Vitamin E. An inhibitor of the platelet release reaction

Vitamin E deficiency in variant angina

BACKGROUND

Oxidative modification of LDL has been suggested to increase coronary vasoreactivity to agonists. A deficiency of vitamin E, a major antioxidant, may be related to the occurrence of coronary artery spasm.

METHODS AND RESULTS

Vitamin E levels were determined with the use of high-performance liquid chromatography in normolipidemic subjects, including 29 patients with active variant angina (group 1), 13 patients with inactive stage of variant angina without anginal attacks during the past 6 months (group 2), 32 patients with a significant (>75%) organic coronary stenosis and stable effort angina (group 3), and 30 patients without coronary artery disease (group 4). Total lipid levels in blood were calculated as total cholesterol plus triglyceride levels. The plasma alpha-tocopherol levels as well as alpha-tocopherol/lipids were significantly lower in group 1 than in groups 2 through 4. Also, the plasma gamma-tocopherol levels were significantly lower in group 1 than in groups 2 through 4. The vitamin E levels were not significantly different between group 1 patients with and those without a significant organic stenosis. In group 1, both alpha- and gamma-tocopherol levels were significantly elevated after a > or = 6-month angina-free period. The alpha-tocopherol levels in the LDL fraction were significantly lower in group 1 than in group 4. Plasma alpha-tocopherol levels were significantly correlated with those in the LDL fractions. In 6 patients of group 1 still having anginal attacks while receiving calcium channel blockers, the addition of vitamin E acetate (300 mg/d) significantly elevated plasma alpha-tocopherol levels and inhibited the occurrence of angina.

CONCLUSIONS

Plasma vitamin E levels were significantly lower in patients with active variant angina than in subjects without coronary spasm, suggesting an association between vitamin E deficiency and coronary artery spasm.

Effects on health of dietary supplementation with 100 mg d-alpha-tocopheryl acetate, daily for 6 years

To evaluate the clinical antioxidant effects of vitamin E, 161 healthy volunteers aged 39 to 56 years, were given 100 or 3 mg of d-alpha-tocopheryl acetate orally daily for 6 years using a randomized, double-blind design. Among the 147 volunteers who qualified for the analysis, seven of the 73 volunteers receiving 3 mg d-alpha-tocopheryl acetate daily and none of the 74 volunteers receiving 100 mg had coronary disorders including myocardial damage (P < 0.02). ST or T wave abnormalities on electrocardiograms were considered to indicate coronary disorders (four volunteers). The mean serum total tocopherol (TOC) concentration in the 100-mg group was significantly higher than that in the 3-mg group 6 months after the start of the study, and this raised value was maintained throughout the study; the level in the 3-mg group did not change significantly from the baseline value. The low-density lipoprotein cholesterol/total TOC ratio, a parameter of the inhibition of peroxidation of low-density lipoprotein cholesterol, was the only serum lipid parameter that was significantly different, at baseline, in the volunteers with coronary disorders compared with the others. These findings indicate that long-term supplementation with 100 mg tocopheryl acetate daily may prevent the early stages of coronary atherosclerosis by decreasing peroxidation of low-density lipoprotein cholesterol.

Experimental thrombosis model induced by free radicals. Application to aspirin and other different substances

A large number of experimental studies suggests that oxygen free radicals play a major role in the pathogenesis of the myocardial lesions observed during the sequence ischemia-reperfusion. The purpose of this study was to determine whether oxygen free radicals can induce thrombosis. In so doing we have developed a new experimental thrombosis model. Reproducible focal thrombosis has been achieved by irradiating mesenteric arterioles of rat for variable time with green filtered light issuing from a mercury lamp after systemic injection of different rose bengal doses. The number of emboli that remove in the blood (N), the duration of total occlusion (T) and the number of emboli per minute were then measured. As control, no rose bengal administration was done and the vessels were exposed to the filtered light. In comparison with this control, results clearly showed that free radicals always induced thrombosis and the induced thrombus was mainly composed of platelets. In this new thrombosis model induced by free radicals antithrombotic drugs (aspirin, 200 mg/Kg, heparin, 2 mg/Kg) and antioxidants (vitamin C, 10 and 20 mg/Kg, allopurinol, 200 and 300 mg/Kg, vitamin E, 500 and 1000 mg/Kg) have been tested. Results have shown that only heparin and vitamin E had an antithrombotic effect on thrombus formation induced by free radicals. This model should be useful in studying the effects of different drugs and could lead to new treatment modalities for ischemic accident and other cardiovascular diseases.

Platelet function defects associated with hemorrhage or thrombosis

Platelet dysfunction, especially acquired forms, is a common cause of hemorrhage, especially when associated with trauma or surgery. Although the hereditary platelet function defects are generally quite rare, hereditary storage pool disease is common enough to be suspected in an individual, usually a child, with characteristic historical and clinical findings. The acquired platelet function defects, especially those resulting from drugs, are common and should promptly be suspected in patients developing easy and spontaneous bruising, mild-to-moderate mucosal membrane hemorrhage, or unexplained bleeding associated with trauma or surgery. The template bleeding time is generally useful as a screening test of platelet function, but a normal template bleeding time, in the face of a suggestive history, suggestive clinical findings, or in a patient frankly bleeding, is not reliable, and platelet aggregation or lumiaggregation should be done in appropriate clinical situations. Also, prolongation of the template bleeding time is an unreliable predictor of clinical bleeding propensity. The mainstay of therapy for almost all these defects, if bleeding is significant, is the liberal infusion of appropriate numbers of platelet concentrates. The acquired platelet function defects should also be managed by attempts to treat or control the underlying disease, if possible, and offending drugs or potentially offending drugs should immediately be stopped.

Vitamin E and the hypercoagulability of neonatal blood

This study examines whether vitamin E deficiency has any role in the hypercoagulability of neonatal blood. Blood was collected from mothers and their full-term placental cords. Vitamin E was measured by high-pressure liquid chromatography and whole blood clotting time was measured by recalcification. Cord plasma had significantly lower vitamin E (P < 0.0001) compared with maternal plasma. Whole blood clotting time of cord blood was significantly (P < 0.002) shorter compared with the clotting time of maternal blood. There was a significant correlation between plasma vitamin E and whole blood clotting time (r = 0.54, P < 0.04) of cord blood. The addition of standard vitamin E to cord blood in vitro resulted in prolongation of whole blood clotting time. This suggests that a deficiency of plasma vitamin E can shorten whole blood clotting time in newborns, which may have a role in the disseminated intravascular coagulation frequently experienced by newborn infants.

Prevention of platelet dysfunction by vitamin E in diabetic atherosclerosis

Premature atherosclerosis and other vascular disorders are serious complications of diabetes mellitus. Contributing factors include (i) increased peroxidation of LDL leading to foam cell formation, fatty streaks and plaque formation in the arterial wall, and (ii) hyperreactivity of blood platelets leading to increased platelet adhesion and aggregation. Vitamin E may play a protective role as an antioxidant and/or membrane stabilizing agent in either mechanism. In platelets it appears to regulate arachidonic acid metabolism. Decreased vitamin E levels in platelets are associated with increased aggregation. This is reversible by correction of the vitamin E status. In diabetics, platelet vitamin E levels tend to be reduced with concomitant increase in platelet aggregation. Several studies in patients with insulin-dependent diabetes mellitus and, to some extent, in those with non-insulin-dependent diabetes mellitus have shown that supplementation with several hundred IU vitamin E significantly reduced platelet aggregation and lipid peroxidation. In healthy volunteers high-dose supplementation had no notable effect on platelet aggregation. However, doses as low as 200 IU vitamin E significantly reduced platelet adhesion and inhibited the formation of protruding pseudopods typically occurring in activated platelets. In diabetic patients a decrease in the nonenzymatic glycation of proteins by vitamin E supplementation has been observed. Controlled studies are needed to confirm the effect of vitamin E on platelet function in well-defined groups of diabetics, followed by large-scale trials investigating the prevention of diabetic vascular complications as clinical end point.

Why don't we use vitamin E in dermatology?

OBJECTIVE

To review the possible uses of topical and systemic tocopherols as therapy for skin conditions in light of the widespread use of vitamin E by patients.

DATA SOURCES

Index Medicus was searched for articles published from 1922 (when vitamin E was discovered) to 1966 (the beginning of MEDLINE). MEDLINE was searched for articles in English and French on vitamin E or tocopherol in relation to dermatology. Additional original articles were identified from the reference lists of the review articles.

STUDY SELECTION

Only well-designed controlled studies were accepted; anecdotes and open studies are cited for completeness and as direction for future research.

DATA SYNTHESIS

There was some weak or conflicting evidence that vitamin E is of value in yellow nail syndrome, vibration disease, epidermolysis bullosa, cancer prevention, claudication, cutaneous ulcers, and collagen synthesis and wound healing. It was of no use in atopic dermatitis, dermatitis herpetiformis, psoriasis, subcorneal pustular dermatosis, porphyrias and skin damage induced by ultraviolet light.

CONCLUSIONS

After 44 years of research there is still scant proof of vitamin E's effectiveness in treating certain dermatologic conditions. Further research in well-designed controlled trials is needed to clarify vitamin E's role.

Vitamin E: more than an antioxidant

The effect of vitamin E on platelet function has been reviewed. Although vitamin E inhibits platelet aggregation in vitro, in vivo it has no significant effect when administered in doses up to 1200 U/day. Platelet adhesion, on the contrary, is strongly inhibited by alpha-tocopherol. Doses of 400 IU/day provide greater than 75% inhibition of platelet adhesion to a variety of adhesive proteins when tested at low shear rate in a laminar flow chamber. The antiadhesive effect of vitamin E appears to be related to a reduction in the number and size of pseudopodia upon platelet activation. This effect of vitamin E may be related to changes in fatty acylation of structural platelet proteins. Clinical studies of vitamin E supplementation in patients with thromboembolic disease are in progress.

Vegetarians have higher plasma alpha-tocopherol relative to cholesterol than do nonvegetarians

Biological antioxidants are thought to play a protective role in certain disease processes, including atherosclerosis. To compare the relative antioxidant/atherogenic risk between vegetarians (presumed lower risk) and omnivores (higher risk), the alpha-tocopherol, total cholesterol and fatty acid (FA) profiles were determined in the plasma of 79 vegetarians (28 males, 51 females) and 79 age- and sex-matched nonvegetarians. In the vegetarian group, mean (+/- SEM) plasma alpha-tocopherol was 714 +/- 46 micrograms/dl for males and 725 +/- 24 for females; corresponding cholesterol values were 122 +/- 5 mg/dl and 138 +/- 3, respectively, which were significantly lower than the respective control values (928 +/- 38; 883 +/- 23 and 206 +/- 6; 188 +/- 4). However, when plasma tocopherol was expressed in terms of cholesterol, the tocopherol: cholesterol molar ratio was significantly enhanced for both male (27%) and female (11%) vegetarians. Vegetarians also had a lower atherosclerosis risk based on their plasma FA profile (higher linoleic:oleic acid ratio) which correlated well (r = 0.72; p less than 0.001) with plasma alpha-tocopherol:cholesterol molar ratio. Since the bulk of tocopherol is transported in low-density lipoprotein, this lipoprotein in vegetarians may be better protected against lipid peroxidation, a process believed to be important in the pathogenesis of atherosclerosis.

Influence of vitamin E on platelet function in humans

alpha-tocopherol, a natural antioxidant, has been found to inhibit platelet aggregation and release when tested in an in vitro system. This effect of vitamin E was thought to be due to a slight reduction of platelet cyclooxygenase activity and inhibition of lipid peroxide formation. Aggregation of platelets derived from individuals on a dietary supplementation of alpha-tocopherol ranging from 400 to 1200 IU/day showed no significant reduction. The discrepancy between the effectiveness of alpha-tocopherol in vitro and ex vivo is probably related to the levels of alpha-tocopherol attainable in platelets and plasma. Investigation of the effect of alpha-tocopherol on platelet adhesion showed a major inhibitory activity at doses of vitamin E as low as 200 IU/day. Measurements were performed in a laminar flow chamber at both high and low shear rates. Reduced platelet adherence to collagen, fibrinogen, and fibronectin could be documented. alpha-tocopherol-enriched platelets that adhered to adhesive surfaces failed to show the usual long thin pseudopodia but exhibited short, rounded, blunt projections. The reason for this shape change is still unclear, but we speculate that it may be causing the vitamin E-induced reduction of platelet adhesiveness. We believe that dietary supplementation with this vitamin could play a role in the treatment of thromboembolic disease, especially when given in conjunction with an inhibitor of platelet aggregation.

Cardiac beta-adrenoceptor sensitivity and Parkinson's disease

Certain clinical manifestations of Parkinson's disease (PD) (speech or/and balance disturbances) are not linked to brain dopamine deficiency. The purpose of the present study was to search for a possible relationship between those so-called "non-dopamine-dependent" extrapyramidal manifestations and the sensitivity of cardiac beta-adrenoceptors. Fourteen patients aged 51 to 69 were included in the study after having given their informed consent. Any factor or pathology susceptible to modify receptor sensitivity entailed exclusion. In the absence of a reference model for measuring the reactivity of central beta-adrenoceptors, a computation of the isoprenalin dose necessary to increase the resting heart rate by 20 bpm was used as an index for beta-adrenergic system reactivity. In addition to that test, other parameters were recorded: disease duration, motor status scale (Columbia), some cognitive functions (MMS and image differed recall). The cardiac beta-receptor decrease in reactivity to isoproterenol is correlated to PD duration (r = 0.8, P less than 0.001). Conversely, the sensitivity of these receptors appeared to be unrelated to the extrapyramidal severity of the disease, hence to the degree of the so-called "non dopamine-dependent" disturbances. Furthermore, such results raise the meaning of the impairment of peripheral aminergic receptors in the cognitive disturbances linked to ageing and/or PD.

Inhibitory effect of vitamin E (alpha-tocopherol) on spontaneous platelet aggregation in whole blood

Vitamin E (D-alpha-tocopherol) inhibited spontaneous human platelet aggregation in whole blood in the 20-200 micrograms/ml range. When alpha-tocopherol (20 micrograms/ml) and aspirin (0.5 mM), or alpha-tocopherol and the mixture of phosphocreatine (1.5 mM) and creatine phosphokinase (50 U/ml) (CP/CPK) were added to this reaction system, a synergic inhibitory effect on aggregation was observed. On the other hand, when both alpha-tocopherol and the specific inhibitor of platelet activating factor (CV-3988; 0.38 mM) were added to this system, the inhibition was the same as that caused by the addition of CV-3988 alone, suggesting there was no synergism, i.e., that the effect of alpha-tocopherol is related to the inhibition of platelet activating factor (PAF)-induced platelet aggregation in whole blood. However, alpha-tocopherol (20 or 50 micrograms/ml) did not inhibit PAF (10 nM) induced platelet aggregation in platelet rich plasma (PRP). These results suggest that the inhibition of platelet aggregation in whole blood by alpha-tocopherol is due to the inhibition of PAF synthesis, and is unrelated to adenosine diphosphate (ADP) or thromboxane A2.

The effect of sulphinpyrazone and alpha-tocopherol on platelet activation and function in haemodialysed patients

In 30 patients on chronic haemodialysis treatment the platelet activity and function were studied before and during antiplatelet therapy with alpha-tocopherol and sulphinpyrazone. In both kinds of treatment a significant decrease of ADP-induced and spontaneous aggregation was observed. Sulphinpyrazone exerts an inhibitory effect not only on platelet aggregation but also on platelet factor 3 and provokes a significant prolongation of the bleeding time.

Phospholipases in biology and medicine

Phospholipases, a group of enzymes that catalyze the hydrolysis of membrane phospholipids, are classified according to the bond cleaved in a phospholipid into PLA1 (EC 3.1.1.3), PLA2 (EC 3.1.1.4), PLB (EC 3.1.1.5), PLC (EC 3.1.4.3), and PLD (EC 3.1.4.4). This paper reviews source and structure of PLA2 and the involvement of PLA2 and PLC in several biological phenomena, such as, signal transduction, photoreception, biosynthesis of lung surfactant, sperm motility, and fertilization. New assays for PLA2 activity and concentration in biological fluids are discussed. Phospholipases are involved in many inflammatory reactions by making arachidonate available for eicosanoid biosynthesis. The determination of PLA2 activity and mass concentration in plasma is useful in the diagnosis and prognosis of pancreatitis and of septic shock. Naturally occurring phospholipase inhibitors, such as lipocortins act as second messengers in the anti-inflammatory response to steroids. Lipocortins may be valuable therapeutic agents, because they are more specific in their anti-inflammatory action than glucocorticoids; therefore, they are less likely to produce harmful side effects.

Short term vitamin E supplementation has no effect on platelet function, plasma phospholipase A2 and lyso-PAF in male volunteers

1. Based largely upon in vitro studies, vitamin E has been reported to inhibit phospholipase A2 activity, to alter phospholipid metabolism and reduce platelet aggregation. 2. The effect of dietary supplementation with D-alpha-tocopherol (1500 iu/day for 14 days) was studied in nine males, 41-63 years old, comparing active treatment with a preceding placebo period. 3. Despite an increase from 2.6 +/- 0.8 (s.d.) x 10(-5) mol/L to 6.0 +/- 1.8 10(-5) mol/L in plasma vitamin E there were no significant changes in the aggregation of diluted whole blood or platelet rich plasma to adenosine diphosphate (ADP) or collagen, in plasma phospholipase A2 activity or plasma lyso-platelet-activating factor (lyso-PAF) (bioassay after in vitro acetylation to PAF). 4. High dose vitamin E dietary supplementation had no effect on these phospholipid and platelet parameters.

Inhibition of cyclooxygenase-independent platelet aggregation by low vitamin E concentration

Platelet aggregation induced by threshold concentrations of agonists such as collagen, PAF or epinephrine was inhibited in vitro by 100 microM aspirin but was restored by stimulating platelets with high concentrations of collagen, PAF or by a combination of epinephrine and PAF. Incubating aspirin-treated platelets with 50-100 microM vitamin E or vitamin E acetate inhibited platelet aggregation by high concentrations of collagen and PAF and by the combination of epinephrine and PAF; platelet thromboxane A2 formation was less than 10% in samples incubated with 100 microM aspirin. Apyrase, added to aspirin-treated platelet, did not influence platelet aggregation induced by epinephrine and PAF. The present study suggests that concentrations of vitamin E as low as 50-100 microM inhibit cyclooxygenase-independent platelet aggregation when combined with an inhibitor of the arachidonate pathway.

Influence of vitamin E on phosphatidylethanolamine lipid polymorphism

The effect of vitamin E, in its major form alpha-tocopherol and its synthetic analog alpha-tocopheryl acetate, on phosphatidylethanolamine lipid polymorphism has been studied by mean of differential scanning calorimetry and 31P-nuclear magnetic resonance techniques. From the interaction of these tocopherols with dielaidoylphosphatidylethanolamine it is concluded that both molecules promote the formation of the hexagonal HII phase at temperatures lower than those of the pure phospholipid. When the tocopherols were incorporated in the saturated dimiristoylphosphatidylethanolamine, which has been shown not to undergo bilayer to hexagonal HII phase transition, up to 90 degrees C, they induce the phospholipid to partially organize in hexagonal HII phase. From our experiments it is shown that alpha-tocopherol is more effective than its analog in promoting HII phase in these systems. It is also shown that, while alpha-tocopheryl acetate does not significantly perturb the gel to liquid-crystalline phase transition of dimirystoylphosphatidylethanolamine, alpha-tocopherol does so and more than one peak appears in the calorimetric profile, indicating that lateral phase separations are taking place.

Ascorbic acid, glutathione and synthetic antioxidants prevent the oxidation of vitamin E in platelets

An earlier report from this laboratory showed that tocopherol in human platelets is oxidized when the platelets are incubated in vitro in Tyrode medium with arachidonate (or other oxidants). Arachidonate is a more potent oxidizing agent in 50 mM potassium phosphate buffer at pH 7.4 with 0.1 mM ethylenediaminetetraacetic acid (EDTA) than in Tyrode medium. Forty to fifty percent of total platelet tocopherol was oxidized upon incubation with 40-50 microM arachidonate in the phosphate-buffered medium. The tocopherol oxidation took place within 15 min after the addition of arachidonate. Preincubation of platelets with ascorbate blocked the oxidation of tocopherol. This is one of the first direct in vitro demonstrations of the vitamin E-sparing action of vitamin C in media containing biological cellular material. Other compounds which blocked the oxidation of platelet tocopherol were ascorbyl palmitate, propyl gallate, butylated hydroxytoluene, hydroquinone and glutathione. If ascorbate or glutathione was added after the tocopherol was oxidized to the quinone there was no reversal of the oxidation.

Vitamin E. deficiency increases the synthesis of platelet-activating factor (PAF) in rat polymorphonuclear leucocytes

Vitamin E deficiency was found to stimulate FMLP (N-formyl-L-methionyl-L-leucyl-L-phenylalanine)-induced biosynthesis of PAF (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) in polymorphonuclear leucocytes (PMN) from rat peritoneum. In three separate experiments each, the amounts of PAF synthesized during 6 min and 12 min incubation of PMN cells from control, vitamin E-supplemented, and vitamin E-deficient rats were 129-240, 131-227 and 248-354 pmol/10(6) cells, respectively. The activity of the acetyl-transferase, which transfers the acetyl moiety of [3H]acetyl-CoA to 2-lysoPAF (1-O-alkyl-sn-glycero-3-phosphocholine) to form [3H]PAF, was higher in PMN homogenates from vitamin E-deficient rats (2.28 +/- 0.07 nmol/min/mg protein) than in those from E-supplemented rats (1.06 +/- 0.10 nmol/min/mg protein). However, there was no difference between the two groups in the activity of acetylhydrolase (4.26 +/- 0.71 and 4.26 +/- 0.06 nmol/min/mg protein, respectively), measured as degradation of [3H]PAF to [3H]lysoPAF. In vitro addition of alpha-tocopherol did not inhibit the increased activity of acetyl-transferase in vitamin E-deficient rats, indicating that the enzyme in vitamin E-supplemented rats was not directly inhibited by alpha-tocopherol. The acetyl-transferases of the two groups showed similar Km values for acetyl-CoA, but different Vmax values (225 microM and 6.4 nmol/min/mg protein in vitamin E-deficient rats, and 216 microM and 3.6 nmol/min/mg protein in vitamin E-supplemented rats), suggesting that the enzyme was not activated but increased in amount in vitamin E deficiency.

Alpha-tocopherol, a potent modulator of endothelial cell function

The effect of alpha-tocopherol on various endothelial cell functions was evaluated in vitro using cultures of human umbilical vein endothelial cells. Prostacyclin synthesis, plasminogen activator activity and von Willebrand factor activity were evaluated in control and alpha-tocopherol supplemented cultures. Alpha-tocopherol produced stimulation of prostacyclin production which peaked at or near 0.5 mM. Plasminogen activator activity was markedly reduced by the addition of alpha-tocopherol. Von Willebrand factor activity showed a significant decrease of processed and cellular forms that was dose-dependent. The incorporation of alpha-tocopherol into endothelial cells could be demonstrated by measuring intracellular levels of the vitamin in washed endothelial cells. Our results show that alpha-tocopherol has a profound effect on multiple endothelial cell functions which are intimately involved in hemostasis.

Reduction of platelet adhesiveness by vitamin E supplementation in humans

Platelet adhesion was tested ex vivo in a group of 7 normal individuals on varying doses of vitamin E. Adhesion to glass slides coated with fibrinogen, fibronectin, collagen I and collagen V was studied by perfusing platelet-rich plasma through a flow chamber. Time- and space-resolved observations of platelet adhesion were made in an area of parallel flow lines and low shear rate under standardized conditions before and after dietary supplementation with vitamin E. The doses varied from 400 I.U./day to 1600 I.U./day in 400 I.U. increments. A statistically significant reduction in platelet adhesion was noted on all four adhesive surfaces at the 400 I.U. level of vitamin E supplementation. This reduction varied in magnitude depending on the adhesive surface. As vitamin E supplementation was increased, no dose-dependent downward trend in adhesion rate was observed although the platelet alpha-tocopherol content progressively increased. Based on our results, we suggest that 400 I.U./day may be a near optimal dose of vitamin E to reduce platelet adhesivity as evaluated in our flow chamber system.

Ethanol stimulates prostacyclin biosynthesis by human neutrophils and potentiates anti-platelet aggregatory effects of prostacyclin

Previous reports on the direct effects of ethanol on human platelet aggregation function have been inconsistent. Ethanol ingestion produces vasodilation and raises intracellular cyclic AMP concentrations, effects similar to those of prostacyclin. We, therefore, hypothesized that ethanol may influence biosynthesis and/or bioactivity of prostacyclin. In our experiments, ethanol in concentrations up to 400 mg% had no consistent inhibitory effect on platelet aggregation in response to epinephrine, ADP, or combination of subthreshold concentrations of epinephrine plus ADP. However, ethanol in concentrations as low as 10 mg% potentiated the platelet aggregation inhibitory effects of prostacyclin. In addition, ethanol (20 mg%) decreased formation of thromboxane A2 in whole blood by 41% and stimulated formation of prostacyclin by 160% (both P less than 0.01). Additional studies using isolated human cells demonstrated synthesis of prostacyclin by neutrophils in the presence of platelets, and this neutrophil prostacyclin formation was enhanced in the presence of ethanol. These effects of alcohol in concentrations achieved after moderate intake may relate to the hemodynamic, biochemical, and cardioprotective effects of ethanol.

Platelet alpha-adrenoceptor function and aging

The effect of aging on platelet alpha-adrenoceptor binding of 3H-dihydro-alpha-ergocryptine (3H-DHE) and alpha-adrenoceptor response expressed as cAMP decrease, after the addition of noradrenaline 10 microM to intact platelets in vitro were examined and correlated with adrenaline induced platelet aggregation in a group of twelve young volunteers (mean age 21 years), and twelve old volunteers (mean age 88 years). The binding by platelets of 3H-DHE was considerably higher in the young than in the old group (mean 292.70 +/- 40.79 and 167.90 +/- 18.30 fmol/mg protein respectively, p less than 0.02). The affinity (Kd) was also influenced with values of 1.49 +/- 0.21 in the young and 3.32 +/- 0.45 nM in the old, p less than 0.01). Stimulation of platelets with noradrenaline caused a greater decrement of cAMP in the old than in the young group (mean 4.44 +/- 0.87 as compared with 0.32 +/- 0.75 pmol/10(9) platelets. Platelet sensitivity to adrenaline, when expressed as aggregation increased in the old group. These data suggest that the enhanced sensitivity to adrenaline as observed in old people is not the result of increased alpha-adrenoreceptor number of affinity, but related to changes in the platelet membrane possibly not related to the alpha-adrenoceptors.

Aging and platelet beta-adrenoceptor function

Ten young volunteers (mean age 20 years) and ten elderly volunteers (mean age 89 years), including equal numbers of healthy men and women, were tested regarding adrenaline-induced platelet aggregation in vitro, platelet cyclic AMP content before and after beta-adrenoceptor stimulation with isoprenaline, as well as binding by platelet membranes of 125I-hydroxybenzylpindolol. In the aged patients there was a highly significant decrease in the concentration of adrenaline needed to produce irreversible platelet aggregation, as compared with the young. Platelet basal cyclic AMP content did not differ but the response to isoprenaline stimulation, expressed as the percentage rise of cyclic AMP content above the unstimulated level, was significantly decreased in the old subjects. The beta-adrenoceptor number was unchanged but the affinity decreased significantly in the old group. The data suggest that the increased aggregation response to adrenaline in old people could be due in part to a diminished functional capacity of the platelet beta-adrenoceptor.

Effects of selenium, vitamin E and vitamin C on human prostacyclin and thromboxane synthesis in vitro

The effects of three antioxidants, selenium, vitamin E and vitamin C, on the production of anti-aggregatory prostacyclin (PGI2) by human endothelial cells and lung tissue as well as on the production of pro-aggregatory thromboxane A2 (TxA2) by human platelets and lung tissue in vitro were studied. Selenium had no effect on endothelial PGI2 synthesis but it dose-dependently inhibited the platelet production of TxA2 (84% at 10(-3) mol/l) and productions of PGI2 (64%) and TxA2 (72%) by human lung. Selenium inhibited platelet TxA2 synthesis much less in the presence of exogenous arachidonic acid (AA) (14% instead of 84%), suggesting that selenium exerts its effect predominantly on phospholipase A2. Vitamin E had no effect on endothelial PGI2, pulmonary PGI2 or TxA2 syntheses but it strongly inhibited platelet TxA2 production (50% at 10(-3) mol/l). This inhibition was only partly (from 50% to 30%) counteracted by exogenous AA suggesting that vitamin E affects both phospholipase A2 and a further step(s) in the TxA2 synthetizing cascade. Vitamin C stimulated endothelial cell PGI2 production (80% at 10(-2) mol/l) but it could not counteract the inhibition of PGI2 production (83%) exerted by 10(-2) mol/l tert-butyl hydroperoxide (TBHP), a promoter of lipid peroxidation. Vitamin C had no effect on pulmonary PGI2 production but it inhibited pulmonary TxA2 formation (38% at 10(-3) mol/l). Slight inhibition of platelet TxA2 production from endogenous AA (6%) turned to slight stimulation (8%) with exogenous AA. The in vivo significance of these results is not yet known.

Pharmacology of platelet inhibitors

Although many drugs have inhibitory effects on platelet function, none of them inhibits all of the mechanisms that may be involved in the various forms of thrombosis. Choice of suitable drugs is hampered by lack of full knowledge concerning the reactions that make the major contributions to the formation of arterial thrombi at sites of repeated vessel wall injury or on atherosclerotic lesions. Drugs such as aspirin that inhibit the arachidonate pathway in platelets can only be expected to be effective against thromboembolic events in which the generation of thromboxane A2 plays a major part. If thrombin and fibrin formation are dominant, oral anticoagulant agents or heparin should be beneficial; thus, experimental evidence indicates that with repeated vessel wall injury, the formation of platelet fibrin thrombi on the vessel wall is probably influenced more by inhibitors of thrombin generation than by the subendothelial constituents such as collagen. Agents like prostacyclin that raise platelet cyclic adenosine monophosphate (AMP) levels in platelets by stimulating adenylate cyclase are potent inhibitors of the reaction of platelets to all aggregating and release-inducing stimuli, but these agents are not suitable for long-term administration. The effect of dipyridamole on platelet cyclic AMP levels is weak, and this drug may act through other effects on platelets or on other cells. Indeed, several of the drugs that have been tested in clinical trials may exert their effects through unrecognized mechanisms. Many combinations of drugs have been used to affect platelets or platelets and coagulation. This practice has been based on the theory that because several mechanisms may be involved in thrombus formation, combinations of drugs that inhibit different mechanisms may be beneficial.

Vitamin E inhibits platelet phospholipase A2

One of the most important functions of phospholipase A2 is the release of arachidonic acid from membrane phospholipids for the synthesis of biologically active eicosanoids. We have demonstrated in our laboratory that vitamin E inhibits platelet phospholipase A2 in a dose-dependent manner. Rats fed a 100 ppm or a 1000 ppm vitamin E diet exhibit diminished phospholipase A2 activity compared to those fed a vitamin E-free diet. Addition of vitamin E to a sonicated platelet suspension resulted in further suppression of the phospholipase A2 activity in all groups of rats. In order to gain insight into the mechanism of vitamin E inhibition of platelet phospholipase A2, we partially purified this enzyme by gel filtration chromatography. Enzyme activity was localized in the soluble supernatant fraction of a high-speed spin. This partially purified rat platelet phospholipase A2 had an absolute requirement for Ca2+ and was inhibited by various forms of tocopherol. Tocol inhibited the enzyme to a greater extent than either D- or DL-alpha-tocopherol, while there was little or no effect from DL-alpha-tocopherol acetate. These results emphasize the importance of the hydroxyl moiety on the chromanol of the vitamin E molecule for its inhibitory action, compared to that of the methyl groups which are absent in tocol. This inhibitory action of vitamin E on platelet phospholipase A2 suggests a crucial function for vitamin E in regulating arachidonate release from the membrane phospholipids and its subsequent metabolism.

Effects of linoleic acid and gamma-linolenic acid intake on platelet functions in elderly people

Sixteen old subjects were given daily dietary supplement of lg of linoleic acid and lg of gamma-linolenic acid (primerose oil) or 2g of linoleic acid (sunflower oil) for periods of two months. Haemostatic parameters, platelet aggregation, exogenous and endogenous arachidonic acid metabolism were investigated before and after the intake. Diets did not induce any significant change in haemostatic parameters (bleeding time, levels of anti-thrombin III, plasminogen and plasma beta-TG and PF4). Platelet rich plasma aggregation induced by collagen and arachidonic acid were significantly reduced after linoleic acid (18:2n-6) intake. In contrast, gamma-linolenic acid (18:3n-6) supplement did not alter aggregation. However, thromboxane B2 formation (under stimulation) and vitamin E level in platelets (but not in plasma) were decreased after 18:3n-6 as compared to 18:2n-6 intake. The mechanism of thromboxane B2 decrease is unclear. Nevertheless, we may speculate that beneficial effect of this decrease could be counterbalanced by the decreased platelet vitamin E. We conclude that intake of 18:2n-6 or 18:3n-6 does not affect much platelet functions in elderly people.

Vitamin E consumption by human blood platelets activated by latex particles

Human blood platelet activation elicited by latex particles is associated to a 30% decrease in the cellular content of vitamin E. The vitamin E consumption is inhibited by the addition of catalase (500 U/ml) and azide (1 mM), but it is not affected by potassium cyanide (1 mM). It may be proposed that the challenge of platelets with particulate stimuli causes generation of oxygen reduction products, which leads to vitamin E depletion.

Deficiency of vitamin E in the alveolar fluid of cigarette smokers. Influence on alveolar macrophage cytotoxicity

Cigarette smoking produces oxidant-mediated changes in the lung important to the pathogenesis of emphysema. Since vitamin E can neutralize reactive oxygen species and prevent peroxidation of unsaturated lipids, it may constitute an important component of the lung's defense against oxidant injury. To better characterize the antioxidant protective role of vitamin E, young asymptomatic smokers and nonsmokers were evaluated by bronchoalveolar lavage before and immediately after a 3-wk course of oral vitamin E (2,400 IU/d). Smoker alveolar fluid at baseline was relatively deficient in vitamin E compared with nonsmoker fluid (3.1 +/- 0.7 ng/ml vs. 20.7 +/- 2.4 ng/ml, P less than 0.005). Although smoker alveolar fluid vitamin E levels increased to 9.3 +/- 2.3 ng/ml after supplementation, the levels remained significantly lower than nonsmoker baseline levels (P less than 0.01). This deficiency was explained, in part, by the increased oxidative metabolism of vitamin E to the quinone form in the lungs of smokers compared with nonsmokers. Although the significance of a lower concentration of alveolar fluid vitamin E is unclear, it may compromise the antioxidant protection afforded by the alveolar fluid as it coats the lung's epithelial surface. The protective role of vitamin E was assessed by cytotoxicity experiments, which demonstrated that the killing of normal rat lung parenchymal cells by smoker alveolar macrophages was inversely related to the vitamin E content of the parenchymal cells. These findings suggest that vitamin E may be an important lower respiratory tract antioxidant, and that the deficiency seen in young smokers may predispose them to an enhanced oxidant attack on their lung parenchymal cells.

Vitamin E exerts antiaggregatory effects without inhibiting the enzymes of the arachidonic acid cascade in platelets

Vitamin E (alpha-tocopherol) and tocopherol acetate produced a slightly increased amount of thromboxane in treated compared to untreated platelets. In tocopherol acetate-treated platelets significantly more lipoxygenase products were produced. alpha-tocopherol induced an increased, but not significant, production of thromboxane B2 during blood clotting. alpha-tocopherol was not found to affect platelet phospholipase activity as determined by its effect on the release of labelled arachidonic acid from platelet phospholipids by challenging the platelets with calcium ionophore A23,187. alpha-tocopherol potentiated the incorporation of labelled arachidonate in the platelet phospholipids. Inspite of having no effect on the arachidonic acid cascade in platelets, alpha-tocopherol inhibited aggregation induced by several aggregating agents including A23,187. Inhibition of aggregation may be explained by the ability of alpha-tocopherol to inhibit intracellular mobilization of sequestered calcium from the dense tubular system to the cytoplasm.

Hydrogen peroxide has a role in the aggregation of human platelets

The aggregation of platelets induced by soluble and particulate stimuli is enhanced by the addition of minute amounts of H2O2. Externally added catalase strongly inhibits the aggregation induced by particulate stimuli and by phorbol myristate acetate (PMA). The addition of aminotriazole to stimulated platelets causes a significant inhibition of intracellular catalase. This indicates the formation of H2O2 inside the platelets during activation. No effects were observed when the platelets were stimulated by the ionophore A23187.

Regional blood flow in compression-induced brain edema in rats: effect of dietary vitamin E

Regional cerebral blood flow (rCBF) was studied autoradiographically in a murine model of focal epidural brain compression, and the effect of vitamin E administration was investigated. Mean cortical CBF was reduced to 0.48 to 0.50 ml/gm/min following 2 or 24 hours of compression. Early (2 hours) following subsequent decompression, a mixed pattern of hypoperfusion and hyperperfusion was observed. Twenty-four hours later, rCBF heterogeneities were less marked. Comparisons among animal groups raised on vitamin E-supplemented, vitamin E-normal, and vitamin E-deficient diets 2 hours after decompression revealed marked reductions in rCBF in the previously compressed cortex of the last two groups and hyperemia of the underlying hippocampus. The vitamin E-supplemented rats showed increased flow in the previously compressed cortex. In addition, vitamin E supplementation tended to eliminate rCBF gradients between subjacent zones. These data may help explain our previous observations of the beneficial effects of vitamin E on compression-induced brain edema.

Vitamin E treatment of haemolytic uraemic syndrome

Because low plasma vitamin E concentrations have been reported in patients with haemolytic uraemic syndrome and there is accumulating evidence of lipid peroxidation in this disease, treatment with the antioxidant vitamin E was undertaken in 16 consecutive children with the syndrome. Twelve children had features at presentation suggesting a poor prognosis for recovery but despite this all 16 patients survived and are well three months later. Fifteen children now have normal values for serum creatinine, blood pressure, and urinalysis for protein but one has slight renal impairment. Although this is not a report of a controlled trial, it seems that patients treated with vitamin E have fared considerably better than our previously treated patients with haemolytic uraemic syndrome, even in the presence of early, adverse prognostic features. We suggest that vitamin E alters the natural history of the disease, and in view of the absence of any observed side effects further experience with this treatment is being sought.

Vitamin therapy in the absence of obvious deficiency. What is the evidence?

Vitamins are a group of organic compounds occurring naturally in food and are necessary for good health. Lack of a vitamin may lead to a specific deficiency syndrome, which may be primary (due to inadequate diet) or secondary (due to malabsorption or to increased metabolic need), and it is rational to use high-dose vitamin supplementation in situations where these clinical conditions exist. However, pharmacological doses of vitamins are claimed to be of value in a wide variety of conditions which have no, or only a superficial, resemblance to the classic vitamin deficiency syndromes. The enormous literature on which these claims are based consists mainly of uncontrolled clinical trials or anecdotal reports. Only a few studies have made use of the techniques of randomisation and double-blinding. Evidence from such studies reveals a beneficial therapeutic effect of vitamin E in intermittent claudication and fibrocystic breast disease and of vitamin C in pressure sores, but the use of vitamin A in acne vulgaris, vitamin E in angina pectoris, hyperlipidaemia and enhancement of athletic capacity, of vitamin C in advanced cancer, and niacin in schizophrenia has been rejected. Evidence is conflicting or inconclusive as to the use of vitamin C in the common cold, asthma and enhancement of athletic capacity, of pantothenic acid in osteoarthritis, and folic acid (folacin) in neural tube defects. Most of the vitamins have been reported to cause adverse effects when ingested in excessive doses. It is therefore worthwhile to consider the risk-benefit ratio before embarking upon the use of high-dose vitamin supplementation for disorders were proof of efficacy is lacking.

Potential clinical applications for high-dose nutritional antioxidants

High but well-tolerated doses of the nutritional antioxidants selenium and vitamins E and C have significant immunostimulant, anti-inflammatory, and anti-carcinogenic effects which are well documented in the existing biomedical literature. In addition, these antioxidants help to protect the structural integrity of ischemic or hypoxic tissues, and may have useful anti-thrombotic actions as well. Supplementation with high-dose nutritional antioxidants may eventually gain a broad role in the prevention, treatment, or palliation of cancer, cardiovascular disease, infection, inflammatory disorders, and certain diabetic complications.