Platelet function in hyperlipoproteinemia

Influence of increased membrane cholesterol on membrane fluidity and cell function in human red blood cells

Cholesterol and phospholipid are the two major lipids of the red cell membrane. Cholesterol is insoluble in water but is solubilized by phospholipids both in membranes and in plasma lipoproteins. Morever, cholesterol exchanges between membranes and lipoproteins. An equilibrium partition is established based on the amount of cholesterol relative to phospholipid (C/PL) in these two compartments. Increases in the C/PL of red cell membranes have been studied under three conditions: First, spontaneous increases in vivo have been observed in the spur red cells of patients with severe liver disease; second, similar red cell changes in vivo have been induced by the administration of cholesterol-enriched diets to rodents and dogs; third, increases in membrane cholesterol have been induced in vitro by enriching the C/PL of the lipoprotein environment with cholesterol-phospholipid dispersions (liposomes) having a C/PL of greater than 1.0. In each case, there is a close relationship between the C/PL of the plasma environment and the C/PL of the red cell membrane. In vivo, the C/PL mole ratio of red cell membranes ranges from a normal value of 0.09--1.0 to values which approach but do not reach 2.0. In vitro, this ratio approaches 3.0. Cholesterol enrichment of red cell membranes directly influences membrane lipid fluidity, as assessed by the rotational diffusion of hydrophobic fluorescent probes such as diphenyl hexatriene (DPH). A close correlation exists between increases in red cell membrane C/PL and decreases in membrane fluidity over the range of membrane C/PL from 1.0 to 2.0; however, little further change in fluidity occurs when membrane C/PL is increased to 2.0--3.0. Cholesterol enrichment of red cell membranes is associated with the transformation of cell contour to one which is redundant and folded, and this is associated with a decrease in red cell filterability in vitro. Circulation in vivo in the presence of the spleen further modifies cell shape to a spiny, irregular (spur) form, and the survival of cholesterol-rich red cells is decreased in the presence of the spleen. Although active Na-K transport is not influenced by cholesterol enrichment of human red cells, several carrier-mediated transport pathways are inhibited. We have demonstrated this effect for the cotransport of Na + K and similar results have been obtained by others in studies of organic acid transport and the transport of small neutral molecules such as erythritol and glycerol. Thus, red cell membrane C/PL is sensitive to the C/PL of the plasma environment. Increasing membrane C/PL causes a decrease in membrane fluidity, and these changes are associated with a reduction in membrane permeability, a distortion of cell contour and filterability and a shortening of the survival of red cells in vivo.

Pilot epidemiological studies in thrombosis

Some differences in the blood of farmers in Nurmijarvi, Finland, Canino, Italy, and Beltsville, Maryland in the United States apparently were associated with differences, among the areas, in the farmers' diets. Those associations suggested that diets that are high in saturated fats (Nurmijarvi) could predispose humans to develop intravascular disease. Such predisposition has been observed in experimental animals. Low levels of the parameters that are considered active in such predisposition apparently were associated with diets that were low in saturated fats (Canino) or with diets that were low in saturated and high in unsaturated fats (Beltsville). Within the limits of the experimental design, the data from the three population groups indicated that a more comprehensive study might establish a relation between diet and intravascular disease in humans.

Experimental atherosclerosis in rabbits: platelet aggregation, thromboxane A2 generation and anti-aggregatory potency of prostacyclin

Experimental atherosclerosis in rabbits was associated with increased aggregation of their platelets to arachidonic acid, and with increased generation of thromboxane A2 by their platelet-rich plasma. A heightened susceptibility of platelets to the anti-aggregatory action of prostacyclin against the ADP-induced aggregation was also observed. It is concluded that in advance atherosclerosis the platelet system is hypersensitive to biologically active metabolites of arachidonic acid.

Heparin neutralizing activity and coronary artery disease

It has been previously shown and confirmed in the present investigation that the disaggregation of adenosine diphosphate (ADP)-induced platelet aggregates occurs at a slow rate more frequently in the platelet-rich plasma (PRP) of men with coronary artery disease. ADP-induced platelet aggregation was studied in the citrated PRP of 32 men (21 with and 11 without coronary artery disease) to determine the relation between release of heparin neutralizing activity (HNA) from platelets and the rate of platelet diaggregation. Each of the five PRP with slow (less than 10 per cent) disaggregation were from men with coronary artery disease. Platelets from these five PRP released from 34 to 51 per cent of their content of HNA during ADP-induced aggregation in contrast to the 27 PRP with more rapid disaggregation, only three of which had a detectable release of HNA. Of the latter 27 PRP, 21 had a second phase of aggregation which usually reached a peak of light transmission less than that of the first phase. These data are consistent with (but do not prove) the hypothesis that HNA released during aggreation may be one of the factors tending to prevent disaggregation of ADP-induced platelet aggregates.

Concomitants of atherosclerotic carotid artery stenosis

To identify metabolic and other concomitants of a single important type of atherosclerotic cerebrovascular disease, 75 patients with angiographically and surgically proven internal carotid stenosis were compared with age and sex matched control subjects with respect to plasma cholesterol, triglycerides and glucose. They were also compared for blood pressure, cigarette smoking, evidence of ischemic heart disease, peripheral vascular disease and for a family history of these diseases. Patients with carotid stenosis had higher systolic and diastolic blood pressures and higher plasma cholesterol and triglyceride concentrations than the control groups. They had, as well, a far greater likelihood of being cigarette smokers and a greater likelihood of having diabetes mellitus and previous evidence of coronary and peripheral vascular disease. Patients with carotid stenosis were far more likely to have 2 or more of these common concomitants of atherosclerosis than were the control subjects. The data suggest that the precursors of carotid stenosis are similar to those of coronary atherosclerosis and raise the hope that modification of these factors may decrease the incidence of this highly prevalent form of cerebrovascular disease.

The role of the platelet in coagulation and clinical thrombotic events

There have been exciting advances in our understanding of platelet function in recent years. Platelet physiology, drugs that inhibit platelet function, and the current evidence supporting the use of platelet-inhibiting drugs in the prevention of clinical thrombotic events are reviewed.

Transient monocular blindness

This paper is a review of the causes of intermittent monocular blindness. The nature of cholesterol and platelet retinal emboli is discussed. Their sources, the frequency with which they may cause transient or fixed blindness and the association between these emboli and pathology of the major cerebral vessels and other organs is discussed. Consideration is given to the equally important abnormalities of platelet behavior and to some of the physiology of retinal blood flow and non-embolic blindness. The current treatment of this symptom may be anticoagulation, surgical correction of a stenotic artery or both. The effect of treatment is unpredictable and in some situations the rationale is suspect. This review may provide a summary on which to base future studies of the effectiveness of various therapeutic agents.

Effect of clofibrate on intravascular coagulation in hyperlipoproteinemia

Intravascular coagulation (IVC) was evaluated in 19 patients with type II and 11 with type IV hyperlipoproteinemia before and after clofibrate therapy by measurements of soluble fibrin complexes (SFC) in plasma; fibrinolysis was estimated by quantitation of fibrin (ogen) degradation products in serum. Untreated type II and type IV patients had increased SFC (P less than 0.01). The former also had activation of the intrinsic coagulation pathway as evidenced by decreased plasma prekallikrein (P less than 0.001), kallikrein inhibitors (P less than 0.001), and factor XII (P less than 0.02). Although clofibrate treatment of the type II patients did not change plasma lipids, it decreased intravascular coagulation, apparently via decreased factor XII activation and stimulation of fibrinolysis. In contrast, treated type IV patients had unchanged SFC and FDP levels, despite decreased plasma triglycerides (P less than 0.01). Clofibrate-induced changes in blood coagulation are independent of lipid-lowering. Clofibrate therapy decreases intravascular coagulation in type II patients and may help to prevent thromboembolic sequelae.

Phase separation in phosphatidylcholine bilayers as a predictor of inhibition of blood platelet aggregation by amantadines

The ability of eleven amantadine derivatives to induce phase separation in dipalmitoyl phosphatidylcholine bilayers was studied by differential scanning calorimetry. The relative potency varied with the shape and size of the hydrocarbon cage. These agents also markedly inhibited blood platelet aggregation. The relative potencies of these compounds to induce phase separation showed a significant correlation (r = 0.70) with their platelet inhibitory activity suggesting that their pharmacologic action may be at the level of the platelet membrane. The effective concentration of the parent component amantadine is similar to its pharmacologic concentration suggesting its use as an anti-platelet drug.

Platelet aggregation and fibrinolytic activity in transient cerebral ischemia

The present study was performed in 34 patients with transient cerebral ischemia, TCI. Twenty-four of the patients were examined angiographically. Atherosclerotic abnormalities were demonstrated in 13 and a total occlusion of the interior carotid artery was found in one patient. The angiograms were normal in 10 patients. One patient suffered from hyperlipoproteinemia, type IV, and one from diabetes mellitus. The platelet aggregation in vitro was increased significantly, as more patients than normal controls showed secondary aggregation with low ADP-concentration: less than or equal to 1 mumol (p less than 0.001). The fibrinolytic capacity was significantly reduced (p less than 0.01) but not particularly in the patients with increased tendency for platelet aggregation. No correlation found between changes in platelet aggregation, the fibrinolytic activity and the angiographic findings. The results described may favor the concept that a prophylactic use of drug excerting an antiaggregation effect on platelets might be useful in patients suffering from TCI.

Determination of antiplatelet antibody activity in sera cytotoxic for human lymphocytes

Pooled serum aliquots obtained from sensitized potential renal allograft recipients on chronic hemodialysis were evaluated for their lymphocytotoxicity titers against the lymphocytes and then for alloantibodies against the platelets of 7 random donors by 5 methods. Platelet donor specific lymphocytotoxicity was present in 93% of 42 combinations. Of the positive combinations, 57% had a positive test for antiplatelet activity by the 14C serotonin release assay, 16% by the platelet aggregation method, and 19% as judged by acid phosphatase availability on the platelet membrane. No serum tested released beta-glucoronidase or lactic dehydrogenase. No correlation of the height of the titer of antiplatelet activity with that for lymphocytoxicity was detected. Thus, even in sera demonstrating significant activity against donor lymphocyte antigens, detection of associated platelet antibody activity is not uniform. Thus, a positive lymphocytoxic titer does not necessarily predict detectable antiplatelet activity. Therefore, additional tests for detection of antiplatelet activity should also be considered. This study shows that of the tests evaluated, the 14C serotonin release assay is the most sensitive for detection of antiplatelet antibodies.

[The influence of insulin, glucose and other monosaccharides on the platelet functions (author's transl)]

In vitro investigations were carried out on the action of insulin, glucose, xylose, galactose, fructose and sorbitol on the platelet aggregation test according to Breddin, on the ADP- and collagen-inducced aggregation and the release of platelet factor 4 as well as on the retraction. When incubating platelet-rich plasma with insulin and glucose simultaneously, a marked inhibition of the ADP- or collagen-induced platelet aggregation and release reaction results. Insulin as well as glucose impaired the platelet function only at high concentrations, this inhibition however did not reach that of a combination of both. Fructose, xylose and sorbitol exerted no significant inhibitory effects. In contrast to the prevented aggregation, the retraction was enhanced. As the causal mechanism for the inhibition of the platelet function the Crabtree effect is discussed.

Platelet coagulant activities and serum lipids in transient cerebral ischemia

To determine whether platelets play a part in the pathogenesis of transient cerebrovascular ischemia, we studied 22 patients with transient ischemia, 18 control patients and 38 normal subjects. Platelet aggregation and [14C]-serotonin release by ADP, epinephrine and collagen were normal in all patients, as were plasma coagulation assays, except for shortened partial thromboplastin times in the patients with transient ischemia. Platelet coagulant activities concerned with initiation and early stages of intrinsic coagulation were increased two to three times in 12 patients with transient ischemic attacks with normal serum lipids and normal in the 10 others with Type IV hyperlipoproteinemia. These results indicate an association between platelet coagulant hyperactivity and transient ischemic attacks in a group of patients with normal serum lipids.

Hyperlipidemia and atherosclerosis

These studies provide new insight into the complex mechanisms wherby hyperlipidemia causes progressive atherosclerosis. It has been shown that physical injury to the endothelial lining of arteries sets off a process which probably is an attempt at healing the injury but which can lead to atherosclerosis. It has also been found that chemical agents such as homocystine can produce a similar series of events leading to atherosclerosis. These events include focal loss of endothelium, exposure of subendothelial connective tissue, and adherence of platelets followed by release of factors that stimulate intimal smooth muscle proliferation. The present studies indicate that the effects of chronic hyperlipidemia are complex in that the condition results not only in the deposition of lipids in the atheromatous lesions but that it may produce the primary endothelial injury that initiates the process of atherosclerosis as well.

Therapy of the hyperlipidemias

Although the diagnosis of hyperlipidemia is somewhat arbitrary, in that the upper limits of normal are not universally agreed upon, it is clear that the risk of atherosclerosis increases with plasma cholesterol concentration; it may also increase in hyperglyceridemia. We take 220 mg/100 ml as the upper limit of normal for plasma cholesterol and 140 mg/100 ml as the upper limit for triglycerides. Treatment of hyperlipidemia is aimed at reducing the risk of atherosclerosis or arresting its course. Alteration of diet is the first step. If maximal diet therapy does not lower plasma lipids to acceptable levels, drug therapy is added. The decision to treat hyperlipidemia with drugs should be based on a careful weighing of risk vs. benefit for each patient. Because of the rapid change in our knowledge of both the risks and the benefits of therapy, that decision must be reviewed and updated at regular intervals.

Hereditary antithrombin III deficiency. Effect of antithrombin III deficiency on platelet function

Antithrombin III (AT III) is the main physiologic inhibitor of thrombin, and activated factors X and IX as well. Normal levels of AT III appear to be necessary to maintain blood fluidity and to prevent thrombosis. Four families with AT III deficiency and recurrent venous thromboembolism have been reported on. We present an additional family with AT III deficiency and a high incidence of thromboembolism. AT III levels were determined by both a functional and an immunologic assay. Results of platelet function tests, not previously reported in persons with AT III deficiency, were found to be normal. Following gel filtration, the platelets were very sensitive to thrombin. Thrombin-induced platelet aggregation appears to be dependent on a balance between the amount of thrombin and AT III present.

Thrombosis and intravascular coagulation

Stress, including trauma and sepsis, is associated with a state of hypercoagulability. In these circumstances the patient is at risk of generalized or local thrombotic complications. New laboratory investigative procedures facilitate diagnosis and permit improved assessment of therapy, which at present remains of unproven efficacy both in the general and local situation.

Plasma heparin neutralizing activity in coronary artery disease

Platelets contain heparin neutralizing activity that is released into plasma after platelet aggregation. Increased amounts of plasma heparin neutralizing activity were found in patients with acute myocardial infarction, unstable angina pectoris and stable arteriographically confirmed coronary artery disease. Plasma heparin neutralizing activity levels provide additional evidence for a role of platelet aggregation in coronary artery disease.

Platelet hypersensitivity and intravascular coagulation in paroxysmal nocturnal hemoglobinuria

The patient described had paroxysmal nocturnal hemoglobinuria associated with recurrent arterial as well as venous thrombosis. Study of platelet function revealed hypersensitivity to epinephrine, adenosine 5'phosphate (ADP) and collagen as judged by their ability to aggregate platelets as well as to release 14C serotonin. The release of total nucleotides was also markedly increased over normal with all aggregating agents. The abnormality was localized to the platelet since aggregation occurred when the patient's platelets were resuspended in normal plasma but not when normal platelets were incubated in the patient's plasma. Presumptive evidence for ongoing intravascular coagulation was an increase in fibrinogen derivatives of heavier molecular weight than the native protein presumably a result of thrombin action. However, factor XII was not activated and fibrinolysis was not increased. Complement component levels and antithrombin concentrations were also normal. The findings in this case suggest that hypersensitive platelets may contribute to the intravascular coagulation that is manifested by the increased incidence of thrombosis in patients with paroxysmal nocturnal hemoglobinuria.

Effects of S-8527 (1, 1-bis (4'-(1"-carboxy-1"-methylpropoxy) phenyl) cyclohexane), a new hypolipidemic compound, on platelet aggregation, adhesiveness and blood coagulation in rats

Effects of S-8527 (1,1-bis[4'-(1"-carboxy-1"-methylpropoxy) phenyl] cyclohexane) on platelet aggregation, adhesiveness and blood coagulation were examined in rats. In animals maintained on a semisynthetic diet containing sucrose (62%) as the only carbohydrate source, platelet adhesiveness increased as compared with that in rats fed a normal chow pellet. Under these experimental conditions, oral dose of S-8527 (30-300 mg/kg) for 14 days decreased platelet adhesiveness and ADP-induced platelet aggregation, but did not decrease collagen-induced platelet aggregation. S-8527 also showed a slight but significant increase of R value of thrombelastograph. In rats which were fed a normal chow pellet oral dose of S-8527 for 14 days did not significantly affect the several tests of platelet function and blood coagulation. These results suggest that S-8527 normalizes the platelet function in a hyper-adhesive state, but does not affect the platelet function in a normal state.

Platelet hypersensitivity induced by cholesterol incorporation

Platelets from individuals with familial hypercholesterolemia show increased sensitivity to the aggregating atents, epinephrine and ADP. Since the mechanism of this abnormal sensitivity is unknown, we examined, in vitro, the influence of the plasma lipid environment on the function of platelets. The composition of plasma lipids was altered by the addition of sonicated cholesterol-dipalmitoyl lecithin liposomes which were "cholesterol normal" (cholesterol-phospholipid mole ratio [C/P] equals 1.0, "cholesterol rich" (C/P eauals 2.2), or "cholesterol poor" (C/P equals 0). Cholesterol-normal liposomes had no influence on platelet lipids or platelet function. In contrast, after incubation for 5 h at 37 degrees C with cholesterol-rich liposomes, normal platelets acquired 39.2% excess cholesterol with no change in phospholipids or protein. The percent increase in platelet membrane cholesterol was three-fold that of the granule fraction. The acquisition of cholesterol by platelets was associated with a 35-fold increase in sensitivity to epinephrine-induced aggregation (P less than 0.001) and 15-fold increase to ADP aggregation (P less than 0.001), as determined both by aggregometry and by [13C]serotonin release. Response to thrombin or collagen was unchanged. Platelets incubated with cholesterol-poor liposomes underwent a selective loss of 21.4% cholesterol and this was associated with an 18-fold reduction in their sensitivity to epinephrine. These studies demonstrate that the cholesterol content of platelets is dependent on the lipid composition of the milier. Cholesterol acquired by platelets may exert its effect on platelet function by a modification of the platelet membrane.

Clofibrate reversal of platelet hypersensitivity in hyperbetalipoproteinemia

Platelet function was evaluated in 29 patients with familial hyperbetalipoproteinemia; 17 were untreated and 12 were receiving clofibrate (Atromid-S). In comparison with 26 normal subjects, the untreated patients aggregated in response to 1/3 the concentration of adenosine diphosphate (ADP), 1/25 the concentration of epinephrine, and 1/4 the concentration of collagen. Treatment with clofibrate, 2 g daily, returned the ADP sensitivity of the platelets to normal and returned epinephrine and collagen sensitivity towards normal, even though it did not alter total cholesterol, low-density-lipoprotein (LDL) cholesterol, or triglyceride concentrations significantly.

Platelet nucleotide release was also 3-to-5 fold increased in untreated type II patients. Clofibrate treatment failed to diminish the elevated platelet nucleotide release.

Incubation of clofibrate at therapeutic concentrations (100-200 µu/ml) with normal platelets in vitro decreased their sensitivity to ADP and epinephrine and reduced 14 C-serotonin release by these agents. The effect on the release was probably mediated through decreased platelet sensitivity since higher doses of epinephrine could reverse the inhibitory effect of clofibrate.

Clofibrate may decrease the incidence of thrombotic complications of atherosclerosis by altering platelet sensitivity to aggregation.