Vitamin E and ventricular fibrillation threshold in myocardial ischemia

Possible mechanisms for the protective action of alpha-tocopherol in vascular hypoxia

1. The mechanism of the protective action of alpha-tocopherol (vitamin E) in sustaining noradrenaline-induced responses in vascular hypoxia was investigated using pharmacological methods. 2. Four vascular spasmogenic agents, methoxamine, acetylcholine, histamine and potassium, each with a different mode of action were used to produce responses in guinea-pig isolated portal vein. In each case the responses were significantly reduced by hypoxia or hypoxia and a substrate-free environment. 3. Pre-incubation of the vein with alpha-tocopherol protected the noradrenaline-induced responses against hypoxia in the substrate-free environment, However, at the EC50 concentration for protection of noradrenaline, alpha-tocopherol failed to protect the responses of each agent from the inhibitory effects of hypoxia, suggesting a mechanism of protection involving noradrenaline. 4. Drugs known to interfere with the disposition of noradrenaline in sympathetically innervated tissues, cocaine, hydrocortisone and tyramine did not affect the response to alpha-tocopherol. 5. Responses to calcium were unaffected by alpha-tocopherol in normoxia and hypoxia. 6. The protective action of alpha-tocopherol was not mimicked by the chromanol ring of the vitamin structure, Trolox C, suggesting that the vascular protection in hypoxia was not dependent on an antioxidant mechanism. 7. However, the glycolytic enzyme inhibitor, iodoacetic acid, prevented the protective action of the vitamin in hypoxia, suggesting that alpha-tocopherol enhanced noradrenaline-mediated activity in hypoxia through an iodoacetic acid-sensitive pathway.

Characteristics of the protective action of alpha-Tocopherol in vascular hypoxia

The functioning of the guinea-pig isolated portal vein was monitored by measuring spontaneous mechanical activity, responses to electrical stimulation and administered noradrenaline in normoxic conditions. The effect of hypoxia, induced by bubbling the physiological bathing solution with a 95% N(2)/5% CO(2) gas mixture, on the mechanical performance of the vein was then assessed. Spontaneous activity declined in hypoxia, with mean contraction tension reduced by 55 + or - 8.8%. The responses to electrical field stimulation (2-32 Hz, 0.7 msec. 70 V) were lowered by 14 + or - 4.6% but contractions produced by a range of noradrenaline concentrations (0.01-160 mu M) were unaffected by hypoxia. Substitution of glucose in the bathing solution with sucrose, a substrate unavailable to the cells for energy generation, produced a marked enhancement of the effect of hypoxia. Spontaneous activity was reduced by 76 + or - 8.3%, electrically-induced activity by 80 + or - 14.4% and noradrenaline-induced responses by 85 + or - 6.8%. Although in normoxia the activity and responses of the portal vein were unaffected by the presence of alpha-tocopherol, it significantly protected the functioning of the vein in hypoxic conditions. This effect was concentration-dependent within the range 10-160 mu M and was most marked when glucose was replaced by sucrose in the bathing solution.

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.

Therapeutic potential of vitamin E against myocardial ischemic-reperfusion injury

Myocardial ischemia is a disease process characterized by reduced coronary flow such that the supply of nutritive blood to heart muscle (myocardium) is insufficient for normal myocardial aerobic metabolism. Prompt reestablishment of coronary flow by invasive and noninvasive clinical procedures is the most direct and effective means of limiting myocardial damage in ischemic heart disease patients, although reperfusion carries with it an injury component which may reflect, at least to some degree, the toxic effects of partially reduced oxygen species and their participation in degenerative cellular processes such as membrane lipid peroxidation. Vitamin E, a lipophilic, chain-breaking antioxidant, is a prominent membrane constituent in heart muscle, where it modulates/regulates various aspects of heart muscle-cell metabolism and function. Vitamin E's beneficial effects against experimentally induced oxidative damage to the heart, along with inverse epidemiological correlations between plasma vitamin E level and either anginal pain or mortality due to ischemic heart disease, suggest that vitamin E might have protective and therapeutic roles against myocardial ischemic-reperfusion injury. Laboratory investigations aimed at addressing this possibility have demonstrated that vitamin E supplementation protects isolated hearts against ischemic-reperfusion injury, and relatively more inconsistent and limited data document cardioprotective effects of vitamin E in some animal models of myocardial ischemia-reperfusion, especially when administered prior to the ischemic period. Clinical attempts to establish whether vitamin E has therapeutic benefit in ischemic heart disease patients remain inconclusive, having relied upon a variety of nonuniformly controlled protocols and a single, rather subjective endpoint (anginal pain). Consequently, although laboratory data constitute a conceptual context for and indirect support of the idea that vitamin E could be a cardioprotectant against ischemic-reperfusion injury, compelling clinical evidence regarding vitamin E's therapeutic potential in the ischemic heart-disease patient is lacking. Elective coronary revascularization would appear to provide an attractive clinical setting for evaluating the therapeutic efficacy of vitamin E in the context of cardiac ischemia-reperfusion. Further biochemical work would still be required to define how vitamin E exerts any cardioprotective effect observed in these patients.

Vitamin E and oxidative stress

Oxidative stress can result from or be enhanced by a large variety of conditions, including nutritional imbalance, exposure to chemical and physical agents in the environment, strenuous physical activities, injury, and hereditary disorders. While many enzymes and compounds are involved in protecting cells from the adverse effects of oxidative stress, vitamin E occupies an important and unique position in the overall antioxidant defense. The antioxidant function of vitamin E is closely related to the status of many dietary components. Vitamin E-depleted animals are generally more susceptible to the adverse effects of environmental agents than supplemented animals. Also, vitamin E supplementation is beneficial to certain groups of the population. However, supplementing vitamin E in experimental subjects maintained on a nutritionally adequate diet does not always provide additional protection. Differential metabolic responses in various organs and differences in experimental conditions often contribute in the discrepancies in the literature. The lack of clear evidence for the occurrence of lipid peroxidation or antioxidant function of vitamin E in vivo can be attributed partly to the presence of active pathways for metabolizing hydroperoxides, aldehydes, and other oxidation products. Specific and sensitive techniques for measuring lipid peroxidation products in biological systems are essential for understanding the role of free radical-induced lipid peroxidation in tissue damage and antioxidant function of vitamin E in vivo.