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.     

Changes in serum thyrotropin (TSH) in man during halofenate administration

Halofenate, a serum lipid-lowering agent which inhibits binding of thyroid hormone to thyroxine-binding globulin (TBG), was administered daily for 14 days to 8 hypothyroid subjects with elevated TSH concentrations as a result of incomplete thyroxine (T4) therapy. Drug administration resulted in mean increases in serum dialyzable fraction T4 (DFT4) of 52% over pretreatment levels (P less than 0.01) and in dialyzable fraction triiodothyronine (DFT3) of 26% in 7 subjects, (P less than 0.01). During halofenate treatment in these 7 subjects, serum TSH concentrations decreased significantly (mean = 39%, P less than 0.01) when DFT4 and DFT3 were increased by halofenate. In only two subjects was there a convincing temporal relationship between increased serum absolute free T4 (AFT4) and decreased serum TSH concentrations. Contrary to what would be predicted from the "free hormone hypothesis", changes in serum TSH concentration in these hypothyroid patients appeared to relate primarily to changes in the free fraction of circulating T4 and T3 (DFT4, DFT3), rather than to alterations in AFT4 or AFT3. Halofenate did not alter serum TBG binding capacity. An eighth subject did not show increased DFT4 and DFT3 during halofenate treatment despite achievement of therapeutic serum levels of the agent; in this patient, serum TSH levels rose progressively throughout the period of inadequate T4 replacement and halofenate administration. In hypothyroid patients, short-term halofenate use suggests that the pituitary-thyroid hormone feedback circuit can respond to increases in serum DFT4 and DFT3 in the absence of detactable increases in absolute free hormone concentrations.     

Inhibition of collagen-induced platelet activation by arachidonyl trifluoromethyl ketone

Collagen-induced platelet activation is associated with, and markedly potentiated by, the release of arachidonic acid and its subsequent conversion to thromboxane A2. The precise mechanism of arachidonic acid release is unknown. An inhibitor of isolated cytosolic phospholipase A2 (cPLA2), arachidonyl trifluoromethyl ketone (AACOCF3), was used to examine the role that cPLA2 plays in this process. AACOCF3 inhibited platelet aggregation in response to collagen and arachidonic acid but not to thrombin, calcium ionophore, phorbol ester, or a thromboxane mimetic. Thromboxane formation stimulated by thrombin or collagen was inhibited by AACOCF3. However, AACOCF3 did not inhibit collagen-induced [14C]arachidonic acid release. These data are consistent with the inhibitory effects of AACOCF3 on collagen-induced aggregation involving an action on the conversion of arachidonic acid to thromboxane.     

The effect of the chronic internal administration of the new Russian n-3 polyunsaturated fatty acid concentrate--epaden--on blood coagulation system indices in rabbits

Daily oral 6-week administration of epaden in a dose containing 0.3 g eucosopentanoic acid and 0.05 g docosahexaenoic acid caused decrease in collagen-induced platelet aggregation in rabbits in vivo and in the activity of the tissue type plasminogen activator, as well as reduction in the level of antithrombin III cofactor activity. No changes were encountered in ADP-induced aggregation, in the platelet count, in platelet adhesion to collagen, and in activated partial thromboplastin time.     

Different roles of endothelium-derived substances on inhibiting platelet aggregation in whole blood

1. The inhibitory effects of adenosine, nitroprusside (a nitric oxide donor) and prostacyclin on collagen-induced rabbit platelet aggregation were studied under two different conditions: in whole blood with an impedance method and in platelet-rich plasma (PRP) with a turbidimetric method. 2. All substances tested were less potent in whole blood than in PRP, and the differences in IC50 value between whole blood and PRP were not of the same order of magnitude; adenosine (669-fold), nitroprusside (54-fold) and prostacyclin (2-fold). 3. These results imply that (a) some other, as yet unknown, factors in blood modulate the platelet aggregation; (b) adenosine and nitric oxide act close to the endothelium, and (c) prostacyclin acts as a relatively long lasting circulating hormone.     

Synthesis and antiplatelet effects of omega-aminoalkoxylxanthones

A series of omega-aminoalkoxylxanthones were synthesized and tested in-vitro for their ability to inhibit aggregation of rabbit washed platelets and human platelet-rich plasma (PRP) induced by various inducers. Nine of these compounds showed more potent antiplatelet effects than natural norathyriol tetraacetate on collagen-induced aggregation. The various omega-aminoalkoxyl side chains of the synthesized compounds modified the antiplatelet effects. All the compounds tested in human PRP showed significant inhibition of secondary aggregation induced by adrenaline, suggesting that the antiplatelet effects of these compounds is mainly due to an inhibitory effect on thromboxane formation. These compounds at high concentration also cause vasorelaxing action in rat thoracic aorta.     

Inhaled nitric oxide inhibits human platelet aggregation, P-selectin expression, and fibrinogen binding in vitro and in vivo

BACKGROUND: Recent data suggest that inhaled NO can inhibit platelet aggregation. This study investigates whether inhaled NO affects the expression level and avidity of platelet membrane receptors that mediate platelet adhesion and aggregation. METHODS AND RESULTS: In 30 healthy volunteers, platelet-rich plasma was incubated with an air/5% CO2 mixture containing 0, 100, 450, and 884 ppm inhaled NO. ADP- and collagen-induced platelet aggregation, the membrane expression of P-selectin, and the binding of fibrinogen to the platelet glycoprotein (GP) IIb/IIIa receptor were determined before (t0) and during the 240 minutes of incubation. In addition, eight patients suffering from severe adult respiratory distress syndrome (ARDS) were investigated before and 120 minutes after the beginning of administration of 10 ppm inhaled NO. In vitro, NO led to a dose-dependent inhibition of both ADP-induced (3+/-3% at 884 ppm versus 70+/-6% at 0 ppm after 240 minutes; P<.001) and collagen-induced (13+/-5% versus 62+/-5%; P<.01) platelet aggregation. Furthermore, P-selectin expression (36+/-7% of t0 value; P<.01) and fibrinogen binding (33+/-11%; P<.01) were inhibited. In patients with ARDS, after two who did not respond to NO inhalation with an improvement in oxygenation had been excluded, an increase in plasma cGMP, prolongation of in vitro bleeding time, and inhibition of platelet aggregation and P-selectin expression were observed, and fibrinogen binding was also inhibited (19+/-7% versus 30+/-8%; P<.05). CONCLUSIONS: NO-dependent inhibition of platelet aggregation may be caused by a decrease in fibrinogen binding to the platelet GP IIb/IIIa receptor.     

Effect of xanthinol nicotinate treatment on platelet aggregation

Treatment of 24 male patients with 3 g/day of xanthinol nicotinate did not change the in vitro measurements of ADP-induced platelet aggregation but produced a marked inhibition of collagen-induced platelet aggregation. This effect may be connected with the drug-induced depression of the ATP level in platelet-rich plasma. Changes in the platelets in the patients' blood or in the lipid composition and the concentration of uric acid in their serum were ruled out as reasons for the decrease of the collagen-induced aggregation. The activity of the three serum enzymes y-GT, GOT, and GPT and the concentration of the blood sugar did not change.     

Alpha-linolenic acid and marine long-chain n-3 fatty acids differ only slightly in their effects on hemostatic factors in healthy subjects

The effects of alpha-linolenic acid (ALA, 18:3n-3), eicosapentaenoic acid (EPA, 20:5n-3), and docosahexaenoic acid (DHA, 22:6n-3) on hemostatic factors were compared. Healthy subjects (29 women and 17 men aged 20-44 y) received either linseed oil (average ALA intake: 5.9 g/d) or fish oil plus sunflower oil (average EPA + DHA intake: 5.2 g/d) for 4 wk. The supplemented amount of fat was 1.19 mg/kJ (1 g/200 kcal) calculated energy expenditure. Stability of habitual diets was monitored. Blood samples were collected at baseline, at the end of the experimental period, and after a 12-wk follow-up period. Different changes in the study groups were seen only in serum cholesterol and triacylglycerols, platelet fatty acid composition, and ADP-induced platelet aggregation. The treatments did not differ in their effects on collagen-induced platelet aggregation and thromboxane production, aggregation to the thromboxane A2 mimic I-BOP, urinary excretion of 11-dehydro-thromboxane B2 and beta-thromboglobulin, bleeding time, plasma fibrinogen concentration, antithrombin III activity, factor VII coagulant activity, or activity of plasminogen activator inhibitor 1. The results indicate that supplemented ALA from vegetable oil and EPA and DHA from a marine source have largely parallel effects on hemostatic factors.     

Clopidogrel inhibition of stent, graft, and vascular thrombogenesis with antithrombotic enhancement by aspirin in nonhuman primates

BACKGROUND: A recent study showed that clopidogrel reduces thrombo-occlusive complications in patients with symptomatic atherosclerosis more effectively than aspirin. METHODS AND RESULTS: The effects of clopidogrel and aspirin have been compared, singly and in combination, for measurements of 111In-labeled platelets and 125I-labeled fibrin deposition in baboon models of arterial thrombosis and related to platelet aggregation and expression of activation epitopes induced by ADP, collagen, and thrombin receptor agonist peptide (TRAP) and to template bleeding times (BTs). Low-dose oral clopidogrel (0.2 mg. kg-1. d-1) produced cumulative (1) intermediate decreases in 111In-platelet and 125I-fibrin deposition for segments of prosthetic vascular graft, deployed endovascular metallic stents, and endarterectomized aorta (P<0.009 in all cases); (2) elimination of ADP-induced platelet aggregation (P<0.001); (3) modest inhibition of collagen-induced platelet aggregation (P<0.01); (4) no reduction in TRAP-induced platelet aggregation; and (5) minimal prolongation of BTs (P=0.03). High-dose oral clopidogrel (>/=2 mg/kg) produced the same effects within 3 hours. The effects of clopidogrel dissipated over 5 to 6 days. Aspirin 10 mg. kg-1. d-1 alone did not decrease 111In-platelet and 125I-fibrin deposition on segments of vascular graft but detectably decreased 111In-platelet and 125I-fibrin accumulation on stents (P<0.01), minimally inhibited ADP- and collagen-induced platelet aggregation (P<0.05 in both cases), and minimally prolonged BTs (P=0.004). Within 3 hours of aspirin administration, the antithrombotic effects of acute high-dose or chronic low-dose clopidogrel were substantially enhanced, and BTs were modestly prolonged without inhibiting platelet aggregation induced by TRAP (P<0.001 in all cases compared with clopidogrel alone). CONCLUSIONS: Clopidogrel produces irreversible, dose-dependent, intermediate reduction in thrombosis that is substantially enhanced by the addition of aspirin. The effects of combining aspirin and clopidogrel need to be evaluated in patients at risk of vascular thrombosis.     

Inhibitory effects of Acanthopanax gracilistylus saponins on human platelet aggregation and platelet factor 4 liberation in vitro

AIM: To study the effects of Acanthopanax gracilistylus var pubescens Li saponins (AGVPS) on human platelet aggregation and platelet factor 4 (PF4) liberation in vitro. METHODS: Human platelet aggregations induced by ADP, adrenaline, and collagen were measured turbidimetrically. The aggregation curve was recorded on a platelet aggregometer and the maximal aggregation rate (ARmax), effective deaggregation rate in 5 min (DR5 min) and lag time (LT) were autocalculated by the built-in microcomputer; PF4 liberation from human platelets stimulated by ADP and collagen was determined by recording the heparin thrombin clotting time (HTCT). Thrombosis was tested by weighing the wet and dry thrombi formed in a siliconized revolving ring. RESULTS: AGVPS inhibited in vitro the ARmax with IC50 of 1.33 (95% confidence limits: 1.09-1.63, ADP-induced), 1.66 (1.54-1.79, adrenaline-induced), and 4.2 g.L-1 (0.6-29, collagen-induced). The DR5 min (on ADP-induced aggregation) and LT (collagen-induced) were also increased as well. Meanwhile, AGVPS 0.63-2.50 g.L-1 prolonged HTCT on ADP- and collagen-stimulated PF4 liberation. At 0.34-1.39 g.L-1, AGVPS reduced the wet and dry weight of thrombi formed in vitro. CONCLUSION: AGVPS inhibits human platelet aggregation, liberation, and thrombosis in vitro, suggesting its possible antithrombotic action in man.     

Proliferative responses to PHA, anti-CD3 and antigens in patients with lymphoproliferative disease of granular lymphocytes. Mannoproteins of Candida albicans induce proliferation and cytotoxicity

The effect of the specific PAF-antagonist WEB 2086, a thieno-triazolo-diazepine, and ketoprofen, a NSAID, was investigated on PAF-induced bovine platelet aggregation measured ex vivo in platelet-rich plasma (PRP). WEB 2086 was infused intravenously over 1 min followed immediately by ketoprofen administration over 1 s (both drugs = 3 mg/kg), in a group of six healthy male Friesian calves. Depending on the PAF concentration, a reversible (10(-8)-10(-9) mol/l) and irreversible (10(-5)-10(-7) mol/l) platelet aggregation was observed. The reversible aggregation was completely blocked by pretreatment of the animal with WEB 2086 and ketoprofen. The inhibitory effects observed during the irreversible aggregation were 47.22%, 54.00% and 88.00% at 10(-5), 10(-6) and 10(-7) mol/l PAF, respectively. Moreover, the aggregation obtained in these condition became reversible. Maximal inhibitory effect of WEB 2086 and ketoprofen on PAF-induced platelet aggregation in calves was observed within 30 min after administration of both drugs. This inhibition persisted even after 24 h and was significantly different from control with P < 0.05. The combined effect of both drugs exceeded the sum of the individual effects (synergism). It was concluded that WEB 2086 and ketoprofen very effectively blocked PAF-induced bovine platelet aggregation in platelet-rich plasma. The study also suggested a synergism between both substances.     

Specific interaction of HLA antibodies (eluates) with washed platelets

The mechanism of complement-independent action of HLA-A2 antibodies (eluates) on washed platelets was investigated. HLA-specific alteration was confirmed by serological (platelet micro-complement fixation), morphological (platelet spreading) and functional parameters (platelet aggregation, inhibition of collagen-induced platelet aggregation, [14C]serotonin release). In the presence of fibrinogen and calcium ions, HLA antibodies induced instantaneous platelet aggregation and release. Although no morphological (spreading) and functional changes (collagen-induded aggregation) were seen, these platelets did not aggregate or release when fibrinogen was subsequently added. When platelets--in the presence of fibrinogen--were incubated with antibody concentrations too low to induce platelet aggregation or release, specific reduction of platelet reactivity was observed by subsequent collagen aggregation. HLA-specific action of antibodies on washed platelets was inhibited by apyrase and acetyl-salicylic acid, indicating an active participation of platelets in HLA antibody-induced platelet alteration.     

Inhibition of platelet aggregation by acetylsalicylic acid and other inhibitors

Effects on platelet aggregation were examined of acetylsalicylic acid (ASA), indomethacin and a number of other agents including dipyridamole, phenylbutazone and sulfinpyrazone under standardized conditions. The Born turbidometric method of measuring platelet aggregation was used with collagen as the stimulus for aggregation. ASA and indomethacin were shown to be among the most potent inhibitors of aggregation, being active at minimal effective concentrations of 1-3 mug/ml using a 10 min time of pre-incubation with the platelet-rich plasma (degree of aggregation inhibition was time dependent). Most of the other agents tested were also active in vitro and both prostaglandin E1 and adenosine were more potent than ASA or indomethacin. However, these agents were shown not to exert significant inhibitory effects when administered orally to rats (dose 10 and 30 mg/kg). ASA proved to be effective in doses as low as 3 mg/kg, and indomethacin in doses as low as 1 mg/kg orally. The inhibitory effects of ASA on aggregation remained for several days after a single oral dose, whereas the effects of indomethacin disappeared within 24 h.     

Evaluation of platelet function by Sonoclot analysis compared with other hemostatic variables in cardiac surgery

Platelet function can be easily measured as time to peak (TP) by Sonoclot Coagulation & Platelet Function Analyzer (Sienco Inc., Morrison, CO) analysis. However a correlation between Sonoclot analysis and platelet aggregation, which is accepted as a test of platelet function, has not been established. In this study, we compared TP and collagen-induced whole blood platelet aggregation in 15 patients undergoing cardiac surgery. Two or three blood samples were randomly obtained from each patient before and after cardiopulmonary bypass (CPB). Sonoclot analysis, collagen-induced whole blood aggregation, and laboratory measurement (including platelet count and coagulation profile) were measured. Seventy-two samples were obtained (35 before CPB and 37 after CPB). TP was correlated with collagen-induced whole blood aggregation (r = -0.652), platelet count (r = -0.671), fibrinogen level (r = -0.598), prothrombin time (r = 0.394), activated partial thromboplastin time (r = 0.486), and use of CPB (r = 0.380). Significant predictors of TP for multiple linear regression modeling were collagen-induced whole blood aggregation, platelet count, and fibrinogen level (r = 0.742). In conclusion, Sonoclot analysis TP predicts approximate platelet function in patients undergoing cardiac surgery. IMPLICATIONS: Approximate platelet function can be easily measured as time to peak by Sonoclot analysis. In this study, time to peak was predicted by platelet count, whole blood platelet aggregation, and fibrinogen level for multiple linear regression modeling.     

The role of pulmocutaneous baroreceptors in the control of lymphatic heart rate in the toad Bufo marinus

We examined the effect of NO on collagen-induced whole blood aggregation and platelet activation in whole blood by using impedance aggregometry and flow cytometry. For the extracellular generation of NO, we chose sodium nitroprusside dihydrate (SNP), and as intracellular generators of NO, L-arginine and isosorbide dinitrate (ISDN). The latter two significantly inhibited whole blood aggregation, whereas SNP had no such effect. The inhibitory effect of ISDN was diminished by addition of methylene blue (MB) or 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-l-oxyl 3-oxide (carboxyl-PTIO), and the inhibitory effect of L-arginine was diminished by addition of N(G)-monomethyl-L-arginine monoacetate (L-NMMA). Although the addition of ISDN increased the cyclic guanosine monophosphate (cGMP) level in whole blood and in the suspension of platelets and white blood cells (PLTs + WBCs), no increase was found in platelet-rich plasma (PRP). The P-selectin expression on the platelet surface in whole blood was reduced by ISDN and L-arginine. These findings suggest that the intracellular generation of NO inhibits whole blood aggregation, and this mechanism may play an important role in its antithrombotic effect in whole blood.     

Seasonal variations in density of questing Ixodes ricinus (Acari: Ixodidae) nymphs and prevalence of infection with B. burgdorferi s.l. in south central Sweden

The pharmacokinetics and effects on platelet function of dipyrone (1.0 g; 2.5 g; i.v.) and ketorolac tromethamine (30 mg; i.m.) were studied in a three-way crossover study in twelve healthy subjects. The biosynthesis of thromboxane A2 in clotting whole blood ex vivo as well as collagen-induced platelet aggregation were determined before and up to 48 h after administration. Both prostanoid biosynthesis and platelet aggregation were inhibited by ketorolac tromethamine for a significantly longer period of time than by both doses of dipyrone. The changes in platelet functions correlated well with the serum concentrations of ketorolac or 4-methylaminoantipyrine and 4-aminoantipyrine. Using the sigmoidal Emax model the mean serum concentration (SD) of ketorolac, 4-methylaminoantipyrine and 4-aminoantipyrine inhibiting platelet TXB2 generation by 50% (EC50) in vitro was found to be 0.088 +/- 0.031, 1.2 +/- 0.3 and 10.2 +/- 3.4 micrograms ml-1, respectively. In conclusion the recovery of platelet function after dipyrone administration is faster as compared to ketorolac tromethamine. This is in line with clinical observations and may be an advantage when these drugs are given as postoperative analgesics at the doses tested.     

The solution structure of human endothelin-2 a 1H-NMR and CD study

OBJECTIVE: Peroxynitrite (ONOO-) is an oxidant formed from the rapid reaction of superoxide and nitric oxide (NO) at sites of inflammation. The literature reports conflicting data on the effects of ONOO- in biological systems, with both NO- and oxidant-dependent effects having been demonstrated. The aim of this study was to investigate these distinct mechanisms through examining molecular aspects of the effects of ONOO- on human platelets, a system in which we have previously shown that ONOO- has both pro- and anti-aggregatory effects. METHODS: Platelet function was assessed by measuring platelet P-selectin expression flow cytometrically, intraplatelet Ca2+ concentrations, and by light aggregometry. A colorimetric method was used to measure extracellular platelet membrane thiols. The contribution of NO and cGMP to the pharmacological effects of ONOO- was investigated using an inhibitor of the soluble guanylate cyclase (sGC), 1H-[1,2,4]oxadiazolo[4,3-alpha]quinoxalin-1-one (ODQ), and the NO scavenger oxy-haemoglobin. RESULTS: Peroxynitrite (50-400 microM) caused a concentration-dependent increase in the number of platelets expressing P-selectin, an increase in intraplatelet Ca2+ concentrations and a decrease in platelet membrane thiols. Peroxynitrite-induced P-selectin expression was augmented by ODQ. In contrast, when P-selectin expression was elicited by collagen, ONOO- acted as an inhibitor of this process, an effect that was further enhanced by the addition of 1% plasma, ODQ or oxy-haemoglobin abolished this inhibitory effect. Finally, low concentrations (50-100 microM) of ONOO- inhibited collagen-induced platelet aggregation, an effect that was reversed by oxy-haemoglobin. CONCLUSIONS: Peroxynitrite exerts dual effects on platelets, which are either activating or inhibitory due to the conversion of ONOO- to NO or NO donors. Peroxynitrite-induced platelet activation seems to be due to thiol oxidation and an increase in intracellular Ca2+. It is important to note that inhibitory, NO-dependent effects occur at lower concentrations than the activating effects. These data are then consistent with the conflicting literature, showing both damaging and cytoprotective effects of ONOO- in biological systems. We hypothesize that the conversion of ONOO- to NO is the critical factor determining the outcome of ONOO- exposure in vivo.     

Biphasic initial phase of platelet aggregation inhibition after oral aspirin

For one hour after the ingestion of 1 g aspirin the pharmacodynamics of acetylsalicylic acid with regard to the inhibition of platelet aggregation were studied in nine healthy male volunteers. Plasma salicylic acid (SA) and acetylsalicylic acid (ASA) levels were measured, and platelet aggregation was controlled by the collagen-induced aggregation. It took 12 - 24 minutes till the maximum of platelet aggregation inhibition was reached; maximal inhibition was only observed with ASA levels above 4.5 /microgram/ml and total ASA levels above 10 /microgram/ml. At that time already more than 50% of the total ASA were hydrolysed to minimally active SA. In spite of further increasing ASA levels inhibition of platelet aggregation decreased again. The different sensitivity of platelet- and vessel wall cyclooxygenase to aspirin does not explain our findings.     

Enzymatic removal of ADP from plasma: unaltered platelet adhesion but reduced aggregation on subendothelium and collagen fibrils

Subendothelium of rabbit aorta and fibrillar collagen were exposed to citrated human or rabbit blood which was circulated through a perfusion chamber under flow conditions similar to those found in arteries. The resulting platelet adhesion and subsequent formation of platelet microthrombi on the exposed surfaces were measured in 0.8 mum thich sections by a morphometric technique using light microscopy. Removal of plasma ADP by the substrate-enzyme combination CP-CPK (creatine phosphate-creatine phosphokinase; 3 mM and 90 U/ml blood) did not affect the initial attachment and spreading of platelets on subendothelium, whereas platelet thrombus formation was strongly inhibited. On free collagen fibrils CP-CPK was much less inhibitory on platelet thrombus formation but platelet adhesion again was not affected. It is concluded that platelet aggregation induced by thrombogenic surfaces in the presence of arterial blood flow is at least partially governed by ADP released from adhering platelets. Platelet adhesion to the examined surfaces, however, does not seem to be mediated by plasma ADP.