Reversible Platelet Aggregation Research Articles

Interralation of prostaglandin endoperoxide (prostaglandin G 2) and cyclic 3′,5′-adenosine monophosphate in human blood platelets

The prostaglandin endoperoxide, prostaglandin G 2, in platelet-rich plasma may produce reversible platelet aggregation without secretion, irreversible aggregation with secretion of platelet constituents inhibited by indomethacin, or the latter effects despite indomethacin, depending on the concentration of the endoperoxide. Irreversible aggregation and platelet secretion induced by prostaglandin G 2 apparently result from the action of ADP, since these responses are inhibited by 2- n-amylthio-5′-AMP (an inhibitor of the actions of ADP on platelets) and they do not occur in heparinized platelet-rich plasma. Prostaglandin G 2 lowers the platelet level of cyclic 3′,5′-AMP. Its actions are inhibited by elevation of cyclic AMP levels by prostaglandin E 1 or dibutyryl cyclic AMP or adenosine. Like malondialdehyde production induced by thrombin, ADP, or arachidonic acid, prostaglandin G 2-induced malondialdehyde production is reduced by dibutyryl cyclic AMP and prosraglandin E 1. Platelet activation by prostaglandin G 2 is enhanced by the adenylate cyclase inhibitor, 9-(tetrahydro-2-furyl)-adenine. The action of prostaglandin G 2 on platelets is more complex then previously reported.

Reversal of platelet aggregation by aortic microsomes

The microsomal fraction of dog aortas inhibited human platelet aggregation induced by arachidonic acid, ADP, or thrombin. When aortic microsomes were added to a preparation of irreversibly aggregated platelets, the aggregates dispersed after 4–6 minutes. The fact that aortic microsomes inhibit platelet aggregation induced by ADP suggests that its effect is probably on the cellular function of platelets and not in direct competition against thromboxane A 2.

Microembolism in experimental septic shock. Distribution of platelets and fibrinogen after intravenous injection of disintegrated Pseudomonas bacteria to dogs.

Platelets were labelled with 51Cr, fibrinogen with 125I and erythrocytes with 59Fe. Disintegrated Pseudomonas bacteria were injected intravenously and radioactive measurements were made on whole blood; tissue biopsies and clottable fibrinogen. After the infection there was an immediate but transient increase of 51Cr activity in the lung concomitant with a decrease in platelet count and 51Cr activity of blood. In the liver there was a less pronounced increase of 51Cr activity. The fibrinogen concentration decreased slightly, paralleled by the 125I activity of whole blood and of clottable fibrinogen, whereas the 125I activity in the lung and liver remained fairly constant. There was no changes of 51Cr activity or 125I activity in biopsies from muscle, pancreas, small intestine, kidney or spleen. During the experiment (3h) there were no signs of significant disseminated intravascular coagulation other than platelet aggregation. A consumption of fibrinogen related to the formation of fibrin plugs could not be detected. After injection of disintegrated Pseudomonas bacteria reversible platelet aggregates were formed and temporarily trapped in the pulmonary microcirculation. This microembolism might induce tissue damage and could be of importance for the development of septic pulmonary complication.

Platelet Function and Plasma Free Fatty Acids during Acute Myocardial Infarction and Severe Angina Pectoris

The relationship between platelet function and plasma free fatty acid concentration has been studied serially during the initial 24 h in 11 patients suffering from acute myocardial infarction and in 5 patients with severe angina pectoris, Similar results were obtained in the 2 groups. Plasma free fatty acid concentration was initially high, and decreased significantly. The distribution of plasma free fatty acids remained unchanged. Platelet concentration was constant, whereas the percentage of reversible venous platelet aggregates initially was higher than in 11 healthy persons matched for age and sex. Platelet aggregates decreased transiently at 16 h. Venous reversible platelet aggregates correlated significantly with concentration of plasma free fatty acid, thus establishing a possible link between a change in lipid metabolism and platelet function. Plasma concentration of platelet factor 4 increased slightly but significantly during the initial hours. This may indicate an increased platelet release reaction.

Heparin effects on plasma lysolecithin formation and platelet aggregation

1. (1) Intravenous administration of heparin (2,500 or 5,000 units) resulted in a 60–70% increase in the rate of lysolecithin formation in incubated human plasma. 2. (2) This effect has been shown to be due to the release or activation of a plasma enzyme which is distinct from lecithin : cholesterol acyl transferase (LCAT). 3. (3) Both the rate and extent of irreversible platelet aggregation initiated by collagen or adrenalin were reduced in post-heparin platelet samples. By contrast the rate and extent of reversible platelet aggregation induced by adenosine diphosphate was unaltered or even stimulated in post-heparin platelet rich plasma. 4. (4) The rate of dextran-accelerated erythrocyte sedimentation was decreased in post-heparin blood. 5. (5) Heparin added in vitro to plasma, platelet rich plasma or whole blood, at concentrations equivalent to the in vivo levels, did not increase lysolecithin formation neither did it decrease platelet aggregation nor retard red cell sedimentation. This suggests that increased lysolecithin formation activated by heparin in vivo may influence both platelet and erythrocyte behaviour. This is discussed as a mechanism which may be relevant to the prophylactic effects of low dose heparin treatment on the incidence of deep vein thrombosis.

Platelet factor 4 and adenosine diphosphate release during human platelet aggregation.

PLATELET FACTOR 4 (PF4) is a low molecular weight, heat stable protein which neutralizes heparin1,2. Recent reports indicate PF4 is released from human platelets during aggregation by adenosine diphosphate (ADP), adrenaline, thrombin and collagen3,4. It is known that human platelet aggregation in plasma containing citrate proceeds in two phases in response to adrenaline, ADP and thrombin5–8. Reversible platelet aggregation seems to be chiefly the result of changes in the platelet membrane induced by external stimuli. The second phase is accompanied by the selective release of various platelet contents, including adenine nucleotides, 5-hydroxytryptamine and certain lysosomal enzymes9. We have examined the release of PF4 in human platelet-rich plasma with particular reference to the two phases of platelet aggregation. We also compared release of PF4 with ADP release.

In Vitro Inhibition and Reversal of Platelet Aggregation by Splenic Extracts

In Vitro Inhibition and Reversal of Platelet Aggregation by Splenic Extracts

Mechanisms of hemostasis

In vivo experiments were performed on rats. The roles of blood clotting, platelets, and the blood vessel smooth muscle were studied independently or in controlled combinations. Enzymatic reduction of the plasma clotting proteins was achieved by infusing either thrombin or trypsin through a mesenteric vein or into the right external jugular vein. Dibenzyline (phenoxybenzamine) was injected intraperitoneally or intravenously to inhibit constriction of vascular smooth muscle. Thrombocytopenia was obtained by infusion of a combination of adenosine diphosphate and Hageman factor via the right external jugular vein. Hemostasis was attained as long as two of the three components were functional. Removal of any two components resulted in continuous bleeding. Maintenance of hemostasis (permanent hemostasis) required the presence of the clotting mechanisms. These experimental observations lend support to the idea that hemostasis occurs in two phases. The primary phase is initiated by the combined action of reversible platelet aggregation and vascular smooth muscle contraction. Each set of mechanisms enhances the effects of the other set and can initiate hemostasis in severed blood vessels of 50 μ or less in diameter. If the coagulation mechanisms have been suppressed, bleeding will resume after a variable period of closure depending on the strength of the reversible platelet plug and the vascular smooth muscle contraction. The secondary phase must involve the formation of irreversible platelet aggregation with subsequent fibrin formation since the coagulation mechanisms are essential for permanent hemostasis. The coagulation mechanisms alone cannot initiate hemostasis unless some externally applied pressure stops the bleeding long enough for a clot to form.

Observations on the Effect of Glass Beads on Platelet Aggregation and Its Relation to Platelet Stickiness

Summary1. The concept of glass bead induced reversible and irreversible platelet aggregation is presented.2. The effect of glass beads in plasma and whole blood was observed microscopically. The microscopic observations were supported by experimental evidence.3. It is shown that phosphoenol pyruvate (PEP) and pyruvate kinase (PK) are capable of completely preventing aggregation if given before glass bead contact. This indicates that adenosine diphosphate (ADP) is the major factor in stimulating aggregation.4. It is shown that the ADP is released in this system, from the platelets, and not from the red cells.5. Haematocrit changes between 20% and 60% do not affect the platelet stickiness.6. The concentration of ADP apparently determines whether the aggregates become irreversible or not.7. Siliconised glass beads do not prevent aggregation; on the contrary, they increase both the observed rate of formation of aggregates, and the measured platelet stickiness.8. Addition of PEP and PK after glass contact causes the reversible aggregates to disaggregate. However, merely leaving the blood to stand for at least 30 min after contact, produces the same effect. This allows the measurement of pure irreversible aggregation.9. The desirability of measuring the irreversible aggregation is discussed.

PLATELET AGGREGATES AND THE MICROCIRCULATION

Platelet aggregates constitute the first visible evidence of intravascular clotting in flowing blood. Thrombin injection, recalcification of citrated blood used for arterial perfusion, triggering of intrinsic coagulation pathways through addition of long-chain saturated fatty acids to blood, or merely allowing spontaneous clotting to occur in silicone handled blood that contains no added anticoagulant, all lead to this phenomenon. Under these circumstances the initial platelet aggregates slow blood flow, and intravascular coagulation proceeds inexorably to intraluminal fibrin deposition. Thrombin acts upon the platelet, causing the release of adenosine diphosphate (ADP), which is responsible for the platelet aggregation. Adenosine diphosphate added directly to whole blood, on the other hand, induces platelet aggregation which is usually reversible. After some minutes, the platelet masses disaggregate and are not followed by fibrin production. The possible pathologic importance of this reversible platelet aggregation is indicated most clearly by reports from Mustard et al¹that myocardial

Inhibition and Reversal of Platelet Aggregation by Methyl Xanthines

SummaryThe 3 methyl xanthines, caffeine, theobromine, and aminophylline inhibited and reversed platelet aggregation induced by ADP in vitro. Inhibition appeared to be noncompetitive and the effects of the methyl xanthines were not mediated through inhibition of platelet sulphydryl groups, chelation of calcium ions, or plasma pH change. A possible explanation for these properties has been suggested.

Effect of normal and atheromatous aortic tissue on platelet aggregation in vitro.

An investigation was made into the platelet-aggregating ability of human aortic tissue. It was found that potent aggregating substances are present in the vascular wall but that the degree and character of platelet aggregation produced was dependent on the method employed in the preparation of the tissue for study. The supernatants of aortic tissue homogenates were found to produce potent irreversible platelet aggregation that occurred after a latent period of one to two minutes. The activity was heat labile, destroyed by collagenase, sedimented by ultracentrifugation, and was considered to be identical or similar to collagen. An extract obtained by boiling aortic tissue with saline was found to cause less powerful reversible platelet aggregation that occurred within 15 to 30 seconds' contact with platelets; the activity was not affected by heating, incubation with collagenase, or ultracentrifugation, and was considered to be due to adenosine diphosphate. Atheromatous aortic tissue when extracted by either method was found to contain less than one half of the potency of normal tissue in causing platelet aggregation.