Doctor of Philosophy
School of Health Sciences, Faculty of Health & Behavioural Sciences
Abdukeyum, Grace Gulbahar, Dietary omega-3 fatty acids, nutritional preconditioning of the heart against ischemic injury, Doctor of Philosophy thesis, School of Health Sciences, Faculty of Health & Behavioural Sciences, University of Wollongong, 2010. http://ro.uow.edu.au/theses/3114
Cardiovascular disease now ranks as the leading cause of death, resulting in one third of all deaths in the world, among which ischemic heart disease is projected to be the number one cause of death globally. It has been long suggested that (n-3) polyunsaturated fatty acids provide cardiovascular protection, with regular intake of fish or fish oil associated with reduced mortality from heart diseases in both clinical trials and epidemiological studies. One major observation in these studies is that fish oil has been found to reduce mortality during or following ischemic events. While ischemia damages the heart, short bursts of ischemia paradoxically protect the heart from the damaging effects of more prolonged ischemia. This powerful cardioprotective phenomenon is termed ischemic preconditioning. The protective influences of ischemic preconditioning include reduction of infarct size, prevention of life-threatening arrhythmias in ischemia and reperfusion, reduced myocardial oxygen demand, and improved recovery of post-ischemic cardiac pumping function. However, the promise of ischemic preconditioning has not yet been realized in bench to bedside application. The characteristics of cardioprotection afforded by feeding (n-3) PUFA suggests preconditioning-like effects, related to incorporation of (n-3) PUFA into heart membranes. However, the long chain highly unsaturated (n-3) PUFA in the membranes are highly susceptible to peroxidation, perhaps making cardiac membranes more susceptible to free radical generation and cellular damage. While free radicals are thought to be involved in the damaging effects of ischemia and reperfusion, they also play a role in protective mechanisms of ischemic preconditioning.
This study evaluated the effects of fish oil on (n-3) PUFA incorporation into myocardial membrane and examined the susceptibility to oxidative damage and myocardial injury in terms of infarct size and postischemic cardiac function. Further, it compared dietary fish oil with ischemic preconditioning and assessed their interaction for effects on heart function and injury during myocardial ischemia and reperfusion. It tested the hypothesis that the susceptibility to peroxidation may provide an ever-present preconditioning stimulus that protects the heart against the damaging effects of a major ischemia reperfusion insult.
Male Wistar rats were fed one of three fully fabricated diets containing 10% fat by weight varying only in the types of fat. The (n-3) PUFA diet contained 7% fish oil + 3% olive oil; The (n-6) PUFA diet contained 5% sunflower seed oil + 5% olive oil; The saturated fatty acid (SF) diet contained 7% saturated fat-rich beef tallow +3% olive oil. Heart function was examined after six weeks feeding using Langendorff-perfused isolated isovolumic heart preparation. In control experiments, isolated perfused hearts were subjected to 30 minutes regional is chemia by occluding the left anterior descending coronary artery, then reperfused for 120 minutes. Ischemic preconditioning consisted of three cycles of five minutes global ischemia before the 30 minutes regional ischemia and 120 minutes reperfusion. Heart function was assessed during perfusion by ECG and by measurement of intraventricular pressure. Infarct size was measured at completion of perfusion in control and ischemic preconditioned hearts as a percent of the ischemic zone at risk. Lipid peroxidation products and antioxidant concentrations were measured in normoxic heart and in ischemic and non-ischemic regions of hearts with or without ischemic preconditioning.
Control (n-3) PUFA hearts had significantly lower spontaneous heart rate, coronary flow, end diastolic pressure, maximum relaxation rate, and fewer ischemic reperfusion arrhythmias than did (n-6) PUFA hearts or SF hearts. In reperfusion (n- 3) PUFA hearts maintained greater developed pressure and maximum rate of relaxation and developed smaller infarcts (10.9±3.6% ischemic zone, n=6) than (n-6) PUFA hearts (47.4±2.3%, n=6) or SF hearts (50.3±4.3%, n=6).
Ischemic preconditioning significantly improved heart function and reduced infarct size in (n-6) PUFA hearts (11.8±5.4%, n=6) and SF hearts (13.1±4.2%, n=6). Heart function and infarct size did not differ between control and ischemic preconditioned hearts (9.6±4.2%) with (n-3) PUFA diet. Arrhythmias were significantly reduced by ischemic preconditioning in (n-6) PUFA hearts or saturated fatty acid hearts towards levels observed in (n-3) PUFA hearts.
Myocardial membranes showed high incorporation of long chain docosahexaenoic acid (DHA) (22:6,n-3), predicting increased risk of peroxidation. The concentration of lipid hydroperoxides and malondialdehyde were higher in normoxic and nonischemic regions of control (n-3) PUFA hearts than in (n-6) PUFA or SF hearts. The concentration of the endogenous antioxidant superoxide dismutase was higher in normoxic and nonischemic regions of control (n-3) PUFA hearts and was increased after ischemic preconditioning in saturated fatty acid and (n-6) PUFA hearts. Both (n-3) PUFA diet and ischemic preconditioning inhibited the ischemiainduced rise in the oxidation products lipid hydroperoxides and malondialdehyde.
This thesis demonstrated that the harmful effects of myocardial ischemia and reperfusion, such as infarct size, poor relaxation, cardiac arrhythmia and poor recovery of contractile function were largely curtailed by feeding an (n-3) PUFA rich diet. The effects of SF and (n-6) PUFA diet on heart function and ischemia susceptibility were largely indistinguishable, indicating that the effects of fish oil were specifically related to its (n-3) PUFA content and not due to either a reduction in saturated fat intake or a non-specific role of polyunsaturated fatty acids. Regular consumption of dietary fish oil induced sustained changes in membrane fatty acid composition and produced cardioprotection that appears similar to late ischemic preconditioning. The continuous presence of (n-3) PUFA in myocardial membranes suggests that, as reported for late ischemic preconditioning, this is not subject to desensitization.
Dietary treatment of rats with (n-3) PUFA caused an increase in peroxidation index suggesting an increase in susceptibility of the membrane to oxidative damage, which might be expected to enhance ischemic damage. An increase in membrane lipid peroxidation was indeed observed in fish oil treated rat hearts, however, it was associated with increased antioxidant activity and reduced lipid oxidation under stress and instead of causing lasting damage to heart function, beneficial effects on arrhythmia, contractile function, and myocardial infarct size were observed. These protective effects are demonstrated as powerful as ischemic preconditioning.
Therefore, in light of cardioprotective effects of (n-3) PUFA to reduce the consequences of ischemic events in the human population when a regular part of the diet, the present thesis demonstrated that (n-3) PUFA induces a form of preconditioning in the heart, which this thesis has termed, nutritional preconditioning. The (n-3) PUFA limit ischemic cardiac injury and myocardial infarction and endow cardioprotection as powerful as ischemic preconditioning under these experimental conditions. Nutritional preconditioning by membrane incorporation of (n-3) PUFA may underpin the low cardiovascular morbidity and mortality associated with regular fish and fish oil consumption.