpha-chymotrypsin for periods of 10 and 40 min. Under such conditions, platelet spreading (S) was substantially inhibited so that the predominant platelet interaction (greater than 80%) on the subendothelium was platelet contact (C). Values of platelet adhesion (C + S) in von Willebrand's disease were significantly reduced (p less than 0.05) compared with normal values at both exposure times. Thus we conclude that the defect in platelet adhesion (C + S) in von Willebrand's disease appears to be associated with a reduced ability of platelets to attach to the surface rather than their inability to spread on the surface.

Heterogeneous abnormalities in the multimeric structure, antigenic properties, and plasma-platelet content of factor VIII/von Willebrand factor in subtypes of classic (type I) and variant (type IIA) von Willebrand's disease

FVIII/VWF in plasma and platelets was studied by various methods in 16 patients with von Willebrand's disease. These methods included measurements of both VIIIR:Ag and VIIIR:RCo levels, radio-crossed immunoelectrophoresis, analysis of the dose-response curves with both fluid-phase and two-site immunoradiometric assays, and SDS-agarose-acrylamide gel electrophoresis. Studies of normal plasma and platelets by the last-named method disclosed the presence of nine to 10 clearly resolvable bands with molecular weights of approximately 1 to 10 X 10(6) and unresolved higher-molecular-weight material, consistent with the previously described multimeric structure of FVIII/VWF. The multimeric structure and antigenic reactivity of FVIII/VWF were normal in 11 patients with type I von Willebrand's disease. However, measurements of the VIIIR:Ag content in plasma and platelets disclosed the presence of three subgroups. In one (type I-1), the VIIIR:Ag content of both plasma and platelets was decreased; in type I-2, VIIIR:Ag was decreased in plasma but normal in platelets, whereas the reverse was found in two patients with type I-3. In five patients with type IIA von Willebrand's disease we observed various abnormalities in the multimeric structure and antigenic reactivity of FVIII/VWF. The distinguishing features in two patients, designated type IIA-1 and IIA-2, were a decreased amount of high-molecular-weight FVIII/VWF and an impaired antigenic reactivity in both plasma and platelets; the defects in type IIA-2 were qualitatively different and more strikingly abnormal than those in type IIA-1. In three other patients, designated type IIIA-3, a less severe deficit of high-molecular-weight FVIII/VWF in plasma was observed and, in addition, the multimeric structure of FVIII/VWF in platelets was normal. The findings in this study demonstrate that a variety of defects in the synthesis, release, antigenic reactivity, and multimerization of FVIII/VWF are probably responsible for the heterogeneous findings in patients with von Willebrand's disease.

Thrombin and ionophore A23187-induced dense granule secretion in storage pool deficient platelets: evidence for impaired nucleotide storage as the primary dense granule defect

The secretion of the dense granule constituents ATP, ADP, calcium, pyrophosphate (PPi), and orthophosphate (Pi), and the release of magnesium induced by thrombin and the divalent cation ionophore A23187 have been quantitated directly in gel-filtered platelets from patients with storage pool deficiency (SPD). Both the contents and the maximal amounts of the dense granule constituents secretable by thrombin were decreased in all the patients studied, while the nonsecretable, retained amounts of these substances were identical in SPD and normal platelets. In response to both thrombin and A23187, the amounts of secretable ATP and ADP were strongly correlated in the platelets of individual patients; in contrast, secretable calcium showed no correlation with the nucleotides, and significant amounts of calcium were secreted in the total absence of nucleotide secretion in the platelets of several patients. The contents of magnesium were normal in all patients, and approximately 12% of platelet magnesium was liberated by thrombin in both SPD and normal platelets. A23187 induced the release of up to 70% of the magnesium content of normal platelets, but released significantly less (46%) magnesium from SPD platelets. Platelet aggregation induced by A23187 in platelet-rich plasma was also markedly decreased in SPD platelets. The correlations among secretable dense granule constituents suggest the presence in SPD platelets of abnormal dense granule structures that sequester calcium and other constituents but little or no adenine nucleotides, and are thus consistent with a hypothesis that impaired nucleotide transport and/or storage may be the primary dense granule defect in this disorder. In addition, these results demonstrate that certain responses to A23187 are impaired in SPD platelets.