Platelets, thrombosis and atherosclerosis

Dose of Aspirin in the Treatment and Prevention of Cardiovascular Disease: Current and Future Directions

In meta-analyses of randomized trials of aspirin among patients with prior occlusive vascular disease events (secondary prevention), doses from 75 mg to more than 1500 mg daily provide similar benefits on myocardial infarction, stroke, and cardiovascular death. In acute myocardial infarction and during acute occlusive stroke, a loading dose of 162.5 to 325 mg is necessary to achieve a rapid clinical antithrombotic effect. In primary prevention trials, predominantly among men, aspirin (75 mg daily to 325 mg on alternate days) reduced the risk of a first myocardial infarction. In a large-scale trial in women, aspirin (100 mg on alternate days) reduced risk of a first stroke. In subgroup analyses of women older than age 65, aspirin significantly reduced first myocar-dial infarction and ischemic stroke. Direct comparisons of higher doses may yield additional cardiovascular benefits. At present, daily doses of 75 to 325 mg aspirin are sufficient for long-term treatment and prevention of cardiovascular disease.

Isradipine decreases arterial thrombogenicity in rabbits. A morphometric and radioisotopic study

The effect of isradipine, a calcium antagonist, on aortic and iliac wall thrombogenicity was examined in rabbits. After one week of dosing, the abdominal aortic and iliac artery endothelium was abraded using a Fogarthy catheter. One group of animals (n = 8) was dosed for one week with isradipine 0.3 mg/kg. A second group of animals received 10 mg acetylsalicylic acid (ASA)/kg daily in addition, while a third group received the vehicle only. Finally, a fourth group of animals (n = 8) was treated with ASA only. The percentage denuded surface covered with contact (unspread) platelets decreased significantly (p < 0.01) from 14.7 +/- 2.0 to 9.3 +/- 2.1 (6.2 +/- 0.8 to 3.7 +/- 0.4). The amount of contact and spread platelets was diminished from 84.9 +/- 5.6 to 71.4 +/- 4.4 (91.8 +/- 5.3 to 75.2 +/- 4.6). Platelet thrombi decreased from 7.4 +/- 0.9 to 4.6 +/- 1.4 (9.4 +/- 1.9 to 5.2 +/- 0.7) in the aortic and the iliac artery, respectively. In-platelet deposition decreased by 39.9 and 41.9%. Concomitant ASA therapy not only abolished the effect of isradipine but enhanced thrombogenicity, probably as a result of almost complete blockade of vascular PGI2-production.

Eicosanoids, other fatty acid metabolites and the cardiovascular system: are the present antithrombotic approaches rational?

Antiplatelet +/- anticoagulant drugs are currently used as the standard treatment to prevent and treat thrombosis. While this approach is beneficial, it is not optimal. Recent evidence suggests that constituents of the vascular endothelium play an important role in regulating vessel wall thrombogenecity, thereby inhibiting thrombogenesis. These include constituents such as PGI2, tissue plasminogen activator, thrombomodulin and the lipoxygenase fatty acid metabolite derived from linoleic acid, 13-hydroxyoctadecadienoic acid (13-HODE). Consequently, new strategies have been developed to maximize the use of these agents for antithrombotic therapy. We will review these different approaches, discuss their rationale, and based upon recent experimental data, introduce an alternative approach for antithrombotic therapy which may circumvent a number of limitations and side-effect of the currently used drugs.

The role of apoE in inhibitory effects of apoE-rich HDL on platelet function

Apolipoprotein E- (ApoE-) rich high-density lipoprotein (HDL), which was prepared from the bound fraction of normolipemic volunteers on heparin-Sepharose and from a hyperalphalipoproteinemic patient, potently inhibited aggregation of human platelets in a dose-dependent fashion. Dimyristoyl phosphatidylcholine liposome with apoE (apoE.DMPC) also inhibited platelet aggregation, and incubation of washed platelets with apoE.DMPC resulted in the release of cholesterol into the supernatant in a time- and dose-dependent manner. These results suggest that apoE plays a major role in the inhibitory effect of apoE-rich HDL in platelet function, presumably due to the release of cholesterol from the plasma membrane.

Selective effects of dietary fats on vascular 13-HODE synthesis and platelet/vessel wall interactions

Fish oil (FO) diets are associated with decreased thrombosis, which is though to be related, in part, to changes in platelet and vessel wall prostanoid synthesis. Recently, we found that 13-hydroxyoctadecadienoic acid (13-HODE) synthesized in the vessel wall from linoleic acid (LA, 18:2 n-6) via the lipoxygenase pathway, also decreases platelet/vessel wall interactions. Thus, we determined whether diets containing fish oil, walnut oil (rich in linoleic acid), black currant seed oil (rich in both linoleic and gamma linolenic acids, 18:3 n-6), or lard influenced vessel wall 13-HODE synthesis and platelet/vessel wall adhesion in rabbits. In vivo, vessel wall thrombogenicity was decreased in animals fed the black currant seed oil rich diet for 4 weeks as compared to the control "LARD" diet. This latter effect was better obtained when gamma linoleic acid was present suggesting a secondary effect of this fatty acid. The decreased vessel wall thrombogenicity in those animals, was associated with increased vessel wall 13-HODE synthesis. In contrast, ex vivo platelet adhesivity was significantly decreased in the fish oil diet fed animals, as compared to the control "LARD" diet and correlated with decreased platelet 12-HETE synthesis. We conclude that both fish oil and black currant seed oil rich diets inhibit platelet/vessel wall adhesion; the black current seed oil diet by increasing the availability of linoleic acid for 13-HODE synthesis and inhibiting vessel wall thrombogenicity; the fish oil diet, by inhibiting platelet 12-HETE synthesis and subsequent platelet adhesion.

Endothelium and underlying membrane reactivity with platelets, leukocytes and tumor cells: regulation by the lipoxygenase-derived fatty acid metabolites, 13-HODE and HETES

We hypothesize that the ratio of intracellular 13-hydroxy-octadeca-dienoic acid (13 HODE) and hydroxy-eicosatetraenoic acid (5-, 12- and/or 15-HETE) influences the expression or presentation of adhesive moieties on platelets, leukocytes, malignant cells and endothelial cells, thereby influencing their subsequent adhesive interactions. Thus, we demonstrate that under unstimulated conditions, these cells preferentially synthesize linoleic acid via their lipoxygenase enzymes into 13-HODE, the intracellular level of which is associated with limited or no cell adhesion, while following stimulation, the same cells preferentially metabolize arachidonic acid via the lipoxygenase enzyme into HETEs, the production of which is associated with enhanced adhesion. Which metabolite is synthesized by these cells and the subsequent adhesivity of these cells appear to be dependent upon both the intracellular level of cAMP and the ratio of linoleic and arachidonic acid substrates. This suggests that manipulation of this ratio will have significant effects on the adhesive events involved in the pathogenesis of thrombosis, inflammation and metastasis.

Atrial thrombosis, abnormal electrocardiograms and sudden death in mice due to copper deficiency

Approximately 20 years ago a diet high in lard and sucrose was described that produced extensive cardiovascular damage in adult mice. Atrial thrombosis, myocardial necrosis and sudden death were frequent. These experiments were repeated as closely as possible; the adverse effects were prevented by a drinking solution containing 10 micrograms copper/ml. Lack of copper also was associated with anemia, cardiac enlargement and abnormal electrocardiograms. Bradycardia, coupled beats, ectopic ventricular foci, premature atrial beats and prolonged PR interval were found. Lack of copper had no effect on cholesterol in plasma. The results may be germane to ischemic heart disease and the thrombotic susceptibility of women who use oral contraceptives or are pregnant frequently, because copper metabolism is altered in these conditions.

Platelet factor 4 release during exercise in patients with coronary artery disease

Many recent studies provide evidence that increased platelet activation occurs in a significant number of patients with atherosclerotic coronary artery disease. The mechanisms responsible for this activation are unknown, although there have been studies suggesting a correlation with abnormal lipoproteinemia, acute myocardial infarction, unstable angina, and exercise-induced myocardial ischemia. We studied 84 patients undergoing standardized treadmill exercise using either a Bruce [N = 63] or symptom-limited Naughton protocol [N = 21]. In contrast to ten healthy volunteer subjects, the patient group demonstrated a significant increase in plasma concentrations of platelet factor 4 [PF4] between pre- and postexercise blood samples confirming earlier reports of exercise-induced platelet activation and secretion. As with previous studies, however, only a subset of patients demonstrated this response. When the entire group was analyzed for the presence or absence of electrocardiographic ischemic changes and the presence of documented versus suspected coronary artery occlusions, there were no differences noted between groups that explained the variable responses measured. However, there was a significant difference between patient groups when analyzed by whether or not they were being treated with beta-blocking agents. Patients who were being treated with propranolol or one of the longer-acting beta-blocking agents did not have a significant increase in plasma PF4 following exercise, in contrast to patients who were not beta-blocked. Plasma concentrations of epinephrine, norepinephrine, and lactic acid were measured in 49 patients and all normal subjects. There was no correlation between the changes in plasma PF4 concentrations and any of these three variables, suggesting that platelet activation was not occurring through direct platelet activation by circulating catecholamines. This study provides further evidence that there is a subset of CAD patients with platelet hyperactivity. This is the first time that beta-blockade has been demonstrated to modify this platelet response. The effectiveness of beta-blocking agents in CAD may be in part related to their antiplatelet effect.

Racial differences in platelet survival time in patients with symptomatic coronary atherosclerosis

Platelet survival times were studied in 40 patients (21 white and 19 black) with coronary artery disease and stable effort induced angina pectoris. The platelet survival times of 19 white controls (9.27 +/- 0.49 days; mean +/- SD) were not significantly different from those of 12 black controls (8.88 +/- 0.81 days), and the platelet survival times for 21 white patients with coronary artery disease (8.46 +/- 0.65 days) were lower than the times for both the white controls (p less than 0.01) and the combined control group (p less than 0.01). However, the difference between the mean platelet survival times of 19 black patients (9.22 +/- 0.68) and the control groups was not significant, and the difference between the mean platelet survival times of the 21 white patients and the 19 black patients was significant (p less than 0.01). Stepwise multiple linear regression analysis indicated that race was the most significant factor in predicting shortened platelet survival (r = 0.4783; p less than 0.01). It is concluded that racial background should be considered in the interpretation of platelet studies and that reported racial differences in the rate and extent of atherosclerotic lesions may be related to racial differences in platelet consumption.

Platelet function and survival in patients wit severe hypercholesterolemia

Platelet aggregation and survival were measured in twelve subjects with severe hypercholesterolemia. There was a small increase in sensitivity to epinephrine and adenosine diphosphate among the most hypercholesterolemic patients, but this did not correlate with reduced platelet lifespan. Platelet survival was normal or only moderately reduced in the markedly hypercholesterolemic homozygous subjects. However, the incidence of reduced platelet survival was significantly increased (p less than 0.05) among the older patients with more extensive atherosclerotic vascular disease compared to the younger patients with limited vascular disease. Marked hypercholesterolemia in the absence of atherosclerosis does not appear to accelerate platelet destruction, although a modest increased aggregability is present.