A Study of the Cation-and pH-Dependent Stability of Factors V and VIII in Plasma*

The in vitro lability of factors V and VIII in plasma has been studied. In agreement with previous reports, an increase in anticoagulant concentration renders both factors more labile (cation-deficient decay), as does an increase in the pH above 7.3 (alkaline-decay). Calcium appears to be the plasma cation which protects factors V and VIII against in vitro loss of activity. The protection obtained by the addition of other divalent cations depended on the type of plasma used. When resin-EDTA plasma was made cation free by dialysis at 4° C and then incubated at 37° C, the rapid loss of factors V and VIII activity could be prevented by prior addition of strontium, manganese and magnesium. In oxalate plasma, nickel, manganese, cadmium and strontium were effective.

The alkaline decay of both factors V and VIII is irreversible. Partial reversibility of the cation-deficient decay was demonstrated for factor V, but not for factor VIII. The temperature coefficient for both the cation-deficient and alkaline decay is 2-3, suggesting an enzymatic rather than a physical reaction.

There was no evidence to implicate thrombin, plasmin or trypsin since inhibitors of these enzymes failed to modify either type of decay.

PRECLINICAL CHARACTERIZATION OF 2-[3-[3-[(5-ETHYL-4′-FLUORO-2-HYDROXY[1,1′-BIPHENYL]-4-YL)OXY]PROPOXY]-2-PROPYLPHENOXY]BENZOIC ACID METABOLISM: IN VITRO SPECIES COMPARISON AND IN VIVO DISPOSITION IN RATS

Assessment of the pharmacokinetics of [14C]2-[3-[3-[(5-ethyl-4'-fluoro-2-hydroxy[1,1'-biphenyl]-4-yl)oxy]propoxy]-2-propylphenoxy-]benzoic acid ([14C]LY293111), an experimental anti-cancer agent, suggested long-lived circulating metabolites in rats. In vivo metabolites of LY293111 were examined in plasma, bile, urine, and feces of Fischer 344 (F344) rats after oral administration of [14C]LY293111. Metabolites were profiled by high-performance liquid chromatography-radiochromatography, and identified by liquid chromatography (LC)/mass spectrometry and LC/NMR. The major in vivo metabolites of LY293111 identified in rats were phenolic (ether), acyl, and bisglucuronides of LY293111. Measurement of radioactivity in rat plasma confirmed that a fraction of LY293111-derived material was irreversibly bound to plasma protein and that this bound fraction increased over time. This was consistent with the observed disparity in half-lives between LY293111 and total radioactivity in rats and monkeys, and is likely due to covalent modification of proteins by the acyl glucuronide. In vitro metabolism of [14C]LY293111 in liver slices from CD-1 mice, F344 rats, rhesus and cynomolgus monkeys, and humans indicates that glucuronidation was the primary metabolic pathway in all species. The acyl glucuronide was the most prevalent radioactive peak (16% of total 14C) produced by F344 rat slices, whereas the ether glucuronide was the major metabolite in all other species (26-36% of total 14C). Several minor hydroxylated metabolites were detected in F344 rat slice extracts but were not observed in other species. The data presented suggest that covalent modification of proteins by LY293111 acyl glucuronide is possible in multiple species, although the relative reactivity of this metabolite appears to be low compared with those known to cause adverse drug reactions.

Increased level of von Willebrand factor is significantly and independently associated with diabetes in postinfarction patients. THROMBO Investigators

Diabetes is an established risk factor for reinfarction and cardiac death in postinfarction patients. Since the underlying mechanism of diabetes-related risk is not fully understood we aimed to evaluate the association between lipids, thrombogenic factors and diabetes in postinfarction patients. The study population consisted of 1,045 postinfarction patients (846 non-diabetic, 125 non-insulin- and 74 insulin-requiring diabetics) with the following blood tests performed 2 months after an index myocardial infarction: lipoprotein (a), apolipoprotein-B, apolipoprotein-A, cholesterol, HDL cholesterol, triglycerides, insulin, von Willebrand factor (vWF), fibrinogen, factor VII, D-dimer, and plasminogen activator inhibitor (PAI-1). After adjustment for relevant clinical covariates, non-insulin-requiring diabetes was significantly (p < 0.05) associated with elevated levels of (odd ratios per 1 log unit increase in parenthesis) vWF (1.74) and PAI-1 (1.42) whereas insulin requiring diabetes was associated with even more elevated levels of vWF (4.68), but not with increased levels of PAI-1. No significant differences in lipid levels were observed among three groups. In conclusion, increased level of von Willebrand factor is significantly and independently associated with diabetes in postinfarction patients, suggesting that endothelial damage is the primary mechanisms contributing to an increased occurrence of vascular and cardiac events in diabetic postinfarction patients.

The storage defects in grey platelet syndrome and alphadelta-storage pool deficiency affect alpha-granule factor V and multimerin storage without altering their proteolytic processing

Among proteins stored in alpha-granules, multimerin and factor V share unusual features: they bind to each other, are proteolysed to unique forms and are stored eccentrically in alpha-granules. These unique features of their processing led us to study these proteins in alpha delta storage pool deficiency (alphadelta-SPD) and grey platelet syndrome (GPS, alpha-SPD), two conditions known to impair alpha-granule protein storage. Platelet factor V and multimerin were severely reduced in GPS, whereas they ranged from reduced to normal in alphadelta-SPD. The platelet levels of factor V and multimerin in these disorders indicated multimerin deficiency was not predictive of platelet factor V deficiency, although it reduced the amount of multimerin associated with platelet factor V. In GPS only, the defect in storing proteins was associated with increased multimerin and multimerin-factor V complexes in plasma. Like normal platelets, GPS and alphadelta-SPD platelets contained factor V mainly in granules. Platelet factor V and multimerin were proteolysed to normal platelet forms in GPS and alphadelta-SPD platelets, indicating that these conditions preserve some aspects of normal alpha-granule protein processing. Although we found factor V can be stored in platelets deficient in multimerin, our data indicate that multimerin storage influences the point at which multimerin binds factor V.

Greater inhibition of platelet procoagulant activity by antibody-derived glycoprotein IIb--IIIa inhibitors than by peptide and peptidomimetic inhibitors

Antibody-derived GPIIb-IIIa antagonists, such as the c7E3 Fab fragment abciximab, have been shown to inhibit platelet procoagulant activity as well as platelet aggregation. Whether low-molecular-weight peptide-derived and peptidomimetic antagonists also inhibit platelet procoagulant activity in a similar manner has not been fully investigated. We compared the effects of the antibody-derived antagonists c7E3 Fab and m10E5 IgG, the peptide-derived antagonists eptifibatide, MK-852 and RGDS, and the peptidomimetic RO44--9883 on platelet procoagulant activity and on the stimulated cytosolic calcium increases that promote procoagulant activity. Procoagulant activity was measured as prothrombinase activity in gel-filtered platelets, activated by collagen plus thrombin or collagen alone, with and without stirring. The stimulated increases in cytosolic calcium were measured in parallel samples of platelets loaded with fura-2AM. Both c7E3 and m10E5 inhibited prothrombinase activity by 40--50% under all conditions of activation tested and inhibited cytosolic calcium increases to a similar extent in stirred, but not unstirred, platelets. The low-molecular-weight antagonists caused significantly less inhibition of prothrombinase activity in collagen plus thrombin-stimulated platelets, and produced no inhibition but rather a slight enhancement of activity in platelets stimulated by collagen alone. These antagonists also had little or no effect on the cytosolic calcium increases in stirred platelets. These differential effects of antibody-derived versus non-antibody GPIIb-IIIa antagonists on procoagulant activity may be a factor contributing to the differing anti-thrombotic effects of these antagonists seen in clinical trials.

Platelet factor V New York: a defect in factor V distinct from that in factor V Quebec resulting in impaired prothrombinase generation

Studies were performed on a patient with a longstanding bleeding disorder whose major defects were impaired platelet prothrombinase activity in the absence of added factor Va, and a platelet factor V value that was either decreased or at the lower limit of normal when assayed on multiple occasions. In contrast, plasma factor V values were consistently normal. Unlike Scott Syndrome, in which platelet prothrombinase activity is decreased in both the presence and absence of added factor V, her platelets appeared to utilize added factor Va normally in supporting the generation of prothrombinase activity. These findings suggest an intrinsic defect in platelet factor V as the basis of her platelet prothrombinase defect. This defect appears to be different than that described in the Quebec platelet disorder (factor V Quebec). Immunoblot analyses of washed platelet lysates demonstrated a pattern of variably sized factor V molecules that was entirely similar to that observed in normal platelets, and both the heavy and light chains of her factor V after thrombin cleavage were of the same size as that observed in normal platelets. In addition, her platelet multimerin was normal and immunoblot analysis excluded the type of generalized granular protein defect and pathological proteolysis that has been suggested to explain the factor V defect in the Quebec platelet disorder. The findings in this patient thus suggest a new type of platelet factor V defect as the basis for the impaired capacity of her activated platelets to support prothrombinase generation. The findings further support an important role for platelet factor V in hemostasis.

5'-UTR structural organization, transcript expression, and mutational analysis of the human Rab geranylgeranyl transferase alpha-subunit (RABGGTA) gene

Hermansky-Pudlak syndrome (HPS) is a recessively inherited disease with dysfunction of several related subcellular organelles including platelet-dense granules, melanosomes, and lysosomes. Our recent identification of the mutation in murine Rab geranylgeranyl transferase alpha-subunit gene (Rabggta) in one mouse model of HPS, the gunmetal mouse, suggested that human patients with similar phenotypes might have mutations in the human orthologous RABGGTA gene. This prompted reanalysis of the 5'-untranslated structure of the human RABGGTA gene in normal individuals and in patients with deficiencies of platelet-dense granules (alphadelta-SPD), alpha granules (alpha-SPD or gray platelet syndrome, GPS) or alpha plus dense granules (alphadelta-SPD). We report the complete sequence of intron alpha of RABGGTA and demonstrate that exon alpha is immediately upstream of intron alpha. The exon/intron structural organization of the 5'-untranslated region (UTR) of human RABGGTA was found to be similar to that of the mouse Rabggta gene. However, exons alpha and introns alpha are not homologous between mouse and human. Features of the 5'-UTR of RABGGTA suggest it is a housekeeping gene. While obvious disease-causing mutations of human RABGGTA were not found in our existing SPD patients by sequencing its entire coding region, several polymorphisms of RABGGTA including a putative cryptic splicing mutation in intron 4 were identified. Knowledge of the 5'-UTR structure of RABGGTA and its common polymorphisms will be useful for mutation screening or linkage analysis in patients with albinism, thrombocytopenia, or platelet SPD.

Gender-related differences in thrombogenic factors predicting recurrent cardiac events in patients after acute myocardial infarction. The THROMBO Investigators

Thrombosis contributes to recurrent coronary events in patients after acute myocardial infarction (AMI), but prognostic significance of thrombogenic factors by gender is unknown. This study aimed to determine gender-related differences in the prognostic significance of thrombogenic factors for predicting cardiac events (nonfatal reinfarction or cardiac death) in postinfarction patients. Blood levels of the following factors were measured 2 months after AMI in 791 men and 254 women: fibrinogen, von Willebrand factor, factor VII and VIIa, plasminogen activator inhibitor, D-dimer, cholesterol, apolipoprotein A-1, apolipoprotein B, lipoprotein(a), triglycerides, and high-density lipoprotein cholesterol. After adjustment for clinical covariates, levels of apolipoprotein A, high-density lipoprotein cholesterol, fibrinogen, and factor VIIa were significantly higher in postinfarction women than men. During a mean 26-month follow-up, there were 67 cardiac events (8.5%) in men and 14 (5.5%) in women (p = 0.11). In the multivariate Cox model, elevated levels of factor VIIa were a significant predictor of cardiac events in women (p = 0.022) but not in men (p = 0.80), with significant gender-related effect (hazard ratio 2.80 vs 0.92, respectively; p <0.05). D-dimer had prognostic value in men (p = 0. 006) but not in women (p = 0.36), although the difference between hazard ratios for men and women was not significant (2.35 vs 1.58, respectively; p = 0.49). In conclusion, elevated levels of factor VIIa are associated with an increased risk of recurrent cardiac events in postinfarction women, but not in men. D-dimer is more predictive for cardiac events in postinfarction men than women. These observations indicate possible gender-related differences in the pathophysiologic mechanisms of recurrent cardiac events.

Thrombogenic factors and recurrent coronary events

BACKGROUND

Thrombosis is a pivotal event in the pathogenesis of coronary disease. We hypothesized that the presence of blood factors that reflect enhanced thrombogenic activity would be associated with an increased risk of recurrent coronary events during long-term follow-up of patients who have recovered from myocardial infarction.

METHODS AND RESULTS

We prospectively enrolled 1045 patients 2 months after an index myocardial infarction. Baseline thrombogenic blood tests included 6 hemostatic variables (D-dimer, fibrinogen, factor VII, factor VIIa, von Willebrand factor, and plasminogen activator inhibitor-1), 7 lipid factors [cholesterol, triglycerides, HDL cholesterol, LDL cholesterol, lipoprotein(a), apolipoprotein (apo)A-I, and apoB], and insulin. Patients were followed up for an average of 26 months, with the primary end point being coronary death or nonfatal myocardial infarction, whichever occurred first. The hemostatic, lipid, and insulin parameters were dichotomized into their top and the lower 3 risk quartiles and evaluated for entry into a Cox survivorship model. High levels of D-dimer (hazard ratio, 2.43; 95% CI, 1.49, 3.97) and apoB (hazard ratio, 1.82; 95% CI, 1.10, 3.00) and low levels of apoA-I (hazard ratio, 1.84; 95% CI, 1.10, 3.08) were independently associated with recurrent coronary events in the Cox model after adjustment for 6 relevant clinical covariates.

CONCLUSIONS

Our findings indicate that a procoagulant state, as reflected in elevated levels of D-dimer, and disordered lipid transport, as indicated by low apoA-1 and high apoB levels, contribute independently to recurrent coronary events in postinfarction patients.

Secreted dense granule adenine nucleotides promote calcium influx and the maintenance of elevated cytosolic calcium levels in stimulated human platelets

Evidence that secreted dense granule adenine nucleotides mediate part of the agonist-induced cytosolic calcium ([Ca2+]i) responses in human platelets was obtained from comparisons of fura-2-loaded platelets from normal subjects and from patients with a form of platelet storage pool deficiency (SPD) in which the secretory dense granules and their contents are virtually absent. SPD platelets had normal initial [Ca2+]i increases induced by thrombin and the endoperoxide analog U46619, but a significantly enhanced decay of elevated [Ca2+]i levels following the initial increases. With thrombin, this enhanced [Ca2+]i decay was associated with decreased Ca2+ influx, as measured by Mn2+ quench of fura-2 fluorescence. Addition of micromolar concentrations of ADP, alone or together with ATP, after stimulation reversed the enhanced [Ca2+]i decay and increased Mn2+ quench in SPD platelets, but had no effect on these responses in normal platelets, while addition of 100-fold higher concentrations of ATP or apyrase before stimulation increased [Ca2+]i decay and decreased Mn2+ quench in normal platelets, but had little effect in SPD platelets. ATP and alpha,beta-methylene ATP, a specific agonist for P2X1 receptors, at micromolar concentrations also increased Mn2+ quench, but to lesser extents than did ADP, in SPD platelets isolated and loaded with fura-2 in the presence of apyrase. Similar effects of ADP and excess ATP were seen in U46619-stimulated platelets, but decreased Ca2+ influx could not be measured directly in SPD platelets, presumably due to the very transient influx response seen with U46619. These results suggest that secreted dense granule ADP and ATP contribute to the maintenance of elevated [Ca2+]i levels, but not to the initial [Ca2+]i increases, in stimulated human platelets, most likely via a nucleotide-specific component of Ca2+ influx which may be mediated by interactions with both P2X1 and P2Y1 purinoceptors.

Impaired Ca2+-induced tyrosine phosphorylation and defective lipid scrambling in erythrocytes from a patient with Scott syndrome: a study using an inhibitor for scramblase that mimics the defect in Scott syndrome

Scott syndrome is an hereditary bleeding disorder characterized by a deficiency in platelet procoagulant activity. Unlike normal blood cells, Scott platelets, as well as erythrocytes and lymphocytes, are strongly impaired in their ability to scramble their membrane phospholipids when challenged with Ca2+. In normal cells this collapse of membrane asymmetry leads to surface exposure of phosphatidylserine. Here we report that Scott erythrocytes show an apparent defect in tyrosine phosphorylation on treatment with Ca2+-ionophore. Diminished tyrosine phosphorylation was also apparent in activated Scott platelets, but much less pronounced than observed in red blood cells. On the other hand, tyrosine phosphorylation profiles observed in Scott red blood cell ghosts after sealing in the presence of adenosine triphosphate (ATP) were indistinguishable from those obtained from normal ghosts. Several observations argue in favor of a mechanism in which tyrosine phosphorylation in red blood cells is facilitated by, rather than required for scrambling of membrane lipids. Staurosporin blocks tyrosine phosphorylation in normal red blood cells, but does not inhibit the lipid scrambling process. White ghosts from normal erythrocytes, resealed in the absence of ATP, exhibit Ca2+-induced lipid scrambling without tyrosine phosphorylation. A selective inhibitor of Ca2+-induced lipid scrambling also showed an apparent inhibition of tyrosine phosphorylation in ionophore-treated normal red blood cells, similar to that observed in Scott erythrocytes. While this inhibitor also suppressed Ca2+-induced lipid scrambling in ghosts that were sealed in the presence of ATP, it did not inhibit tyrosine kinase activity. We conclude that the apparent deficiency in tyrosine phosphorylation in Scott cells is an epiphenomenon, possibly associated with a defect in phospholipid scrambling, but not causal to this defect.

Glycoprotein IIb Leu214Pro mutation produces glanzmann thrombasthenia with both quantitative and qualitative abnormalities in GPIIb/IIIa

Glanzmann thrombasthenia is an inherited bleeding disorder due to a functional reduction or absence of platelet GPIIb/IIIa (alphaIIbbeta3) integrin receptors. Based on a prolonged bleeding time and absence of platelet aggregation in response to physiologic agonists, a 55-year-old white man was diagnosed as having Glanzmann thrombasthenia. The patient's platelet fibrinogen level was approximately 5% of normal. As judged by complex-dependent monoclonal antibody (MoAb) binding, surface expression of platelet GPIIb/IIIa receptors was less than 5.5% of normal, whereas the binding of an anti-GPIIIa specific MoAb (7H2) was approximately 12% of normal. Immunoblot analysis of the patient's platelet lysates showed approximately 35% of normal levels of GPIIIa, approximately 30% of normal levels of GPIIb, and an abnormally migrating fragment of GPIIb. Biotinylation of the surface proteins on the patient's platelets followed by immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed only GPIIb and GPIIIa subunits of normal size. Surface expression of platelet alphavbeta3 receptors was 192% of normal, suggesting that the patient's' defect was in GPIIb. Sequence analysis of the patient's GPIIb cDNA identified a T to C transition at nucleotide 643, predicting a Leu214Pro substitution. Direct sequencing of GPIIb exon 6 indicated that the patient is homozygous for the mutation. The nature of the Leu214Pro mutation was analyzed by expression in Chinese hamster ovary (CHO) cells. As judged by subunit-specific MoAb binding, surface expression of mutant receptors was approximately 60% of normal, but these receptors were not recognized by the complex-dependent monoclonal antibodies, 10E5 and 7E3. In addition, mutant receptors pretreated with the ligand-induced binding site MoAb AP5 were not recognized by the activation-dependent MoAb PAC-1 and mutant expressing CHO cells did not adhere to immobilized fibrinogen. These data suggest that the Leu214Pro mutation in GPIIb disrupts the structural conformation, and either directly or indirectly, the ligand binding properties of the heterodimeric complex. This is in accord with studies from other integrins that have implicated a beta-turn in a homologous region as important in ligand binding. Thus, the Leu214Pro mutation appears to produce the Glanzmann thrombasthenia phenotype by both qualitative and quantitative abnormalities. In addition, the mutation appears to confer susceptibility of the GPIIb subunit to proteolysis.

Scott syndrome erythrocytes contain a membrane protein capable of mediating Ca2+-dependent transbilayer migration of membrane phospholipids

Phospholipid (PL) scramblase is a plasma membrane protein that mediates accelerated transbilayer migration of PLs upon binding Ca2+, facilitating rapid mobilization of phosphatidylserine to the cell surface upon elevation of internal Ca2+. In patients with Scott syndrome, a congenital bleeding disorder related to defective expression of membrane coagulant activity, circulating blood cells show decreased cell surface exposure of phosphatidylserine at elevated cytosolic [Ca2+], implying an underlying defect or deficiency of PL scramblase. To gain insight into the molecular basis of this disorder, we compared PL scramblase in Scott erythrocyte membranes to those of normal controls. Whereas membranes of Scott cells were unresponsive to Ca2+-induced activation of PL scramblase at neutral pH, apparently normal PL scramblase activity was induced at pH < 6.0. After extraction with octylglucoside, a membrane protein was isolated from the Scott cells which exhibited normal PL scramblase activity when reconstituted in vesicles with exogenous PLs. Like PL scramblase from normal erythrocytes, PL scramblase from Scott erythrocytes was maximally activated either by addition of Ca2+ (at pH 7.4) or by acidification to pH < 6.0, and similar apparent affinities for Ca2+ and rates of transbilayer transfer of PLs were observed. This suggests that the defect in Scott syndrome is related to an altered interaction of Ca2+ with PL scramblase on the endofacial surface of the cell membrane, due either to an intrinsic constraint upon the protein preventing interaction with Ca2+ in situ, or due to an unidentified inhibitor or cofactor in the Scott cell that is dissociated by detergent.

Platelet alpha granule deficiency associated with decreased P-selectin and selective impairment of thrombin-induced activation in a new patient with gray platelet syndrome (alpha-storage pool deficiency)

We report studies on a new patient with gray platelet syndrome (GPS, alpha-storage pool deficiency). Her lifelong bleeding history is associated with platelet abnormalities characteristic of GPS including mild to moderate thrombocytopenia, a population of abnormally large platelets, and specific deficiencies of alpha-granule constituents and morphologically typical alpha-granules. Platelet function studies showed normal aggregation responses to adenosine diphosphate, epinephrine, collagen, arachidonate, and ristocetin but impaired activation responses to thrombin and a thrombin receptor-activating peptide (T1 peptide). These impaired responses included T1 peptide-induced aggregation, thrombin-induced adenine nucleotide secretion, and thrombin-induced (Ca2+)i increases. The impairment of the thrombin-induced (Ca2+)i increase was observed as a substantially slower initial rise in (Ca2+)i levels and a smaller maximum (Ca2+)i increase compared with the responses obtained in normal platelets and are thus similar to those reported previously in another patients with GPS. Flow cytometric measurements of the binding of two distinct monoclonal antibodies against the functional thrombin receptor indicated the presence of a normal number of receptors and normal receptor cleavage by thrombin in the GPS platelets, providing additional support for the hypothesis presented in previous studies that the thrombin activation defect in GPS platelets occurs subsequent to the interaction of thrombin with its receptor. The alpha-granule deficiency in this patient was associated with an approximately 50% decrease in the content and surface expression of the alpha-granule membrane-specific protein P-selectin in contrast to a previous report of normal amounts of P-selectin in the platelets of two related patients with GPS. This finding raises the possibility that the alpha-granule deficiency in GPS may be expressed in different phenotypes characterized by differences in the amount or constitution of residual alpha-granule membranes present in GPS platelets.

Platelet prothrombinase activity and intracellular calcium responses in patients with storage pool deficiency, glycoprotein IIb-IIIa deficiency, or impaired platelet coagulant activity--a comparison with Scott syndrome

The procoagulant activity of platelets induced by collagen, thrombin, and collagen plus thrombin, measured as their capacity to promote the conversion of prothrombin to thrombin in the presence of factors Va and Xa, was studied in patients with alpha, alpha delta, and delta storage pool deficiency (SPD), thrombasthenia, and in two new patients with isolated defects in platelet coagulant activity, and compared with that in Scott syndrome. The most significant abnormality in the new patients, whose defect may differ from that in Scott syndrome, is an impairment in collagen plus thrombin-induced prothrombinase activity in the absence of added factor Va. In one of these patients this may be caused by an abnormality in platelet alpha-granule factor V distinct from that described for factor V Quebec, alpha delta-SPD, or alpha-SPD (gray platelet syndrome). Prothrombinase activity in response to all agonists was impaired in delta-SPD and was associated with an inability of these platelets to maintain elevated intracellular calcium levels. Both the rapid decline in agonist-induced [Ca2+]i levels and the impaired prothrombinase activation in delta-SPD platelets were corrected by the addition of adenosine diphosphate (ADP) after stimulation. These findings suggest that secreted ADP may play an important role in the generation of prothrombinase activity by contributing to the maintenance of a critical [Ca2+]i level necessary to maintain aminophospholipids on the outer surface of the platelet membrane, and provide evidence that dense granules may be a major source of ADP which can contribute to calcium influx in stimulated platelets. Parallel alterations, including both increases and decreases, in the [Ca2+]i and prothrombinase responses were also observed in thrombasthenia, depending on the agonist and stirring conditions. Both responses were increased in collagen-stimulated, unstirred platelets, whereas an inability to maintain increased [Ca2+]i levels, associated with decreased prothrombinase activity in all but one atypical patient, was seen in stirred collagen plus thrombin-activated platelets. Although the parallel alterations in these responses in thrombasthenia, as in SPD, further show the close association between the generation of prothrombinase activity and the maintenance of increased intracellular Ca2+ levels, the specific role that GPIIb-IIIa may play in both these events remains unresolved. Our findings of both enhancement and inhibition of these activation-related events in thrombasthenic platelets may be related to previous conflicting reports on the promotion or inhibition of fibrin formation by GPIIb-IIIa, and could be relevant to the use of specific inhibitors of GPIIb-IIIa as antithrombotic agents. In addition, the study provides further support for the concept that the development of agents that could induce a Scott syndrome defect in normal platelets may provide a new approach to antithrombotic therapy.

Enhanced increases in cytosolic Ca2+ in ADP-stimulated platelets from patients with delta-storage pool deficiency--a possible indicator of interactions between granule-bound ADP and the membrane ADP receptor

The possible involvement of secreted platelet substances in agonist-induced [Ca2+]i increases was investigated by comparing these increases in aspirin-treated, fura-2-loaded normal platelets and platelets from patients with storage pool deficiencies (SPD). In the presence and absence of extracellular calcium, the [Ca2+]i response induced by 10 microM ADP, but not those induced by 0.1 unit/ml thrombin, 3.3 microM U46619, or 20 microM serotonin, was significantly greater in SPD platelets than in normal platelets, and was increased to the greatest extent in SPD patients with Hermansky-Pudlak syndrome (HPS), in whom the dense granule deficiencies are the most severe. Pre-incubation of SPD-HPS and normal platelets with 0.005-5 microM ADP produced a dose-dependent inhibition of the [Ca2+]i response induced by 10 microM ADP, but did not alter the [Ca2+]i increases induced by thrombin or U46619. Within a limited range of ADP concentrations, the dose-inhibition curve of the [Ca2+]i response to 10 microM ADP was significantly shifted to the right in SPD-HPS platelets, indicating that pre-incubation with greater amounts of ADP were required to achieve the same extent of inhibition as in normal platelets. These results are consistent with a hypothesis that the smaller ADP-induced [Ca2+]i increases seen in normal platelets may result from prior interactions of dense granule ADP, released via leakage or low levels of activation, with membrane ADP receptors, causing receptor desensitization. Addition of apyrase to platelet-rich plasma prior to fura-2 loading increased the ADP-induced [Ca2+]i response in both normal and SPD-HPS platelets, suggesting that some release of ADP derived from both dense granule and non-granular sources occurs during in vitro fura-2 loading and platelet washing procedures. However, this [Ca2+]i response was also greater in SPD-HPS platelets when blood was collected with minimal manipulation directly into anticoagulant containing apyrase, raising the possibility that release of dense granule ADP resulting in receptor desensitization may also occur in vivo. Thus, in addition to enhancing platelet activation, dense granule ADP could also act to limit the ADP-mediated reactivity of platelets exposed in vivo to low levels of stimulation.

Correction of the platelet adhesion defect in delta-storage pool deficiency at elevated hematocrit--possible role of adenosine diphosphate

Previous studies on patients with storage pool deficiency (SPD) who are specifically deficient in platelet dense granules (delta-SPD) have suggested a role for dense granule substances, in all likelihood adenosine diphosphate (ADP), in mediating thrombus formation on subendothelium at high shear rates. The role of dense granule substances in mediating platelet adhesion appears to be more complicated Previous studies in delta-SPD suggested an adhesion defect that was strongly influenced by the patient's hematocrit (Hct) value. To explore further the possibility that red blood cells (RBCs) may influence the role that platelet storage granules play in mediating adhesion at high shear rates, we have measured adhesion (and thrombus formation) throughout a preselected range of Hct values (30% to 60%) in normal subjects and in patients with delta-SPD. The present studies confirm the defect in platelet adhesion in patients with delta-SPD, most significantly at Hct values of 30% to 40%. This defect (but not that of thrombus formation) can be completely corrected by the addition of RBCs. The correction of the platelet adhesion defect by RBCs was specific for delta-SPD; it was not observed in either von Willebrand's disease or thrombasthenia. Studies performed on normal blood under conditions that could be expected to block any effect of ADP on adhesion and an analysis of the type of adhesion defect in delta-SPD suggest that ADP may be involved in the process required for platelet spreading on the subendothelium. The corrective effect of RBCs on platelet adhesion in delta-SPD appears to be chemical rather than physical in nature, possibly due to shear-induced release of RBC ADP or to other recently described properties of RBCs that enhance collagen-induced platelet interactions.

The complex of phosphatidylinositol 4,5-bisphosphate and calcium ions is not responsible for Ca2+-induced loss of phospholipid asymmetry in the human erythrocyte: a study in Scott syndrome, a disorder of calcium-induced phospholipid scrambling

Elevation of cytoplasmic Ca2+ levels in human erythrocytes induces a progressive loss of membrane phospholipid asymmetry, a process that is impaired in erythrocytes from a patient with Scott syndrome. We show here that porcine erythrocytes are similarly incapable of Ca2+-induced redistribution of membrane phospholipids. Because a complex of phosphatidylinositol 4,5-bisphosphate (PIP2) and Ca2+ has been proposed as the mediator of enhanced transbilayer movement of lipids (J Biol Chem 269:6347,1994), these cell systems offer a unique opportunity for testing this mechanism. Analysis of both total PIP2 content and the metabolic-resistant pool of PIP2 that remains after incubation with Ca2+ ionophore showed no appreciable differences between normal and Scott erythrocytes. Moreover, porcine erythrocytes were found to have slightly higher levels of both total and metabolic-resistant PIP2 in comparison with normal human erythrocytes. Although loading of normal erythrocytes with exogenously added PIP2 gave rise to a Ca2+-induced increase in prothrombinase activity and apparent transbilayer movement of nitrobenzoxadiazolyl (NBD)-phospholipids, these PIP2-loaded cells were also found to undergo progressive Ca2+-dependent cell lysis, which seriously hampers interpretation of these data. Moreover, loading Scott cells with PIP2 did not abolish their impaired lipid scrambling, even in the presence of a Ca2+-ionophore. Finally, artificial lipid vesicles containing no PIP2 or 1 mole percent of PIP2 were indistinguishable with respect to transbilayer movement of NBD-phosphatidylcholine in the presence of Ca2+. Our findings suggest that Ca2+-induced redistribution of membrane phospholipids cannot simply be attributed to the steady-state concentration of PIP2, and imply that such lipid movement is regulated by other cellular processes.