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.

Pseudo-von Willebrand's disease. An intrinsic platelet defect with aggregation by unmodified human factor VIII/von Willebrand factor and enhanced adsorption of its high-molecular-weight multimers

Four members (from four generations) of a family with a mild bleeding disorder and intermittent thrombocytopenia had decreased plasma levels of properties related to factor VIII/von Willebrand factor (FVIII/VWF), an absence of high-molecular-weight forms of FVIII/VWF in the plasma (but normal multimeric structure in the platelets), and increased ristocetin-induced platelet aggregation, as in Type IIB von Willebrand's disease. However, unlike the abnormality in FVIII/VWF in Type IIB disease, the basic defect in this family was in their platelets, which absorbed FVIII/VWF high-molecular-weight multimers at lower concentrations of ristocetin than did normal platelets. In addition, either in platelet-rich plasma or suspended in buffer, their platelets were aggregated by unmodified normal human FVIII/VWF without ristocetin. Since the abnormalities of plasma FVIII/VWF in this family may be secondary to the platelet abnormalities, the term "pseudo-von Willbrand's disease" may be suitably descriptive of their disorder.

Platelet malondialdehyde production and aggregation responses induced by arachidonate, prostaglandin-G2, collagen, and epinephrine in 12 patients with storage pool deficiency

We assessed the integrity of the prostaglandin synthetic pathway by measuring malondialdehyde (MDA) production and studied platelet aggregation responses to arachidonic acid and PGG2 in 12 patients with storage pool deficiency (SPD). Eight patients were deficient only in dense granules (delta-SPD) and four were deficient in both dense and alpha-granules (alpha delta-SPD). Production of MDA in response to arachidonic acid (AA), epinephrine, and collagen suggested that the transformation of AA to prostaglandin metabolites was normal in delta-SPD but abnormal in alpha delta-SPD and that the liberation of AA from phospholipids were abnormal in the majority of patients with SPD. Since the content of secretable adenosine diphosphate (ADP) is diminished in SPD platelets, the aggregation responses of these platelets to AA and PGG2 were studied to help answer the question whether these agents aggregate platelets directly or through release of endogenous ADP. Among patients with delta-SPD, aggregation by both AA and PGG2 was decreased in four albinos whose platelets were markedly deficient in ADP. In contrast, normal, or less strikingly abnormal, responses were observed in patients whose platelets either contained higher levels of platelet ADP or showed increased sensitivity to ADP. The more marked impaired responses to AA and PGG2 in patients with alpha delta-SPD suggest that substances derived from alpha-granules may also play a role in platelet aggregation by these agents. The aggregation responses in these patients with various types of SPD is consistent with a theory that granule-derived ADP mediates platelet aggregation by AA and PGG2.

Effect of aspirin and dipyridamole on the interaction of human platelets with sub-endothelium: studies using citrated and native blood

The effect of aspirin and dipyridamole ingestion on the interaction of platelets with the subendothelium was studied using both citrated blood and directly sampled (native) blood. After obtained control studies, normal human subjects ingested 0.6 g of aspirin, 150 mg of dipyridamole, or a placebo and studies were repeated 1 1/2 hrs later. Subjects continued on placebo, aspirin (0.6 g b.i.d.) or dipyridamole (100 mg q.i.d.) for 6 days and studies were obtained 1 1/2 hrs after the last dose. Blood was circulated through an annular chamber on whose inner core were mounted everted segments of de-endothelialized rabbit aorta. The wall shear rate was 2,600 sec(-1). Surface coverage with adherent platelets and platelet thrombi, as well as several parameters of thrombus dimensions, were evaluated morphometrically. Aspirin ingestion markedly reduced platelet thrombi in citrated blood,--but had a much lesser inhibitory effective in native blood. Platelet adhesion was unaffected in native blood, in contrast to previous findings in which a lower shear rate (800 sec (-1)) was used. Ingestion of dipyridamole did not inhibit platelet adhesion or thrombi in either citrated or native blood. The studies indicated that, with the flow conditions used, aspirin is a relatively weak inhibitor of platelet thrombus formation in directly sampled human blood.

Dependence of human platelet functional responses on divalent cations: aggregation and secretion in heparin- and hirudin-anticoagulated platelet-rich plasma and the effects of chelating agents

The dependence of ADP- and epinephrine-induced platelet aggregation and secretion on extracellular divalent cations was examined by quantitating these responses in citrate-, heparin-, and hirudin-anticoagulated platelet-rich plasma. ADP-induced 14C-5HT secretion in heparin-PRP and hirudin-PRP was generally decreased, relative to that in citrate-PRP, without corresponding reductions in aggregation, whereas in response to epinephrine, both aggregation and secretion were decreased in heparin-PRP, and abolished in hirudin-PRP. In heparin-PRP, but not in hirudin-PRP, the degree to which these responses were altered was highly variable among normal subjects, and was dependent on the anticoagulant concentration. Addition of citrate restored the extent of ADP-induced secretion and of epinephrine-induced aggregation and secretion in heparin-PRP to that observed in citrate-PRP, and increased the extent of ADP-induced secretion in hirudin-PRP. Addition of EDTA or EGTA, however, had no effect of ADP-induced secretion in heparin-PRP. These results suggest that ADP-induced aggregation and secretion, as well as responses to ADP vs. epinephrine, have different dependencies on extracellular or surface-bound divalent cations. The variable responses observed in heparin-PRP may reflect direct interactions of heparin with platelets, and this variability may account for the conflicting results of previous studies.

Impaired platelet response to thromboxane-A2 and defective calcium mobilization in a patient with a bleeding disorder

Platelet aggregation, secretion, and thromboxane formation induced by various agonists, including arachidonate, prostaglandin-G2 (PGG2), and thromboxane-A2 (TxA2), were examined in a patient with a bleeding disorder who was previously reported to have a TxA2-related defect. Aggregation and 14C-5HT secretion were decreased, and no TxB2 formation occurred in response to adenosine diphosphate (ADP), epinephrine, or collagen. Arachidonate-induced aggregation and TxB2 formation, and PGG2-induced aggregation (but not TxB2 formation) were impaired at low agonist concentrations. The patient's platelets did not aggregate in response to TxA2 generated from arachidonate in normal platelets, but were capable of synthesizing TxA2 from both arachidonate and PGG2. In addition, aggregation and secretion induced by low concentrations of the ionophore A23187 were impaired in platelet-rich plasma (PRP) and in gel-filtered platelets in the absence of extracellular calcium; these responses became normal at higher A23187 concentrations or, in GFP, at low A23187 concentrations in the presence of exogenous calcium. These findings indicate that the TxA2 defect in this patient does not result from a thromboxane synthetase deficiency, but may be due to impaired mobilization of platelet calcium, and thus are consistent with the possibility that TxA2 may act as a calcium ionophore.