Cardiovascular actions of dietary polyunsaturates and related mechanisms. A state-of-the-art-review

Cyclic nucleotides in platelets of genetically hypertriglyceridemic and hypertensive rats. Thrombin and nitric oxide responses are unrelated to plasma triglyceride levels

Prague hereditary hypertriglyceridemic (HTG) rats constitute a genetic model of hypertension associated with hyperlipidemia and insulin resistance. Various cell alterations, including changes in membrane dynamics, ion transport, and decreased platelet responses to thrombin have been observed in this strain. As hypertriglyceridemia appears to be associated with reduced endothelium-dependent vasodilation and platelet aggregation, we examined whether triglycerides could modulate cell responsiveness through changes in cyclic nucleotides in platelets of HTG rats. From the age of 6 weeks, these hypertensive animals were subjected for 10 weeks to interventions that modified circulating triglycerides levels (2.17+/-0.09 mmol/l), leading to their reduction (gemfibrozil treatment, 0.87+/-0.05 mmol/l) or elevation (high fructose intake, 3.23+/-0.07 mmol/l). Basal cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) contents were 15% and 48% lower in isolated platelets of HTG rats than in those of Lewis controls. cAMP level was further reduced in HTG rats subjected to high fructose intake. Irrespective of their plasma triglyceride levels, the thrombin-induced increase in platelet cGMP levels present in Lewis rats was absent in platelets of HTG rats. In contrast, no strain- or treatment-related differences were observed in the magnitude or kinetics of cGMP response to exogenous nitric oxide (NO). NO-induced cGMP and cAMP changes were associated in an opposite manner with trimethylamino-diphenylhexatriene (TMA-DPH) anisotropy, a biophysical parameter that reflects the microviscosity of the outer part of the cell membrane. Our results indicate that the attenuation of platelet responsiveness to thrombin in HTG rats represents a strain difference that cannot merely be due to a difference in plasma triglyceride levels. Platelet hyporesponsiveness to agonists such as thrombin in HTG rats cannot be explained by a change in levels of inhibitory cyclic nucleotides, since they were actually found to be low and not high.

Ontogenetic aspects of hypertension development: analysis in the rat

In this review, we attempt to outline the age-dependent interactions of principal systems controlling the structure and function of the cardiovascular system in immature rats developing hypertension. We focus our attention on the cardiovascular effects of various pharmacological, nutritional, and behavioral interventions applied at different stages of ontogeny. Several distinct critical periods (developmental windows), in which particular stimuli affect the further development of the cardiovascular phenotype, are specified in the rat. It is evident that short-term transient treatment of genetically hypertensive rats with certain antihypertensive drugs in prepuberty and puberty (at the age of 4-10 wk) has long-term beneficial effects on further development of their cardiovascular apparatus. This juvenile critical period coincides with the period of high susceptibility to the hypertensive effects of increased salt intake. If the hypertensive process develops after this critical period (due to early antihypertensive treatment or late administration of certain hypertensive stimuli, e.g., high salt intake), blood pressure elevation, cardiovascular hypertrophy, connective tissue accumulation, and end-organ damage are considerably attenuated compared with rats developing hypertension during the juvenile critical period. As far as the role of various electrolytes in blood pressure modulation is concerned, prohypertensive effects of dietary Na+ and antihypertensive effects of dietary Ca2+ are enhanced in immature animals, whereas vascular protective and antihypertensive effects of dietary K+ are almost independent of age. At a given level of dietary electrolyte intake, the balance between dietary carbohydrate and fat intake can modify blood pressure even in rats with established hypertension, but dietary protein intake affects the blood pressure development in immature animals only. Dietary protein restriction during gestation, as well as altered mother-offspring interactions in the suckling period, might have important long-term hypertensive consequences. The critical periods (developmental windows) should be respected in the future pharmacological or gene therapy of human hypertension.

Enhanced nitric oxide synthesis reverses salt-induced alterations in blood flow and cGMP levels

To understand the role of nitric oxide in salt-induced hypertension, we evaluated cardiovascular, hemodynamic and biochemical parameters in Dahl salt-sensitive rats fed low (0.3%) and high (8.0%) sodium diets. Two high salt groups received 1.25 and 2.5 g/L l-arginine in their drinking water. After three weeks of treatment, blood pressure was greater in the high salt groups. l-arginine did not modify salt-induced hypertension. Eicosapentaenoic acid (EPA) caused a smaller depressor response compared to normotensive rats. The increase in blood pressure was associated with decreases in aortic and renal blood flows. In renal artery, the reduction was counteracted by both l-arginine doses; whereas in the aorta, only the higher l-arginine one restored blood flow. The salt-induced reduction in aortic cyclic GMP level was only overcome by the higher l-arginine treatment. These data suggest that at the dose levels tested, nitric oxide reverses the reduction in cGMP and blood flow, but not the blood pressure changes associated with salt-induced hypertension.

In vivo plasma lipid oxidation in sugar-induced rat hypertriglyceridemia and hypertension

Fe(II) and Fe(III) are required for the catalysis of lipid peroxidation through generation of reactive oxygen species that damage cell membranes. This study investigated the effect of free radicals and lipid peroxidation, induced by intraperitoneal injection of iron-dextran in vivo, in the plasma of the sugar-induced hypertriglyceridemic and hypertensive male and female rats. Lipid peroxidation was measured by the malondialdehyde (MDA) equivalent, using a fluorescence method of 2-thiobarbituric acid reactive substances (TBARS). Iron increased TBARS generation by fourfold (P<.0001) in male control rats and by twofold (P<.01) in female control rats, and the difference between TBARS concentration in female as compared with male animals was statistically significant (P<.05). In the case of the sugar-fed group, iron-dextran produced an increase of TBARS concentration by twofold in both male (P<.001) and female rats (P<.01), and no significant difference in TBARS concentration was observed between sugar-fed female and male rats. The analysis of fatty acid composition by gas chromatography showed a significant diminution of 50% in the proportion of arachidonic acid (C20:4n-6) in the male control group in comparison with the female group (P<.0001). In female control rats, a small diminution in the proportion of C20:4n-6 and in the other polyunsaturated fatty acids was observed (P<.05). A significant difference in the C20:4n-6 proportion was found between the male and female group of control rats. In the sugar-fed group, iron induced a significant diminution of arachidonic acid (P<.001) in both female and male rats in comparison with the sugar-fed group without iron.

Renal effects of a high unsaturated fat diet in renal artery stenosis in rats

The renal effects of an unsaturated fat (UNSAT) diet in mild to moderate two-kidney, one-clip (2K1C) renovascular hypertension were evaluated. An UNSAT diet (37% by energy) prevented the development of hypertension compared to 2K1C rats fed a high saturated fat (SAT) (37% by energy) and a normal fat (CONTROL) (11% by energy) diet. Urinary sodium and fractional sodium excretion increased in 2K1C rats as compared to SHAM operated controls, regardless of the diet received. In the early weeks of the experiment (weeks 2-4 post-surgery to induce hypertension), an enhanced natriuresis occurred in the 2K1C UNSAT as compared to the 2K1C CONTROL and SAT diet groups. This resulted from an increase in the glomerular filtration rate (GFR in mls.min-1) as measured using the single-injection [51Cr] EDTA method (2K1C UNSAT; 1.99 +/- 0.18 versus 2K1C SAT; 1.27 +/- 0.09, p < 0.02; and versus SHAM CONTROL; 1.45 x 0.01; p < 0.02). The increased GFR was not associated with alterations in effective renal plasma flow (ERPF) as measured using the single-injection [125I] Na hippurate method. No differences in sodium excretion; GFR; ERPF or renal blood flow (microsphere technique) were noted between the 2K1C UNSAT and SAT diet groups at weeks 6-8 post-surgery, despite a continued antihypertensive effect of the UNSAT diet. Hence, the antihypertensive effect of an unsaturated fat diet in 2K1C renovascular hypertension in rats is associated with transient glomerular changes leading to an enhanced natriuresis.

Effects of MaxEPA on salt-induced hypertension: relationship to [3H]nitrobenzylthioinosine binding sites

We investigated the effects of dietary MaxEPA (a major source of eicosapentaenoic acid in fish oil) supplementation on blood pressure (BP) responses and heart rate (HR) of Dahl salt-sensitive (SS) rats fed low (0.4% NaCl) and high (8.0% NaCl) sodium diets. During a four week treatment period, BP remained normotensive in rats on low salt diet but was significantly elevated in those on high salt diet, causing 50% mortality. MaxEPA diminished the BP elevation and prevented the high salt-induced mortality. HR was not affected by either salt diet alone but was reduced in the presence of MaxEPA. At the end of the treatment period, the distribution of [3H]nitrobenzylthioinosine ([3H]NBMPR) binding, a putative marker of adenosine transport and metabolism, was estimated in selected rat tissues in order to evaluate the role of the purinergic system in the BP lowering effect of MaxEPA. Maximal [3H]NBMPR binding capacity (Bmax) in the kidney and platelets were 39% and 82% lower, respectively, in rats on high salt diet than in those on low salt diet. MaxEPA significantly blunted the decrease in Bmax in the kidney but not in platelets and increased Bmax in heart (48%) of low salt group. There were no changes in dissociation constants (Kd). The results suggest that MaxEPA can attenuate salt-induced hypertension, reduce salt-induced mortality and protect the integrity of kidney NBMPR binding sites in salt-induced hypertension.

Abnormal membrane concentrations of 20 and 22-carbon essential fatty acids: a common link between risk factors and coronary and peripheral vascular disease?

Although elevated levels of cholesterol are associated with increased risks of coronary and peripheral vascular disease, the association frequently fails to provide a causative explanation at the individual level. New hypotheses are required which, whether or not they are correct, will provide new lines of research. It is proposed here that the causes of vascular disease are abnormal membrane phospholipid concentrations of the 20-carbon and 22-carbon essential fatty acids (EFAs) of the n-6 and n-3 series. These levels become abnormal with ageing, with stress and in response to smoking, high cholesterol levels and high saturated fat intakes. They are also abnormal in patients with diabetes and hypertension. The effects of these EFAs and their metabolites include lowering of triglycerides, elevation of high-density lipoprotein (HDL)-cholesterol, reduction of blood pressure, vasodilatation, reduction of fibrinogen levels and inhibition of platelet aggregation and of cardiac arrhythmias. Prospective studies have shown that abnormal levels of these fatty acids are predictive of future coronary death. Controlled trials of treatment have demonstrated that provision of the fatty acids reduces both coronary and total mortality. Further experimental and clinical investigations of the roles of appropriate membrane concentrations of these fatty acids are justified.

Effects of fish oil supplementation on the changes in myocardial cyclic adenosine monophosphate, inositol 1,4,5-triphosphate and mitochondrial calcium levels during acute coronary occlusion-reperfusion in cholesterol-fed rabbits

We studied the changes in myocardial second messengers and mitochondrial calcium levels during acute coronary occlusion-reperfusion in New Zealand white male rabbits fed a high cholesterol diet with or without fish oil supplementation. Group I, control rabbits, were fed a standard laboratory rabbit chow. In addition to the standard chow, Group II rabbits received a 1% cholesterol-enriched diet for 2 weeks, while Group III rabbits were fed a 1% cholesterol and 10% fish oil supplemented diet for 2 weeks. Acute coronary occlusion for 10 min or 1 h was induced by ligating the marginal branch of the left circumflex coronary artery. The vessel was then reperfused for 1 or 4 h in short- and long-term ischemia studies respectively. In the short-term ischemia study, myocardial samples taken from the cholesterol-fed rabbits had the highest cyclic adenosine monophosphate, inositol 1,4,5-triphosphate and mitochondrial calcium levels among the normal (nonischemic) and the ischemic areas of the three groups. The cholesterol and fish oil treated rabbits significantly suppressed the elevation of cyclic adenosine monophosphate (P < 0.05 compared with the cholesterol-fed rabbits in normal and ischemic areas respectively), but did not significantly attenuate the elevation of inositol 1,4,5-triphosphate and calcium levels. In the long-term ischemia study, the cholesterol-fed rabbits had the highest levels of these three messengers among the normal areas. However, only inositol 1,4,5-triphosphate level reached statistical significance (P < 0.05 compared with control). This group of rabbits had the lowest level of cyclic adenosine monophosphate, but the highest inositol 1,4,5-triphosphate and calcium levels among the ischemic areas.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of dietary fat source on sudden death syndrome and cardiac sarcoplasmic reticular calcium transport in broiler chickens

Wheat and soybean diets supplemented with either tallow or sunflower oil (SFO) were fed to broiler chicks. Variables examined included performance, incidence of sudden death syndrome (SDS), and cardiac sarcoplasmic reticular (SR) calcium transport. The phospholipid content of heart tissues was also determined. Birds fed the SFO diet gained significantly (P < .05) more weight over the first 21 days of age and had a significantly better feed:gain ratio (P < .01). The incidence of SDS mortality up to 39 days of age was also lower (P < .05) for SFO-fed birds than for those fed the tallow diet. Calcium (45Ca2+) uptake and calcium-magnesium 5'-adenosinetriphosphatase (Ca2+ + Mg(2+)-ATPase) activity in cardiac SR vesicles did not differ due to diet (P > .05). However, compared with similar weight pen-mates showing no disease signs, SDS birds had depressed 45Ca2+ uptake (P < .01) and Ca2+ + Mg(2+)-ATPase (P < .05) of cardiac SR vesicles. The phosphatidylcholine concentration in the cell membranes of heart tissue of tallow-fed birds was significantly higher (P < .05) than in SFO-fed chicks. No differences were seen in other phospholipid constituents. The SDS birds, however, had significantly (P < .05) lower phosphatidylethanolamine plus phosphatidylglycerol, sphingomyelin, and total phospholipid concentrations in the heart tissues than the pen-mate controls. The results support the hypotheses that SDS in broilers is a cardiac dysfunction associated with defective cardiac SR membrane function and that dietary fat type is implicated with the syndrome.

Effect of a diet rich in linseed oil on complex viscosity and blood pressure in spontaneously hypertensive rats (SHR)

The influence of a diet rich in linseed oil (10% in weight) with a content of 61.2% of alpha-linolenic fatty acid on blood pressure and complex blood viscosity was investigated in spontaneously hypertensive rats. A decrease in blood pressure by 59 mm Hg was found compared to the age-matched, untreated control group. The viscous (eta') and elastic (eta") components of viscosity were also reduced at various shear rates (gamma). The same applies to the aggregation index, which is a measure of the aggregation tendency of red blood cells (RBC). Of course, the effects on blood viscosity cannot explain the observed degree of blood-pressure-lowering. Nevertheless, a decrease in the viscosity can improve the flow conditions in the microcirculation, which may lead to a better oxygen supply.

Effects of a diet rich in Q-3 fatty acids on left ventricular geometry and dynamics in spontaneously hypertensive rats

The aim of the present study was to investigate the chronic effects of a dietetic antihypertensive treatment on blood pressure, ventricular dynamics and geometry of the pressure loaded heart. Spontaneously hypertensive rats (SRH) received a standard diet enriched with 10% mackerel oil, containing 30% polyunsaturated Q-3 fatty acids, over a period of 70 days. As described previously the diet reduced blood pressure permanently by 40-50 mm Hg. Despite this reduction, the degree of left ventricular hypertrophy was only slightly (statistically insignificantly) reduced. This was probably a result of an increase in sympathetic tone as indicated by a raised pulse rate. On the other hand, the treatment prevented the development of eccentric hypertrophy, typical to the SHR, without changing the elastic material properties of the myocardium. Since the age-matched controls did not show significant degenerative alterations, protective effects at the level of myocardial tissue could not be demonstrated. Furthermore, future investigations need to investigate why myocardial contractility of the treated animals diminishes as observed in this study.

Copper intake affects rat heart performance during ischemia-reperfusion: possible relation to altered lipid and fatty acid metabolism

Hearts from rats fed low copper (1.3 mg copper/kg diet) or a copper-supplemented diet (243 mg copper/kg diet) were perfused for 90 min according to the Langendorff method. The perfusion protocol included 30 min normoxia, 30 min ischemia and 30 min reperfusion. After 90 min perfusion, hearts from the low copper group had gained more weight, had lower coronary perfusion pressure, developed less force of contraction and secreted less 6-keto PGF1 alpha into the perfusate than hearts from the copper-supplemented group. After perfusion, the major lipid change in the hearts from both groups was a 85-90% decrease in total triacylglycerol. In both groups, stearic acid and arachidonic acid (mg%) were increased in the triacylglycerol fraction after heart perfusion. The quantitative (mg/g) decrease in the triacylglycerol content of stearic acid and arachidonic acid was significantly less in the copper-supplemented group. After perfusion, dihomo-gamma-linolenic acid (mg/g) was lower in heart phospholipids from the low copper group. Dihomo-gamma-linolenic/arachidonic acid (microgram/mg) was significantly decreased after perfusion only in the hearts from the low copper group. Lipid and fatty acid changes in the hearts of the rats fed low dietary copper may contribute to abnormal heart function in this group.

Dietary fatty acids and myocardial function

It is widely recognized that dietary polyunsaturated fatty acids (PUFA's) and cholesterol can profoundly influence the development of atherosclerotic plaques in coronary vessels, which may lead to myocardial infarction. The possibility that dietary fatty acids may also directly influence cardiac function has received less attention. We therefore reviewed the evidence of the effects of dietary fatty acids, in particular n-3 and n-6 PUFA's, on myocardial phospholipid fatty acid composition and cardiovascular performance. Heart organelles appear to incorporate uncommon fatty acids like 22:1 and trans- 18:1. Diets enriched with 22:1 induce myocardial lipidosis. N-9, n-6 and n-3 families compete among membrane C20 and C22 acids. Several studies have dealt with the relation between diet-induced changes of cardiac membrane (sarcolemma, sarcoplasmic reticulum and mitochondria) phospholipids and membrane function. In view of the variety of diets used and of the membrane functions studied, the results do not permit equivocal interpretation. Several investigators have reported an altered stress response of the heart due to a change of PUFA's in the diet. In rats fed with a low 18:2n-6/18:3n-3 ratio combined with relatively low amounts of saturated fatty acids, a high incidence of myocardial lesions has been observed. Pigs are less sensitive but more susceptible to the development of vitamin E deficiency, when the dietary PUFA content is high. Increased contractility and coronary flow rate have been reported for Langendorff-perfused hearts of rats fed 18:2n-6-rich diets. The effects on coronary flow rate are possibly related to alterations in eicosanoid synthesis, which may also contribute to the reduction by n-6 or n-3 PUFA's in infarct size, magnitude of recovery of function and suppression of reperfusion arrhythmias following release of a coronary artery ligation. On the other hand, increased peroxidation of membrane lipids, due to their high content of n-3 PUFA, may be deleterious.

Comparative study of the blood pressure effects of four different vegetable fats on young, spontaneously hypertensive rats

Following the suckling period, four groups of male four-week-old spontaneously hypertensive rats (SHR) were fed semisynthetic diets with 14% (by weight) of either sunflower seed oil [46% 18:2(n-6); linoleic acid (LA)-rich], linseed oil [62.5% 18:3(n-3) + 12.9% 18:2(n-6); alpha-linolenic acid (LNA)-rich], evening primrose oil [9.2% 18:3(n-6) + 71% 18:2(n-6); gamma-linolenic acid (LNA)-rich] or hydrogenated palm kernel fat [1.5% 18:2(n-6); polyunsaturated fatty acid (PUFA)-deficient], respectively, up to an age of 18 wk. All diets enriched with PUFA provoked an attenuation of hypertension development. The effect was lowest in the LA-rich group and highest in the gamma-LNA-rich group. Differences in fatty acid composition of renal phospholipids between groups reflect the fatty acids present in the respective dietary fats. Renomedullary production of PGF2 alpha was significantly reduced in alpha-LNA-rich and slightly diminished in gamma-LNA-rich fed rats. Aortic formation of 6-keto-PGF1 alpha and TXB2 was increased in animals fed the gamma-LNA-rich diet. Thus, the attenuation of hypertension development cannot be explained only by changes in prostanoid formation. Other mechanisms possibly involved should be pursued.