Release Of Dense Granule Contents Research Articles

Mechanism of Abnormal Bleeding in Patients Undergoing Cardiopulmonary Bypass: Acquired Transient Platelet Dysfunction Associated With Selective α-Granule Release

The hemostatic alterations underlying the occasional bleeding diathesis associated with cardiopulmonary bypass surgery have been defined in 31 selected patients by simultaneously measuring the serial changes in concentration, function and kinetics of platelets, coagulation factors, and fibrinolytic components. The effects of uncomplicated bypass surgery were evaluated in 21 elective patients. Immediately upon bypass, the circulating levels of platelets, coagulation factors, and plasminogen regularly fell about 50% due to dilution with nonblood prime in the oxygenator apparatus. However, none of these components was reduced below established hemostatic levels at any time during or following the procedure. Furthermore, the functional transformation of fibrinogen to fibrin was not measurably impaired. Heparin effectively blocked both fibrinolysis and fibrinogen consumption during bypass and was consistently neutralized with protamine without rebound at the end of bypass. Circulating platelets were uniformly activated in elective patients during bypass through a process that involved release and partial depletion of α-granules, but not release of dense granule constituents. Platelet activation was associated with transient marked impairment of function as evidenced by striking prolongation of the bleeding time and defective aggregation in vitro. In uncomplicated patients, platelet function largely normalized within 1 hr after bypass. Controlled studies in baboons demonstrated that platelet dysfunction was produced by either cardiopulmonary bypass under normothermic conditions or by hypothermia alone without bypass. In the baboon model, defective platelet function was also transient and characterized by selective depletion of platelet a-granule contents without release of dense granule contents. By contrast, in all of 10 patients in whom bypass surgery was complicated by substantial abnormal bleeding, there was persistent bleeding time prolongation to greater than 25 min that persisted for hours after bypass despite platelet counts above 100,000/µl. With platelet transfusions, the bleeding time shortened rapidly, followed by cessation of bleeding. From these studies we conclude that cardiopulmonary bypass patients demonstrate abnormal clinical bleeding when there is a persistence of platelet dysfunction manifested as a bleeding time of greater than 20 min after protamine administration. Such bleeding patients with a prolonged bleeding time should receive platelet transfusions even when the platelet count is greater than 100,000/µl.

Proteomic analysis of platelet alpha-granules using mass spectrometry.

Platelets have three major types of secretory organelles: lysosomes, dense granules, and alpha-granules. alpha-Granules contain several adhesive proteins involved in hemostasis, as well as glycoproteins involved in inflammation, wound healing, and cell-matrix interactions. This article represents the first effort to define the platelet alpha-granule proteome using mass spectrometry (MS). We prepared a subcellular fraction enriched in intact alpha-granules from human platelets using sucrose gradient ultracentrifugation. alpha-Granule proteins were separated and identified using sodium dodecylsulfate polyacrylamide gel electrophoresis and liquid chromatography-tandem MS. In the sucrose fraction enriched in alpha-granules, we identified 284 non-redundant proteins, 44 of which appear to be new alpha-granule proteins, on the basis of a literature review. Immunoelectron microscopy confirmed the presence of Scamp2, APLP2, ESAM and LAMA5 in platelet alpha-granules for the first time. We identified 65% of the same proteins that were detected in the platelet releasate (J. A. Coppinger et al. [Blood 2004;103: 2096-104]) as well as additional soluble and membrane proteins. Our method provides a suitable tool for analyzing the granule proteome of patients with storage pool deficiencies.

Dynamic Alterations in the Expression of P2Y12 Receptor but Not COX-1 in Platelets from Streptozocin-Induced Diabetic Rats.

In patients or animals with established diabetes mellitus, platelets have been shown to be hyperreactive to ADP. This alteration of the platelet function occurs independent of activation of the arachidonate pathway or release of dense granule contents. In the present study, we examined the expression of the platelet ADP-binding receptor, P2Y12, in early stage of streptozocin(STZ)-induced diabetes in rats. Platelet messenger RNA (mRNA) levels for P2Y12 and cyclooxygenase-1 (COX-1) were determined by comparative RT-PCR in 7 control rats on day 0, 6 diabetic rats each on days 2, 3, 4, and 4 diabetic rats on day 7 after intraperitoneal injection of streptozocin (50 mg/kg). The plasma glucose level was 406 ± 10 mg/dl in diabetic rats, significantly greater than 151 ± 12 mg/dl in control rats. After induction of diabetes, the expression of P2Y12 mRNA significantly decreased to 76 ± 9% of the level of control on day 2, 54 ± 6% of control on day 3, reached a nadir of 28 ± 6% of control on day 4, and rose to 43 ± 10% of control on day 7. These dynamic changes of P2Y12 mRNA in platelets reflected the expression of P2Y12 mRNA in megakaryocytes isolated to a high state of purity (>96 %) from rat bone marrow. Specifically, the megakaryocytic P2Y12 mRNA expression in the diabetic rats on days 1, 2 and 3 were 74 ± 8 %, 93 ± 1%, and 117 ± 6% of those in control rats, respectively. Treatment of diabetic rats with insulin reduced the trend of decline in the platelet P2Y12 expression. By contrast, the platelet COX-1 mRNA expression measured on days 2, 3, 4 and 7 after the induction of diabetes was similar to that in the control rats. These results in diabetic rats demonstrate dynamic alterations of the mRNA expression of megakaryocyte-platelet P2Y12, but not of COX-1. The decrease in the P2Y12 receptor expression in early stage of STZ-induced diabetes may serve to attenuate the hyperaggregability of platelets and reduce the risk of vascular thrombosis.

Akt Activation in Platelets Depends on Gi Signaling Pathways

The serine-threonine kinase Akt has been established as an important signaling intermediate in regulating cell survival, cell cycle progression, as well as agonist-induced platelet activation. Stimulation of platelets with various agonists including thrombin results in Akt activation. As thrombin can stimulate multiple G protein signaling pathways, we investigated the mechanism of thrombin-induced activation of Akt. Stimulation of platelets with a PAR1-activating peptide (SFLLRN), PAR4-activating peptide (AYPGKF), and thrombin resulted in Thr308 and Ser473 phosphorylation of Akt, which results in its activation. This phosphorylation and activation of Akt were dramatically inhibited in the presence of AR-C69931MX, a P2Y12 receptor-selective antagonist, or GF 109203X, a protein kinase C inhibitor, but Akt phosphorylation was restored by supplemental Gi or Gz signaling. Unlike wild-type mouse platelets, platelets from Galphaq-deficient mice failed to trigger Akt phosphorylation by thrombin and AYPGKF, whereas Akt phosphorylation was not affected by these agonists in platelets from mice that lack P2Y1 receptor. However, ADP caused Akt phosphorylation in Galphaq- and P2Y1-deficient platelets, which was completely blocked by AR-C69931MX. In contrast, ADP failed to cause Akt phosphorylation in platelets from mice treated with clopidogrel, and thrombin and AYPGKF induced minimal phosphorylation of Akt, which was not affected by AR-C69931MX in these platelets. These data demonstrate that Gi, but not Gq or G12/13, signaling pathways are required for activation of Akt in platelets, and Gi signaling pathways, stimulated by secreted ADP, play an essential role in the activation of Akt in platelets.

Targeting of tissue plasminogen activator into the regulated secretory pathway of neuroendocrine cells ,

Plasma levels of tissue plasminogen activator (tPA) increase rapidly in response to specific vasoactive agents, trauma, and neural stimulation. This response has been attributed to acute release of tPA from stored pools within the vascular endothelium and from catecholamine storage vesicles of chromaffin cells. We have tested directly whether tPA can be sorted into the regulated secretory pathway using the murine pituitary-derived neuroendocrine cell line AtT-20 transfected with tPA cDNA. Clones of AtT-20 cells expressing tPA were isolated, and targeting of tPA into the regulated secretory pathway was demonstrated by (1) stimulation of tPA secretion with 8-bromo-cAMP, the secretagogue which promotes the release of dense granule contents; (2) colocalization with ACTH, an endogenous protein that is stored in dense core granules; and (3) retention of newly synthesized tPA in the cell for prolonged periods of time. Laser scanning confocal microscopy analysis of cells immunostained with antibodies to tPA and ACTH showed colocalization at the tips of the neuritic processes under the cytoplasmic membrane, a region where dense granules are known to migrate after maturation. Treatment of the cells with 5 mM 8-bromo-cAMP for 30 min resulted in a 2.41±0.36-fold increase in tPA secretion. Both the magnitude of the stimulatory effect and the fraction of the intracellular tPA released were the same regardless of the tPA expression level in the various clones. Pulse-chase experiments showed that a portion of newly synthesized tPA is retained in the cell for at least 4 h and is released into the culture medium in response to 8-bromo-cAMP. These studies indicate that tPA, under the appropriate conditions, can be targeted into the regulated secretory pathway and can be stored for later release by cellular stimuli.

The interaction of ancrod with human platelets

Ancrod, a serine protease purified from the venom of Agkistrodon rhodostoma, has been used as a therapeutic anticoagulant for a number of indications, including replacement of heparin in patients with heparin-induced thrombocytopenia. Ancrod has similar fibrinolytic activity to thrombin, but ancrod specifically cleaves only the alpha chain of fibrinogen, producing the characteristic fibrinopeptides A, AP and AY. Because ancrod has been used in patients with heparin-induced thrombocytopenia, it is important to ensure that ancrod does not directly affect the platelets and potentially increase the hemostatic effect. The effect of ancrod on platelets has not been well established, and there is not agreement in published studies. Additionally, some of the studies are over 15 years old and pre-date sensitive assays such as glycoprotein analysis. For these reasons, we investigated the interaction of ancrod with human platelets using direct and indirect, functional and biochemical techniques. Incubation of platelets with ancrod alone did not induce platelet aggregation or the release of dense-granule contents. Pre-incubation of platelets with ancrod did not augment or inhibit the maximal aggregation achieved with thrombin, nor did it affect the amount of serotonin release from dense granules caused by activation by thrombin. Studies of ancrod-treated platelets using monoclonal antibody-mediated radioimmunoprecipitation demonstrated that high concentrations of ancrod did not cause measurable cleavage of either the glycoproteins Ib-IX or IIb-IIIa. Incubation of radiolabeled platelets with ancrod-treated plasma also had no effect on the platelet glycoproteins, indicating that ancrod does not indirectly affect the major surface receptors. Direct binding studies using radiolabeled ancrod did not demonstrate specific binding to the platelet surface. Together these studies indicate that ancrod does not directly affect nor bind to platelets in vitro . The hypocoagulant state and subsequent platelet function defect resulting from the use of ancrod appears to be limited to the removal of fibrinogen from the circulation.

Role of chondroitin 4‐sulphate as a receptor for polycation induced human platelet aggregation

1. Proteoglycans provide negatively charged sites on the surface of platelets, leukocytes and endothelial cells. Since chondroitin 4-sulphate is the main proteoglycan present on the platelet surface, the role of this molecule in mediating the activation of human platelets by polylysine was studied. 2. Platelets were desensitized with phorbol 12-myristate 13-acetate (PMA, 10 nM) 5 min before the addition of polylysine to platelet-rich plasma (PRP). Changes in the intracellular Ca2+ concentration were measured in fura2-am (2 microM) loaded platelets and protein phosphorylation was assessed by autoradiography of the electrophoretic profile obtained from [32P]-phosphate labelled platelets. The release of dense granule contents was measured in [14C]-5-hydroxytryptamine loaded platelets and the synthesis of thromboxane (TXA2) was assessed by radioimmunoassay. Surface chondroitin 4-sulphate proteoglycan was degraded by incubating platelets with different concentrations of chondroitinase AC (3 min, 37 degrees C). The amount of chondroitin 4-sulphate remaining in the platelets was then quantified after proteolysis and agarose gel electrophoresis. 3. The addition of PMA to PRP before polylysine inhibited the aggregation by 88 +/- 18% (n = 3). Staurosporine (1 microM, 5 min) prevented the PMA-induced inhibition. Chondroitinase AC (4 pu ml-1 to 400 muu ml-1, 3 min) abolished the polylysine-induced aggregation in PRP but caused only a discrete inhibition of ADP-induced aggregation. The concentration of chrondroitin 4-sulphate in PRP (0.96 +/- 0.2 microgram/10(8) platelets, n = 3) and in washed platelets (WP; 0.35 +/- 0.1 microgram/10(8) platelets, n = 3) was significantly reduced following incubation with chondroitinase AC (PRP = 0.63 +/- 0.1 microgram/10(8) platelets and WP = 0.08 +/- 0.06 microgram/10(8) platelets). 4. Washed platelets had a significantly lower concentration of chondroitin 4-sulphate than platelets in PRP. The addition of polylysine to WP induced a rapid increase in light transmission which was not accompanied by TXA2 synthesis or the release of dense granule contents. This effect was not inhibited by sodium nitroprusside (SNP), iloprost, EDTA or the peptide RGDS. This event was accompanied by the discrete phosphorylation of plekstrin and myosin light chain, which were inhibited by staurosporine (10 microM, 10 min). The hydrolysis of platelet surface chondroitin 4-sulphate strongly reduced the polylysine-induced phosphorylation. 5. Our results indicate that polylysine activates platelets through a specific receptor which could be the proteoglycan chondroitin 4-sulphate present on the platelet membrane.

Conditions influencing release of granule contents from human platelets in citrated plasma induced by ADP or the thrombin receptor activating peptide SFLLRN: Direct measurement of percent release of β‐thromboglobulin and assessment by flow cytometry of P‐selectin expression

Contrary to a recent report [Rinder et al.: Blood 82:505, 1993], aspirin does inhibit the release of α-granule contents as well as inhibiting the release of dense granule contents by human platelets during ADP-induced aggregation in citrated platelet-rich plasma (PRP). Measurements were: percent release of 14C-serotonin from prelabeled platelets, radio-immunoassay of β-thromboglobulin (βTG), and expression on the platelet surface of the α-granule constituent, P-selectin, by flow cytometry. During the second phase of ADP-induced aggregation, 69.0 ± 8.3% of βTG and 54.1 ± 4.6% of 14C-serotonin were released (means ± SEM, n = 13); aspirin treatment reduced these values to 6.0 ± 1.2 and 1.0 ± 0.3%, respectively. In contrast, incubation of platelets with ADP without stirring caused only 6.7 ± 1.7% release of βTG and 2.1 ± 0.4% release of 14C-serotonin; these low values were not appreciably affected by aspirin. During ADP-induced primary aggregation in PRP anticoagulated with FPRCH2Cl (PPACK), only 4.7 ± 0.9% release of βTG and no detectable release of 14C-serotonin occurred; aspirin had no effect. In both stirred and unstirred PRP, the thrombin receptor activating peptide, SFLLRN (50 μM), caused at least 75% release of the contents of both granules, which was partially inhibited by aspirin. Upon incubation of platelets with ADP (2–10 μM), the mean fluorescence intensity due to P-selectin was <14% of that induced by SFLLRN. In this unstirred system used for flow cytometry, aspirin treatment caused no significant inhibition of P-selectin expression. Thus, under conditions in which ADP does not cause secondary aggregation (physiological Ca2??? concentration or unstirred citrated PRP) release of the contents of both types of granules is less than 7% and aspirin is not inhibitory; the P-selectin expression associated with this low percent release is also unaffected by aspirin. However, aspirin does strongly inhibit the extensive release of both α-granule and dense granule contents during ADP-induced secondary aggregation in citrated PRP. © 1996 Wiley-Liss, Inc.

Conditions influencing release of granule contents from human platelets in citrated plasma induced by ADP or the thrombin receptor activating peptide SFLLRN: direct measurement of percent release of beta-thromboglobulin and assessment by flow cytometry of P-selectin expression.

Contrary to a recent report [Rinder et al.: Blood 82:505, 1993], aspirin does inhibit the release of alpha-granule contents as well as inhibiting the release of dense granule contents by human platelets during ADP-induced aggregation in citrated platelet-rich plasma (PRP). Measurements were: percent release of 14C-serotonin from prelabeled platelets, radio-immunoassay of beta-thromboglobulin (beta TG), and expression on the platelet surface of the alpha-granule constituent, P-selectin, by flow cytometry. During the second phase of ADP-induced aggregation, 69.0 +/- 8.3% of beta TG and 54.1 +/- 4.6% of 14C-serotonin were released (mean +/- SEM, n = 13); aspirin treatment reduced these values to 6.0 +/- 1.2 and 1.0 +/- 0.3%, respectively. In contrast, incubation of platelets with ADP without stirring caused only 6.7 +/- 1.7% release of beta TG and 2.1 +/- 0.4% release of 14C-serotonin; these low values were not appreciably affected by aspirin. During ADP-induced primary aggregation in PRP anticoagulated with FPRCH2CI (PPACK), only 4.7 +/- 0.9% release of beta TG and no detectable release of 14C-serotonin occurred; aspirin had no effect. In both stirred and unstirred PRP, the thrombin receptor activating peptide, SFLLRN (50 microM), caused at least 75% release of the contents of both granules, which was partially inhibited by aspirin. Upon incubation of platelets with ADP (2-10 microM), the mean fluorescence intensity due to P-selectin was < 14% of that induced by SFLLRN. In this unstirred system used for flow cytometry, aspirin treatment caused no significant inhibition of P-selectin expression. Thus, under conditions in which ADP does not cause secondary aggregation (physiological Ca2+ concentration or unstirred citrated PRP) release of the contents of both types of granules is less than 7% and aspirin is not inhibitory; the P-selectin expression associated with this low percent release is also unaffected by aspirin. However, aspirin does strongly inhibit the extensive release of both alpha-granule and dense granule contents during ADP-induced secondary aggregation in citrated PRP.

Differential secretion of blood platelet storage granules

Platelets contain three types of secretory granules, dense granules, α-granules and lysosomes, which are characterized by their different contents. Dense granule and α-granule secretion appear to be similar in responsiveness to dose and types of agonists, whereas lysosomal secretion is observed only with higher doses of strong agonists such as thrombin. Recently, with the advent of flow cytometry, surface expression of membrane granule proteins, which are claimed to be specific for granule type, has come into use as a monitor for secretion. Expression of CD62 (PADGEM) in particular has become synonymous with α-granule secretion, based on comparisons with measurements of β-thromboglobulin release by a method in which secretion is not stopped by fixation. We have now developed an immunoassay for fibrinogen that tolerates fixation stopping and have compared the release of dense and α-granule markers in the same platelet supernatants with the expression of CD62 and CD63 in gel-filtered platelets. At thrombin concentrations less than 0.04 U/ml, secretion of α-granule fibrinogen was both more rapid and quantitatively greater than that of dense granule serotonin, ATP and ADP. Comparison of the secretion of granule markers (contents) with the expression of granule membrane markers on the platelet surface showed that surface expression of CD62 (P-selectin, PADGEM) corresponded to fibrinogen secretion, and CD63 correlated reasonably well with the release of dense granule contents. Pretreatment of platelets with acetylsalicylic acid (ASA) before gel-filtration moderately inhibited thrombin-induced dense and α-granule release in GFP at a concentration range of 0.01-0.03 U/ml. The agonist effect of a thrombin receptor agonist peptide (TRAP) was comparable to that of thrombin with respect to all measured markers except for β-hexosaminidase release, which was significantly less with TRAP.

Rabbit and Rat Platelets Do Not Respond to Thrombin Receptor Peptides That Activate Human Platelets

AbstractHuman platelets are aggregated and induced to release their granule contents and form thromboxane by peptides as short as 6–amino acid residues (SFLLRN) corresponding to the newly released N-terminus of the thrombin receptor that is cleaved by thrombin. Using washed platelets, we found that these responses to SFLLRN (2 to 6 µmol/L) were enhanced by fibrinogen. However, neither SFLLRN norSFLLRNPNDKYEPF had any effect on washed rabbit or rat platelets, although they were fully responsive to human thrombin. Concentrations of the peptides as high as 100 µmol/L did not cause the platelets of rabbits or rats to change shape, aggregate, release granule contents, or form thromboxane. SFLLRN did not affect the extent of aggregation induced by adenosine diphosphate (ADP) or a low concentration of thrombin. Pig platelets responded to 50 µmol/L SFLLRN with reversible aggregation, which was enhanced by fibrinogen, but not accompanied by the release of dense granule contents. Guinea pig platelets aggregated and released granule contents in response to 25 or 50 µmol/L of SFLLRN, but responded with only shape change to lower concentrations. Thus, these experiments indicate that rabbit and rat platelets lack a functional response to human thrombin receptor peptides that fully activate the previously described human thrombin receptor, despite a full response of both rabbit and rat platelets to human thrombin, and that pig and guinea pig platelets have incomplete responses to these human thrombin receptor peptides. The results suggest that platelets of rabbits and rats, and perhaps guinea pigs and pigs, respond to thrombin through an alternative receptor that has also been suggested to be present on human platelets.

Protein kinase C activation without membrane contact in platelets stimulated by bryostatin.

The tumor promoter phorbol myristate acetate (PMA) directly activates protein kinase C (PKC) and, in human platelets, induces aggregation, release of granular contents, mobilization of intracellular Ca2+ as detected by the photoprotein aequorin, and phosphorylation of the 47-kDa substrate (p47) of PKC. Whether PKC activation by PMA or other agonists requires translocation of PKC from the cytoplasm to the lipids of the platelet surface membrane, however, is not known. Bryostatin-1, a macrocyclic lactone that binds to PKC at or near the PMA-binding site and thus blocks PMA's proliferative effects, induced concentration-dependent p47 phosphorylation, platelet aggregation, release of dense granule contents, and a rise in cytoplasmic Ca2+ (ED50 = 2.0 nM) in intact aequorin-loaded platelets. Neither alpha-, beta-, delta-, or zeta-PKC isozymes translocated to the plasma membrane following bryostatin-1, although translocation of both alpha- and beta-PKC isozymes was seen in PMA-stimulated platelets. In a cell-free assay, bryostatin-1 activated PKC purified from platelets if the membrane-derived lipid phosphatidyl serine was present; however, if 12(S)-hydroxy-5,8,10,14-eicosatetraenoic acid or arachidonic acid, two lipid products formed in the cytoplasm of activated platelets, were present, no membrane-derived lipids were required for bryostatin-induced PKC activation. Thus, phosphorylation of PKC substrates and associated functional changes do not require isozyme translocation to the surface membrane, even when these events are induced by a direct activator of PKC.

Hypersensitivity to ADP of platelets from diabetic rats associated with enhanced fibrinogen binding.

Platelets from diabetic humans and animals are hypersensitive to ADP. The hypersensitivity to ADP of platelets from diabetic rats occurs independently of activation of the arachidonate pathway or release of dense granule contents. During platelet aggregation by ADP, fibrinogen binds to its receptor on platelets. We examined if the hypersensitivity to ADP of platelets from diabetic rats is associated with enhanced early binding of fibrinogen to its receptor on these platelets. Fibrinogen association with platelets from rats with spontaneous or streptozotocin-induced diabetes was significantly greater 10 s or 1 min after addition of ADP (10 microM) than with platelets from their corresponding control rats. Since enhanced fibrinogen association occurred with platelets from insulin-treated rats with spontaneous diabetes, and from rats with streptozotocin-induced diabetes that did not receive insulin, the enhanced fibrinogen binding is likely due to the diabetic state rather than to the administration of insulin or the mechanism responsible for the diabetes. Therefore, enhanced early fibrinogen association with platelets from diabetic rats is associated with their hypersensitivity to ADP.

Eicosapentaenoic Acid Interferes with U46619-Stimulated Formation of Inositol Phosphates in Washed Rabbit Platelets

Eicosapentaenoic acid (EPA) inhibits platelet responsiveness to aggregating agents. To investigate the reactions that are affected by EPA, we examined the effect of preincubating aspirin-treated rabbit platelets with EPA on stimulation of inositol phosphate formation in response to the TXA2 analogue U46619. Stimulation of platelets with U46619 (0.5 microM) caused aggregation and slight release of dense granule contents; aggregation and release were inhibited by preincubation of the platelets with EPA (50 microM) for 1 h followed by washing to remove unincorporated EPA. Incubation with EPA (50 microM) for 1 h did not cause a detectable increase in the amount of EPA in the platelet phospholipids. When platelets were prelabelled with [3H]inositol, stimulation with U46619 of control platelets that had not been incubated with EPA significantly increased the labelling of inositol phosphates. The increases in inositol phosphate labelling due to U46619 at 10 and 60 s were partially inhibited by preincubation of the platelets with 50 microM EPA. Since the activity of cyclo-oxygenase was blocked with aspirin, inhibition of inositol phosphate labelling in response to U46619 indicates either that there may be inhibition of signal transduction without a detectable change in the amount of EPA in platelet phospholipids, that changes in signal transduction require only minute changes in the fatty acid composition of membrane phospholipids, or that after a 1 h incubation with EPA, activation of phospholipase C is affected by a mechanism that is not directly related to incorporation of EPA.

Conditions Affecting the Responses of Human Platelets to Epinephrine

Conditions affecting the responses of human platelets to epinephrine were examined. In platelet-rich plasma prepared from blood anticoagulated with hirudin or PPACK (D-phenylalanyl-L-prolyl-L-arginine chloromethyl ketone), epinephrine did not cause shape change or aggregation. In a Tyrode-albumin-apyrase solution containing a concentration of Ca2+ in the physiological range, and fibrinogen, epinephrine in concentrations as high as 40 microM did not induce platelet shape change, caused either no primary aggregation or very slight primary aggregation, and did not induce thromboxane formation, release of dense granule contents, or secondary aggregation. In contrast, in citrated platelet-rich plasma, epinephrine induced two phases of aggregation. This is not attributable to the generation of traces of thrombin since the same effects were evident when blood was taken into a combined citrate-hirudin anticoagulant or a combined citrate-PPACK anticoagulant. In a modified Tyrode-albumin-apyrase solution containing approximately 20 microM Ca2+, 1 mM Mg2+, and fibrinogen, epinephrine induced extensive aggregation after a lag phase, but no primary phase was evident; thromboxane formation and release of dense granule contents accompanied the aggregation response. These responses were also observed when PPACK was included with the acid-citrate-dextrose anticoagulant, and in the washing and resuspending fluids. In the presence of aspirin or the thromboxane receptor blocker BM 13,177 a few small aggregates were detected by particle counting and by scanning electron microscopy; with the latter inhibitor, the platelets in the aggregates retained their disc shape; secondary aggregation and the responses associated with it did not occur.(ABSTRACT TRUNCATED AT 250 WORDS)