Fish oil and the platelet-blood vessel wall interaction

n-3 polyunsaturated fatty acids and coronary thrombosis

Studies of Greenland Eskimos showed that a very high intake of marine n-3 fatty acids markedly inhibited platelet reactivity and suggested that intake of these fatty acids might prevent coronary thrombosis. Later studies with lower, more practical doses of n-3 fatty acids also have shown a platelet inhibitory effect of n-3 fatty acids, albeit fairly marginal. Furthermore, n-3 fatty acids have little effect on measures of blood coagulability and may slightly decrease fibrinolysis. In animal models, n-3 fatty acids often have been shown to inhibit thrombosis, but again the doses have tended to be very high. Finally, there has been little effect of (low-dose) n-3 fatty acids in clinical trials in humans on the incidence of myocardial infarction. Overall, there is little evidence for a major antithrombotic effect of practical doses of n-3 fatty acids on coronary thrombosis. This does not exclude a beneficial effect of n-3 fatty acids on coronary heart disease as suggested from clinical trials, but the major effect may be antiarrhythmic rather than antithrombotic.

Endothelin-1 attenuates omega3 fatty acid-induced apoptosis by inhibition of caspase 3

Endothelin-1 (ET-1) may be involved in the induction of vascular hypertrophy in hypertension. ET-1 may also modulate vascular growth through the exertion of antiapoptotic effects. The omega3 fatty acids (omega3 FAs), which have antiproliferative effects in various cell types, may have a beneficial role in hypertension. We tested the hypothesis that ET-1 could act as a survival factor against omega3 FA-induced apoptosis and attempted to elucidate possible molecular mechanisms underlying the protective action of ET-1 on docosahexaenoic acid (DHA)-induced apoptosis. Mesenteric vascular smooth muscle cells were stimulated with DHA, a representative omega3 FA. Dose-response curves of DHA at different apoptotic stages were assessed with the use of flow cytometry: (1) very early: plasma membrane phosphatidylserine (PS) translocation; (2) early: change in mitochondrial transmembrane potential (DeltaPsim); and (3) late: cell cycle analysis. Expression of the proapoptotic protein bax and the antiapoptotic protein bcl-2 was determined with Western blot assay. The activity and the expression of caspase 3, which is a critical proteolytic enzyme involved in the death-signaling pathway, were evaluated with a fluorometric immunosorbent enzyme assay and Western blot analysis, respectively. Apoptosis, which was detected with PS translocation, DeltaPsim disruption, and cell cycle analysis, was increased dose dependently by DHA. DHA-induced apoptosis was attenuated through exposure to ET-1 for 1 hour before DHA in cell cycle analysis. The interference of ET-1 with DHA-induced apoptosis, as detected with cell cycle analysis, was not apparent at the membrane (PS translocation) or the mitochondrial (DeltaPsim) level. The increase in bax/bcl-2 ratio in DHA-stimulated cells was not affected by ET-1. However, DHA increased both caspase 3 activity and the active forms of caspase 3 (20 and 17 kDa), resulting in enhanced DNA fragmentation as shown through Hoechst staining and fluorescence microscopy, which were attenuated by ET-1 pretreatment. In conclusion, DHA, an omega3 FA, induced apoptosis in vascular smooth muscle cells in a dose-dependent manner. ET-1 exerted important protective effects through the attenuation of DHA-induced caspase 3 activation and subsequent DNA fragmentation in the late stages of apoptosis.

n-3 fatty acids do not decrease plasma endothelin levels in healthy individuals

The effect of three different doses of n-3 polyunsaturated fatty acids (PUFA) on endothelin-1 (ET-1) was studied. Study 1 included 40 healthy volunteers randomized to a single supplement of 20 g of n-3 or n-6 PUFA. Plasma ET-1 was measured 14 h after ingestion, and no changes in plasma ET-1 after intake of n-3 PUFA were observed, compared to baseline values. In study 2, 32 subjects had 0.65 g of n-3 PUFA or a fat mixture per day for 12 weeks. No changes in plasma ET-1 were found after the oil supplements. Finally, 22 persons had 4 g of n-3 PUFA for 6 weeks. A significant increase in plasma ET-1 was seen in this group after the supplement. Thus, n-3 PUFAs do not lower plasma levels of ET-1, the most potent vasoconstrictor known.

Potentiation of the hyporeactivity induced by in vivo endothelial injury in the rat carotid artery by chronic treatment with fish oil

1. The present study investigates whether or not chronic feeding of rats with a diet enriched in fish oil affects the reactivity of balloon-injured carotid arteries. The left carotid arteries were injured in vivo by the repeated passage of a balloon catheter. Both the right (control artery) and the left carotid arteries were excised 24 h after the injury, and suspended in organ chambers for the measurement of changes in isometric tension in the presence of indomethacin. 2. Phenylephrine evoked similar concentration-contraction curves in the right (control) carotid arteries without endothelium from control and fish oil-fed rats. Balloon injury decreased the contractility of carotid arteries to phenylephrine in both types of rats and the pEC50 for phenylephrine was significantly decreased in balloon-injured arteries from control rats compared to those obtained in arteries from fish oil-fed rats (pEC50 7.59 +/- 0.1 and 7.28 +/- 0.06, respectively) while maximal contractions were similar (1.93 +/- 0.15 g and 1.79 +/- 0.12 g, respectively). 3. The treatment of control right carotid arteries without endothelium with either NG-nitro-L-arginine (an inhibitor of nitric oxide synthase) or superoxide dismutase (which protects nitric oxide from degradation) did not affect significantly the contractions to phenylephrine in either group. In these preparations, methylene blue (an inhibitor of soluble guanylate cyclase) decreased slightly but significantly maximal contractions to phenylephrine in both groups. The treatment of balloon-injured carotid arteries with NG-nitro-L-arginine or methylene blue partly restored contractions to phenylephrine in arteries from both types of rat. Superoxide dismutase further depressed the contractility to the alpha l-adrenoceptor agonist in balloon-injured arteries from control diet-fed rats but had no effect in balloon-injured preparations from fish oil-fed rats.4. 3-Morpholino-sydnonimine (SIN-1, a donor of nitric oxide) evoked similar concentration-dependent relaxations in control and balloon-injured carotid arteries from both types of rat.5. Balloon injury caused an increase in the tissue content of cyclic GMP in carotid arteries from control diet-fed rats. This production of cyclic GMP was abolished by N0-nitro-L-arginine. Superoxide dismutase potentiated significantly the production of cyclic GMP caused by balloon injury in control but not in fish oil-fed rats.6 These observations confirm that in vivo balloon injury causes the production of nitric oxide in the injured blood vessel wall. This production of nitric oxide from L-arginine accounts for the decreased contractility to phenylephrine and the accumulation of cyclic GMP in balloon-injured arteries. They further indicate that chronic feeding of rats with fish oil potentiates the L-arginine-nitric oxide pathway in the injured vessel leading to an enhanced hyporeactivity to phenylephrine.

n-3 fatty acids and renal diseases

There are numerous biologic rationales for the use of n-3 fatty acids in renal diseases, including a possible increase in the renal vasodilatory capacity by a rearrangement of renal prostanoid production, a reduction in the production of proinflammatory leukotrienes, a reduction in the transcapillary escape rate of albumin, and actions limiting cyclosporine-related nephrotoxicity. Studies of animal models of renal disease, mostly of immune-renal disease, support the idea of the possible usefulness of these compounds. The most promising areas of clinical investigation include the reduction of proteinuria in some chronic glomerular diseases, the treatment of immunoglobulin A nephropathy, and the prevention of cyclosporine-induced nephrotoxicity. However, the results of larger clinical studies, some of which are ongoing, are necessary to support the use of n-3 fatty acids in human renal diseases.

The influence of different amounts of n-3 polyunsaturated fatty acids on bleeding time and in vivo vascular reactivity

Mesenteric bleeding time, mesenteric vascular reactivity, platelet and erythrocyte lipid fatty acid composition were measured at 2-3 weeks, 5-6 weeks and 11-22 weeks in normotensive Wistar rats, fed on high (6.5% energy) or moderate (1.6% energy) intakes of eicosapentaenoic acid (20:5n-3; EPA) as fish oil, compared with controls fed on a diet devoid of EPA. All diets contained the same level of linoleic acid (4% energy): the moderate- and high-EPA diets also contained 1.1 and 4.4% of the energy as docosahexaenoic acid (22:6n-3) respectively. Moderate, but not high, intakes of EPA increased mesenteric bleeding time. Similar reductions in erythrocyte and platelet arachidonic acid (20:4n-6) occurred in animals fed on either high or low amounts of EPA, but the proportion of EPA increased dose-dependently. At high intakes of EPA the proportion of oleic acid in platelets and erythrocytes was decreased. Blood pressure platelet counts, mesenteric vessel diameter and mesenteric vascular reactivity to vasopressin were unaffected by treatment. High intakes of fish oil led to a slight fall in packed cell volume. In a second experiment bleeding time and mesenteric vascular reactivity to noradrenaline were increased 2-4 weeks after receiving a moderate intake of EPA and these effects persisted 5-21 d after switching to a control diet. A similar increase in vascular reactivity to noradrenaline was observed in animals given indomethacin (6 mg/kg) but not in those given aspirin (20 mg/kg).

Vascular relaxation to omega-3 fatty acids: comparison to sodium nitroprusside, nitroglycerin, papaverine, and D600

The vasorelaxant activity of the omega-3 fatty acids--docosahexaenoic (DHA) and eicosapentaenoic (EPA) acids--in comparison with other known vasodilators--sodium nitroprusside, nitroglycerin, papaverine, and D600--were studied in the isolated rat aorta. The relaxant responses of these vasodilators and fatty acids at concentrations of 1-100 microM were assessed in aortic rings contracted with norepinephrine (NE 10(-6) M) or with KCl (30 mM). Cyclic nucleotide enhancers (sodium nitroprusside, nitroglycerin, papaverine) were more effective in producing relaxation, regardless of the contractile mechanism, i.e., alpha-adrenoceptor stimulation or depolarization. In contrast, the omega-3 fatty acids produced augmented relaxation in NE-contracted vessels. Relaxations produced by DHA (15 +/- 2% to 45 +/- 10%) were similar to D600 (16 +/- 2% to 60 +/- 7%) in NE-contracted rings, but not in KCl contracted rings. The responses to D600 and DHA in KCl-contracted vessels were 79 +/- 2% to 104 +/- 3% and 5 +/- 1% to 21 +/- 3%, respectively. In another set of experiments, the effects of omega-3 fatty acids in the presence of albumin were examined; no significant differences in the induced relaxant responses were noted. These results suggest that the mechanisms of vascular relaxation, such as cyclic nucleotide elevation and calcium antagonism of potential-operated channels, are different from those induced by the omega-3 fatty acids.