| BackgroundObesity, which is associated with metabolic syndrome, has become an epidemic problem worldwide. Patients with obesity and metabolic syndrome have a three-fold increased risk of coronary heart diseases. Following an acute myocardial infarction, obese patients experience worse clinical outcomes. Thus, using any effective cardioprotective strategy to reduce the consequences of coronary artery disease is important in this population.Volatile anesthetics are reported to produce pharmacological preconditioning and reduce ischemia-reperfusion (I/R) injury in the heart in a variety of experimental animal models as well as in humans. In normal animals, activation of AMP-activated protein kinase (AMPK) is reported to be involved in anesthetic-induced cardioprotective signaling; and this activation requires production of reactive oxygen species (ROS). However, a recent study demonstrated that anesthetic preconditioning against myocardial I/R injury is abolished in diet-induced obesity, and the failure to precondition the obese myocardium is probably due to increased basal myocardial oxidative stress, which impairs ROS-mediated AMPK activation during anesthetic preconditioning.Exercise training has been shown to reduce age-related myocardial oxidative stress and restore ischemic-preconditioning in the aging heart. However, effects of exercise training on anesthetic preconditioning in obese heart are unclear. The aim of this study is to examine whether exercise training would normalize ROS-mediated AMPK signaling pathway by reducing basal myocardial oxidative stress and prevent the attenuation of anesthetic preconditioning-induced cardioprotection in obesity.Part 1 The effects of exercise training on metabolism and basal myocardial oxidative stress in diet-induced obese rats.Objective:To investigate the effects of exercise training on metabolism and basal myocardial oxidative stress in diet-induced obese rats.Methods:Male Sprague-Dawley rats weighing 82.7±1.8 g were randomly divided into four groups(n=8 for each group):(1) L-SED, after 4 weeks of feeding with control diet containing 10% of kcal from fat (designated as low-fat), rats were assigned to sedentary conditions for 8 weeks; (2) L-EXE, after 4 weeks of feeding with control diet, rats were assigned to treadmill exercise for 8 weeks; (3) O-SED, after 4 weeks of feeding with high-fat diet containing 45% of kcal from fat, rats were assigned to sedentary conditions for 8 weeks; (4) O-EXE, after 4 weeks of feeding with high-fat diet, rats were assigned to treadmill exercise for 8 weeks. After 12 weeks of control diet or high-fat diet feeding and 8 weeks of exercise training, rats were sacrificed. Blood samples and heart tissues were collected to assess the biomarkers for metabolism and basal myocardial oxidative stress.Results:1. Metabolic effects of exercise trainingBefore exercise training (4 weeks after control diet or high-fat diet feeding), obese rats displayed high body weight compared with lean rats (180±9 vs. 156±6, p <0.05; obese rats vs. lean rats). At this time point, body weight was similar between lean sedentary and lean exercise-trained rats and also between obese sedentary and obese exercise-trained rats. At the end of the study, obese sedentary rats were markedly heavier than lean sedentary rats. In addition, plasma levels of insulin, leptin, and total cholesterol were significantly higher in obese sedentary rats than those in lean sedentary rats. Exercise training significantly reduced these parameters in obese exercise-trained rats but had no effects in lean exercise-trained rats. The levels of blood glucose and plasma triglycerides were similar among groups.2. Effects of exercise training on basal oxidative stressAfter 12 weeks of control diet or high-fat diet feeding and 8 weeks of exercise training, the basal level of superoxide production in the heart was greater in obese sedentary rats than that in lean sedentary rats. Superoxide production was unchanged in lean exercise-trained rats, but it was reduced in obese exercise-trained rats.Western blot analysis revealed that protein levels of antioxidant enzyme superoxide dismutase (SOD) and antioxidant master regulator Nrf2 in the heart were lower in obese sedentary rats than those in lean sedentary rats. Exercise training reversed protein levels of SOD and Nrf2 in obese exercise-trained rats but did not alter those in lean exercise-trained rats. There was no difference in protein levels of antioxidant enzyme catalase across 4 groups.Part 2 Exercise training prevents the attenuation of anesthetic preconditioning-mediated cardioprotection in diet-induced obese ratsObjective:To examine whether exercise training would normalize ROS-mediated AMPK signaling pathway by reducing basal myocardial oxidative stress and restore anesthetic sevoflurane preconditioning in obesity.Methods:After 12 weeks of control diet or high-fat diet feeding and 8 weeks of exercise training, sedentary rats and exercise-trained rats were randomly divided into eight groups(n=11-13 for each group) for myocardial ischemia-reperfusion (I/R) as follows:(1) L-SED, lean sedentary rats subjected to I/R alone; (2) L-SED+SEV, lean sedentary rats received sevoflurane preconditioning before I/R; (3) L-EXE+SEV, lean exercise-trained rats received sevoflurane preconditioning before I/R; (4) L-EXE, lean exercise-trained rats subjected to I/R alone; (5) O-SED, obese sedentary rats subjected to I/R alone; (6) O-SED+SEV, obese sedentary rats received sevoflurane preconditioning before I/R; (7) O-EXE+SEV, obese exercise-trained rats received sevoflurane preconditioning before I/R; (8) O-EXE, obese exercise-trained rats subjected to I/R alone.10 min after preconditioning, rats were subjected to 25 min of regional myocardial ischemia followed by 120 min of reperfusion. At the end of the protocol, the hearts were removed for assessments of area at risk and infarct size. To examine the effects of exercise training on sevoflurane preconditioning-induced AMPK activation and nitrite and nitrate (NOx) production, some rats from each group (n= 5-6 for each group) were sacrificed at the end of ischemia and myocardial samples were collected from ischemic left ventricular areas.Results:1. Effect of sevoflurane preconditioning and exercise training on infarct sizeThere was no difference in infarct size between lean sedentary and obese sedentary rats following I/R. Sevoflurane preconditioning significantly reduced infarct size in lean sedentary rats, but it failed to protect obese sedentary rats against myocardial infarction. After 8 weeks of exercise training, sevoflurane preconditioning did not further decrease infarct size in lean exercise-trained rats; however, this strategy significantly reduced infarct size in obese exercise-trained rats to the same extent as that observed in lean rats. Exercise training alone did not reduce infarct size in both lean and obese rats. In addition, the ratio of area at risk following I/R was similar among all groups.2. Effect of sevoflurane preconditioning and exercise training on AMPK activationAt the end of ischemia, AMPK activation (phosphorylation of AMPK/total AMPK) did not differ between lean sedentary and obese sedentary rats. Sevoflurane preconditioning similarly increased AMPK activation in lean sedentary and lean exercise-trained rats, whereas it had no effect in obese sedentary rats. However, after exercise training, sevoflurane preconditioning significantly increased AMPK activation in obese exercise-trained rats compared with obese sedentary rats. Exercise training alone did not change AMPK activation in either lean rats or obese rats as compared to their sedentary rats.3. Effect of sevoflurane preconditioning and exercise training on endothelial nitric oxide synthase (eNOS) activation and myocardial nitrite and nitrate (NOx) productioneNOS activation (phosphorylation of eNOS/total eNOS) and NOx level were similar between two sedentary groups. Sevoflurane preconditioning significantly augmented eNOS activation and NOx level in lean sedentary and lean exercise-trained rats but not in obese sedentary rats, whereas eNOS activation and NOx level were enhanced in response to sevoflurane preconditioning in obese exercise-trained rats. Neither eNOS activation nor NOx level was altered in lean exercise-trained rats or obese exercise-trained rats without sevoflurane preconditioning.4. Effect of sevoflurane preconditioning and exercise training on superoxide anionMyocardial superoxide production, measured at the end of ischemia, was significantly higher in obese sedentary rats than that in lean sedentary rats. Sevoflurane preconditioning augmented superoxide production equally in lean sedentary and lean exercise-trained rats, but it had no effect in obese sedentary rats. However, superoxide production was increased in obese exercise-trained rats in response to sevoflurane preconditioning. Exercise training alone did not alter superoxide production in lean rats, but it reduced that in obese rats as compared to their sedentary rats.Conclusion:1. The results suggest that exercise training can ameliorate metabolic syndrome, reduce myocardial oxidative stress, and improve antioxidant systems in diet indued obese rats.2. Exercise training was found to restore sevoflurane preconditioning-mediated cardioprotection in obesity. Restoration of this protective strategy appears to be related to improvement in basal oxidative stress, which normalizes ROS-mediated AMPK pathway. |
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