The effect of hypercholesterolemia on guinea pig platelets, erythrocytes and megakaryocytes

LOX-1: Its cytotopographical variance and disease stress

Lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) is a canonical receptor for oxidized LDL (oxLDL) among the known modified LDL particles. Topographical variance on LOX-1 expression in different cell types and its influence on the atherogenic potential of the particular cell type is the main focus of this review. Characteristic features of LOX-1 on the atherogenic potential of aortic endothelial cells, macrophages, platelets, and vascular smooth muscle cells have been discussed. Nonspecificity of ligands, besides oxLDL, is also the highlight of this review to show the chameleon characteristics in the functional activity of the receptor protein. Induction of LOX-1 has been reported in diseases like atherosclerosis, diabetes, and hypertension, as well as in the inflammatory response of immune reactions. The expression of LOX-1 is upregulated by the vicious cycle of stimulatory response from proatherogenic molecules.

The effects of L-carnitine on some hematological parameters in rats fed a cholesterol-rich diet

We evaluated the effects of L-carnitine on the hematological characteristics of rats fed a cholesterol-rich diet. Healthy male Wistar Albino rats were assigned to four equal groups. During the 40 day experiment, group 1 was fed standard rat pellets, group 2 was fed standard rat pellets containing 7.5 % cholesterol powder, group 3 was fed standard rat pellets and water that contained 75 mg/l L-carnitine, and group 4 was fed standard rat pellets that contained 7.5% cholesterol and water that contained 75 mg/l L-carnitine. Blood samples were analyzed for red (RBC) and white (WBC) blood cells, hemoglobin, hematocrit, granulocytes, lymphocytes, monocytes, mean cell volume (MCV), mean corpuscular hemoglobin (MCH), mean corpuscular hemoglobin concentration (MCHC) using an automated cell counter. The RBC count in the group that received the 7.5% cholesterol diet was decreased significantly compared to the other groups. The hematocrit of the cholesterol group was lower than for the L-carnitine + cholesterol and L-carnitine groups. The MCV in the cholesterol group was significantly higher than the control group. The MCH in the cholesterol group was higher than for the other groups. There was no significant difference among the groups with regard to hemoglobin, MCHC, WBCs and leukocyte types. L-carnitine appears to have beneficial effects on erythrocyte stability, erythropoiesis and hyperlipidemia.

Increased platelet cholesterol and decreased percentage volume of platelets as a secondary risk factor for coronary artery disease

Platelet hyperactivity is likely to contribute to the progression of atherogenesis and organized thrombus formation on vascular surfaces. The purpose of this study was to examine the effect of hypercholesterolemia on the cholesterol content of platelets, on platelet responsiveness and other platelet indices using platelets from 5 groups of age-matched subjects (n = 30 each), which includes healthy controls. All groups except controls had a high plasma lipid profile. While subjects in group I had only hyperlipidemia, those in groups II and III had hyperlipidemia in conjunction with diabetes mellitus and hypertension, respectively. The fourth group consisted of patients with confirmed coronary artery disease (CAD). The parameters studied include packed cell volume of platelets (platelet crit), platelet distribution width (PDW), platelet cholesterol and platelet aggregation in response to adenosine diphosphate and collagen. All the patient groups showed increased platelet aggregation (p < 0.05) and low platelet crit compared with controls (p < 0.05). In addition, platelet cholesterol was increased in patients with coronary disease, hyperlipidemia and diabetes mellitus (p < 0.05) but not in patients with hypertension (p > 0.05); PDW was high only in CAD (p < 0.05). A higher PDW indicated a prothrombotic tendency in CAD patients. Our data suggest that hyperlipidemia increases the lipid content in platelets and enhances their reactivity. Hyperactive platelets with increased platelet cholesterol may contribute to accelerated atherogenesis associated with CAD.

Prothrombotic factors enhance heparin-induced thrombocytopenia and thrombosis in vivo in a mouse model

BACKGROUND

Heparin-induced thrombocytopenia/thrombosis (HIT/T) is a common cause of life- and limb-threatening thrombosis. The development of antibodies that react with complexes of heparin and platelet factor 4 (PF4) is fundamental to the development of the disease. However, anti-PF4/heparin antibodies are far more common than is HIT/T and there is less understanding of the factors that contribute to thrombosis in only a subset of patients.

OBJECTIVES

Both qualitative and quantitative differences in multiple factors (e.g. antibodies, heparin and platelets) may influence the clinical course of patients who develop anti-PF4/heparin antibodies. We examined the hypothesis that host-specific factors, such as comorbid prothrombotic conditions, would exacerbate the pathologic effects of anti-PF4/heparin antibodies.

METHODS AND RESULTS

A mouse model transgenic for human Fcgamma RIIa and PF4 and null for mouse PF4 was used to study the influence of prothrombotic conditions on the effects of anti-PF4/heparin antibodies in vivo. To simulate a prothrombotic milieu, mice were fed a hypercholesterolemic diet (HD). HD-fed mice had elevated plasma cholesterol, increased platelet reactivity and increased endothelial activation relative to mice fed a standard diet (SD). Age- and sex-matched mice from each diet group were treated with an anti-PF4/heparin antibody and heparin. HD-fed mice developed more severe thrombocytopenia than similarly treated SD-fed mice. Mice with moderate to severe thrombocytopenia had elevated plasma levels of thrombin-antithrombin complexes, indicative of increased thrombin generation in vivo. Platelet-fibrin thrombi were observed in multiple organs of HD-fed mice that developed severe thrombocytopenia.

CONCLUSIONS

Host-specific factors, such as prothrombotic changes in platelet reactivity and/or endothelial activation, may influence the development of thrombosis in a subset of patients who develop anti-PF4/heparin antibodies.

Damage to the structure of erythrocyte plasma membranes in patients with type-2 hypercholesterolemia

BACKGROUND

Hypercholesterolemia may decrease the deformability of red blood cells which impairs their hemorheological behavior and promotes atherosclerosis. The study involved 60 hypercholesterolemic patients and 30 healthy individuals as the control group.

METHODS

The membrane fluidity of erythrocytes was estimated by a spin-label method (5-doxylstearic acid (5-DSA)). The ratio of weakly to strongly (W/S) immobilized residues of erythrocyte membrane-bond maleimide-tempo spin label was studied in oxidative damage to membrane protein. Damage to erythrocyte proteins was also indicated by means of Na(+) K(+) ATPase activity.

RESULTS

The membranes of hyperlipidemia (hlp) patients contain larger concentrations of cholesterol 2.16+/-0.24 than do those of the normolipemic individuals 0.31+/-0.24 (P<0.001). The level of Na(+) K(+) ATPase in the erythrocyte membrane from the control group was higher 103.4+/-1.3 (nmolPi/(mgproteinsh)) than in the patient group 93.6+/-3.2 (nmolPi/(mgproteinsh)) (P<0.001). The order parameter S 5-DSA in the control group was 0.745+/-0.009 and in hlp patients was 0.755+/-0.009 (P<0.001). The W/S ratio in the control group amounted to 2.00+/-0.09 and in the hlp patient group was 2.50+/-0.11 (P<0.001).

CONCLUSION

Type-2 hypercholesterolemia causes changes in the structure and fluidity of erythrocyte plasma membranes since the excess of cholesterol affects the normal rheology of blood through its interaction with erythrocytes. It also impairs the function and structure of plasma membrane proteins.

Influences of hypercholesterolemia on red cell indices and erythrocyte sedimentation rate in elderly persons

BACKGROUND

We investigated whether hypercholesterolemia influenced the values of mean corpuscular volume (MCV) and erythrocyte sedimentation rate (ESR).

METHODS

A total of 463 nonanemic elderly persons were evaluated regarding red cell indices, ESR, and ESR-related parameters, such as fibrinogen, albumin, and C-reactive protein (CRP).

RESULTS

There were no significant differences in MCV between elderly men with and without hypercholesterolemia (>/=240 mg/dl) nor between the subjects with a marked increase of serum cholesterol concentrations (>/=260 mg/dl) and with severely lowered cholesterol concentrations (<155 mg/dl). ESR in elderly men with hypercholesterolemia averaged 12.3+/-6.8 mm/h, which were significantly higher than in those without hypercholesterolemia (6.0+/-4.7 mm/h, p<0.01). ESR averaged threefold higher in the elderly men with serum cholesterol concentration >/=260 mg/dl versus those with serum cholesterol concentrations <155 mg/dl, although no significant differences were observed in fibrinogen, albumin, and CRP values between the two groups. Serum cholesterol concentrations were higher in elderly men with ESR>/=15.0 mm/h (248.9+/-43.5 mg/dl), compared to those with ESR<2.0 mm/h (199.5+/-31.7 mg/dl, p<0.01). Serum cholesterol concentrations showed no associations with red cell indices but correlated significantly with ESR in elderly men (r=0.24, p<0.01) and postmenopausal women (r=0.21, p<0.01).

CONCLUSION

Hypercholesterolemia does not appear to influence MCV but significantly accelerates ESR, especially in elderly men.

Lipid rafts are critical membrane domains in blood platelet activation processes

Among the various hematopoi;etic cells, platelets are critical for maintaining the integrity of the vascular system. They must be rapidly activated by sequential and coordinated mechanisms in order to efficiently prevent haemorrhage upon vascular injury. Several signal transduction pathways lead to platelet activation in vitro and in vivo, among them, several are initiated via receptors or co-receptors containing immuno-receptor tyrosine-based activation motifs (ITAM) which trigger downstream signalling like the immune receptors in lymphocytes. However, in contrast to immune cells for which the role of lipid rafts in signalling has largely been described, the involvement of laterally segregated membrane microdomains in platelet activation has been investigated only recently. The results obtained until now strongly suggest that early steps of platelet activation via the collagen receptor GpVI or via FcgammaRIIa occur preferentially in these microdomains where specific proteins efficiently organize key downstream signalling pathways. In addition, lipid rafts also contribute to platelet activation via heterotrimeric G-protein-coupled receptors. They are sites where the phosphoinositide (PI) metabolism is highly active, leading to a local generation of lipid second messengers such as phosphatidylinositol 3,4,5-trisphosphate. Here, evidence is accumulating that cholesterol-enriched membrane microdomains are part of a general process that contributes to the efficiency and the coordination of platelet activation mechanisms. Here we will discuss the biochemical and functional characterizations of human platelet rafts and their potential impact in platelet physiopathology.

Polyploidy: occurrence in nature, mechanisms, and significance for the megakaryocyte-platelet system

OBJECTIVE

Polyploidy, the state of having greater than the diploid content of DNA, has been recognized in a variety cells. Among these cell types, the megakaryocytes are classified as obligate polyploid cells, developing a polyploid DNA content regularly during the normal life cycle of the organism, while other cells may become polyploid only in response to certain stimuli. The objective of this review is to briefly describe the different cell cycle alterations that may lead to high ploidy, while focusing on the megakaryocyte and the importance of high ploidy to platelet level and function.

MATERIALS AND METHODS

Relevant articles appearing in scientific journals and books published in the United States and in Europe during the years 1910-1999 were used as resources for this review. We selected fundamental studies related to cell cycle regulation as well as studies relevant to the regulation of the endomitotic cell cycle in megakaryocytes. Also surveyed were publications describing the relevance of high ploidy to high platelet count and to platelet reactivity, in normal situations and in a disease state.

RESULTS

Different cells may achieve polyploidy through different alterations in the cell cycle machinery.

CONCLUSIONS

While upregulation of cyclin D3 further augments ploidy in polyploidizing megakaryocytes in vivo, future investigation should aim to explore how normal megakaryocytes may initiate the processes of skipping late anaphase and cytokinesis associated with high ploidy. In humans, under normal conditions, megakaryocyte ploidy correlates with platelet volume, and large platelets are highly reactive. This may not apply, however, to the disease state.

Lipoproteins and the haemostatic system in atherothrombotic disorders

The remarkable extent to which interactions between the plasma lipoproteins, inflammatory factors and the haemostatic system contribute to the response to injury and growth of the plaque in atherosclerosis is being increasingly documented. High plasma concentrations of very-low density (VLDL) and low-density lipoproteins (LDL), together with oxidatively modified LDL and lipoprotein (a), can induce responses in vascular endothelial cells, smooth muscle cells, monocytes/macrophages, platelets, neutrophils and humoral factors that are in a variety of ways both procoagulant and antifibrinolytic. Plasma high-density lipoproteins appear to promote anticoagulant mechanisms. Post-prandial lipaemia is associated with transient changes in factor VII which may be indicative of temporary hypercoagulability. The cellular and humoral effects of LDL and VLDL on the haemostatic system appear to be largely reversible, which may help to explain the prompt improvement in the atherothrombotic state gained by correction of hyperlipidaemia.

Arachidonic acid of platelet phospholipids is decreased after extracorporeal removal of plasma low density lipoproteins in patients with familial hypercholesterolemia

Platelet phospholipid composition was analyzed before and after extracorporeal removal of low density lipoproteins (LDL) by LDL apheresis in six patients with familial hypercholesterolemia. Elevated levels of total plasma cholesterol and the portion of plasma cholesterol carried by LDL were reduced by 56 and 66% after LDL apheresis. Platelet cholesterol contents remained unaffected. While the phosphatidylcholine (PC):sphingomyelin (SM) ratio in plasma lipoproteins was increased by 22% following apheresis, the same parameter was lowered by 14% in platelets. LDL apheresis induced decreases in the percentages of distinct molecular species containing arachidonic acid in platelet diacyl subgroups of PC, phosphatidylinositol (PI) and phosphatidylserine (PS) as well as in alkenylacyl (plasmalogen) phosphatidylethanolamine (PE). Directly after apheresis, the percentages of molecular species with arachidonic acid of diacyl PC, diacyl PI and alkenylacyl PE were reduced by 20, 23 and 8%, respectively. Two days after the procedure, total arachidonic acid of diacyl PC, diacyl PS and alkenylacyl PE was lowered by 11, 20 and 8%. Overall, the amount of phospholipid bound arachidonic acid was reduced by 16% after apheresis (from 79.1 to 66.4 nmol/10(8) platelets). The results are thus in agreement with previous data indicating decreased phospholipid bound arachidonic acid in red blood cells after apheresis (Engelman B. Bräutigam C, Kulschar R et al. Biochim Biophys Acta 1994:1196:154). Urinary 2,3-dinor thromboxane B2, an estimate of platelet thromboxane A2 (TXA2) production, tended to be decreased following the procedure. The percentage change in the TXA2 metabolite was positively related to the magnitude of change induced by apheresis in phospholipid bound arachidonic acid. In summary, the results suggest that in patients with hypercholesterolemia, the level of plasma LDL is an important determinant of the arachidonic acid content of several platelet phospholipids.

Megakaryocytes and platelets in vascular disease

Platelets are anucleate cells with no DNA. They are derived from their precursor, the megakaryocyte (MK), whose differentiation is characterized by nuclear polyploidization through a process called endomitosis. Changes in the MK-platelet-haemostasis axis may precede acute thrombotic events. Changes in MK ploidy distribution may be associated with the production of large platelets. Mean platelet volume (MPV) is an important biological variable as it is a determinant of platelet reactivity. Large platelets are denser and more active haemostatically. MPV is increased in patients after myocardial infarction (MI) and is a predictor of a further ischaemic event and death when measured after MI. It has been suggested that changes not only in platelets but also in the parental MK are associated with chronic and acute vascular events. The regulation of megakaryocytopoiesis depends on several haematopoietic factors such as thrombopoietin. An understanding of the signalling system that controls platelet number and size might give insight into a role of platelet production in thrombogenesis and atherogenesis.

Effect of beraprost, a stable prostacyclin analogue, on red blood cell deformability impairment in the presence of hypercholesterolemia in rabbits

We evaluated the effects of orally administered beraprost, a stable prostacyclin analogue, on the rheological behavior of red blood cells (RBC) in the presence of hypercholesterolemia. Rabbits fed a cholesterolrich diet were administered various doses of beraprost or pravastatin. We evaluated rheological behavior of RBC by assessing RBC deformability, using a positive-pressure filtration method. The maximum pressure generated by passing a suspension of RBC through a membrane filter was used as an index of RBC deformability. After animals were fed cholesterol for 16 weeks, the maximum pressure increased significantly from 172 +/- 15 mm Hg at baseline to 261 +/- 18 mm Hg (p < 0.01, n = 24). The reduction in RBC deformability associated with hypercholesterolemia improved dose dependently during 1-h incubation with various doses of beraprost. In ex vivo study, beraprost markedly restored RBC deformability 3 h after its oral administration to 218 +/- 17 mm Hg (n = 9) at a low dose and to 215 +/- 20 mm Hg (n = 9) at a high dose. The effect persisted for at least 2 h. Pravastatin failed to reduce the increased maximum pressure. Findings suggest that beraprost treatment may improve the microcirculation by restoring RBC deformability in the presence of hypercholesterolemia.

Lipoproteins and the haemostatic system in atherothrombotic disorders

The early belief that the haemostatic system has no active role in the formation of the atheromatous plaque is no longer tenable. Rather, the association between hypercholesterolaemia and atherosclerosis appears to arise in part because of various effects of high concentrations of LDL and VLDL particles on the cellular and humoral components of the system, thereby promoting plaque growth and thrombosis. These may be summarized as follows: 1. High concentrations of native LDL have been reported to promote the adhesion of monocytes to the endothelial cell, suggesting that the latter undergoes a form of activation upon such exposure. Oxidized LDL is more potent in this respect, and persistent exposure of endothelium to such particles can eventually lead to cell injury. 2. Activated endothelial cells acquire characteristics on their luminal surface conducive to thrombin generation and fibrin production. Thrombin has several actions on the endothelial cell, monocyte, smooth muscle cell and platelet which in the presence of hypercholesterolaemia will promote the formation of atheroma. 3. Oxidatively modified LDL can activate circulating monocytes, when they also acquire procoagulant properties which favour thrombin production. 4. Platelets show an increased tendency to aggregate when exposed to hypercholesterolaemic plasma. This effect may arise in part because the platelet of the hypercholesterolaemic patient expresses an increased number of fibrinogen binding sites on its surface following activation by agonists such as ADP. These hyperaggregable platelets adhere to activated endothelial cells which express von Willebrand factor on their surface, and to subendothelial proteins exposed in the gaps that open between injured endothelial cells. Platelets exposed to raised LDL levels also show a reduced sensitivity to prostacyclin, an antiaggregatory agent. Oxidatively modified LDL has been reported to stimulate aggregation of platelets in the absence of other agonists such as ADP or thrombin (spontaneous aggregation). 5. Platelet aggregation and fibrin deposition at sites of endothelial injury will create microthrombi which become incorporated into the lesion by organization, thereby increasing the fibrous and cellular content of the atheromatous plaque. 6. Lipolysis of triglyceride-rich lipoproteins at the endothelial cell surface leads to transient activation of the coagulation mechanism with activation of factor VII. Activated factor VII is a potent procoagulant when it forms a complex with tissue factor in the atheromatous lesion. Persistent hypertriglyceridaemia is accompanied by raised concentrations of factor X, factor IX, factor VII and prothrombin. 7. Hypertriglyceridaemia is associated with an increased plasma concentration of PAI-1 and a reduction in plasma fibrinolytic activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Platelet LDL receptor recognizes with the same apparent affinity both oxidized and native LDL. Evidence that the receptor-ligand complexes are not internalized

We have recently demonstrated that the platelet low-density lipoprotein (LDL) receptor is immunologically different from the "classic" receptor of nucleated cells. We undertook the current studies to investigate the interaction of this receptor with oxidized LDL and to determine whether an endocytosis-mediated response is involved in the binding of LDL to platelets. The platelet LDL receptor recognized with the same affinity both native and oxidized LDL particles (IC50, 0.045 and 0.054 g/L; Kd, 45.8 and 65.9 nmol/L, respectively). The Hill coefficients of the displacement of 125I-LDL binding were -1.10 and -1.05 for unlabeled native and oxidized LDL, respectively, thereby suggesting a single set of binding sites. To ascertain whether human platelets bind oxidized LDL, we performed ligand binding assays with 125I-oxidized LDL. Saturation curves of 125I-oxidized LDL binding at 22 degrees C showed that human platelets bound these modified particles to a class of saturable binding sites, numbering approximately 3895 +/- 241 per platelet with a dissociation constant (Kd) of 96.2 +/- 10.3 nmol/L. Displacement experiments showed that 125I-oxidized LDL binding was inhibited with the same affinity by both oxidized and native LDL (IC50, 0.055 and 0.065 g/L; Kd, 88 and 64 nmol/L, respectively). The Hill coefficients of the displacement of the 125I-oxidized LDL binding were -1.02 and -1.07 for unlabeled oxidized and native LDL, respectively, suggesting that a single set of binding sites is implicated. Moreover, oxidized LDL- at a protein concentration of 0.5 g/L enhanced ADP- and collagen-induced platelet aggregation in a manner similar to native LDL.(ABSTRACT TRUNCATED AT 250 WORDS)

The megakaryocyte platelet system and vascular disease

Platelets form a heterogeneous population of cells produced from the uniquely large polyploid cell found in the bone marrow, the megakaryocyte. The platelet megakaryocyte axis forms a dynamic equilibrium varying in normal biology and in disease. Prolonged platelet destruction leads to the production of large platelets from large, high ploidy megakaryocytes. In vivo and ex vivo studies show that such platelets have more haemostatic potential than smaller less dense platelets. The evidence suggesting that prothrombotic changes in the megakaryocyte platelet axis precede coronary artery thrombosis and the importance of platelet reactivity in atherosclerosis will be reviewed.

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)

Etofibrate increases binding of low and high density lipoprotein to human platelets of patients with type II hyperlipoproteinemia

Previous work suggested an influence of etofibrate, a diester of nicotinic acid and clofibric acid, on lipoprotein receptors. Besides its beneficial effects on plasma lipoprotein levels of decrease in total cholesterol, LDL-cholesterol and triglycerides and increase in HDL-cholesterol, etofibrate was shown to inhibit platelet function. In order to further evaluate platelet-lipoprotein interactions, the effects of etofibrate on plasma lipids and lipoproteins on the specific binding of normal [111In]LDL and [111In]HDL onto platelets as well as its effect on platelet function were evaluated in 8 patients affected by Type II hyperlipoproteinemia (HLP). In all patients binding was saturable and indicated high affinity binding sites capable of binding 927 +/- 233 ng protein of [111In]LDL/10(9) platelets (Kd 12 +/- 3 micrograms protein/ml) and 1496 +/- 435 ng protein of [111In]HDL/10(9) platelets (Kd 14 +/- 3 micrograms protein/ml). The capacity of native LDL (HDL) to displace bound [111In]LDL ([111In]HDL) by half (IC50) amounted to 22 +/- 9 micrograms protein/ml (26 +/- 8 micrograms protein/ml). Following a 6-week treatment period with etofibrate (500 mg twice daily), decrease in plasma total cholesterol, LDL-cholesterol and apolipoprotein (apo) B and increase in HDL-cholesterol and apo AI was correlated to a significant (P < 0.01) increase in LDL- as well as HDL-receptor binding. The platelet binding capacity increased to 1085 +/- 212 ng protein/10(9) platelets (Kd 8 +/- 3 micrograms protein/ml) for [111In]LDL and to 1867 +/- 266 ng protein/10(9) platelets for [111In]HDL (Kd 11 +/- 3 micrograms protein/ml). Platelet function studies demonstrated significantly (P < 0.01) reduced platelet aggregation in response to ADP and thromboxane formation after 6 weeks of etofibrate therapy. These findings in patients with HPL Type II indicate in vivo upregulation of specific [111In]LDL as well as [111In]HDL binding sites on human platelets associated with reduced platelet activation following etofibrate therapy.

The regulation of megakaryocyte polyploidization and its implications for coronary artery occlusion

Polyploidization is a distinctive feature of megakaryocyte differentiation. The physiological meaning and the regulation of this process are obscure. Megakaryocyte ploidy varies in normal biology and in disease. Here we review the evidence suggesting that ploidy changes may have a role in the determination of platelet reactivity and in the aetiology of coronary artery occlusion. We also present a hypothesis that may serve as a framework to explore the regulation of megakaryocyte polyploidization at the molecular level and also may provide a rational basis to explain the occurrence of ploidy changes in ischaemic heart disease.

Ex vivo investigation of blood monocyte and platelet behaviour in pigs maintained on an atherogenic diet

Domestic pigs aged 4 months were fed for 16 weeks an atherogenic diet rich in cholesterol and saturated fatty acid. The increase of plasma cholesterol and triacylglycerol levels was found to be accompanied by a significant increase in the number of blood monocytes and platelets when compared to control animals. Furthermore, the atherogenic diet produced a small but significant reduction in the blood monocyte phagocytic capacity and adhesion to plastic surface. No significant differences between both groups were found when spontaneous platelet aggregation in whole blood was studied. However, platelets from pigs fed the atherogenic diet had a smaller mean cell volume compared to controls. The results indicate than an atherogenic diet may affect blood monocytes and platelets in pigs.

Effects of hypercholesterolemia on the nucleotide content in human blood cells

In hypercholesterolemia significant changes in the nucleotide pattern of erythrocytes and lymphocytes as determined by high performance liquid chromatography were found. The decrease in ATP of lymphocytes in hypercholesterolemia from 10.4 +/- 0.3 to 7.0 +/- 0.4 nmol mg-1 protein (n = 8) was associated with an increase in ADP from 2.2 +/- 0.2 to 4.0 +/- 0.2 nmol mg-1 protein (P less than 0.005). The pattern of guanosine phosphates likewise was found to be changed in hypercholesterolemia. Akin to lymphocytes, red blood cells displayed marked changes in nucleotide levels. No such changes were observed in platelets. Cultured lymphocytes incubated with human plasma low density lipoproteins (LDL) (140 mg cholesterol dl-1) displayed a reversible fall in ATP and an increase in ADP by about 40% and 160%, respectively, with high density lipoproteins (HDL) or very low density lipoproteins (VLDL) being essentially ineffectual. It is concluded that in hypercholesterolemia a significant change in the nucleotide pattern of blood cells is exerted by the increase in LDL. Possible pathophysiological implications are discussed.

Evidence for an altered lipid metabolic state in circulating blood monocytes under conditions of hyperlipemia in swine and its implications in arterial lipid metabolism

Circulating blood monocytes were isolated from normal and hypercholesterolemic swine, and the monocyte lipid compositions and lipid biosynthesis profiles were assessed. The data indicate that monocytes freshly isolated from hyperlipemic swine have increased phospholipid and cholesterol contents and have increased biosynthetic capability for synthesizing phospholipids, triglycerides, and cholesteryl esters, but not cholesterol. The profile of the stimulated lipid synthesis capability is similar to that of the swine aortic intima undergoing atherogenic change. These studies indicate that circulating blood monocytes in hyperlipemic swine, which are known to give rise to intimal foam cells in the early fatty streak lesion, can contribute to altered vessel lipid metabolism without a requirement for in situ modification by wall factors.

Lipid and membrane fluidity abnormalities in platelets and megakaryocytes of the hereditary macrothrombocytopenic Wistar Furth rat

Biochemical and functional abnormalities of megakaryocytes and platelets were studied in Wistar Furth (WF) rats which have genetically determined macrothrombocytopenia and megakaryocytopenia, and were compared with their counterparts in Sprague-Dawley (SD) rats. Both megakaryocytes and platelets synthesized phospholipids from [14C]acetate. WF and SD megakaryocytes incorporated 0.27 and 0.29 nmol acetate per 10(6) cells, respectively. Phosphatidylcholine (PC) accounted for 64% and 58% of the PL radioactive label in megakaryocytes of SD and WF rats, respectively, (P less than 0.05), while 69% of labeled activity was associated with PC of SD platelets compared to 60% found in PC of WF platelets (P less than 0.01). In WF platelets a significant increase in the levels of lysophosphatidylcholine (6.1% vs. 3.0%) was observed. WF platelets had substantially higher levels of esterified cholesterol, triglycerides, ceramides and a 3-fold increase in the total protein per platelet compared to SD platelets. The fatty acid composition of WF platelet PC showed quantitative abnormalities. Plasma lecithin-cholesterol acyl transferase activity and platelet function monitored by the uptake and release of [14C] serotonin showed nonsignificant variations between SD and WF rats. Compared with the control, platelet membrane fluidity, measured by fluorescence polarization using platelets labeled with 1,6-diphenyl-1,3,5-hexatriene, was significantly decreased in the WF rats.

Effect of ethanol on low density lipoprotein and platelet composition

The present study was designed to investigate the effect of ethanol (EtOH) dose on low density lipoprotein (LDL) and platelet composition. Male squirrel monkeys were divided into three groups designated Control, Low, and High EtOH, and fed isocaloric liquid diets containing 0%, 12%, and 24% of calories as EtOH, respectively. After four months of treatment, monkeys fed the 12% alcohol dose had LDL and platelet cholesterol concentrations similar to Controls. By contrast, platelet membranes from High EtOH animals contained significantly more cholesterol which was associated with higher levels of plasma LDL cholesterol and apolipoprotein B. Blood platelet count, size, and mass were similar for all groups and circulating platelet aggregates were absent in the two alcohol cohorts. Despite elevations in platelet cholesterol mass and thromboxane A2 (TXA2) precursor, phospholipid arachidonate, platelet responsiveness, measured as thromboxane formed in response to a collagen challenge in vitro, and the cholesterol/phospholipid molar ratio, were not significantly altered by high dose alcohol. Normal platelet activity in High EtOH monkeys may have resulted from a significant increase in the platelet phospholipid polyunsaturated/saturated fatty acid ratio and a non-significant increase in platelet phospholipid mass, both of which would have a fluidizing effect on platelet membranes. Our data indicate that low EtOH intake has no effect on platelet composition and function while unfavorable platelet cholesterol enrichment following consumption of high dose ethanol may arise from elevations in plasma LDL.(ABSTRACT TRUNCATED AT 250 WORDS)

Lovastatin therapy in hypercholesterolemia: effect on fibrinogen, hemorrheologic parameters, platelet activity, and red blood cell morphology

The effect of lovastatin therapy on blood rheology was investigated in 26 hypercholesterolemic patients. Treatment with lovastatin was associated with a significant improvement in whole blood filtration time and a tendency toward normalization in red blood cell morphology. A significant increase was observed in fibrinogen level, in ADP-induced platelet aggregation, in the percentage of "big" platelets, and in platelet count. The viscosity of whole blood and plasma and the percentage of aggregated platelets did not change significantly. The cause for these hemorrheologic changes and their role in influencing the coronary risk of lovastatin-treated hypercholesterolemic patients should be further investigated.

Platelet function in Watanabe heritable hyperlipidemic rabbits. Decreased sensitivity to thromboxane A2

The characteristics of platelets from seven 5-7-month-old homozygous Watanabe heritable hyperlipidemic (WHHL) rabbits (plasma cholesterol, 13.9 +/- 1.7 mM, mean +/- SEM) were compared with those of platelets from normocholesterolemic age/weight- and sex-matched control rabbits (plasma cholesterol, 2.2 +/- 0.3 mM). Whole-blood platelet count and platelet size and protein content were not different in the two groups of rabbits, and the platelets from the WHHL rabbits were not enriched in cholesterol as indicated by identical mean cholesterol:phospholipid molar ratios (C/P). Responses of washed platelets stimulated with various agonists were studied to determine the effects of the genetically determined hypercholesterolemia on the various pathways of platelet aggregation in the absence of plasma components. In platelets from WHHL rabbits compared with controls, aggregation induced by ADP (0.5-5 microM) did not differ; collagen-induced (0.25-1.5 micrograms/ml) responses (aggregation, secretion of carbon-14-labeled serotonin from the amine storage granules of prelabeled platelets, and thromboxane A2 [TxA2] formation) were significantly less extensive; with aspirin-treated platelets, aggregation and secretion of granule contents induced by the TxA2 mimetic U46619 (0.25-1 microM) were significantly less extensive; and thrombin-induced (0.005-0.1 unit/ml) responses of untreated platelets (aggregation, secretion of granule contents, and TxA2 formation) or aspirin-treated platelets (aggregation and secretion of granule contents) did not differ. These observations are in direct contrast with previous studies of platelets from rabbits with diet-induced hypercholesterolemia, in which responses to TxA2 and thrombin were enhanced. Platelets from WHHL rabbits are hyposensitive to aggregation induced by TxA2.

Effect of diet-induced hypercholesterolemia on proteoglycan metabolism in guinea pig megakaryocytes and platelets

Proteoglycan metabolism was evaluated in megakaryocytes and platelets from guinea pigs fed a 1% cholesterol diet for 3 or 7 weeks. The animals were injected with a single dose of [35S]sulfate at the end of the feeding period, and megakaryocytes and platelets were isolated after 3 hours and then daily for 4 days thereafter. Proteoglycans were extracted from the cells of each animal and analyzed by ion-exchange chromatography, gel filtration, and electrophoresis. The maximal labeling of platelets occurred 2 days after [35S]sulfate injection as compared with 3 days in controls. A proteoglycan that eluted at Kav 0.2 from the Sepharose CL-6B column appeared 1 day after labeling. Additional proteoglycans of Kav 0.15 appeared at subsequent time points. The labeling profile for cholesterol-fed animals was unchanged from 2-4 days, unlike profiles from controls, which had exhibited a gradual increase in mean proteoglycan size. Thus, the progressive change in size of proteoglycans synthesized during normal megakaryocyte maturation was altered. The mean chain length of the proteoglycan-associated glycosaminoglycans from cholesterol-fed animals was increased relative to that of controls. In conjunction with the twofold increase in mean megakaryocyte size induced by cholesterol feeding in guinea pigs, the changes in proteoglycan synthesis suggest a state of stimulated megakaryocytopoiesis.

Hemostatic variables in homozygous familial hypercholesterolemia. Effect of regular plasma cholesterol removal by low density lipoprotein apheresis

Plasma levels of tissue plasminogen activator (t-PA) and plasminogen activator inhibitor (PAI) and the in vitro ability of platelets to aggregate and of monocytes to express procoagulant (tissue factor) activity (PCA) were evaluated in five patients who are homozygous for familial hypercholesterolemia (FH) before and after a single and a regular 5-month cholesterol removal by low density lipoprotein (LDL) apheresis. The biweekly procedure resulted in a 25% to 30% reduction (approximately 150 mg/dl) in total and LDL cholesterol (both were greater than 550 mg/dl at the beginning of the study). The basal levels of t-PA antigen and fibrinolytic activity before and after 10 minutes of venous stasis, basal PAI activity, and PAI-1 antigen were comparable to controls and were not affected by LDL apheresis. Likewise, regardless of the cholesterol removal, the PCA of freshly isolated monocytes and that of monocytes incubated with lipopolysaccharide did not differ from control values. Finally, the pre-apheresis sensitivity of platelets to adenosine diphosphate, arachidonic acid, and collagen was 1.5 to 2 times the normal value. This ratio was unchanged throughout the 5-month procedure. We conclude that fibrinolysis and monocyte PCA are normal in FH patients, whereas platelet aggregation is abnormally high, and none of these parameters is significantly affected by a 25% to 30% reduction in total and LDL cholesterol by LDL apheresis. Furthermore, our data suggest that removal of cholesterol from plasma by LDL apheresis is important for gaining insight into the mechanisms involved in the ischemic complications of arteriosclerosis in FH patients.

The control of megakaryocyte ploidy and platelet production: biology and pathology

Following experimental platelet destruction in animals, large platelets, which are more hemostatically active, are produced before any change in bone marrow megakaryocyte DNA content. When platelet production is stimulated by administration of i.v. vincristine in rats, megakaryocyte ploidy is increased, but mean platelet volume is unchanged. When platelet production and destruction are both stimulated by chronic hypoxia or administration of anti-platelet serum, mean platelet volume and megakaryocyte DNA content are both increased. Since platelet volume is determined primarily at thrombopoiesis, these results imply that mean platelet volume and megakaryocyte DNA content are under separate hormonal control. Therefore, it has been postulated that changes in mean platelet volume occur following changes in platelet production rate, whereas changes in megakaryocyte ploidy are associated with an increased rate of platelet production. In myocardial infarction, platelets have increased mean volume and reduced bleeding time more than in controls. In addition, men with myocardial infarction have increased megakaryocyte size and increased DNA content when compared to controls. These changes are similar to those observed in rabbits following cholesterol feeding. If megakaryocyte polyploidy and mean platelet volume are under separate hormonal control, this suggests that in myocardial infarction, both hormones are active--one stimulating an increased platelet size, the other stimulating the increased megakaryocyte DNA content. In contrast, patients with lymphoma exhibiting a secondary thrombocytosis have no change in mean platelet volume. However, these subjects also have larger bone marrow megakaryocytes when compared to controls. The relation between megakaryocyte size and ploidy implies that the DNA content of these cells is increased in lymphoma.(ABSTRACT TRUNCATED AT 250 WORDS)

Accelerated atherogenesis occurs following platelet destruction and increases in megakaryocyte size and DNA content

Platelets and megakaryocytes have a role in atherogenesis. In this study the effect of platelet destruction, caused by injection of serum containing anti-platelet antibodies, on the development of atheroma and on megakaryocyte size and DNA content was studied in rabbits on a high cholesterol diet. Seven days after destruction of the circulating platelets a significant (150% and 300% in two separate studies) increase in the area of atheroma in the aorta was demonstrated. Histological examination revealed that large quantities of extracellular lipid and connective tissue were present in these plaques. The acceleration of atherogenesis in this model was found to be preceded by a significant increase in megakaryocyte size and in the frequency of megakaryocytes with a high DNA content. Platelets derived from large, high ploidy megakaryocytes may be of importance for the development of the atherosclerotic lesion. Neither the endothelial permeability as assessed by injection of Evans blue nor the prostacyclin production by the aorta of animals injected with anti-platelet serum were significantly different from the aortas of control animals.

Biochemical and functional abnormalities in hypercholesterolemic rabbit platelets

This study was designed to elucidate changes in rabbit platelet lipids induced by a cholesterol rich diet and to explore the possible correlation of these lipid changes with platelet abnormalities. Pronounced biochemical alterations were observed when serum cholesterol levels of 700-1000 mg% were reached. Hypercholesterolemic (HC) platelets contained 37% more neutral lipids and 16% less phospholipids than the controls. Lysolecithin, cholesterol esters and phosphatidylinositol (PI) levels were increased in HC platelets, and the levels of phosphatidylcholine (PC) were decreased. The cholesterol/phospholipid molar ratio of lipidemic platelets increased from 0.55 +/- 0.011 to 0.89 +/- 0.016 (P less than 0.01) in eight weeks. HC platelets had 90% more arachidonic acid (AA) in the PI than normal platelets. No significant changes in AA of PC were observed. Platelet function was monitored by the uptake and release of [14C]serotonin in platelet rich plasma (PRP), using varying concentrations of collagen as an aggregating agent. The uptake of [14C]serotonin in HC and normal platelets ranged from 78-94%. The percent of [14C]serotonin released from normal and HC platelets was proportional to the concentration of collagen. However, lipidemic platelets were hyperreactive to low concentrations of collagen. Incorporation of 50 microM acetylsalicylic acid into the aggregating medium suppressed the release of [14C]serotonin in normal PRP by more than 90%, but had only a partial effect on lipidemic PRP.

Platelet function and survival in rats with genetically determined hypercholesterolaemia

Platelets from rats with genetically determined hypercholesterolaemia are hypersensitive to aggregation induced by thrombin compared with platelets from their genetic controls without hypercholesterolaemia. Aggregation or release induced by thrombin of platelets from hypercholesterolaemic and control rats correlated significantly with plasma cholesterol concentrations. Platelet responses to ADP or collagen were not different between the groups. The hypersensitivity to thrombin-induced aggregation was independent of released ADP or products of arachidonic acid metabolism. The changes in platelet sensitivity occurred with only moderate increases in plasma cholesterol concentration and with no detectable changes in total platelet cholesterol. The hypersensitivity of platelets from hypercholesterolaemic rats was not associated with a reduction in platelet survival or any significant injury to the aortic endothelium in these animals. Platelets from hypercholesterolaemic rats were smaller than platelets from controls. Thus, platelets from rats with genetically determined hypercholesterolaemia have alterations in function similar to those found with platelets from rats with diet-induced hypercholesterolaemia indicating that this strain can be used to study the mechanisms by which cholesterol can change platelet function without the possible complicating effects of dietary factors. Since platelet hypersensitivity occurred in rats with genetically determined hypercholesterolaemia without a reduction in platelet survival, these studies are also consistent with the reduced platelet survival found in animals with diet-induced hypercholesterolaemia being independent of platelet changes.

Cytotoxicity of cholesterol oxidase to cells of hypercholesterolemic guinea pigs

1. A high cholesterol diet caused guinea pig erythrocytes to become sensitive to lysis by cholesterol oxidase (CO), a protein not hemolytic to normal cells. 2. Lysis was associated with conversion of membrane cholesterol to its oxidation product (delta-4-cholesten-3-one). 3. Intravenous injection of CO to hypercholesterolemic guinea pigs produced a reduction in serum cholesterol, but was not lethal as it was in rabbits. 4. Homogenized spleen, liver and kidney from the hyperlipidemic animals were sensitive to in vitro cholesterol oxidation while tissues from non-lipemic animals were resistant to modification.

Thrombin binding to platelets from hypercholesterolaemic rats

Platelets from rats made hypercholesterolaemic with a diet enriched with milk fat and cholesterol and containing taurocholate to promote hypercholesterolaemia aggregated more extensively to a low concentration of thrombin than platelets from rats given a milk fat-enriched diet containing sitosterol. Total and specific binding of thrombin to platelets from hypercholesterolaemic rats was significantly greater than in controls when expressed per mg platelet protein, per mumol platelet cholesterol, or per unit relative surface area. Total and specific binding of thrombin per platelet were not different between the groups. However, platelets from hypercholesterolaemic rats had less protein and cholesterol, were smaller and had less surface area than control platelets; platelet cholesterol content expressed per mg platelet protein was not different. Thus, the increase in thrombin-binding to the smaller platelets from hypercholesterolaemic rats during the first 10 s after its addition may be responsible, at least in part, for the hypersensitivity of these platelets to thrombin.

Megakaryocyte and vascular changes in rabbits on a short-term high cholesterol diet

The effect of a short-term high cholesterol diet on thrombopoiesis and vascular ultrastructure was evaluated in rabbits. Six pairs of male litter-mate rabbits were randomized pairwise to feeding with either 2 g of cholesterol daily in addition to their normal diet or normal diet alone for 7 days. A significant 12-fold increase in median serum cholesterol (P less than 0.035) and an insignificant decrease in platelet count (P = 0.07) were found in the animals fed a high cholesterol diet. In these animals the total and cytoplasmic megakaryocyte size measured as planimetric areas in bone marrow sections were significantly decreased (P less than 0.035). No statistically significant difference in the megakaryocyte DNA content measured by flow cytometry in marrow suspensions enriched for megakaryocytes by density gradient centrifugation and centrifugal elutriation was observed between the cholesterol-fed animals and controls. Light microscopic, transmission and scanning electron microscopic examination of the aorta in both groups of animals showed a morphologically intact endothelium without any adhesion of blood-borne cells to the luminal surface. Transmission electron microscopic studies showed that cells with ultrastructural features resembling smooth muscle cells were present in the intima of the aortas of the animals on the high cholesterol diet, but not in control animals. A decrease in the size of bone marrow megakaryocytes and the occurrence of intimal smooth muscle cells are found in rabbits fed a high cholesterol diet for 7 days. These cellular events may be important features in early atherogenesis.

Altered platelet deformability in patients with type IIa and type IV hyperlipoproteinemia

Platelets from subjects with hyperlipoproteinemia (HLP) differ from normal platelets in lipid composition and function depending upon the phenotypic classification of the HLP. The present study has evaluated the deformability of platelets from human subjects with type IIa and type IV HLP. Platelets suspended in autologous plasma diluted 30-fold with buffer were aspirated into micropipettes 0.7-0.8 microns in diameter by step-wise increment in tension, and the resulting extension lengths were recorded. Platelets from type IIa subjects could not be aspirated as far into the micropipettes as normal platelets. However, less tension was required to reach maximum cell extension than with normal platelets, and the initial extension lengths and slopes of the stress responses were the same as the control. In contrast, platelets from subjects with type IV HLP showed a generalized increase in deformability. The initial cell extensions aspirated from type IV platelets were longer than normal, and larger maximum cell extensions were achieved at lower tensions than control platelets. The type IV platelets were also mechanically fragile and fragmented at lower tensions than control or type IIa platelets. The variance in platelet deformability between subjects of the same phenotype was not directly correlated to plasma lipid or lipoprotein concentrations. This study confirms alterations in the structural organization of platelets from subjects with type IIa and type IV HLP.

Rapid activation of human platelets by low concentrations of low-density lipoprotein via phosphatidylinositol cycle

The interaction of low-density lipoprotein (LDL) with the human platelet was investigated with regard to saturable high-affinity binding, shape change, cytosolic free Ca2+ concentration, phosphatidylinositol (PtdIns) turnover, and thromboxane B2 biosynthesis. The experiments show that LDL, at a concentration approximately 100 times lower than in plasma, causes platelet activation concomitantly with stimulation of the PtdIns cycle and thromboxane B2 formation, similarly to other activators of platelets. The effects of LDL were inhibited by high-density lipoprotein. The results suggest that activation of platelets by low concentrations of LDL may play a role in pathophysiological conditions and that platelet can serve as a model for studying the influence of LDL on various target cells.

Low density lipoprotein causes general cellular activation with increased phosphatidylinositol turnover and lipoprotein catabolism

Low density lipoprotein (LDL), at concentrations high enough for receptor binding but not high enough to saturate the receptor, induces activation of phosphatidylinositol (PtdIns) turnover in a variety of cell types with various biological functions. Using both biochemical and electron microscopic studies, we have shown that blood platelets take up and degrade LDL in a manner reminiscent of phagocytic cell types. The activation of both PtdIns turnover and LDL metabolism is inhibited by high density lipoprotein. Thus, LDL at hormonal concentrations causes general cellular activation. Since all cell types studied responded to LDL with increased PtdIns turnover and uptake of LDL cholesterol, the PtdIns cycle may also be involved in the cellular regulation of LDL cholesterol metabolism.

Platelet activation by plasma lipoproteins

Enhanced platelet activity in patients with hypercholesterolemia and moderate hypertriglyceridemia can be attributed to increased LDL and VLDL and/or decreased HDL concentrations. In marked hypertriglyceridemia, where there is an accumulation of chylomicrons, platelet function is reduced. Treatment, whether by diet, medication, or plasmapheresis, that will result in a change in lipoprotein pattern is accompanied by a parallel change in platelet responsiveness. Incubation of lipoproteins with isolated platelets results in enhancement of platelet activation by LDL and VLDL and suppression of activity by HDL and chylomicrons. These findings have in vivo confirmation. They are even more pronounced and sometimes altered when the lipoproteins are derived from hyperlipidemic subjects. The effects of the lipoproteins on platelet activity appear to be dependent on lipoprotein composition and on factors such as cholesterol:protein ratio, apo C-III0:apo C-III2 ratio, apo B concentration, and triglyceride:protein ratio. The lipoproteins interact with platelets at specific receptor sites. Rapid change in platelet composition, particularly with regard to cholesterol, phospholipid, and fatty acid content, might ensue, with consequent alterations in membrane fluidity and enzyme activities and either suppression or activation of platelet function. This review has indicated that lipoproteins have a clear influence on platelet function. This interaction could be well of paramount importance in determining atherogenic risk.

Enhanced accumulation and turnover of esterified cholesterol in injured rabbit aorta

Arterial injury exacerbates experimental atherogenesis. This report evaluates cholesteryl ester influx and loss during the first 5 days after arterial injury. Selected areas of aortas from rabbits made hypercholesterolemic by 12 to 16 days of cholesterol feeding were injured with a balloon catheter. This allowed measurements and a relatively precise comparison of cholesterol and cholesteryl ester influx into the uninjured and injured arterial wall within the same animal. The animals received oral doses of 3H- or 14C-cholesterol either just before injury or 1 day later. Most animals were given the other isotope of cholesterol 1 day before sacrifice, which took place 2 to 5 days after injury. Measurement of accumulated labelled esterified cholesterol within the same animal during two different time periods allowed the estimation of total influx and fractional loss of entered esterified cholesterol within the artery. Between 2 and 5 days after injury, total influx into areas of injured artery averaged 30 to 60 times the total influx into the adjacent uninjured areas. By the fifth day after injury, the esterified cholesterol content was about 10 times greater in injured areas than in adjacent, uninjured areas. The nonesterified cholesterol content of injured areas was about 70% greater than that of adjacent noninjured areas. Total influx and fractional loss of arterial cholesteryl ester calculated from radioactivity data could account for 117 +/- 6 (mean +/- SEM) percent of the cholesteryl ester mass that accumulated during the interval from injury to sacrifice. The total influx rates and daily increments in cholesteryl ester content of injured areas were positively related.

Megakaryopoiesis disturbances in atherosclerotic rabbits

Rabbits given a hypercholesterolemic diet (500 mg/day) for 6 months and then maintained for another 6 months on a normal diet were found to have developed fibrous lipidic lesions in the aorta. Although circulating platelet levels in these animals were normal there was a reduction in mean megakaryocyte ploidy. The high concentrations of megakaryoblasts in all the sedimentation fractions collected by the 'STAPUT' system suggested an increase in megakaryocyte turnover with activation of committed stem cells. In addition, other defects in maturation of megakaryocytes were observed, such as abnormalities in the demarcation membrane system and granule number. These data reveal that defects in megakaryocyte maturation and turnover may occur during the process of reparative fibrosis of the arterial tree following a period of moderate hypercholesterolemic diet in the rabbit.

Platelet activation in normo- and hyperlipoproteinemias

In the last few years it became obvious that platelets are involved in the development of atherosclerotic diseases. This involvement of platelets has been taken into account in the "response to injury" hypothesis of atherosclerosis. The hypothesis is based on the assumption that atherosclerotic lesions result from endothelial injury, followed by the interaction of vessel wall constituents with lipoproteins, macrophages, and platelets. In the first part of this review, general aspects of platelet activation are summarized and the pathways of platelet aggregation as well as their involvement in blood coagulation are discussed. The second part of this paper describes the influence of cholesterol, lipoproteins, and apolipoproteins upon the activation and metabolic behavior of platelets. Physiological and pathophysiological processes particularly occurring in different types of hyperlipoproteinemias and atherosclerotic disorders are discussed in this context.

Cholesterol exchange in platelets, erythrocytes and megakaryocytes

Cholesterol exchange between plasma and human platelets and erythrocytes and guinea pig platelets, erythrocytes and megakaryocytes was studied. The characteristics of exchange of cholesterol between [3H]cholesterol-labeled plasma and human platelets and erythrocytes were similar: exchange per cell was independent of cell concentration in whole plasma, decreased only 2-fold over a wide range of cell concentrations in low concentrations of plasma and approached a plateau at 1/3 normal plasma cholesterol concentration, and there was no net change in the cholesterol content of either cell. The activation energy for exchange for both cells was 47 kJ/mol. In all experiments, erythrocyte cholesterol was labeled to approximately twice the specific activity of platelet cholesterol. Guinea pig megakaryocyte cholesterol exchanged at 25-33% of the rate of guinea pig platelet cholesterol in vitro. Similarly, when guinea pigs were fed [3H]cholesterol, erythrocyte cholesterol specific activity after 24 h was 90%, platelet 50-65%, and megakaryocyte 20-26% that of plasma. Guinea pig platelets incubated with plasma radiolabeled in free and esterified cholesterol incorporated radioactivity from free but not esterified cholesterol. The similarity of free cholesterol exchange in platelets and erythrocytes in vitro and in vivo and the apparent inability of platelets to take up cholesterol esters from lipoproteins suggest that the interaction between normal platelets and normocholesterolemic plasma is limited to cholesterol exchange.