The influence of red blood-cells on platelet adhesiveness

Inhibition of red blood cell-induced platelet aggregation in whole blood by a nonionic surfactant, poloxamer 188 (RheothRx injection)

RheothRx Injection, an aqueous solution of a nonionic block copolymer (poloxamer 188) formulated for intravenous administration, was investigated as an inhibitor of red blood cell (RBC)-induced platelet aggregation at plasma concentrations of 0.05-5mgmL-1. Platelet aggregation was determined by measuring the fall in single platelet counts after mechanical agitation of 2mL aliquots of citrated whole blood in a 37 degrees C shaking waterbath. Inhibition of RBC-induced platelet aggregation of > 95% was observed for poloxamer 188 at a concentration of 1mgmL-1, and 41% inhibition was observed at 0.05mgmL-1. Poloxamer 188 was observed to be a more effective inhibitor of RBC-induced platelet aggregation than 2-chloradenosine (2-ClAd) or phosphoenolpyruvate/pyruvate kinase (PEP/PK). Studies using platelet rich plasma (PRP) showed that platelet aggregation could not be induced by shaking in the absence of RBC, though aggregation was induced by the addition of exogenous adenosine diphosphate (ADP). Poloxamer 188 did not inhibit ADP-induced platelet aggregation. We propose that poloxamer 188 protects RBC from mechanical trauma by non-specific adsorption of copolymer to the RBC surface (via the hydrophobic polyoxypropylene moiety), and that this effect prevents mechanical damage and hence leakage of ADP from RBC. RheothRx Injection has been shown to have value in the treatment of acute ischemic disorders such as myocardial infarction. The observation of significant inhibition of RBC-induced platelet aggregation at clinically relevant concentrations suggests that RheothRx Injection may have antithrombotic properties in vivo, and may therefore have potential not only in acute ischemia but also to prevent thrombosis within vascular prostheses or to prevent rethrombosis after angioplasty or endarterectomy.

Polycythaemia vera and cerebral blood flow: a preliminary study with transcranial Doppler

OBJECTIVES

The purpose of this study has been to investigate by ultrasonographic methods the flow velocities of cerebral arteries because increased blood viscosity due to haematocrit elevation can cause neurological symptoms in polycythaemia vera patients, because of the resulting decrease in cerebral flow.

SUBJECTS AND DESIGN

Twenty newly diagnosed patients, with haemoglobin values of > 18 g dl-1 and/or an haematocrit of > 50%, were examined by transcranial Doppler. Recordings were performed in basal conditions and after pharmacological and/or phlebotomic treatment, when haematocrit values were < or = 50%. Blood velocities were evaluated in middle (MCA), anterior (ACA), posterior (PCA) cerebral arteries and in the basilar (BA) artery.

RESULTS

Basal recordings showed decreased velocities (MCA: 39.40 +/- 9.34 cm s-1; ACA: 34.05 +/- 10.25 cm s-1; PCA: 31.46 +/- 5.97 cm s-1; and BA: 27.47 +/- 7.42 cm s-1); pre- and post-treatment value differences observed in MCA, ACA and BA were highly significant (P < 0.001).

CONCLUSIONS

A decrease in cerebral flow could be a risk for multifocal micro-ischaemic cerebral infarctions leading, after several years, to a multi-infarct dementia; an early reduction in erythrocyte burden should be very useful in polycythaemic patients in preventing lacunar lesions.

Platelet function in uncomplicated insulin-dependent diabetic patients at rest and following exercise

There are conflicting reports of platelet function abnormalities in diabetic patients without vascular complications. We have studied in vitro platelet aggregation, using platelet rich plasma and whole blood techniques, in 18 patients with uncomplicated insulin-dependent diabetes and a matched group of 24 non-diabetic subjects. In addition we measured plasma beta-thromboglobulin levels in these groups, as an index of in vivo platelet activation, and compared the indices of in vitro and in vivo platelet function before and after maximal bicycle exercise. Before exercise plasma beta-thromboglobulin levels and platelet sensitivities to ADP, collagen or adrenaline, as assessed by both methods of platelet aggregation, were the same in diabetic and control subjects. Both groups showed similar increases in beta-thromboglobulin levels and in platelet sensitivity to all agonists in whole blood following exercise. Using platelet rich plasma there were no changes in platelet sensitivity in either group after exercise. In non-diabetic subjects, increases in noradrenaline levels after exercise correlated with increases in platelet sensitivity to adrenaline in whole blood. This was not observed in the diabetic group. Abnormalities of platelet function, using the techniques described here, are not present in diabetic patients who do not have clinical evidence of vascular disease.

Red blood cells mediate spontaneous aggregation of platelets in whole blood

The influence of red blood cells on spontaneous platelet aggregation (SPA) has been studied ex vivo. Platelet aggregation was quantified by measuring the fall in single platelet count using a new whole blood platelet counter. When aliquots of whole blood and autologous platelet rich plasma (PRP) were roller-mixed at 37 degrees C, a marked fall in platelet count occurred in whole blood due to SPA but platelet count remained almost unchanged in PRP. When blood from healthy young controls, aged 20-35 years, was compared with healthy old controls, aged 48-80 years, and patients with thrombotic complications, the extent of SPA was in the order: thrombotic patients greater than old controls greater than young controls. Prostacyclin and the new stable prostacyclin analogue Iloprost, at 8 nM effectively inhibited SPA. 2-Chloroadenosine (10 microM) which is an inhibitor of ADP-induced platelet aggregation was also an effective inhibitor of SPA. Acetylsalicylic acid (56 microM) and the thromboxane A2 receptor blocker BM13.177 (0.5 microM) only partially inhibited SPA. ADP from red blood cells is suspected to mediate red cell-induced SPA. However, the possibility that the red cells have an important physical role in SPA cannot be ruled out.

Enhancement of collagen-induced aggregation of platelets in whole blood

In order to elucidate the features of platelet aggregation in whole blood, studies were carried out on human blood. The platelet aggregation reaction was monitored by counting the residual free platelet number with an electronic particle counter (Coulter). Platelets in citrated whole blood were aggregated by a very small amount of collagen which did not aggregate platelets in citrated plasma. Such enhancement was not observed if ADP or epinephrine was used. By addition of isolated erythrocytes to platelet rich plasma, enhancement of the platelet response to collagen was obtained. The erythrocyte membrane stabilizer, Dilazep, abolished the enhancement effect of erythrocytes. Those results indicated that erythrocytes enhanced the platelet response to collagen. Although participation of ADP from the erythrocytes in the enhancement was suggested, the results of ADP determinations on suspensions of erythrocytes indicated that other factors of the erythrocytes might be involved in the enhancement.

A study of the haematological and haemorheological consequences of venesection

A study has been made of a number of haematological and haemorheological factors following venesection therapy. Haematocrit was very effectively reduced by the venesection regimen and led to a fall in whole blood viscosity in spite of the microcytosis that was eventually induced in all the subjects. No clinically significant coagulation changes were observed. It is concluded that a simple venesection regimen without volume replacement is a safe procedure for improving blood rheology.

Further studies on the role of red blood cells in spontaneous platelet aggregation

The influence of red blood cells on platelet aggregation has recently been a subject of considerable interest. We have studied the effect of red cells on spontaneously formed platelet aggregates in rotating vials at 37 degrees C. Platelet aggregation was quantified by measuring the fall in number of single platelets with a whole blood platelet counter. Autologous packed red cells, platelet rich plasma and platelet free plasma were used to reconstitute aliquots of blood with constant platelet count but 0-60% haematocrit (Hct). The fall in platelet count was minimal at zero Hct, increased markedly with the Hct in the anaemic range and less markedly in the normal to polycythaemic ranges of Hct. Scanning electron microscopic observation of whole blood showed the presence of small platelet aggregates after about 3 mins rotation and very large aggregates after about 12 mins. ADP from red cells has been implicated in triggering platelet aggregation in whole blood. Whether aggregates are formed as a result of ADP leaking from the red cells or by their jostling physical action on the platelets is discussed. The marked effect of the red cells on spontaneous platelet aggregation however, justifies the manipulation of the Hct as a useful therapeutic option in the control of thrombotic and bleeding tendencies.

Effect of L-histidine in vivo on human platelet function and arachidonic acid metabolism

The effect of the amino acid l-histidine on human platelet function and arachidonic acid metabolism was studied in 18 healthy subjects with increased spontaneous platelet aggregation. The participants received placebo or l-histidine 3g/day for 7 days. The intake of l-histidine reduced the degree of spontaneous platelet aggregation (p less than 0.05) and inhibited the generation of platelet TXB2 by 47% (p less than 0.001) whereas platelet PGE2 synthesis was not affected. The mean l-histidine plasma concentration increased from 83.1 +/- 2.4 to 108 +/- 8.1 mumol/l (p less than 0.01) during the study. L-histidine was found to be an effective inhibitor of spontaneous platelet aggregation and platelet TXB2 generation. The present data verify interactions of histidine with human platelet function, probably mediated by arachidonic acid metabolites.

Effects of ethanol on platelet aggregation: an in vitro study

Alcohol ingestion results in the formation of circulating microaggregates in the pig. To investigate the underlying mechanism, the effects of alcohol on platelet aggregation using a Born-aggregometer, was investigated. After incubating unstirred platelet rich plasma (PRP) with moderate concentrations of alcohol (175 mmol/l) the aggregation induced by collagen was reduced. This was probably due to platelet refractoriness caused by platelet ADP release. We could also demonstrate that high concentrations of alcohol (630 mmol/l) caused platelet release in stirred PRP. Release of ADP from red cells, after incubating unstirred whole blood with low concentrations of alcohol (17 mmol/l), was the probable explanation to the observed platelet refractoriness to ADP and collagen. Alcohol causing release of ADP from red cells is likely the cause of platelet aggregation in circulating blood and is probably the mechanism in formation of circulating platelet aggregates after alcohol ingestion.

Effect of haematocrit on carotid stenosis and cerebral infarction

Carotid angiograms in 187 patients presenting with transient ischaemic attacks and minor completed strokes were reviewed. A haematocrit of 50% or more was encountered more frequently in those found to have carotid occlusion. The severity of vessel-wall disease showed no relationship to the level of haematocrit. Computerised axial tomographic to the level of haematocrit. Computerized axial tomographic measurements of the volume of the cerebral infarct seen in 23 patients with carotid occlusion and completed strokes showed a correlation between the size of the infarct and the height of the haematocrit. It is suggested that reduced blood flow resulting from increase blood viscosity associated with a high haematocrit adversely affects collateral flow, thereby increasing the size of the infarct.

New aspects of the pathogenesis of atherosclerosis: possible approaches to prevention and treatment

New techniques in cell biology and recent advances in rheology have opened up fresh perspectives in the understanding of the pathogenesis of atherosclerosis. There is now an increasing amount of experimental evidence to support the hypothesis that arterial thrombi are the first stage in atherogenesis in the context of endothelial lesions and not merely secondary factors without any causal significance. Factors influencing platelet aggregation and adhesiveness are discussed, and the animal and clinical studies carried out with the platelet aggregation inhibitor pentoxifylline and its effects on the microcirculation are reviewed.

Coronary arteries in fatal acute myocardial infarction

The coronary arteries are diffusely involved by atherosclerotic plaques in fatal acute myocardial infarction (AMI). The degree of luminal narrowing may vary but plaques are present in practically every millimeter of extramural coronary artery. Usually the lumens of at least two of the three major coronary arteries are narrowed > 75% by old plaques in patients who die suddenly (< 6 hours) from cardiac disease with or without myocardial necrosis. Coronary thrombi occur in about 10% of patients who die suddenly or in whom necrosis is limited to the left ventricular subendocardium, and in about 50% of patients with transmural myocardial necrosis. Coronary thrombi usually indicate the presence of shock or congestive heart failure or both during the development of myocardial necrosis. The infrequency of coronary thrombi in patients dying suddenly of cardiac disease and in those with transmural necrosis who never have shock or congestive heart failure suggests that the thrombi may be consequences rather than causes of AMI.

Although it may not precipitate AMI, coronary thrombosis may still be the underlying cause of the atherosclerosis. The finding of fibrin deposits in old atherosclerotic plaques and the findings of atherosclerotic-type lesions (cholesterol clefts, foam cells, pultaceous debris, calcific deposits) in organized known thrombi (as in the left atrium in mitral stenosis) suggest a strong relationship between thrombosis and atherosclerosis.

Coronary arterial emboli are not rare; they are located in distal portions of the coronary tree and are present in the small epicardial branches as well as in intramural coronary arteries. In contrast, coronary thrombi are located in proximal portions of major extramural vessels, are infrequent in the small epicardial branches, and are absent in intramural coronary arteries. Coronary atherosclerosis is limited to the extramural coronary arteries and spares the intramural coronary arteries.

The influence of salicylate on platelets and whole blood adenine nucleotides

1. The addition of sodium salicylate to freshly withdrawn human blood or native platelet-rich plasma significantly delays platelet aggregation in vitro.2. The administration of acetylsalicylic acid to human subjects also significantly delays platelet aggregation in their whole blood and there is a short delay in the formation of fibrin but this is not statistically significant.3. Salicylate, whether added to human blood in vitro as sodium salicylate or given by mouth as acetylsalicylic acid, significantly reduces the platelet clumping activity of adenosine diphosphate (ADP) added to whole blood in vitro.4. The administration of aspirin in high doses for several days produces a marked increase in the total adenine nucleotide content of whole blood. The percentage of adenosine triphosphate (ATP) was increased, that of ADP decreased while there is an obvious increase in the ATP: ADP ratio.5. There is little correlation between the plasma salicylate level and the delay aspirin produces in platelet aggregation in vitro or the changes that occur in the levels of ADP or ATP in whole blood during administration of aspirin.6. Significant correlations do occur, however, between the delay in platelet aggregation in vitro and (i) the percentage increase in the ATP concentration, (ii) the percentage decrease in ADP concentration, (iii) the percentage change in the ATP: ADP ratio observed during aspirin administration.

Role of erythrocytes in the acute platelet response to operation

A marked increase in platelet adhesiveness was shown to occur within four hours of operation. Postoperative red cells produced greater adhesiveness in platelets from a common pool than did preoperative red cells. A greater increase in platelet adhesiveness was observed following operation with the whole blood technique than when exogenous adenosine diphosphate was added to platelet-rich plasma. Release of adenosine diphosphate from red cells so facilitated may contribute to the acute platelet-adhesiveness response to operation.

Thrombosis in coronary heart disease

Images