Low density lipoprotein (LDL) from male volunteers stimulated the thromboxane formation by human platelets

Lovastatin increases arachidonic acid levels and stimulates thromboxane synthesis in human liver and monocytic cell lines

The effect of lovastatin (LOV), the inhibitor of 3-hydroxy-3-methyl-glutaryl coenzyme A reductase, on linoleic acid (LA, 18:2n-6) metabolism was examined in human monocytic Mono Mac 6 (MM6) and hepatoma Hep G2 cells. The desaturation of LA was examined after LOV (72 h, 10 microM) or dimethylsulfoxide (LOV carrier, < 0.1%) and [14C]LA (last 18 h, 0.3 microCi, 5 microM). In both cell lines, LOV reduced the percentage of 14C label associated with LA and increased the percentage of label in the 20:4n-6 and the 22:5n-6 fractions. In Hep G2 but not MM6 cells, this effect was fully reversible by means of coincubation with mevalonic acid (500 microM), but not with cholesterol or lipoproteins. In both cell lines, the LOV-mediated increase in LA desaturation resulted in dose-dependent reductions of LA and elevations of AA in cellular phospholipids. The lipids secreted by LOV-treated Hep G2 cells were also enriched in arachidonic acid (AA). In the MM6 cells, LOV increased release of thromboxane upon stimulation with the calcium ionophore A23187. In summary, our findings of higher LA desaturation and AA enrichment of lipids secreted by the Hep G2 cells suggest that LOV treatment may increase the delivery of AA from the liver to extrahepatic tissues. The changes in membrane fatty acid composition can influence a variety of cellular functions, such as eicosanoid synthesis in monocytic cells. The mechanism appears to be related to the reduced availability of intermediates of cholesterogenesis.

Oxidative status and oral contraceptive. Its relevance to platelet abnormalities and cardiovascular risk

Oral contraceptive (OC) use is a risk for thrombogenic events. This paper reviews effects of OC on oxidative status, coagulation, and platelet activity. Complicating effects of cardiovascular risk factors such as smoking, diabetes, hyperpidemia, and hypertension, are discussed. From these data we conclude that: 1. OC use modifies slightly but significantly the oxidative status in women and in animals by decreasing in plasma and blood cells the antioxidant defenses (vitamins and enzymes). 2. The changes in the oxidative status are related to an increase in plasma lipid peroxides apparently responsible for the hyperaggregability and possibly the imbalance in clotting factors associated with the OC-induced prethrombotic state. 3. These effects of OC appear to be increased by a high intake of polyunsaturated fat and counteracted by supplements of vitamin E. 4. The risk factors acting synergistically with OC, have all been shown to increase platelet reactivity. In addition, smoking, diabetes, and, to some extent, dyslipidemia are associated with an increased level of lipid peroxides and concomitant changes in the antioxidant defenses that can be additive to those induced by OC. Thus, free radicals and lipid peroxidation could be the underlying mechanism in the predisposition to thrombosis induced by most risk factors in OC users. 5. Results of epidemiologic and experimental studies in this field will be concordant only when diet and natural antioxidants will be systematically taken into consideration.

Lipids and thrombosis

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

Renal vasoconstriction caused by short-term cholesterol feeding is corrected by thromboxane antagonist or probucol

Recent studies indicate that short-term cholesterol feeding causes vascular hyperreactivity and/or increased tone in certain vascular beds. The present study in rats examined the effect of 3 wk of cholesterol-supplemented diet (CSD) on renal hemodynamics. We tested the hypothesis that LDL oxidized in vivo is causally related to increased renal vascular tone by adding the antioxidant drug probucol to the CSD (CSD + P). Micropuncture of surface nephrons in the CSD rats demonstrated that single nephron glomerular filtration rate (SNGFR) and single nephron afferent plasma flow (QA) were markedly lower than in normal rats, whereas glomerular capillary pressure (PGC), afferent arteriolar resistance (RA), and single nephron filtration fraction (SNFF) were higher. In the CSD + P animals, almost all of these hemodynamic abnormalities were absent. TXB2 and PGE2 were increased in proximal tubule fluid and urine in the CSD rats, but normal in the CSD + P group. Infusion of a TXA2 receptor antagonist into the suprarenal aorta of CSD rats caused a rapid return to normal of RBF (renal blood flow), GFR (glomerular filtration rate), SNGFR, QA, RA, PGC, and Kf (ultrafiltration coefficient). Our observations demonstrate that cholesterol feeding leads to renal vasoconstriction, which appears to be mediated largely by increased TXA2 production. The fact that probucol prevented the hemodynamic abnormalities as well as the increased TX production is consistent with the hypothesis that LDL oxidized in vivo initiates events leading to TX mediated vasoconstriction.

Longitudinal and cross-sectional studies of the relationship between 6-keto PGF1 alpha and high density lipoproteins

Two studies were performed to examine the relationship between Prostacyclin (PGI2) and high density lipoprotein cholesterol (HDL-C). A longitudinal study examined the stable metabolite of PGI2, 6-keto PGF1 alpha, along with HDL-C, and total cholesterol (TC) before (Week 0), during (Week 4), and after (Week 9) an eight week aerobic conditioning program. 6-keto PGF1 alpha was measured by radioimmunoassay using 125I, and HDL-C and TC were spectrophotometrically analysed. Maximal oxygen uptake and resting heart rate data obtained at Week 0 confirmed that the training group (E) was not different from the sedentary group (C). Results obtained at Week 9 indicated that maximal oxygen uptake was higher (p less than 0.05) and resting heart rate lower (p less than 0.05) in the E group, while these variables remained unchanged in group C. Levels of 6-keto PGF1 alpha, HDL-C, and TC did not differ between groups E and C when Week 0 was compared with Week 9. However, within the E group, significantly lower concentrations of 6-keto PGF1 alpha (p less than 0.05) and HDL-C (p less than 0.05) were found at Week 4 compared with Week 0. A cross-sectional study involving 52 subjects examined the relationship between 6-keto PGF1 alpha and several cardiovascular disease risk factors which included HDL-C. The only significant correlation (r = 0.50 p less than 0.05) observed was between 6-keto PGF1 alpha and HDL-C.

Inhibition of thromboxane A2 synthesis in rats treated with phenobarbital

The effect of phenobarbital administration on serum lipoproteins and thromboxane A2 synthesis in platelets was studied in rats. Phenobarbital decreased the serum LDL level by 33% and increased the HDL level by more than 15%. The synthesis of thromboxane A2 in the platelets of the phenobarbital treated animals was found to be reduced by 43%. Thromboxane A2 synthesis in the platelets of the control animals was inhibited by HDL and stimulated by LDL. Hence it appears that the decreased thromboxane A2 synthesis in the platelets of phenobarbital treated rats was at least partly due to the increased HDL and decreased LDL in the serum. Phenobarbital treatment also caused a 15% increase in the serum HDL-cholesterol although it did not have any significant effect on the total serum cholesterol.

Effects of a low-fat, low-cholesterol diet on serum lipids, platelet aggregation and thromboxane formation

The purpose of this study was to test the effects of a low-fat, low-cholesterol diet on serum lipids, platelet aggregation, thromboxane formation and blood pressure. Fifteen hyperlipidemic males with normal blood pressure were treated for 2 weeks. Resting supine blood pressure, total serum cholesterol, HDL-cholesterol, the ratio of total cholesterol/HDL-cholesterol and triglycerides all were significantly reduced. The maximum aggregation and velocity of aggregation for isolated platelets stimulated with either ADP or collagen was significantly reduced. Thromboxane formation during 5 min of platelet aggregation with collagen or ADP was also significantly reduced. These results have important implications for hypertension and atherosclerotic cardiovascular disease.

Thromboxane A2 (TXA2) formation by washed human platelets under the influence of low and high density lipoproteins from healthy donors

Low density lipoprotein (LDL) stimulates TXA2 formation (measured as malondialdehyde) in washed human platelets with active cyclooxygenase, whereas high density lipoprotein (HDL) does not show any influence on TXA2 formation in this system. These results support the hypothesis that the proaggregatory action of a high plasma LDL concentration may be mediated by modification in the biosynthesis of eicosanoids. The mode of action of lipoproteins is discussed.

The influence of lipoproteins (LDL and HDL) on PGI2-formation by isolated aortic preparations of rabbits

The influence of LDL- and HDL-concentrations of 0.5-2.0 mg/ml and 0.25-1.0 mg/ml respectively, from male and female donors on the formation of PGI2 in isolated aortic preparations was investigated under in vitro conditions. LDL of male donors inhibited endogenous PGI2-formation and the conversion of exogenous PBH2 to PGI2. On the other hand the highest concentration of HDL induced an increase in the basal and in the stimulated PGI2-formation. Lipoproteins of female donors showed no or only a weak stimulating effect on PGI2-formation. Human serum albumin in comparable concentrations of 1-50 mg/ml remained without influence. The importance of the results for cardiovascular diseases is discussed.

Endogenous lipoproteins modify the thromboxane formation capacity of platelets

A significantly negative correlation was demonstrated between HDL-cholesterol levels of serum and the thromboxane B2 (TXB2) formation in clotted whole blood, whereas a significantly positive correlation was estimated between the LDL cholesterol or apolipoprotein B (apo B) levels and the TXB2 formation in clotted whole blood. Similar relationships were observed between the HDL and apo B serum levels and the thrombin-induced malondialdehyde formation in platelet-rich plasma. The levels of HDL2 cholesterol, total cholesterol, apo A-I and of triglycerides were not significantly correlated with the TXB2 or malondialdehyde formation in both systems. The results in this study support the hypothesis that the TXB2 formation may be modulated by endogenous lipoproteins: high level of LDL stimulates and high level of HDL inhibits the TXB2 formation.

Influence of platelet activation factor and prostaglandins on cholesterol esterification in human plasma

Prostaglandin E1 (PGE1) and platelet activation factor (PAF) were found respectively to activate or to inhibit cholesterol esterification in whole plasma but not in lipoprotein-deficient plasma. It is suggested that these effects are mediated by the interaction of PGE1 and PAF with high-density lipoproteins [(1984) FEBS Lett. 173, 291-294]. Possible physiological implications of these findings are discussed.

Relationship between platelet phospholipase activity and plasma in ischemic heart disease

Platelet phospholipase plays an important role in the metabolic responses of platelets to exogenous stimuli. The platelet phospholipase activity (PLA) was therefore studied in 38 patients with ischemic heart disease (IHD) and in 26 age-matched normal subjects who served as controls. The mean platelet PLA in the IHD group was 12.72 +/- 1.03 nmol/mg protein/30 sec which was significantly (p less than 0.005) higher than that of the normal controls (8.72 +/- 0.76). When they were classified into acute stage, such as unstable angina or acute myocardial infarction (AMI), and chronic stage, such as stable angina or old myocardial infarction (OMI), there was no significant difference between them. On the other hand, about two-fold activation of platelet PLA was observed in acute stage IHD, and 20-30% inhibition of it was demonstrated in chronic stage IHD following the addition of autologous plasma to washed platelet suspensions, suggesting that certain plasma factor(s) are responsible for such phenomena. In an attempt to identify these plasma factor(s), various substances such as serum albumin, high density lipoprotein, prostaglandin E1 (PGE1) and E2 (PGE2), and platelet activating factor were assessed by in vitro experiments. Only PGE1 and PGE2 revealed a significant effect on the platelet PLA. The relationship between plasma and platelet activity in terms of platelet PLA deserves attention since it varies according to the type and stage of IHD.