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PubMed2006

Imported platelets demonstrate decreased pH and glucose by reagent strip testing when compared to locally derived platelets.

The most common infectious risk from blood transfusion in the United States is bacterial contamination of platelet components. Although detection of bacterially contaminated platelet components is best achieved with a culture system, AABB standards permit alternatives including the use of staining methods or reagent strips. In this study, 13,216 consecutive platelet components were screened using reagent strips for evidence of the presence of bacteria. Testing was performed immediately prior to release of the platelet components for transfusion; 10,836 were collected locally and 2,380 were imported from other blood banks. A mix of whole-blood-derived and apheresis products was included. If either the glucose concentration was <250 mg/ml or the pH was <7.0, the platelet component was quarantined and a specimen was obtained for Gram's stain and culture. Every transfused platelet component that was associated with a reported transfusion reaction was also tested by Gram's stain and culture. Overall, 1.47% of imported platelet components were reactive while only 0.12% of locally collected platelets were reactive. Of 48 reactive platelet components, 44 were tested by Gram's stain and culture. None was found to be bacterially contaminated. In summary, imported platelet components were significantly more likely to be falsely reactive by reagent strip screening as compared to locally prepared platelet components.

Bacteria↗

PubMedDate not supplied

Evaluation of platelet damage in extracorporeal circuits using a visual platelet morphology method.

Platelet damage in extracorporeal circuits occurs as the result of contact with foreign surfaces, shear stress, gas interface, and other nonphysiologic conditions. A scoring method developed to determine platelet activation was modified for evaluating platelet damage in extracorporeal circuits. This method assigns a numerical score to platelet damage, as assessed by direct visualization with phase microscopy. Various commercially available blood oxygenators were tested by using a modified AAMI/ASAIO in vitro blood trauma protocol. Direct gas contact oxygenators showed a marked decrease in total platelet count and significant platelet damage. Platelet depletion and platelet damage were lower in membrane oxygenators than in direct gas contact oxygenators. Differences in platelet damage were observed between membrane oxygenators. The observed differences between devices demonstrate the influence of materials and hemodynamic design on platelet depletion and damage. The method developed allows quantitative evaluation of platelet damage caused by extracorporeal devices and is a sensitive indicator of lethal and sublethal trauma.

Blood Flow Velocity↗

PubMed1987

Thrombin-induced platelet factor Va formation in patients with a gray platelet syndrome.

The present study was initiated to establish the functional factor V concentration in platelets of patients with a mild bleeding disorder ascribed to a gray platelet syndrome. This inherited platelet disorder has been characterized by a specific deficiency of alpha-granules and subsequent deficiencies in the alpha-granule proteins. We found that the concentration of plasma factor V was slightly decreased (70% of normal values). In contrast, platelet factor Va formation was severely impaired. Besides a much lower factor V content than in control platelets (10-20% of normal), the dependency of platelet factor Va formation on thrombin concentration was altered. Increasing the thrombin concentration 4-fold compared to the concentration that results in maximal factor Va generation from normal platelets did not result in a maximal factor Va formation from gray platelets. When a suspension of washed gray platelets was incubated with a prostacyclin analogue prior to the stimulation with thrombin, a 10-fold lower factor Va activity was measured. Thus, thrombin-induced factor Va formation in a suspension of gray platelets is the result of a release reaction, followed by the thrombin-catalyzed activation of released factor V. Whereas the kinetics of the former reaction are apparently impaired, the kinetics of the latter one were found to be identical to those observed for normal platelet and plasma factor V activation.

Blood Platelet Disorders↗

PubMed1988

Uncoupling in the expression of platelet GP IIb/IIIa in human endothelial cells and K562 cells: absence of immunologic crossreactivity between platelet GP IIb and the vitronectin receptor alpha chain.

Platelet glycoproteins (GP) IIb and IIIa exist as noncovalently associated Ca++-dependent heterodimer complexes within the platelet membrane and express the major platelet alloantigens Leka (Baka) and PIA1 (Zwa), which are genetic markers of GP IIb and GP IIIa, respectively. Since heterodimers immunologically related to platelet GP IIb/IIIa have been identified in a number of nucleated cell types, we tested anti-Leka and anti-PIA1 antiserum, polyclonal anti-platelet GP IIb/IIIa IgG, as well as a panel of 28 monoclonal anti-GP IIb, GP IIIa, or complex dependent anti-GP IIb/IIIa antibodies on endothelial cells, peripheral blood mononuclear cells, and the erythroleukemic cells HEL and K562 in order to determine whether nucleated cell GP IIb/IIIa related proteins and platelet GP IIb/IIIa are immunologically related. Using immunofluorescence, immunoblotting, and immunoprecipitation experiments, evidence is presented that (1) the alloantigen Leka is not expressed in endothelial cells of an individual whose platelets are of the Leka/PIA1 phenotype, whereas the PIA1 alloantigen is readily detectable in these cells, (2) that in contrast to HEL cells, which express platelet GP IIb/IIIa and are of the Leka/PIA1 phenotype, platelet GP IIb is immunologically undetectable in 12-O-tetradecanoyl-phorbol-13-acetate (TPA)-treated K562 cells despite the presence of platelet GP IIIa, and (3) that peripheral blood mononuclear cells do not express platelet GP IIb or GP IIIa on their cell surface.

Adult↗

PubMed1989

Role of membrane glycoproteins in the interaction of blood platelets with the vessel wall--the study on platelet adhesion to in vitro cultured subendothelial matrix.

Adhesion of platelets to the subendothelium is an essential step in hemostasis and thrombosis. Several receptors for adhesive macromolecules have been identified on platelets and are included in the integrin family. To clarify the role of platelet membrane glycoproteins in the interaction of platelets with the subendothelium, 51Cr-labeled platelet adhesion assay and antibody-blocking experiments were performed by using in vitro cultured subendothelium under the static condition. The platelet adhesion in this assay was inhibited by anti-GPIa (VLA-2), GPIc (VLA-5) and -GPIc'-(VLA-6) antibodies, while anti-GPIb and -GPIIb/IIIa antibodies had no effect. Platelets from the patients with Glanzmann's thrombasthenia could also attach to the subendothelium, whereas those from a patient whose platelets lacked GPIa failed to attach to the extracellular matrix (ECM). The monoclonal antibodies against fibronectin and laminin which recognized the cell binding domain of these molecules inhibited the platelet adhesion when they were pre-treated with ECM. Furthermore, antibody-blocking experiments revealed that the percent inhibition by the combination of anti-GPIa, -GPIc and -GPIc' antibodies used herein was approximately 75%. They did not completely inhibit the attachment. These results suggest that the interactions of collagen, fibronectin and laminin with their receptors on platelets are involved in the mechanism of platelet adhesion to subendothelium.

Blood Vessels↗

PubMed1988

Differential requirements for platelet aggregation and inhibition of adenylate cyclase by epinephrine. Studies of a familial platelet alpha 2-adrenergic receptor defect.

We describe a family whose members have impaired platelet aggregation and secretion responses to epinephrine with normal responses to adenosine diphosphate and collagen. Platelet alpha 2-adrenergic receptors (measured using 3H methyl-yohimbine) were diminished in the propositus (78 sites per platelet), his two sisters (70 and 27 sites per platelet), and parents (37 and 63 sites per platelet), but not in two maternal aunts (12 normal subjects, 214 +/- 18 sites per platelet; mean +/- SE). However, the inhibition of cyclic adenosine monophosphate (cAMP) levels by epinephrine in platelets exposed to 400 nmol/L PGI2 was similar in the patients and five normal subjects (epinephrine concentration for 50% inhibition, 0.04 +/- 0.01 mumol/L v 0.03 +/- 0.01 mumol/L; P greater than .05). In normal platelets, the concentration of yohimbine (0.18 mumol/L) required for half maximal inhibition of aggregation induced by 2 mumol/L epinephrine was lower than that for inhibition of its effect on adenylate cyclase (1.6 mumol/L). In quin2 loaded platelets, thrombin (0.1 U/mL) stimulated rise in cytoplasmic Ca2+ concentration, [Ca2+]i, was normal in the two patients studied. The PGI2 analog ZK 36,374 completely inhibited thrombin-induced rise in [Ca2+]i; the reversal of this inhibition by epinephrine was normal in the two patients. Thus, despite the impaired aggregation response to epinephrine, platelets from these patients have normal ability to inhibit PGI2-stimulated cAMP levels. These patients with an inherited receptor defect provide evidence that fewer platelet alpha 2-adrenergic receptors are required for epinephrine-induced inhibition of adenylate cyclase than for aggregation.

Adenosine Diphosphate↗

PubMed1988

The human fibroblast class II extracellular matrix receptor mediates platelet adhesion to collagen and is identical to the platelet glycoprotein Ia-IIa complex.

A monoclonal antibody, P1H5, to the human fibroblast class II extracellular matrix receptor (ECMR II) specifically inhibits human fibroblast adhesion to collagen and immunoprecipitates a cell surface receptor containing an alpha and beta subunit of approximately 140 kilodaltons each (Wayner, E. A., and Carter, W. G. (1987) J. Cell Biol. 105, 1873-1884). We report here that P1H5 also specifically inhibits adhesion of unactivated human platelets to type I and III collagens, but not to fibronectin. Immunoprecipitation of the class II ECMR from Triton X-100 detergent lysates of platelets, after cell surface iodination, identified the platelet collagen receptor. Peptide mapping confirmed that the II alpha and II beta subunits immunoprecipitated from platelets are structurally homologous with those derived from fibroblasts. The platelet ECMR II alpha and -beta subunits comigrate with platelet membrane glycoproteins Ia and IIa, respectively, on two-dimensional nonreduced-reduced sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels. These results indicate that platelet and fibroblast adhesion to collagen are both mediated by a similar receptor and that the alpha and beta subunits of this receptor are identical to platelet membrane glycoproteins Ia and IIa, respectively. Although glycoprotein Ia has been previously implicated as a collagen binding protein, our results are the first direct evidence that platelet glycoprotein Ia is associated with glycoprotein IIa in a heterodimer complex and that this complex, by mediating platelet attachment, is an actual receptor for platelet adhesion to collagen.

Antibodies, Monoclonal↗

PubMed1989

Human platelet glycoprotein Ia. One component is only expressed on the surface of activated platelets and may be a granule constituent.

Glycoprotein Ia (GP Ia) is a relatively minor component of human blood platelets thought to be a receptor involved in collagen-induced platelet activation. However, some difficulties exist with the definition of this glycoprotein. The expression of GP Ia on resting (prostacyclin analogue-treated) and thrombin-activated platelets was compared by surface labeling with 125I-lactoperoxidase. Intact platelets or platelets solubilized in sodium dodecyl sulfate were labeled with periodate/[3H]NaBH4. Analysis on two-dimensional isoelectric focusing/sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels showed that GP Ia is very poorly labeled in resting platelets. After activation a new spot (GP Ia*) appears with the same relative molecular mass as GP Ia under reducing conditions. GP Ia and Ia* can be clearly separated by two-dimensional nonreduced/reduced gel electrophoresis. Therefore, two glycoproteins which have been termed GP Ia exist in platelets with similar molecular weight and pI under reducing conditions. One of these (GP Ia*) is only surface-labeled when platelets are activated, indicating that it is only exposed on the surface of activated platelets. Supernatant from activated platelets contains this glycoprotein as well as other granule components. This glycoprotein is missing in platelets from two patients with collagen-response defects.

Autoradiography↗

PubMed1986

Regulation of the phosphoinositide hydrolysis pathway in thrombin-stimulated platelets by a pertussis toxin-sensitive guanine nucleotide-binding protein. Evaluation of its contribution to platelet activation and comparisons with the adenylate cyclase inhibitory protein, Gi.

In platelets activated by thrombin, the hydrolysis of phosphatidylinositol 4,5-bisphosphate by phospholipase C produces inositol 1,4,5-triphosphate (IP3) and diacylglycerol, metabolites which are known to cause Ca2+ release from the platelet dense tubular system and granule secretion. Previous studies suggest that phospholipase C activation is coupled to platelet thrombin receptors by a guanine nucleotide-binding protein or G protein. The present studies examine the contribution of this protein to thrombin-induced platelet activation and compare its properties with those of Gi, the G protein which mediates inhibition of adenylate cyclase by thrombin. In platelets permeabilized with saponin, nonhydrolyzable GTP analogs reproduced the effects of thrombin by causing diacylglycerol formation, Ca2+ release from the dense tubular system and serotonin secretion. In intact platelets, fluoride, which by-passes the thrombin receptor and directly activates G proteins, caused phosphoinositide hydrolysis and secretion. Fluoride also caused an increase in the platelet cytosolic free Ca2+ concentration that appeared to be due to a combination of Ca2+ release from the dense tubular system and increased Ca2+ influx across the platelet plasma membrane. Guanosine 5'-O-(2-thiodiphosphate) (GDP beta S), which inhibits G protein function, inhibited the ability of thrombin to cause IP3 and diacylglycerol formation, granule secretion, and Ca2+ release from the dense tubular system in saponin-treated platelets. Increasing the thrombin concentration overcame the effects of GDP beta S on secretion without restoring diacylglycerol formation. The effects of GDP beta S on platelet responses to thrombin which had been subjected to partial proteolysis (gamma-thrombin) were similar to those obtained with native alpha-thrombin despite the fact that gamma-thrombin is a less potent inhibitor of adenylate cyclase than is alpha-thrombin. Thrombin-induced diacylglycerol formation and 45Ca release were also inhibited when the saponin-treated platelets were preincubated with pertussis toxin, an event that was associated with the ADP-ribosylation of a protein with Mr = 41.7 kDa. At each concentration tested, the inhibition of thrombin-induced diacylglycerol formation by pertussis toxin paralleled the inhibition of thrombin's ability to suppress PGI2-stimulated cAMP formation.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenylate Cyclase Toxin↗

PubMed1987

Unique interactions of asialo von Willebrand factor with platelets in platelet-type von Willebrand disease.

The present studies demonstrate that platelets from patients with platelet-type von Willebrand disease show specific and saturable binding of asialo von Willebrand factor (AS-vWF) under conditions where such binding is not observed with normal platelets. Although specific binding of 125I-AS-vWF to formalin-fixed normal platelets could not be demonstrated, specific binding to fixed patient platelets was seen with an apparent Kd of 1.3 micrograms/mL and specific maximally bound ligand of 0.40 micrograms/10(8) platelets. Preincubation of patient platelets with the antiglycoprotein Ib (anti-GPIb) monoclonal antibody AS-2 reduced total binding close to the level of computer-estimated nonspecific binding. In contrast, binding was not reduced by preincubation with anti-GPIIb/IIIa monoclonal antibody or with 5 mmol/L EDTA. Under stirring conditions, the binding of AS-vWF to fixed patient platelets was accompanied by a strong agglutination response. AS-vWF-induced agglutination was similarly observed in patient but not normal platelet-rich plasma (PRP) in the presence of 5 mmol/L EDTA. In the absence of EDTA, AS-vWF produced a full aggregation response in patient PRP at concentrations as low as 0.1 microgram/mL in contrast to the 2 to 20 micrograms/mL required by normal PRP. Both thromboxane B2 formation and adenosine triphosphate secretion showed an AS-vWF concentration dependence paralleling the aggregation responses. These studies show that a major difference in the platelets from patients with platelet-type von Willebrand disease is the presence of an exposed, high-affinity binding site associated with GPIb that recognizes AS-vWF.

Adenosine Triphosphate↗

PubMed1988

Expression of fibrinogen on the surface of ADP-stimulated platelets: comparison of human and rabbit platelets.

Fibrinogen is a cofactor in the aggregation of human platelets and must be added to suspensions of washed human platelets for extensive aggregation to occur in response to ADP. Although the patterns of fibrinogen binding and dissociation during platelet aggregation and deaggregation are similar for human and rabbit platelets, washed rabbit platelets aggregate extensively when stimulated with ADP even in the absence of added fibrinogen. To determine whether secreted fibrinogen was present on the surface of ADP-stimulated platelets and available to support aggregation, the binding of 125I-F(ab')2 fragments of anti-fibrinogen antibodies to formaldehyde-fixed platelets was measured. Although no fibrinogen was detected on human platelets either before or after treatment with ADP, fibrinogen was expressed on the surface of rabbit platelets after stimulation with ADP. However, this secreted fibrinogen did not dissociate during deaggregation of the platelets. Thus, the aggregation of rabbit platelets in the absence of exogenous fibrinogen may be supported by secreted fibrinogen, but a fibrinogen-independent component has not been completely ruled out.

Adenosine Diphosphate↗

PubMed1984

Platelet cytoskeleton: immunofluorescence studies on thrombin-activated platelets.

Cytoskeletal proteins were isolated from chicken gizzard smooth muscle and from platelets and antibodies prepared against them. It was shown by indirect immunofluorescence technique that actin, alpha-actinin, and vinculin are not present on the surface of platelets. Physiologic concentrations of thrombin (0.04 to 0.5 U/ml) that induce platelet aggregation and release in the presence of calcium from freshly isolated platelets do not induce platelet changes resulting in the availability of the cytoskeleton to antibodies. Because the F(ab')2 fragments of anti-cytoskeletal proteins IgG do not inhibit thrombin-induced aggregation of platelets, the direct role of these proteins in thrombin-induced platelet aggregation, as with ADP and collagen, may be rejected. However, when freshly isolated platelets are treated with thrombin (1 U/ml), antibodies to actin, alpha-actinin, and vinculin stained the platelets; therefore, this demonstrates that thrombin at this high and probably nonphysiologic concentration induces a reorganization of the membrane components with the subsequent exposure of the proteins of the cytoskeleton. We demonstrate interaction between isolated actin and alpha-actinin but not vinculin with fibronectin. After stimulation of platelets by thrombin, certain cytoskeletal proteins may interact with subendothelial fibronectin and thereby promote and consolidate platelet adhesion.

Animals↗

PubMed1983

Arachidonic acid stimulates the formation of 1,2-diacylglycerol and phosphatidic acid in human platelets. Degree of phospholipase C activation correlates with protein phosphorylation, platelet shape change, serotonin release, and aggregation.

Exogenous unlabeled arachidonic acid (AA) added to human platelets prelabeled with [3H]AA induces breakdown of [3H]phosphatidylinositol and the rapid and transient formation of [3H]1,2-diacylglycerol and [3H]phosphatidic acid (PA), indicating activation of phosphatidylinositol-specific phospholipase C. Formation of [3H]1,2-diacylglycerol and [3H]PA is inhibited by pretreatment of platelets with aspirin, which suggests that endoperoxides or thromboxane A2 are responsible for AA-induced stimulation of phospholipase C. Exogenous unlabeled AA also induces the formation of [32P]PA or [14C]PA in platelets that have been prelabeled with 32Pi or [14C]AA, respectively. Increased radioactivity in PA reflects increased content of PA as measured by the fatty acid composition of PA. The relation of PA production, which reflects stimulation of phospholipase C, to specific platelet responses was further investigated. Low concentrations of AA (0.05-0.2 microM) induces platelet shape change in parallel to formation of 50-100% PA and phosphorylation of a 40,000 molecular weight protein. Higher concentrations of AA (0.5-50 microM) stimulate the formation of a further amount of PA (200-250%), and phosphorylation of 40,000 molecular weight protein, platelet aggregation, and serotonin release. Indomethacin inhibits all these observed changes by inhibiting the conversion of AA by platelet cyclooxygenase. In contrast, prostacyclin blocks these responses without affecting conversion of AA by platelet cyclooxygenase and thromboxane synthetase. We conclude that formation of endoperoxides and thromboxane A2 is necessary but not sufficient for platelet activation by AA. Only if PA is formed are platelets activated. The results indicate a central role for the phospholipase C pathway in the process of platelet activation.

Arachidonic Acid↗

PubMed1984

Platelet-platelet recognition during aggregation: distinct mechanisms determined by the release reaction.

Fixed platelets, bearing covalently bound fibrinogen, were previously shown to participate passively in aggregation induced by thrombin or A23187. The bound fibrinogen was specifically required for the interaction. The present study examines the role of the release reaction in controlling the passive participation of the fixed platelets in aggregation. Aggregation of fresh platelets induced by low ADP concentration (less than or equal to 2 microM) was not augmented by the fixed platelets, but was augmented when higher concentrations of ADP were employed. Fixed platelets failed to enhance aggregation of aspirin-treated fresh platelets induced by 10 microM ADP; the augmentation capability was reconstituted by a supernatant fraction from platelets activated by 10 microM ADP in the presence of EDTA. The binding of soluble fibrinogen to activated fresh platelets occurred regardless of the release reaction, but the interaction of fragmented fixed platelets bearing bound fibrinogen with aggregating fresh platelets was apparent only when release took place. It is concluded that fibrinogen covalently bound to fixed platelets is selectively recognized by released compound(s).

Adenosine Diphosphate↗

PubMed1978

A platelet procoagulant activity associated with platelet shape change.

PCA was measured for human PRP by determining recalcification times assayed in a minimal-dilution, controlled PH/PCO2 system in a siliconized cuvette, with the use of light transmission measurements (aggregometry). Platelet shape, aggregation, and plasma clotting end points were assayed photometrically, with platelet morphology and aggregation studied in parallel by light microscopy. With varying concentrations of ADP preincubated with PRP initially containing essentially disc-shaped platelets, it was found that induced shape change in the absence of an aggregation is necessary and sufficient for the development of PCA. This was consistently measurable as a shortening of recalcification times by approximately 50% for suspensions of shape-changed platelets vs. disc-shaped platelets. The pharmacologic inhibition of the endoperoxide pathway-mediated platelet secondary aggregation and release by aspirin administered in vivo does not impair the ability of human platelets to develop this PCA. Inhibition of shape change with amounts of 5'-adenosine monophosphate insufficient to affect coagulation tests in the absence of platelets leads to 80% to 90% inhibition of the ADP-induced PCA. This PCA is shown to be fully reversible, with morphologic reversion of shape-changed platelets to the discoid form, and is shown to be distinct from other PCAs previously described for platelets activated in different ways, such as PF3 activity. It is suggested that the binding of coagulation factors to the platelet membrane may be regulated concomitantly with shape change.

Adenosine Diphosphate↗

PubMed1981

Platelet von Willebrand's antigen II: active release by aggregating agents and a marker of platelet release reaction in vivo.

von Willebrand's antigen II (vW AgII), a plasma and platelet antigen immunologically and biochemically distinct from factor-VIII-related antigen (VIIIR:Ag), is decreased in von Willebrand's disease. vW AgII is released from platelets during aggregation and clotting. The release of platelet vW AgII was studied in washed platelets following aggregation by thrombin, collagen, and adenosine diphosphate (ADP). Total platelet vW AgII was 3.39 U/10(9) platelets. Thrombin and collagen yielded release of 85% and 86% of platelet vW AgII, respectively, at the highest concentrations. Release of platelet vW AgII was correlated with the release of 5-hydroxytryptamine (5HT). Release of vW AgII by collagen and thrombin was inhibited by metabolic inhibitors. In addition, collagen release of vW AgII was inhibited by aspirin. In clinical syndromes associated with intravascular platelet destruction, marked elevations of plasma vW AgII were noted. Thus, vW AgII is released by a metabolically active process from platelets during aggregation. In addition, vW AgII appears to be a marker of intravascular platelet destruction.

Adenosine Diphosphate↗

PubMed1995

In vivo biotinylation demonstrates that reticulated platelets are the youngest platelets in circulation.

In both mice and humans, a subset of platelets can be identified that shows increased labeling with nucleic acid-specific fluorescent dyes, such as thiazole orange. Termed "reticulated platelets," they have been postulated to be platelets that have recently entered the circulation. Their numbers appear to reflect the rate of new platelet production in a number of clinical and experimental situations. To determine whether reticulated platelets really are the youngest platelets in circulation and to estimate the length of time that they are identifiable after entering the circulation, we have employed a technique of "in vivo biotinylation" in mice that labels the entire cohort of circulating cells with covalently bound biotin. Blood samples can then be double-labeled with fluorescent avidin derivatives and thiazole orange, permitting correlated measurement of both surface biotin content and nucleic acid content. The biotinylation occurs rapidly, is complete within 30 minutes, is stable for several days, and does not appear to alter platelet function. The results show that within 24 hours after in vivo biotinylation, platelets appear in the circulation with decreased levels of biotinylation and that these are the reticulated platelets. The estimated lifespan of reticulated platelets is 1.8 days, and the lifespan of all platelets by this method is 4.5 days, which is in agreement with estimates made by other methods.

Animals↗

PubMed1994

Platelet alpha 2-adrenoceptors and diurnal changes of platelet aggregability in hypertensive patients.

OBJECTIVE: To analyse whether platelets from hypertensive patients have an increased responsiveness to aggregating agents during morning hours and whether these changes might be related to concurrent changes in platelet membrane alpha 2-adrenoceptor characteristics, plasma catecholamine and cortisol levels, and blood pressure values. DESIGN AND METHODS: Blood samples from 14 mild-to-moderate essential hypertensive males were collected in the morning (0700-0900 h) and the evening (1900-2100 h) to determine platelet aggregability responses to adrenaline and ADP, platelet alpha 2-adrenoceptor number and binding affinity to [3H]-yohimbine, plasma catecholamines and cortisol. During the same day patients underwent 24-h ambulatory blood pressure monitoring. RESULTS: The lowest concentration of adrenaline required to induce biphasic aggregation was significantly lower in the morning than in the evening, indicating an increased morning platelet aggregability to adrenaline; the minimum ADP concentration inducing aggregation was similar in morning and evening samples. There were no significant differences between morning and evening samples in platelet alpha 2-adrenoceptor number and binding affinity. Plasma adrenaline, noradrenaline and cortisol levels were higher in the morning than in the evening, but no correlation was observed between hormonal changes and the morning increase in platelet sensitivity to adrenaline. Ambulatory blood pressure recording showed abrupt morning elevations in systolic and diastolic blood pressures over sleeping values. However, morning blood pressure readings were not significantly different from those recorded during the rest of the day and in the evening. The morning rise in mean arterial pressure displayed a significant inverse correlation with the increased platelet sensitivity to adrenaline that was observed during the same hours. CONCLUSIONS: The results indicate that the increased morning responsiveness to adrenaline that was observed in platelets obtained from hypertensive patients does not appear to be mediated by changes in the characteristics of platelet membrane alpha 2-adrenoceptors, but morning blood pressure elevations might play some role in inducing this platelet hyper-reactivity.

Adenosine Diphosphate↗