Vitamin E and platelet eicosanoids in diabetes mellitus

The platelet in diabetes: focus on prevention of ischemic events

Accelerated atherosclerosis and the increased risk of thrombotic vascular events in diabetes may result from dyslipidemia, endothelial dysfunction, platelet hyperreactivity, an impaired fibrinolytic balance, and abnormal blood flow. There is also a correlation between hyperglycemia and cardiovascular (CV) events. The importance of platelets in the atherothrombotic process has led to investigation of using antiplatelet agents to reduce CV risk. A meta-analysis conducted by the Antiplatelet Trialists' Collaboration demonstrated that aspirin reduced the risk of ischemic vascular events as a secondary prevention strategy in numerous high-risk groups, including patients with diabetes. Based on results from placebo-controlled randomized trials, the American Diabetes Association recommends low-dose enteric-coated aspirin as a primary prevention strategy for people with diabetes at high risk for CV events. Clopidogrel is recommended if aspirin allergy is present. There is occasionally a need for an alternative to aspirin or for additive antiplatelet therapy. Aspirin in low doses inhibits thromboxane production by platelets but has little to no effect on other sites of platelet reactivity. Agents such as ticlopidine and clopidogrel inhibit ADP-induced platelet activation, whereas the platelet glycoprotein (Gp) IIb/IIIa complex receptor antagonists block activity at the fibrinogen binding site on the platelet. These agents appear to be useful in acute coronary syndromes (ACSs) in diabetic and nondiabetic patients. A combination of clopidogrel plus aspirin was more effective than placebo plus standard therapy (including aspirin) in reducing a composite CV outcome in patients with unstable angina and non-ST segment elevation myocardial infarction. In a meta-analysis of six trials in diabetic patients with ACSs, intravenous GpIIb-IIIa inhibitors reduced 30-day mortality when compared with control subjects. Results from controlled prospective clinical trials justify the use of enteric-coated low-dose aspirin (81-325 mg) as a primary or secondary prevention strategy in adult diabetic individuals (aged >30 years) at high risk for CV events. Recent studies support the use of clopidogrel in addition to standard therapy, as well as the use of GpIIb-IIIa inhibitors in ACS patients.

Dietary polyunsaturated fatty acid and antioxidant modulation of vascular dysfunction in the spontaneously hypertensive rat

Two currently available edible oils-olive and canola-and two oil blends of plant origin having different n-3/n-6 polyunsaturated fatty acid (PUFA) ratios were evaluated for their ability to modify vascular dysfunction in the spontaneously hypertensive rat (SHR). Synthetic diets supplemented with test oils (5% w/w) were fed for 12 weeks, and segments of thoracic aorta used to assess vascular function. Vessels from the SHR displayed a spontaneous constrictor response after the inhibition of endothelial cell nitric oxide (NO) with N(omega)-nitro-L-arginine (NOLA). Dietary alpha -linoleate enrichment led to a reduction (P<0.05) in this abnormality with a dietary n-3/n-6 PUFA ratio of 1.0 (blend-1) yielding the best outcome. Relaxation to acetylcholine (ACh) was unaffected by dietary lipid supplementation. NOLA treated rings also displayed contractions to ACh that were abolished by indomethacin, thromboxane antagonists SQ29548, picotamide and flavonoids kaempferol and quercetin. In contrast, alpha-tocopherol, rutin and the lipoxygenase inhibitor esculetin resulted in only partial (30-55%) inhibition, and were ineffective against the NOLA-induced contraction suggesting the operation of different biochemical mechanisms in mediating the spontaneous and Ach-induced contractions. Results implicate plant-based oils and antioxidants as potential modulators of vascular function.

The effect of modest vitamin E supplementation on lipid peroxidation products and other cardiovascular risk factors in diabetic patients

Among many factors, elevated lipids and lipid peroxide levels in blood are major risk factors in the development of cardiovascular disease in diabetic patients. This study has examined whether oral supplementation of vitamin E, an antioxidant, has any effect on blood lipid peroxidation products (LP) and lipid profile of diabetic patients. Thirty-five diabetics(D) were supplemented with DL-alpha-tocopherol (E) capsule (orally, 100 IU/d) or placebo (P) for three months in double-blind clinical trials. Plasma E was analyzed by HPLC and LP by the thiobarbituric acid-reactivity; serum lipids by auto-analyzer. Data were analyzed using paired t-test and Wilcoxon Signed Rank Test. Vitamin E supplementation significantly lowered LP and lipid levels in diabetic patients; there were no differences in these parameters after P supplementation. There were no differences in the duration of diabetes and ages of D between P- and E- supplemented groups. This study suggests that vitamin E supplementation significantly lowers blood LP and lipid levels in diabetic patients.

Dietary lipid modification of myocardial eicosanoids following ischemia and reperfusion in the rat

Several different edible oils were compared for their ability to modify eicosanoid biosynthesis following experimentally-induced myocardial ischemia and reperfusion in the rat. Two types of palm oil [neutralized, bleached, and deodorized (NBDPO) and refined, bleached, and deodorized (RBDPO)] and partially hydrogenated soybean oil (SBO) were tested against a diet supplemented with sunflower seed oil (SSO) rich in n-6 polyunsaturated fatty acids (PUFA). Fish oil (FO) rich in n-3 PUFA, with its known cardioprotective actions, served as an internal reference point for the study. Test oils were fed as a 12% (w/w) supplement for nine months before the induction of myocardial ischemia and reperfusion. Palm oil diets exerted effects indistinguishable from the SBO group against cardiac arrhythmia, which occurred following alterations to coronary blood flow. Arrhythmic potentials, as expressed by a hierarchical scale (0-9) of arrhythmia score, were: SSO, 1.5 +/- 0.5; FO, 0.9 +/- 0.4; SBO, 3.1 +/- 0.5*; NBDPO, 3.2 +/- 0.5*; RBDPO, 3.3 +/- 0.6*; *P < 0.05 vs. SSO. Following ischemia and reperfusion, both SSO and RBDPO groups tended to show an increase in myocardial prostacyclin, with the effect being more prominent in the RBDPO group (SSO, 10%; RBDPO, 25%). Thromboxane production was reduced in the FO group. Interestingly, cardiac muscle from both FO and palm oil groups displayed a reduced capacity to produce 12-hydroxyeicosatetraenoic acid SSO, 591 +/- 95.8; SBO, 375.5 +/- 48.9; NBDPO, 287.2 +/- 64.7*; RBDPO, 230.9 +/- 80.2**; FO, 203.7 +/- 81.4** (ng/g dry wt, *P < 0.05, **P < 0.01). No clear relationship was seen between the availability of 20:4n-6 in myocardial phospholipids and eicosanoid profile. Data suggests that fatty acid composition of edible oils is not the only determinant of arrhythmic vulnerability and eicosanoid production.

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.

Vitamin and mineral supplementation in patients with diabetes mellitus

Most patients with diabetes generally do not require routine vitamin and mineral supplementation, just like the rest of the population. However, based on the published medical literature, it would appear that some degree of supplementation with certain vitamins (eg, vitamins C and E) and minerals (eg, magnesium) may be worthwhile and become more commonly recommended in the future. The role of vitamins and minerals in controlling blood glucose levels in the patients with diabetes also is discussed.

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.