Nature of the transferable factor which causes abnormal platelet behaviour in vascular disease

Plasma prekallikrein, kallikrein inhibitors, kininogen and lipids during gemfibrozil treatment in type II dyslipidaemia

The effects of gemfibrozil on plasma prekallikrein, kallikrein inhibitors, kininogen and plasma lipids were investigated in 31 male subjects having either type IIA or IIB dyslipidaemia. During gemfibrozil use, plasma prekallikrein and kininogen were increased significantly while kallikrein inhibitors increased only slightly. Total cholesterol and triglycerides decreased while HDL cholesterol was increased. Changes in prekallikrein and HDL cholesterol were correlated, whereas no other significant correlations between changes in lipid and kinin parameters were seen. The observed changes in prekallikrein and kininogen possibly indicate a shift in the thrombo-haemorrhagic balance in favour for increased fibrinolysis. If so, the effects of gemfibrozil in prevention and management of atherosclerosis would not be solely due to correlation of the dyslipidaemia but also to protection against the accelerated coagulation tendency seen in type II dyslipidaemia.

Technical considerations for platelet aggregation and related problems

Platelet aggregation generally is ordered by the physician to evaluate platelet function in hemorrhagic or thrombotic disorders. Malfunction of the platelet may be the result of an intrinsic congenital defect or an acquired problem induced by drugs or certain circulating plasma factors. It is necessary to obtain information from the patient with respect to family history, drug ingestion, physical or mental stress. In addition, other laboratory studies should be obtained to rule out general coagulation disorders affecting the plasma factors. A bleeding time will be helpful in establishing the severity of any platelet dysfunction. Technical considerations with regard to the preparation of the samples are of primary importance in determining platelet aggregation. Aggregating studies require the use of a variety of binding agents. (Studies on shape change, adhesion of platelets, release of platelet granule substance, and or lysis with extrusion of cytoplasmic constituents may be helpful in certain cases.) Instrumentation for platelet aggregation presently is available in many hospitals. The technical factors to be considered for routine aggregation studies include the type and strength of anticoagulant, centrifugation technique used in preparing the platelet-rich and platelet-poor plasma, platelet concentration, time of storage of the sample after venipuncture and after centrifugation, temperature, and the mixing of the sample. In general, critical concentrations of each reagent should be employed to improve the discrimination capability of the assay. Small differences in response may be obliterated by using excessive concentrations of a given reagent. Comparison in response to the test platelets with control platelets is best done at the same time by performing the aggregation in a dual instrument so that handling procedures will be identical and artifactual differences eliminated.

Similar behaviour of lecithin:cholesterol acyltransferase and pseudocholinesterase in liver disease and hyperlipoproteinemia

Using exogenous substrate for its assay, lecithin:cholesterol acyltransferase (LCAT) was found to be decreased in liver disease and higher than normal in endogenous hypertriglyceridemia. LCAT activity was positively correlated with serum cholesterol and triglyceride. However in the six patients with excessive hypertriglyceridemia (type V), LCAT activity was lower than in type IV hyperlipoproteinemia. LCAT activity was not changed significantly in type II-a hyperlipoproteinemia. A striking parallel was noted between plasma LCAT and serum pseudocholinesterase activity. It suggested that both these liver secretion enzymes might be induced by an accelerated turnover of serum lipids and lipoproteins. Pathogenical implications of these findings are briefly discussed.

Pathogenetic mechanisms in atherosclerosis

Four theories of atherogenesis are briefly reviewed and criticized: the degenerative, the thrombogenic, the platelet aggregation and the insudative theory. Evidence is presented in detail to suggest that a modified form of the insudative theory (1) accounts more satisfactorily than the other theories for the known association of risk factors with atherosclerosis and (2) allows one to understand how some of the more important risk factors operate at the level of the arterial wall. It is proposed that atherosclerotic plaques, and also certain extravascular lesions broadly associated with atherosclerosis (corneal arcus, xanthomas), arise because altered endothelial permeability allows certain reactive macromolecular plasma proteins (the plasma low density and very low density lipoproteins and fibrinogen, which are normally largely confined to the circulation) to permeate endothelium and interact with charged components of the connective tissue gel of the arterial wall or other tissues. The effect of hyperlipidemia, hypertension, arterial disease or injury upon this process, and the manner in which these factors interact, is examined in relation to experimental findings and clinical observations.

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.

Blood platelet response to plasma from patients with ischemic heart disease

Blood platelets change shape (from small round spheres to larger spread forms) as they participate in thrombosis. Using an electron microscopic technique, we surveyed 14 patients with both acute and chronic ischemic heart disease; each had increased spread platelet forms (69 plus and minus 22.2 [standard deviation] percent) when compared with 14 asymptomatic control subjects (P less than 0.001). When platelets from these 14 control subjects were exposed to plasma from the patients with ischemic heart disease, spread forms increased from 13.4 plus and minus 9.1 to 44.5 plus and minus 15.5 percent (P less than 0.001). There was no significant increase in spread platelets in these control subjects when their blood was mixed with plasma from another control group. Similar studies were performed in seriously ill noncardiac patients: 9 of 13 had increased spread platelet forms when compared with control subjects, but plasma from only 5 of these 9 subjects caused increased spread forms when mixed with platelets from normal subjects (P less than 0.05). Thus a factor existed in the plasma of these patients with ischemic heart disease that caused normal platelets to become spread. Similarly the plasma of some patients with serious noncardiac disease had a comparable effect on normal platelets. Although the identity of this factor is unknown, it is probably unrelated to hormonal or therapeutic influences occurring either during acute infarction or during the stress of serious illness because (1) the effect of the plasma from patients with acute ischemic heart disease was identical to that of patients with chronic ischemic heart disease, and (2) the effect was not present in all patients with serious noncardiac disease.

Plasma lipids, blood coagulation and fibrinolysis in peripheral arterial disease

In a study relating platelet stickiness, plasma fibrinogen and fibrinolysis to fasting plasma lipids in patients with atherosclerosis of the lower limbs, a significant correlation has been observed between platelet stickiness and plasma phospholipid levels. The possible mechanisms are discussed. No other correlation is observed. Fasting levels of plasma fibrinogen were raised, while levels of plasminogen, and the degree of platelet stickiness in many cases were normal.

Acanthocytosis with normolipoproteinaemia: biophysical aspects

The electrokinetic properties of normal and pathological red cells and platelets are compared.

The electrophoretic mobilities and isoelectric points of normal and pathological erythrocytes do not differ significantly, adsorption of calcium and uranyl ions occur to the same extent and treatment with neuraminidase induces almost identical mobility decreases.

The pathological platelet-plasma system is more sensitive to adenosine diphosphate (ADP) than normal systems; platelet aggregation and electrophoretic mobility increases are induced by much lower concentrations of ADP than those normally required. The pathological platelet system shows a greater tendency to aggregate spontaneously. The abnormal sensitivity to ADP of the pathological platelet-plasma system is considered in terms of plasma components.

The irrelevance of adhesive platelet estimations after thrombosis

It has been found that adhesive platelet counts are not of clinical use either after surgical operation or during long-term anticoagulant therapy in the detection of thrombosis. Calculation of the theoretical risk of development of platelet thrombi related to the platelet count suggests a possible reason for this absence of clinical significance. The evidence that platelet adhesiveness alters because of uncontrollable platelet variables and is controlled at least in part by plasma factors is discussed. The possibility of measuring the plasma factors is considered briefly.

Coumarin therapy and platelet aggregation

Platelet aggregation has been related to blood coagulation studies in patients on nicoumalone, a coumarin anticoagulant. Aggregation studies were performed by means of Chandler's tube and the adenosine diphosphate (A.D.P.)-induced optical density method. Platelet aggregation in Chandler's tube has been shown to be quite different from A.D.P. aggregation and to be dependent on the "intrinsic" (blood) clotting system. When the intrinsic system was depressed by coumarin anticoagulant, aggregation was delayed in Chandler's tube, but patients with a predominantly "extrinsic" (tissue) system defect gave normal results even when their prothrombin time was excessively prolonged. In contrast there was an increased response to A.D.P. in the anticoagulated patients.The study emphasizes the different mechanisms of platelet aggregation, which we have referred to as coagulation-induced and A.D.P.-induced aggregation. It also shows the limitations of routine control of oral anticoagulants by prothrombin time alone, as the coagulation-induced platelet aggregation appears to be quantitatively related to the overall level of clotting factors in the intrinsic system and independent of the extrinsic system.