Cholestyramine treatment of type IIa hypercholesterolaemia normalizes platelet reactivity against prostacyclin

Platelet-dependent thrombin generation in patients with hyperlipidemia

OBJECTIVES

We evaluated coagulability as determined by platelet-dependent thrombin generation in hypercholesterolemic patients before and after treatment with pravastatin and in hypertriglyceridemic patients to investigate the usefulness of coagulability as an index of atherosclerosis and to determine the importance of treating hyperlipidemia.

BACKGROUND

An understanding of the interaction between platelets and the plasma coagulation system is important for clarifying the mechanism of the procoagulant process.

METHODS

We assessed coagulability in 58 patients with hypercholesterolemia (serum total cholesterol level > or = 220 mg/dl, age 56.5 +/- 1.5 years [mean +/- SEM]), 37 patients with hypertriglyceridemia (serum triglyceride level > or = 200 mg/dl, age 59.5 +/- 1.7 years), 13 patients with hypercholesterolemia plus hypertriglyceridemia (age 51.4 +/- 3.1 years) and 75 normal subjects (age 52.2 +/- 1.7 years). We also studied platelet-dependent thrombin generation in patients with hypercholesterolemia before and after treatment with pravastatin. Calcium chloride was added to 0.5 ml of platelet-rich plasma (150 x 10(9)/liter) to initiate coagulation. Ten microliters of the sample was transferred into 90 microliters of 3.8% sodium citrate at 10-min intervals for 30 min. A chromogenic substrate, S-2238, was added to each sample, and absorbance was measured spectrophotometrically at a wavelength of 405 nm to determine thrombin generation.

RESULTS

Platelet-dependent thrombin generation was increased in patients with hypercholesterolemia and patients with hypercholesterolemia plus hypertriglyceridemia (p < 0.01) compared with patients with hypertriglyceridemia and control subjects. Treatment with pravastatin normalized thrombin generation.

CONCLUSIONS

Hypercholesterolemia, but not hypertriglyceridemia, was associated with increased platelet-dependent thrombin generation. Pravastatin normalized the generation of thrombin.

Prostacyclin, nitric oxide, and atherosclerosis

Disorders in arterial production of PGI2 and NO occur in atherosclerosis. Exogenous PGI2 and NO are capable of interacting pharmacologically. We claim that no such direct interactions occur between endogenous endothelial PGI2 and NO. Studying mechanisms of cardiac reactive hyperemia in guinea pigs and of thrombolysis in cats, we surmise that in vivo vascular intima releases PGI2 intraluminally while NO is secreted abluminally and thus these two ephemeral mediators do not see each other. Hence, in any disease, the disturbances in endothelial generation of PGI2 or NO have to be scrutinized separately. It may well be that endogenous PGI2 maintains endothelial thromboresistance while NO controls arterial myocytes and tissues in which microcirculation is embedded. These responsibilities remain unshared. Interactions between PGI2 and NO are confined to pharmacological domains.

The actions of lovastatin on platelet function and platelet eicosanoid receptors in type II hypercholesterolaemia. A double-blind, placebo-controlled, prospective study

We have studied the effects of 12 weeks of lovastatin (20 mg per day) on platelet function and thromboxane formation in 18 patients with type II hypercholesterolaemia in a double-blind, placebo-controlled, prospective study. Lovastatin significantly reduced total serum and LDL-cholesterol by 20% and 25% respectively. Washed platelets of lovastatin-treated patients had significantly reduced collagen-induced aggregation and thromboxane formation ex vivo. There was no change in ADP-induced platelet aggregation, but a significant increase in prostacyclin (iloprost)-stimulated platelet cyclic AMP concentrations in lovastatin-treated patients. This was associated with a significant increase in the number of prostacyclin receptors in platelet membranes prepared from lovastatin-treated patients. There was also an increase in platelet thromboxane receptors. There were no such changes in the placebo group. These data confirm our original observation of normalization of platelet function in hypercholesterolaemia by HMGCoA reductase inhibitors and suggest changes in platelet membrane composition at the megakaryocyte level as a possible site of action.

Influences of lipid-modifying agents on hemostasis

Drugs affecting lipid metabolism may influence, to a variable extent, the hemostatic system, that is, platelet activation, fibrinogen, and fibrinolysis. These effects may or may not be linked to the activity of these compounds on the lipid/lipoprotein profile. For this reason it may be important to consider the effects of hypolipidemic drugs on the different aspects of hemostasis, because this may allow a better understanding of their clinical use, as well as, eventually, a more proper selection in individual patients. Among the major lipid-lowering agents, fibric acids belong to a multifaceted series of abnormal fatty acids known to interact with a liver nuclear receptor, in turn activating fatty acid catabolism. A similar activity may be exerted by n-3 fatty acids from fish, as well as by other chemically related or unrelated compounds. Among fibric acids all but gemfibrozil can reduce fibrinogen levels; this last drug can, however, apparently activate fibrinolysis. Among the selective cholesterol-lowering medications, both resins and HMG CoA reductase inhibitors may reduce, in some patients, over prolonged periods of treatment, platelet sensitivity to major aggregants. This effect may be seen best with non-liver-selective agents (e.g., simvastatin), although recent data cast doubt on its constancy. A direct comparative evaluation of different HMG CoA reductase inhibitors on platelet aggregability has never been carried out. These last drugs may also reduce the circulating levels of the tissue factor pathway inhibitor (TFPI), transported by LDL in plasma, which is a potentially negative effect. A lipid-lowering molecule with antioxidant activity, for example, probucol, may also possibly play a role in controlling platelet activation.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduction of LDL cholesterol by pravastatin does not influence platelet activation in patients with mild hypercholesterolaemia at risk of coronary heart disease

The effect of pravastatin on low density lipoprotein (LDL) cholesterol and platelet activation was studied in 16 patients with mild hypercholesterolaemia who had two or more additional cardiovascular risk factors. Patients were treated with either pravastatin (20-40 mg day-1) or placebo for 1 year. Plasma LDL and urinary excretion of 2,3-dinor-thromboxane B2 (an index of platelet activation in vivo) were determined at 0, 3, 6 and 12 months. There was a significant reduction in LDL at 6 and 12 months (2P less than 0.05) but this was not associated with any significant change in thromboxane metabolite excretion.

Platelet membrane fluidity in type IIA, type IIB and type IV hyperlipoproteinemia

Fluorescence spectroscopy, a very sensitive index for measuring the biophysical properties of living cell systems, was used to examine the structural order of intact, resting, gel-filtered platelets from hyperlipidemic subjects (n = 48, 25-70 years) and normolipemic subjects (n = 34, 19-68 years). Fluorescence anisotropy (r[s]), which is inversely related to membrane fluidity, was estimated using 3 different fluorescent dyes, DPH, TMA-DPH, and 6-AS, known to label different regions of biological membranes. Increased membrane fluidity was observed in type IIB (n = 24, 36-62 yrs; r[s] = 0.0692 +/- 0.09) and type IV (n = 10, 33-57 yrs; r[s] = 0.058 +/- 0.006) hyperlipidemics in comparison to type IIA (n = 14, 25-70 yrs; r[s] = 0.086 +/- 0.019) and control subjects (n = 24, 28-68 yrs; r[s] = 0.079 +/- 0.012). The temperature dependency of r[s]-DPH values was significantly different (P less than 0.01) in platelets from type IIB and type IV patients compared to type IIA and control subjects of similar age. A significant positive correlation (P less than 0.005) between membrane fluidity and age was found only in healthy control subjects (n = 34, 19-68 yrs). Despite significant (P less than 0.01) differences in plasma lipid concentrations in hyperlipidemic patients and controls, significant ex vivo relations between membrane fluidity and lipoprotein concentrations, free fatty acid distribution, and increased age were found only in healthy control subjects. Plasma levels of thromboxane as well as serum selenium concentrations did not significantly differ between hypercholesterolemic, hypertriglyceridemic, and control subjects.

Lipids and thrombosis

There is extensive evidence of important interactions between plasma lipoproteins and platelet function. Some population groups, particularly hypercholesterolaemic patients, have strong evidence of abnormal platelet function which is mediated by the binding of lipoproteins, especially oxidized LDL, to surface receptors. Additionally, abnormal plasma lipid levels precipitate membrane composition changes by increasing the cholesterol:phospholipid ratio. The resulting changes in microviscosity seem to affect transmembrane signalling and might in some cases influence receptor binding. This not only has important therapeutic implications with regard to lipid-lowering drug therapy but also with regard to the potential beneficial effects of dietary therapy.

Effects of synvinolin on platelet aggregation and thromboxane B2 synthesis in type IIa hypercholesterolemic patients

An increased susceptibility of platelets to aggregation induced by various agents and a higher production of active arachidonate metabolism have been described in type IIa hypercholesterolemia. This study was designed to evaluate whether changes in platelet function could be observed in hypercholesterolemic patients after synvinolin therapy. Administration of synvinolin to 12 type IIa hypercholesterolemic patients for 24 weeks had a lipid lowering effect and resulted in a marked reduction of platelet aggregation and thromboxane formation induced by collagen and arachidonate. Maximum response was achieved at 4-8 weeks and lipid lowering effects at 2 weeks. This finding indicates that platelet changes cannot be explained by a direct effect of synvinolin on platelets, and the antiplatelet response may therefore depend on platelet membrane lipid composition changes, particularly in the platelet cholesterol content of platelet membranes, following substantial reductions of total plasma cholesterol and LDL-cholesterol.