Rat platelet aggregation by ATP. Aggregometrical and ultrastructural comparison with aggregations induced by ADP and collagen.

This paper describes the aggregation of rat platelets by adenosine triphosphate (ATP). The aggregometry of ATP-induced aggregation and the ultrastructure of ATP-aggregated platelets were compared and contrasted with those of adenosine diphosphate (ADP)-treated and collagen-treated samples. Human platelets were also studied alongside with rat specimens. Several lines of evidence indicate that the ATP-induced aggregation of rat platelet-rich plasma (PRP) is not a result of contaminating ADP in the ATP preparation. ATP did not cause aggregation of human platelets; it inhibited ADP- and collagen-induced human platelet aggregation. ATP pretreated with a creatine phosphate/creatine phosphokinase system caused similar rat platelet aggregation as did ATP not treated with this system. The aggregometry of ATP-induced aggregation of rat PRP was similar to that of collagen-induced aggregation but markedly different from that of ADP-induced aggregation. However, the nature of ATP-induced aggregation was similar to that induced by ADP. Both ATP- and ADP-induced rat platelet aggregations were not affected by adenosine, adenosine monophosphate, or acetylsalicylic acid. The ultrastructure of ATP-aggregated platelets was similar to that of ADP-aggregated ones. It appears that either platelets of rats possess specific ATP receptors or the rat plasma contains a material, lacking or insufficiently present in human plasma, that converts ATP to ADP in a fashion similar to the release of ADP from platelet storage granules.     

Critical evaluation of platelet aggregation in whole human blood

Platelet aggregation studies generally are performed in platelet-rich plasma (PRP) by the turbidometric method. The authors compared this technic with the recently introduced impedance aggregometry in PRP and whole blood (WB). In healthy controls there was a good correlation between the two technics when aggregation was induced by ADP or collagen. As compared with PRP, platelets in WB were more sensitive to the aggregating effect of thrombin, ristocetin, and arachidonic acid. Platelet sensitivity to prostacyclin was increased in WB. The anti-platelet effect of a single oral dose of aspirin could be detected for a longer period in WB than in PRP. Platelet aggregation tests in WB from patients with platelet dysfunctions showed the same response pattern to different aggregating agents as in PRP. In contrast to turbidometry, the impedance method in PRP and WB enabled registration of platelet aggregation in a dose-dependent fashion in a sample from a patient with severe hyperlipoproteinemia. It is concluded that platelet aggregation can be studied conveniently with the impedance method in the more physiologic medium of WB. Providing the same information as the well-established turbidometry, the time-sparing impedance method needs less citrated blood. Moreover, our results show an increased sensitivity of the WB system to some aggregating and anti-platelet agents.     

Factors influencing platelet aggregation in whole blood

In order to evaluate the interference of blood cells on platelet aggregation, spontaneous platelet aggregation (SPA), ADP, and collagen-induced platelet aggregation were investigated in whole blood by the impedance method and in platelet-rich plasma (PRP) by densitometric and impedance aggregometers. Stirring of the sample induced a significant decrease of neutrophils (P less than 0.001) but no changes of red blood cell (RBC) and platelet count. After collagen addition, a further decrease of neutrophils was observed, while RBC count was unmodified. The occurrence of SPA was not different in whole blood and in PRP. Platelet number and hematocrit did not affect either spontaneous or collagen-induced aggregation. A significant linear relation (r = -0.60, P less than 0.01) between neutrophil count and collagen whole blood platelet aggregation was found. Collagen- and ADP-induced aggregation were significantly higher and lower, respectively, in whole blood than in PRP using the densitometric method. No differences were observed in SPA and collagen platelet aggregation according to age and sex.     

Platelet function and ABO blood group

An influence of the ABO blood group on von Willebrand and Factor VIII:C levels is known. Von Willebrand factor interacts with at least two platelet membrane receptors, but the effect of ABO group on platelet function is an unstudied area. The authors examined platelet function in 40 plateletpheresis donors using an impedance lumi-aggregometer. Aggregation responses to collagen, adenosine diphosphate (ADP), and ristocetin were measured and the adenosine triphosphate (ATP) release to thrombin. Twenty donors were Group O and 20 were Group A. Measurements of von Willebrand factor antigen (vWf:Ag), Factor VIII: C, and ristocetin co-factor (RiCoF) in the same group showed reduced levels of vWf:Ag and Factor VIII:C in Group O, as previously reported. The aggregation response to collagen and ADP and the release of ATP did not differ. The aggregation response to ristocetin, however, was better in Group O than in Group A despite the lower vWf:Ag levels. The explanation for this is unclear, but the data suggest an influence of blood group antigens on the interaction between von Willebrand's factor and platelets.     

Comparison of the effect of heparin and citrate on platelet aggregation

When whole blood was passed through a column of glass beads, heparinized and native blood gave similar results. With citrated blood far fewer platelets were lost on the column. Thus citrate inhibits this test and heparin has no effect.Platelet-rich plasma (PRP) was prepared from citrated and heparinized blood and the responses in a number of aggregation tests were compared. With added adenosine diphosphate and adrenaline (first wave), the response in heparin PRP was greater than that in citrate PRP and was proportional to the amount of free calcium ions. With added collagen and adrenaline (second wave) the response was worse in heparin than in citrate platelet-rich plasma. It is concluded that heparin has no effect on the adhesive forces but inhibits the release mechanism. This may be of therapeutic importance.     

Species variability in platelet aggregation response to different agonists

Conflicting data on platelet function in animal species are reported in the literature. In this study, the response of buffalo, horse, pig and sheep platelets to different agonists was assessed. Blood samples were collected from the jugular vein of six healthy subjects of each species and platelet-rich plasma was obtained by centrifugation. Platelet aggregation responses to increasing doses of adenosine 5'-diphosphate (ADP), arachidonic acid, collagen, platelet activating factor (PAF) and ristocetin were measured by a turbidimetric method. Horse platelets were the most responsive to ADP, collagen and PAF, whereas sheep platelets were the most responsive to ristocetin. The response to arachidonic acid varied least between species. PAF was the most effective agonist, inducing a maximum aggregation response at a concentration of 1 micro M for platelets of each species. Conversely, concentrations of ristocetin higher than 1mg/ml induced a maximum aggregation response only with sheep and horse platelets. The different responses of platelets from the four animal species to various agonists may reflect either (1). structural differences (including composition of the platelet membrane and presence of specific agonist receptors), or (2). activation of distinct signalling pathways by the agonist.     

Adenine nucleotides, serotonin, and aggregation properties of platelets of blue foxes (Alopex lagopus) with the Chediak-Higashi syndrome

Bleeding times, concentrations of serotonin in whole blood, and concentrations of adenine nucleotides as well as aggregation properties of platelets were examined in 18 blue foxes with Chediak-Higashi-like syndrome (CHS) and 16 controls. A claw of each ketamine-sedated fox was cut until bleeding started and the bleeding time was recorded as the time from the first to the last drop. The bleeding time was greatly increased in CHS foxes. Platelet counts of CHS foxes were normal, but aggregation induced by adenosine diphosphate (ADP), serotonin, collagen, and arachidonate was impaired. Adrenaline and serotonin was impaired. Adrenaline and serotonin potentiated the aggregatory effect of ADP on control as well as on CHS platelets. The mean concentration of ADP in CHS platelets was about one-third that in controls, whereas adenosine triphosphate (ATP) was approximately one-half that in controls. Serotonin could not, in most cases, be detected in blood of CHS foxes. These findings suggest that the prolonged bleeding time in the CHS foxes is, at least partly, due to a storage pool deficiency. The drastically reduced, and in some cases absent, aggregation of CHS platelets in response to arachidonate suggests that defective arachidonate metabolism contributes to the impaired hemostasis.     

Spontaneous platelet aggregation in a hereditary giant platelet syndrome (MPS)

The characteristics of spontaneous platelet aggregation (SPA) in a hereditary giant platelet syndrome (Montreal platelet syndrome, MPS) are examined. SPA was quantitated by microscopy from the decrease in single platelets in platelet-rich plasma (PRP). In contrast to normal donors, a significant proportion (20-50%) of platelets in MPS whole blood and PRP occurred in microaggregates typically containing 2-6 disk-shaped platelets. Stirring MPS-PRP at 1000 rpm for 10 minutes further increased the fraction of platelets in aggregates by 10-170%, the percentage increase not being correlated to the donor's platelet count (5000-220,000 microliters-1). Normal platelets resuspended in MPS platelet-poor plasma (PPP) did not undergo SPA, whereas MPS platelets resuspended in normal PPP or Ca2+-free, fibrinogen-free Tyrode's continued to show SPA. The increase in SPA could be inhibited by 10 microM prostaglandin (PG) E1, 150 mM ASA or glutaraldehyde or formaldehyde fixation; however, it was not inhibited by 10 nM PGI2 and was only partially inhibited by 1 microM 2-chloroadenosine and 1-10 units/ml apyrase. SPA in Acid-citrate-dextrose-PRP was much less than in PRP; however, SPA reoccurred on returning the platelets to platelet-free plasma or Tyrode's. Platelet aggregation (PA) could be increased over that due to SPA alone by the addition of adenosine diphosphate, adrenaline, collagen, ionophore A-23187, arachidonic acid and ristocetin, with results suggesting that the response to these agents is normal. The ristocetin-induced increase in PA was completely blocked by an IgG specific for Bernard-Soulier syndrome. In contrast, MPS platelets had a reduced sensitivity to thrombin, which appeared to be more pronounced at low platelet counts. There was no correlation between the thrombin insensitivity and the extent of SPA. Total adenosine triphosphate (ATP) and thrombin-induced release of ATP and platelet factor 4 appeared normal for MPS platelets. The ultrastructural features of MPS platelets were within normal limits except for an increased frequency of giant granules. SPA was observed for 5/5 MPS donors, but only one of three MPS donors' platelets evaluated for glycoprotein I and sialic acid content showed any measurable reduction as compared with normal controls. The above observations point to the existence of an as yet undetermined anomaly of MPS plasma membrane related to a fibrinogen and Ca2+ independent form of platelet aggregation.     

Platelet impedance aggregation in whole blood and its inhibition by antiplatelet drugs.

Platelet aggregation was studied in citrated whole blood by an electrical impedance method. Blood samples from normal volunteers were studied with the aim of finding a suitable method for the routine study of samples from patients. An erratic tracing and low maximum aggregation were seen in samples with a high normal haematocrit. Optimal aggregation was seen when blood was diluted to a haematocrit of .300; isotonic saline was a better diluent than platelet poor plasma. No appreciable differences were seen when the platelet count was diluted down to 50 X 10(9)/l, after which there was a progressive reduction in response. Dose response curves were obtained, and normal ranges for ADP, collagen, and sodium arachidonate were determined. Acetylsalicylic acid had a more pronounced effect on ADP aggregation than on collagen. Prostacyclin (Epoprostenol) and the synthetic prostacyclin analogue ZK 36,374 both showed dose dependent inhibition of aggregation, but the duration of effect of the latter was much longer (greater than 6 h).     

Whole blood aggregation using impedance and particle counter methods

Platelet function in platelet-rich plasma may be evaluated using an optical or electrical impedance method. Platelet function in whole blood may be studied using an impedance or particle counter method. A comparison of these techniques in whole blood has not been performed. Twenty-four healthy subjects were studied using both techniques over a range of concentrations of collagen and adenosine diphosphate (ADP). Although broad similarities exist, discordant patterns occur in individual subjects. The particle counter method is capable of detecting small aggregates and thus is more sensitive. The impedance aggregometer detects only larger aggregate formation. These techniques are complementary rather than competitive and may have considerable potential in profiling platelet function.     

Aggregation of human peripheral blood mononuclear cells by calcium ionophore A23187. Comparison with the aggregation of platelets and defective response in Glanzmann's thrombasthenia

Following exposure to calcium ionophore A23187, washed peripheral blood mononuclear cells (MNC) aggregated and formed thromboxane, like platelets. However, while aspirin strongly inhibited platelet aggregation and thromboxane formation, it had a little effect on the aggregation of MNC. In about 50% of the samples studied, aggregation of MNC was associated with the secretion of ATP. However studies in which exogenous ATP or ADP were used, suggested that the aggregation of MNC is independent of the secretion of nucleotides. MNC from 2 thrombasthenic patients failed to aggregate and bound 9–10 fold less radiolabelled fibrinogen than those from normals when challenged with A23187. However, fibrinogen, which plays a major role in the aggregation of platelets, did not appear to be involved in the aggregation of MNC. A differing behavior of these two types of cells was also found when the effect of plasma was studied on the aggregation response to A23187. Indeed, citrated plasma greatly enhanced the aggregation of platelets while it suppressed the response of MNC. This inhibitory effect of plasma was not detected when heparinized plasma was substituted for citrated plasma. We conclude that the aggregation of MNC in response to A23187 does not involve basic events known to play a major role in the aggregation of platelets. The response to A23187 may be an important probe for understanding basic mechanisms and pathophysiological significance of the aggregation of MNC.     

Lysophosphatidic acids. Influence on platelet aggregation and intracellular calcium flux.

Decanoyl-, palmitoyl-, and oleoyl-lysophosphatidic acid (LPA) were studied for their effects on platelet aggregation and intracellular calcium flux. Palmitoyl-LPA and oleoyl-LPA both caused a concentration-dependent aggregation of human blood platelets at concentrations of 12--300 microM. Aggregation by adenosine diphosphate (ADP) was enhanced at slightly lower concentrations. First-wave aggregation induced by these LPAs was not blocked by aspirin, indomethacin, or heparin, suggesting similarities to ADP aggregation. However, in washed platelets with a high calcium concentration, no serotonin secretion was observed, even though full aggregation occurred, suggesting that aggregation was not due to released ADP. This concept was supported by studies of platelets deficient in the storage pool of ADP and serotonin, which had a normal first-wave aggregation response to palmitoyl-LPA. Aggregation induced by palmitoyl LPA was inhibited by prostaglandin E1 (PGE1), theophylline, and ethylenediaminotetraacetate (EDTA), though in the presence of EDTA shape change occurred. Aggregation stimulated by palmitoyl-LPA or oleoyl-LPA was characterized by changes in the shape of the platelets with development of pseudopods and centralization of granules closely surrounded by contractile microfilaments and supporting microtubules. The addition of palmitoyl-LPA and oleoyl-LPA, but not decanoyl-LPA, caused the release of calcium from a platelet membrane fraction that contains elements of the intracellular calcium storage system and actively concentrates this cation in the presence of adenosine triphosphate (ATP) and magnesium. It is suggested that LPAs cause aggregation by stimulating the release of calcium intracellularly.     

Activation of platelets by microfibrils and collagen. A comparative study

Previous works demonstrated that microfibrils stimulate blood platelets to aggregate. The present study compares the activation of platelets by human placental and bovine aortic microfibrils and by type III collagen. We studied the morphological changes occurring in in platelets during their activation and aggregation, as well as the kinetics of the release reaction and thromboxane B2 formation. As for collagen, the microfibrils-induced platelet aggregation followed a lag phase, during which progressive emission of pseudopodes and centralization of organelles occurred. Aggregation was associated with secretion of beta-thromboglobulin and adenylic adenylic nucleotides, and with formation of thromboxane B2; it was established that the kinetics of secretion and the aggregation curve were parallel. Microfibrils-induced aggregation was also inhibited by ethylenediamine tetraacetic acid, creatine phosphate-creatine phosphokinase, and aspirin, showing that it was calcium-dependent and required a secretion of ADP and formation of endoperoxide and thromboxane. The response to microfibrils was much more rapid than to collagen; placental microfibrils reacted faster than aortic microfibrils. The requirement of plasma in the microfibrils platelets interaction was confirmed: 10 microliters is the minimal amount of plasma necessary for an aggregation of platelets in 400 microliters of buffer. This fact supports the idea of the existence of two different pathways in the interaction between platelet and the subendothelium, depending on the vascular structure (microfibrils or collagen) involved, even though the sequence of events leading to the formation of an aggregate is similar.     

Platelet aggregation and platelet function analyzer 100 (PFA-100) closure time in camels—a comparative study with humans

Despite the very active coagulation system in camels, there are no previous studies on camel platelet functions. It is our aim to study camel platelet function using aggregometry, Platelet Function Analyzer (PFA100), and flow cytometry. A total of 103 camels, 19 males and 84 females, were studied. Their ages ranged from 5 to 20 years (mean±SD: 6.4±4.4 years). The results obtained were compared with healthy humans. Platelet aggregometry was undertaken in platelet-rich plasma in response to adenosine diphosphate (ADP), adrenaline, collagen, arachidonic acid, and ristocetin. Camel platelet function in whole blood was also tested using the PFA-100 and by flow cytometry using three human monoclonal antibodies (CD42, CD61, and CD62). Camel platelets failed to respond to arachidonic acid, adrenaline, and ristocetin. However, responses to ADP and collagen were obtained but were less than the human values. The addition of human plasma caused some enhancement of the aggregation responses to adrenaline and collagen but not ristocetin or arachidonic acid. However, the presence of human serum or heparin resulted in a very marked enhancement of the camel platelet aggregation responses to all agonists, except arachidonic acid. PFA-100 closure times of the collagen–ADP and the collagen–epinephrine cartridges were markedly longer than in humans. In the flow cytometry studies, camel platelets failed to respond to any of the human monoclonal antibodies with or without activation by ADP, thrombin, human plasma, or serum. This first study on camel platelet functions uncovered the distinction between camel and human platelet functions. The lack of platelet responses to certain aggregating agonists, their enhancement with human plasma and serum, as well as the prolongation of the PFA-100 closure times, add other unique characteristics to the biology of this interesting creature.     

Some interactions between human platelets and glass: von Willebrand''s disease compared with normal

If native or heparinized blood is passed slowly through a column of glass beads at room temperature, the number of platelets removed from the initial drop emerging from the column is less than that removed from the final drop. At 4 degrees C. this difference disappears. If the blood is passed rapidly through such columns at room temperature fewer platelets are removed, but the initial-final difference persists. Von Willebrand's platelets are removed normally at slow speeds; at fast speeds abnormally few platelets are removed. Platelets emerging from all such columns are in aggregates.On adding glass beads to normal heparinized plasma, the platelets at once become more rounded and after about 50 seconds' delay they aggregate: the delay and rate of aggregation can be quantitated. Aggregation occurs best at 20 to 30 degrees C. and is not inhibited by the addition of some enzyme inhibitors. In von Willebrand's disease all these glass-induced aggregation phenomena occur normally and aggregation in response to adenosine diphosphate (ADP), serotonin creatinine sulphate (5-HT), adrenaline, collagen, and glass is also normal.     

The effects of in vitro incubation with acetylsalicylic acid on thrombocyte aggregation in whole blood from normal ostriches

Avian thrombocytes are circulating nucleated blood cells which play an active role in both haemostasis and phagocytosis. To better define their role in haemostasis, thrombocyte aggregation was measured in whole blood from healthy adult ostriches using an electrical impedance aggregometer. Thrombocytes were stimulated with collagen (5μg/ml or 1μg/ml), platelet-activating factor (PAF (0.1μ m, 0.01μ m, or 0.001μ m), or ADP (25μ m). Irreversible aggregation occurred consistently in response to collagen or PAF but not to ADP. Aggregation in response to high concentrations of PAF or collagen was not affected by in vitro incubation with 500μ m aspirin. Aggregation in response to low concentrations of PAF or collagen was partially inhibited by in vitro incubation with 500μ m aspirin, suggesting the presence of a prostaglandin-dependent mechanism for regulation of thrombocyte activation similar to that in mammalian platelets.     

The mechanism of adenosine diphosphate induced platelet aggregation: binding to platelet receptors and inhibition of binding and aggregation by prostaglandin E1

1. Normal human blood platelets in stirred plasma were incubated with [(14)C]ADP for 10-360 sec and the aggregation responses were correlated with platelet bound radioactivity, the platelets being separated from the plasma within 25 sec of the end of the experiment.2. The platelet aggregation response, measured as a change in light transmittance through platelet-rich plasma, was related to the plasma [(14)C]ADP concentration and linearly related to the log platelet bound [(14)C]ADP 60-120 sec after addition of nucleotide to plasma.3. Thin layer chromatography of the platelet bound radioactivity showed that 78-90% was unmetabolized ADP, the remainder being AMP. Plasma radioactivity consisted of ADP, AMP and adenosine. There was no detectable radioactive cyclic AMP in either platelets or plasma.4. Further accumulation of radioactivity occurred after 180 sec but was not related to the aggregation response.5. Prostaglandin E(1) inhibited aggregation and platelet [(14)C]ADP accumulation when added to platelet-rich plasma 60 sec before [(14)C]ADP. There was a significant correlation between inhibition of aggregation and inhibition of [(14)C]ADP accumulation.6. Prostaglandin E(1) also reversed ADP aggregation when added to platelet-rich plasma after the nucleotide, with an accompanying decrease in platelet bound [(14)C]ADP.7. It is concluded that ADP induces platelet aggregation by binding to specific receptors probably located on the plasma membrane, and that prostaglandin E(1) inhibits this effect by interfering with the ADP binding.     

The release of nucleotides, 5-hydroxytryptamine and enzymes from human blood platelets during aggregation

1. Adenosine diphosphate (ADP) and adrenaline caused the aggregation of human platelets suspended in plasma containing citrate anticoagulant and stirred at 37 degrees C. The aggregation occurred in two phases and the second phase was associated with the appearance in the plasma of up to 30% of the ATP and 55% of the ADP present in the platelets. The concentration of ADP appearing in the plasma was up to 7 times the concentration added.2. Radioactivity was released by ADP and by adrenaline from platelets labelled with radioactive 5-hydroxytryptamine; this release was closely correlated with the second phase of aggregation and with the release of nucleotides.3. Acid phosphatase, beta-glucuronidase and adenylate kinase were released to a small extent during second phase aggregation by ADP or adrenaline; thrombin and collagen particles caused significantly greater release of beta-glucuronidase than of either acid phosphatase or of adenylate kinase.4. Morphological changes indicating degranulation of the platelets were observed during the second phase of aggregation produced by adrenaline and by ADP.5. The second phase of aggregation, degranulation of platelets, and the release of nucleotides, of labelled 5-hydroxytryptamine and of enzymes, were all inhibited by concentrations of amitriptyline which did not inhibit aggregation.     

Gel-filtered human platelets. Ultrastructure, function, and role of proteins in inhibition of aggregation by aspirin.

Gel filtration of human platelet-rich plasma (PRP) on columns of Sepharose 2B removed at least 99.85% of the plasma proteins from platelets when a column 10 cm in height was used and a plasma volume 11 to 14% of the gel-bed volume was applied. ADP and ATP levels in gel-filtered platelets (GFP) were not significantly different from those in PRP. By transmission electron microscopy, GFP were indistinguishable from PRP. Gel filtration appears to be a highly satisfactory technique of separating platelets from plasma without modifying structure, function, or contents significantly. The roles of several crude protein fractions in platelet aggregation and aspirin's inhibition of aggregation were examined. Fraction I (mostly fibrinogen) enhanced collagen-induced aggregation of gel-filtered platelets; Fraction V (mostly albumin) was inhibitory. Fraction II (mostly gamma-globulin) or gelatin had no significant effect. Aspirin added to gel-filtered platelets inhibited aggregation by 80%. The addition of mixtures of plasma proteins containing albumin increased albumin's inhibitory effect. Incubation of gel-filtered platelets with aspirin labeled in the carboxyl position resulted in no uptake of the label. In contrast, incubation with acetyl-labeled aspirin was followed by uptake of more than 2 X 10(6) acetyl groups per platelet in 1 minute. Incubation for 30 minutes resulted in a five- to sixfold further increase in uptake of the label. Aspirin can acetylate platelets and inhibit aggregation directly. Plasma proteins, in particular albumin or a contaminant of the albumin fraction tested, enhance the inhibitory effect of aspirin on platelet aggregation.     

The disappearance of adenosine from blood and platelet suspension in relation to the platelet cyclic AMP content

Adenosine exerts anti-aggregatory effects on human platelets in vitro, probably by increasing intraplatelet levels of cyclic AMP. In addition, adenosine prevents platelet loss in vivo. We have studied the relationship between the concentration of adenosine in the platelet media and the level of cAMP. In PRP, exogenous adenosine (2-16 microM) was eliminated with a half-life close to 5 min. Approximately half of the added adenosine was deaminated (blocked by 1-2 microM EHNA), and half was eliminated by uptake into platelets (blocked by 2 microM dipyridamole). In whole blood the half-life for adenosine was much shorter, about 15 s. Addition of adenosine deaminase (0.3 microgram ml-1) to PRP resulted in a measured half-life for adenosine approximating that of whole blood. In PRP where adenosine was eliminated as quickly as in whole blood, the adenosine-mediated stimulation of cAMP was 35% lower than in PRP, and the cAMP response lasted 2 min versus 15 min in normal PRP. These results suggest that the magnitude and duration of adenosine's effect on platelets are markedly overestimated by studying platelet suspensions. In blood, the effect of adenosine is smaller in magnitude and very transient. The possibility is discussed that the action of adenosine in vivo on blood platelets can therefore be quite local.