Oxidative stress impairs insulin signal in skeletal muscle and causes insulin resistance in postinfarct heart failure.

Insulin resistance has been shown to occur as a consequence of heart failure. However, its exact mechanisms in this setting remain unknown. We have previously reported that oxidative stress is enhanced in the skeletal muscle from mice with heart failure after myocardial infarction (MI) (30). This study is aimed to investigate whether insulin resistance in postinfarct heart failure is due to the impairment of insulin signaling in the skeletal muscle caused by oxidative stress. Mice were divided into four groups: sham operated (sham); sham treated with apocynin, an inhibitor of NAD(P)H oxidase activation (10 mmol/l in drinking water); MI; and MI treated with apocynin. After 4 wk, intraperitoneal insulin tolerance tests were performed, and skeletal muscle samples were obtained for insulin signaling measurements. MI mice showed left ventricular dilation and dysfunction by echocardiography and increased left ventricular end-diastolic pressure and lung weight. The decrease in glucose level after insulin load significantly attenuated in MI compared with sham. Insulin-stimulated serine phosphorylation of Akt and glucose transporter-4 translocation were decreased in MI mice by 61 and 23%, respectively. Apocynin ameliorated the increase in oxidative stress and NAD(P)H oxidase activities measured by the lucigenin assay in the skeletal muscle after MI. It also improved insulin resistance and inhibited the decrease of Akt phosphorylation and glucose transporter-4 translocation. Insulin resistance was induced by the direct impairment of insulin signaling in the skeletal muscle from postinfarct heart failure, which was associated with the enhanced oxidative stress via NAD(P)H oxidase.

Oxidative stress in skeletal muscle impairs mitochondrial respiration and limits exercise capacity in type 2 diabetic mice.

Insulin resistance or diabetes is associated with limited exercise capacity, which can be caused by the abnormal energy metabolism in skeletal muscle. Oxidative stress is involved in mitochondrial dysfunction in diabetes. We hypothesized that increased oxidative stress could cause mitochondrial dysfunction in skeletal muscle and make contribution to exercise intolerance in diabetes. C57/BL6J mice were fed on normal diet or high fat diet (HFD) for 8 wk to induce obesity with insulin resistance and diabetes. Treadmill tests with expired gas analysis were performed to determine the exercise capacity and whole body oxygen uptake (Vo(2)). The work (vertical distance x body weight) to exhaustion was reduced in the HFD mice by 36%, accompanied by a 16% decrease of peak Vo(2). Mitochondrial ADP-stimulated respiration, electron transport chain complex I and III activities, and mitochondrial content in skeletal muscle were decreased in the HFD mice. Furthermore, superoxide production and NAD(P)H oxidase activity in skeletal muscle were significantly increased in the HFD mice. Intriguingly, the treatment of HFD-fed mice with apocynin [10 mmol/l; an inhibitor of NAD(P)H oxidase activation] improved exercise intolerance and mitochondrial dysfunction in skeletal muscle without affecting glucose metabolism itself. The exercise capacity and mitochondrial function in skeletal muscle were impaired in type 2 diabetes, which might be due to enhanced oxidative stress. Therapies designed to regulate oxidative stress and maintain mitochondrial function could be beneficial to improve the exercise capacity in type 2 diabetes.

Increased myocardial NAD(P)H oxidase-derived superoxide causes the exacerbation of postinfarct heart failure in type 2 diabetes.

Type 2 diabetes adversely affects the outcomes in patients with myocardial infarction (MI), which is associated with the development of left ventricular (LV) failure. NAD(P)H oxidase-derived superoxide (O(2)(-)) production is increased in type 2 diabetes. However, its pathophysiological significance in advanced post-MI LV failure associated with type 2 diabetes remains unestablished. We thus hypothesized that an inhibitor of NAD(P)H oxidase activation, apocynin, could attenuate the exacerbated LV failure after MI in high-fat diet (HFD)-induced obese mice with type 2 diabetes. Male C57BL/6J mice were fed on either HFD or normal diet (ND) for 8 wk. At 4 wk of feeding, MI was created in mice by ligating the left coronary artery. HFD-fed MI mice were treated with either 10 mmol/l apocynin or vehicle. HFD + MI had significantly greater LV end-diastolic diameter (LVEDD; 5.7 +/- 0.1 vs. 5.3 +/- 0.2 mm), end-diastolic pressure (12 +/- 2 vs. 8 +/- 1 mmHg), and lung weight/tibial length (10.1 +/- 0.3 vs. 8.7 +/- 0.7 mg/mm) than ND + MI, which was accompanied by an increased interstitial fibrosis of noninfarcted LV. Treatment of HFD + MI with apocynin significantly decreased LVEDD (5.4 +/- 0.1 mm), LV end-diastolic pressure (9.7 +/- 0.8 mmHg), lung weight/tibial length (9.0 +/- 0.3 mg/mm), and concomitantly interstitial fibrosis of noninfarcted LV to the ND + MI level without affecting body weight, glucose metabolism, and infarct size. NAD(P)H oxidase activity and O(2)(-) production were increased in noninfarcted LV tissues from HFD + MI, both of which were attenuated by apocynin to the ND + MI level. Type 2 diabetes was associated with the exacerbation of LV failure after MI via increasing NAD(P)H oxidase-derived O(2)(-), which may be a novel important therapeutic target in advanced heart failure with diabetes.

Angiotensin II type 1 receptor blocker attenuates myocardial remodeling and preserves diastolic function in diabetic heart.

Blockade of the renin-angiotensin system reduces cardiovascular morbidity and mortality in diabetic patients. Angiotensin II (Ang II) plays an important role in the structural and functional abnormalities of the diabetic heart. We investigated whether or not Ang II type 1 receptor blocker (ARB) could attenuate left ventricular (LV) remodeling in male mice with diabetes mellitus (DM) induced by the injection of streptozotocin (200 mg/kg, i.p.). Diabetic mice were treated with candesartan (1 mg/kg/day; DM+Candesartan, n=7) or vehicle (DM+Vehicle, n=7) for 8 weeks. Heart rate and aortic blood pressure were comparable between the groups. Normal systolic function was preserved in diabetic mice. In contrast, diastolic function was impaired in DM+Vehicle and was improved in DM+Candesartan, as assessed by the deceleration time of the peak velocity of transmitral diastolic flow (40.3+/-0.3 vs. 37.3+/-0.5 ms, p<0.01) and the time needed for relaxation of 50% maximal LV pressure to baseline value (tau; 10.6+/-0.7 vs. 8.7+/-0.6 ms, p<0.05) without significant changes in heart rate and aortic blood pressure. Improvement of LV diastolic function was accompanied by the attenuation of myocyte hypertrophy, interstitial fibrosis and apoptosis in association with the expression of connective tissue growth factor (CTGF) and myocardial oxidative stress. Moreover, candesartan directly inhibited Ang II-mediated induction of CTGF in cultured cardiac fibroblasts. ARB might be beneficial to prevent cardiac abnormalities in DM.

Overexpression of glutathione peroxidase attenuates myocardial remodeling and preserves diastolic function in diabetic heart.

Oxidative stress plays an important role in the structural and functional abnormalities of diabetic heart. Glutathione peroxidase (GSHPx) is a critical antioxidant enzyme that removes H(2)O(2) in both the cytosol and mitochondia. We hypothesized that the overexpression of GSHPx gene could attenuate left ventricular (LV) remodeling in diabetes mellitus (DM). We induced DM by injection of streptozotocin (160 mg/kg ip) in male GSHPx transgenic mice (TG+DM) and nontransgenic wildtype littermates (WT+DM). GSHPx activity was higher in the hearts of TG mice compared with WT mice, with no significant changes in other antioxidant enzymes. LV thiobarbituric acid-reactive substances measured in TG+DM at 8 wk were significantly lower than those in WT+DM (58 +/- 3 vs. 71 +/- 5 nmol/g, P < 0.05). Heart rate and aortic blood pressure were comparable between groups. Systolic function was preserved normal in WT+DM and TG+DM mice. In contrast, diastolic function was impaired in WT+DM and was improved in TG+DM as assessed by the deceleration time of peak velocity of transmitral diastolic flow and the time needed for relaxation of 50% maximal LV pressure to baseline value (tau; 13.5 +/- 1.2 vs. 8.9 +/- 0.7 ms, P < 0.01). The TG+DM values were comparable with those of WT+Control (tau; 7.8 +/- 0.2 ms). Improvement of LV diastolic function was accompanied by the attenuation of myocyte hypertrophy, interstitial fibrosis, and apoptosis. Overexpression of GSHPx gene ameliorated LV remodeling and diastolic dysfunction in DM. Therapies designed to interfere with oxidative stress might be beneficial to prevent cardiac abnormalities in DM.