John S. Charnock

Dietary fish oil prevents ventricular fibrillation following coronary artery occlusion and reperfusion.

Coronary artery occlusion and reperfusion in the anesthetized rat was used as a whole animal model of arrhythmia and sudden cardiac death to examine the influence of long-term dietary lipid modulation of myocardial membrane fatty acids on the development of cardiac arrhythmias. Feeding rats a diet supplemented with tuna fish oil significantly reduced the incidence and severity of arrhythmias, preventing ventricular fibrillation during both occlusion and reperfusion. Dietary sunflower seed oil reduced arrhythmias during occlusion but not in reperfusion. Dietary fat can modify the vulnerability of the myocardium to arrhythmic stimuli. The efficacy of tuna fish oil in reducing vulnerability to both ischemic and reperfusion arrhythmias suggests a potential beneficial effect of dietary n-3 fatty acids in addition to their influence on hemostasis, plasma lipids, and atherosclerosis that may contribute to their proposed role in lowering cardiovascular disease mortality and morbidity.

Dietary lipid modulation of ventricular fibrillation threshold in the marmoset monkey

Programmed electrical stimulation was used to examine the ability of long-term dietary lipid modulation to influence myocardial vulnerability to the induction of ventricular fibrillation in adult marmoset monkeys (Callithrix jacchus). Marmosets fed diets supplemented (to a total of 28.5% of the energy as fat) with polyunsaturated fatty acid (PUFA)-rich tuna fish oil or sunflower seed oil had significantly elevated mean ventricular fibrillation threshold compared with those fed a saturated animal fat supplemented diet or a reference diet not supplemented with fat (11.2% of the energy as fat). Fibrillation threshold was reduced during acute myocardial ischemia induced by coronary artery occlusion but still remained higher in the PUFA-fed animals than either the control or the ischemic threshold in reference or saturated fat supplemented animals. Dietary tuna fish oil was associated with a low incidence of sustained fibrillation episodes and no fatalities. These results indicate that myocardial substrate vulnerability to arrhythmic stimuli is increased during ischemia in a nonhuman primate model but dietary PUFA can reduce vulnerability under both normal and ischemic conditions. Reduced dietary fat intake alone was without effect.

Comparative Changes in the Fatty-Acid Composition of Rat Cardiac Phospholipids after Long-Term Feeding of Sunflower Seed Oil- or Tuna Fish Oil-Supplemented Diets

The fatty-acid composition of rat heart phospholipids was examined after long-term, i.e. more than 12 months, feeding of diets supplemented with n-6 fatty acids as sunflower seed oil (SSO), or n-3 fatty acids as tuna fish oil (TFO) which is a particularly rich source of docosahexenoic acid (DHA). Although some small changes occurred in the relative proportions of palmitic and stearic acids and in the ratio of total saturates to total unsaturates, the most important changes were in the relative proportions of 18:2 n-6 and 20:4 n-6 to 20:5 n-3 and 22:6 n-3. In general, the n-6/n-3 ratio of phosphatidylcholine (PC), phosphatidylethanolamine (PE) and diphosphatidyl glycerol (DPG) was altered in favour of the family of fatty acids administered, although the proportions of the individual long-chain polyunsaturated fatty acids which contributed to this ratio varied from one class of phospholipids to another. In cardiac PC and PE, feeding TFO supplements reduced the proportions of arachidonic acid (AA) and significantly elevated (p less than 0.01) the proportions of DHA but produced relatively little change in those of eicosapentenoic acid (EPA). In DPG, feeding TFO led to a significant increase in the proportion of AA as well as an increase in DHA. The level of EPA was relatively low in PC, PE and DPG even after TFO feeding and never reached comparable levels with that of either AA or DHA. Nevertheless the n-6/n-3 ratio in all these classes of major cardiac phospholipids was significantly reduced by feeding TFO compared to the SSO diet or the commercial rat chow (CC) reference group. In contrast to the reports of other workers who have studied the fatty-acid composition of platelet membranes after feeding various fish oil supplements, in the rat heart the major effect of tuna fish oil is an increase in the proportion of DHA rather than EPA in the cardiac phospholipids.

Homeostatic control of membrane fatty acid composition in the rat after dietary lipid treatment

Diets in which both the lipid content and composition (polyunsaturated to saturated fatty acid ratio) were varied were fed to rats for 20 weeks, and the effects on the tissue lipid profiles were determined. The fatty acid profile of the plasma lipids, and the phospholipid fatty acids of the mitochondrial and microsomal fractions of liver, heart, kidney and brain, as well as erythrocyte membranes were determined. Despite large differences in the level and type of lipid present in the experimental diets and in the proportion of saturated fatty acids in the plasma lipids in response to the various diets, there was little effect on the proportion of saturated to unsaturated fatty acids in the phospholipids of the various membranes examined. The major effect of altering the dietary level of polyunsaturated to saturated fatty acids was on the ratio of the ω6/ω3 series of unsaturated fatty acids in the membrane lipids. This change occurred in all tissues except the brain, in which only a small response to altered dietary lipid intake was observed. The ω6/ω3 ratio was elevated upon feeding a diet rich in ω6 polyunsaturated fatty acids, but decreased when a diet rich in saturated fatty acids was fed. The failure to significantly alter membrane lipid saturation/unsaturation in the tissues examined would suggest that a homeostatic mechanism is operative in biological membranes and may act to buffer membranes from the effects of changes in the nature of the dietary lipid intake.

Dietary modulation of lipid metabolism and mechanical performance of the heart

Sudden Cardiac Death resulting from sustained ventricular fibrillation or malignant cardiac arrhythmia has been linked to the type of dietary fat intake in several economically well developed countries where high levels of saturated fatty acids are common. Experimental studies with the small non-human primate marmoset monkey have clearly demonstrated the health benefit of substituting polyunsaturated fatty acids (PUFAs) for dietary saturated fatty acids. Heart rate and blood pressure are lowered, while the left ventricular ejection fraction and the electrical threshold for the induction of ventricular fibrillation are both increased after prolonged feeding of PUFA enriched diets. All these changes in heart function reduce the risk of developing malignant cardiac arrhythmias.The fatty acid composition of cardiac membrane phospholipids is profoundly altered by these changes in dietary lipid intake. In particular the proportions of arachidonic acid (AA), eicosapentaenoic acid (EPA) and docosahexae noic acid (DHA) are altered in such a way that the production of myocardial eicosanoids is affected. Although the changes in proportion of these long-chain PUFAs in cardiac phosphatidyl ethanolamine and phosphatidyl inositol are not identical, the shift in balance between these substrates or inhibitors of cyclo-oxygenase activity leads to relatively greater production of prostacyclin (PGI2) than thromboxane (TXA2).The effect of the omega-3 PUFAs of fish oil is proportionally greater than that of linoleic acid (LA; 18:2, ω6) rich sunflower seed oil, particularly during ischaemia, and probably reflects the different nutritionally induced changes in cardiac membrane fatty acid composition by these different types of dietary PUFAs. (Mol Cell Biochem 116: 19–25, 1992).

Differences in fatty acid composition of various tissues of the marmoset monkey (Lithrix jacchus after different lipid supplemented diets

1. The fatty acid composition of different muscles, organs and blood components of the marmoset monkey were examined after long-term feeding of several well defined lipid supplemented diets. 2. Similarities between the fatty acid composition of cardiac and skeletal muscles which persisted after all diets suggest that biopsy of skeletal muscle may have an important diagnostic value in this and other primate species. 3. The relationship between the dietary intake of individual fatty acids and their proportions in different tissues is both complex and variable. 4. However, the lipid metabolism of Callithrix jacchus recommends this small non-human primate as a most suitable species for the study of lipid nutrition.

Differential modulation of rat heart mitochondrial membrane-associated enzymes by dietary lipid.

Diets supplemented with high levels of saturated fatty acids derived from sheep kidney (perirenal) fat or unsaturated fatty acids derived from sunflower seed oil were fed to rats and the effect on heart mitochondrial lipid composition and membrane-associated enzyme behaviour was determined. The dietary lipid treatments did not change the overall level of membrane lipid unsaturation but did alter the proportion of various unsaturated fatty acids. This led to a change in the omega 6/omega 3 unsaturated fatty acid ratio, which was highest in the sunflower seed oil fed rats. Arrhenius plots of the mitochondrial membrane associated enzymes succinate-cytochrome c reductase and oligomycin-sensitive adenosinetriphosphatase (ATPase) after dietary lipid treatment revealed different responses in their critical temperature. For succinate-cytochrome c reductase, the critical temperature was 29 degrees C for rats fed the sheep kidney fat diet and 20 degrees C for rats fed the sunflower seed oil diet. In contrast, no shift in the critical temperature for the mitochondrial ATPase was apparent as a result of the differing dietary lipid treatments. The results suggest that the discontinuity in the Arrhenius plot of succinate-cytochrome c reductase is induced by some change in the physical properties of the membrane lipids. In contrast, mitochondrial ATPase appears insensitive, in terms of its thermal behaviour, to changes occurring in the composition of the membrane lipids. However, the specific activity of the mitochondrial ATPase was affected by the dietary lipid treatment being highest for the rats fed the sheep kidney fat diet. No dietary lipid effect was observed for the specific activity of succinate-cytochrome c reductase. This differential response of the two mitochondrial membrane enzymes to dietary-induced changes in membrane lipid composition may affect mitochondrial oxidative phosphorylation.

Diet and Cardiac Arrhythmia: Effects of Lipids on Age-Related Changes in Myocardial Function in the Rat

Male rats were fed for 3-4 months (short-term) or 12-15 months (long-term) on a standard laboratory diet alone (control) or supplemented with sunflower seed oil (SSO, 12% w/w) or sheep kidney fat (SKF, 12% w/w). Papillary muscles were electrically driven (1 Hz, 5 ms, supramaximal voltage) at 37 degrees C in Krebs-Henseleit solution, and contractions were measured isometrically. Both the positive inotropic responses to CA++ and the incidence of spontaneous tachyarrhythmias under catecholamine stress were increased by short-term SKF feeding and with age in control and SKF groups, whereas SSO prevented these changes. The results show a marked effect of age upon ventricular myocardial function in the rat, which appears to be accelerated by the consumption of animal (saturated) fat while polyunsaturated vegetable oil provides some degree of protection. It is suggested that changes in membrane lipid composition can alter the Ca++ handling characteristics of myocardial cells.

The interaction of dietary fatty acid and cholesterol on catecholamine-stimulated adenylate cyclase activity in the rat heart

Diets supplemented with high levels of saturated or unsaturated fatty acids supplied by addition of sheep kidney fat or sunflower seed oil, respectively, were fed to rats with or without dietary cholesterol. The effects of these diets on cardiac membrane lipid composition, catecholamine-stimulated adenylate cyclase and beta-adrenergic receptor activity associated with cardiac membranes, were determined. The fatty acid-supplemented diets, either with or without cholesterol, resulted in alterations in the proportion of the (n-6) to (n-3) series of unsaturated fatty acids, with the sunflower seed oil increasing and the sheep kidney fat decreasing this ratio, but did not by themselves significantly alter the ratio of saturated to unsaturated fatty acids. However, cholesterol supplementation resulted in a decrease in the proportion of saturated and polyunsaturated fatty acids and a dramatic increase in oleic acid in cardiac membrane phospholipids irrespective of the nature of the dietary fatty acid supplement. The cholesterol/phospholipid ratio of cardiac membrane lipids was also markedly increased with dietary cholesterol supplementation. Although relatively unaffected by the nature of the dietary fatty acid supplement, catecholamine-stimulated adenylate cyclase activity was significantly increased with dietary cholesterol supplementation and was positively correlated with the value of the membrane cholesterol/phospholipid ratio. Although the dissociation constant for the beta-adrenergic receptor, determined by [125I](-)-iodocyanopindolol binding, was unaffected by the nature of the dietary lipid supplement, the number of beta-adrenergic receptors was dramatically reduced by dietary cholesterol and negatively correlated with the value of the membrane cholesterol/phospholipid ratio. These results indicate that the activity of the membrane-associated beta-adrenergic/adenylate cyclase system of the heart can be influenced by dietary lipids particularly those altering the membrane cholesterol/phospholipid ratio and presumably membrane physico-chemical properties. In the face of these dietary-induced changes, a degree of homeostasis was apparent both with regard to membrane fatty acid composition in response to an altered membrane cholesterol/phospholipid ratio, and to down regulation of the beta-adrenergic receptor in response to enhanced catecholamine-stimulated adenylate cyclase activity.

Response of rat heart membranes and associated ion-transporting ATPases to dietary lipid

The effects of different dietary fat intake on the lipid composition and enzyme behaviour of sarcolemmal (Na+ + K+)ATPase and sarcoplasmic reticulum Ca2+-ATPase from rat heart were investigated. Rat diets were supplemented with either sunflower seed oil (unsatd./satd. 5.6) or sheep kidney fat (unsatd./satd. 0.8). Significant changes in the phospholipid fatty acid composition were observed in both membranes after 9 weeks dietary lipid treatment. For both membranes, the total saturated/unsaturated fatty acid levels were unaffected by the dietary lipid treatment, however the proportions of the major unsaturated fatty acids were altered. Animals fed the sunflower seed oil diet exhibited an increase in n-6 fatty acids, including linoleic (18:2(n-6] and arachidonic (20:4(n-6] while the sheep kidney fat dietary rats were higher in n-3 fatty acids, principally docosahexaenoic (22:6), with the net result being a higher n-6/n-3 ratio in the sunflower seed oil group compared to sheep kidney fat dietary animals. Fluorescence polarization indicated that the fluidity of sarcoplasmic reticular membrane was greater than that of sarcolemmal membrane, with a dietary lipid-induced decrease in fluidity being observed in the sarcoplasmic reticular membrane from sheep kidney fat dietary animals. Despite these significant changes in membrane composition and physical properties, neither the specific activity nor the temperature-activity relationship (Arrhenius profile) of the associated ATPases were altered. These results suggest that with regard to the parameters measured in this study, the two ion-transporting ATPases are not modulated by changes which occur in the membrane lipid composition as a result of the diet.