Blood pressure and vascular reactivity changes in spontaneously hypertensive rats fed fish oil

1. To examine possible mechanisms of antihypertensive effects of feeding fish oil rich in n-3 fatty acids, we have studied vascular reactivity of aortic rings and perfused mesenteric resistance vessels of spontaneously hypertensive rats (SHR) given such a diet. 2. In two experiments, rats were fed a semi-synthetic diet containing either 'fish oil' (10 and 20% by weight) or hydrogenated coconut oil (control) (10 and 20%) for 4 weeks. 3. Blood pressure rose significantly less in the fish oil group than in controls in both experiments. 4. Aortic rings from control rats showed endothelium-dependent relaxations to low concentrations of acetylcholine (ACh) but relaxed less at higher concentrations. In contrast, rings from the fish oil group had relaxations which increased through the range of concentrations used. Indomethacin (10 microM) also increased the relaxation responses seen in rings from control rats, suggesting that fish oil inhibits a contractile cyclo-oxygenase product. This contractile substance may be thromboxane A2 (TxA2) or its endoperoxide precursor, prostaglandin H2 (PGH2) as aortic incubates and serum levels of TxB2 (the stable product of TxA2) were greatly reduced in fish oil-fed rats, and the decrease of relaxant responses to high concentrations of ACh were also blocked by a TxA2/PGH2 receptor blocker (SQ 29548). 5. In contrast to aortic rings, perfused preconstricted mesenteric resistance vessels of control rats relaxed to ACh in a similar fashion to tissues from fish oil-fed rats. However, in this preparation, fish oil feeding enhanced relaxations to sodium nitroprusside (SNP) and contractile responses to noradrenaline were less than controls. After removal of endothelium with 0.05% saponin, contractile responses to noradrenaline increased in both groups but responses from fish oil-treated rats were still attenuated. This suggests that fish oil feeding alters reactivity of mesenteric resistance vessels at the level of the smooth muscle. 6. The results indicate that fish oil feeding may reduce blood pressure by decreasing vascular smooth muscle reactivity to noradrenaline in resistance vessels. The effect may be enhanced by inhibition of an endothelium-derived cyclo-oxygenase product, such as TxA2 or PGH2 in conduit vessels.

Effect of fish oil feeding on blood pressure and vascular reactivity in spontaneously hypertensive rats

1. To examine possible antihypertensive mechanisms of fish oil feeding, we have studied vascular reactivity of aortic rings and blood pressure of spontaneously hypertensive rats (SHR). 2. SHR were fed a synthetic diet supplemented with either (10% by weight) 'Max EPA' fish oil or hydrogenated coconut oil (saturated fat) for 4 weeks. 3. Mean systolic blood pressure of fish oil fed rats was 9 mmHg lower than saturated fat fed controls. 4. Aortic rings of control SHR had a biphasic response to acetylcholine (ACh), relaxing at lower concentrations but contracting at concentrations higher than 3 x 10(-7) mol/L. No such contractions were seen in tissues of fish oil fed rats. The contractions were abolished by indomethacin, suggesting that they were caused by a cyclo-oxygenase product. 5. Tissue analysis showed that both aortic and serum generation of thromboxane B2(TxB2) was approximately three times less in fish oil fed rats than in control tissues. 6. These results indicate that the lowering of blood pressure in fish oil fed SHR could in part be due to decrease in production of thromboxane (TxA2), a potent vasoconstrictor, hence influencing vascular tone and compliance of the aorta.

Effect of pure eicosapentaenoic acid feeding on blood pressure and vascular reactivity in spontaneously hypertensive rats

1. This study examined the effects of eicosapentaenoic acid (EPA) treatment on vascular reactivity and blood pressure in spontaneously hypertensive rats (SHR). 2. Twenty SHR were given pure EPA as the methyl ester (280 mg/kg) by gavage for 10 days. An equal number of control rats received vehicle alone. EPA treatment had no effect on blood pressure compared with control rats. 3. Aortic rings from EPA-treated rats, precontracted with PGF2 alpha showed increased endothelium-dependent relaxations to acetylcholine. Endothelium-independent relaxations to sodium nitroprusside were not altered. Rings from rats fed pure EPA did not show any differences in vasoconstrictor responses to noradrenaline or serotonin. 4. Serum thromboxane B2 (TXB2) levels fell 17% in animals given pure EPA, but prostacyclin production was not affected. These responses are less than those seen following Max EPA fish oil. 5. Thus, pure EPA treatment did not lower blood pressure, but may have a direct effect on aortic endothelia and cause increased endothelium-dependent relaxations in response to acetylcholine in SHR.