The Krebs cycle plays a fundamental role in cardiac energy production and is often implicated in the energetic imbalance characteristic of heart disease. In this study, we measured Krebs cycle flux in real time in perfused rat hearts using hyperpolarized magnetic resonance spectroscopy (MRS). [2-(13)C]Pyruvate was hyperpolarized and infused into isolated perfused hearts in both healthy and postischemic metabolic states. We followed the enzymatic conversion of pyruvate to lactate, acetylcarnitine, citrate, and glutamate with 1 s temporal resolution. The appearance of (13)C-labeled glutamate was delayed compared with that of other metabolites, indicating that Krebs cycle flux can be measured directly. The production of (13)C-labeled citrate and glutamate was decreased postischemia, as opposed to lactate, which was significantly elevated. These results showed that the control and fluxes of the Krebs cycle in heart disease can be studied using hyperpolarized [2-(13)C]pyruvate.

Elevated levels of cardiac mitochondrial uncoupling protein 3 (UCP3) and decreased cardiac efficiency (hydraulic power/oxygen consumption) with abnormal cardiac function occur in obese, diabetic mice. To determine whether cardiac mitochondrial uncoupling occurs in non-genetic obesity, we fed rats a high fat diet (55% kcal from fat) or standard laboratory chow (7% kcal from fat) for 3 weeks, after which we measured cardiac function in vivo using cine MRI, efficiency in isolated working hearts and respiration rates and ADP/O ratios in isolated interfibrillar mitochondria; also, measured were medium chain acyl-CoA dehydrogenase (MCAD) and citrate synthase activities plus uncoupling protein 3 (UCP3), mitochondrial thioesterase 1 (MTE-1), adenine nucleotide translocase (ANT) and ATP synthase protein levels. We found that in vivo cardiac function was the same for all rats, yet oxygen consumption was 19% higher in high fat-fed rat hearts, therefore, efficiency was 21% lower than in controls. We found that mitochondrial fatty acid oxidation rates were 25% higher, and MCAD activity was 23% higher, in hearts from rats fed the high fat diet when compared with controls. Mitochondria from high fat-fed rat hearts had lower ADP/O ratios than controls, indicating increased respiratory uncoupling, which was ameliorated by GDP, a UCP3 inhibitor. Mitochondrial UCP3 and MTE-1 levels were both increased by 20% in high fat-fed rat hearts when compared with controls, with no significant change in ATP synthase or ANT levels, or citrate synthase activity. We conclude that increased cardiac oxygen utilisation, and thereby decreased cardiac efficiency, occurs in non-genetic obesity, which is associated with increased mitochondrial uncoupling due to elevated UCP3 and MTE-1 levels.

Obesity is associated with risk factors for cardiovascular disease, including insulin resistance, and can lead to cardiac hypertrophy and congestive heart failure. Here, we used the insulin-sensitizing agent rosiglitazone to investigate the cellular mechanisms linking insulin resistance in the obese Zucker rat heart with increased susceptibility to ischemic injury. Rats were treated for 7 or 14 days with 3 mg/kg per os rosiglitazone. Hearts were isolated and perfused before and during insulin stimulation or during 32 min low-flow ischemia at 0.3 ml ⅐ min ؊1 ⅐ grams wet wt ؊1 and reperfusion. D[2-3 H]glucose was used as a tracer of glucose uptake, and phosphorus-31 nuclear magnetic resonance spectroscopy was used to follow energetics during ischemia. At 12 months of age, obese rat hearts were insulin resistant with decreased GLUT4 protein expression. During ischemia, glucose uptake was lower and depletion of ATP was greater in obese rat hearts, thereby significantly impairing recovery of contractile function during reperfusion. Rosiglitazone treatment normalized the insulin resistance and restored GLUT4 protein levels in obese rat hearts. Glucose uptake during ischemia was also normalized by rosiglitazone treatment, thereby preventing the greater loss of ATP and restoring recovery of contractile function to that of lean rat hearts. We conclude that rosiglitazone treatment, by normalizing glucose uptake, protected obese rat hearts from ischemic injury.

BackgroundTo study the pathogenesis of diabetic cardiomyopathy, reliable animal models of type 2 diabetes are required. Physiologically relevant rodent models are needed, which not only replicate the human pathology but also mimic the disease process. Here we characterised cardiac metabolic abnormalities, and investigated the optimal experimental approach for inducing disease, in a new model of type 2 diabetes.Methods and resultsMale Wistar rats were fed a high-fat diet for three weeks, with a single intraperitoneal injection of low dose streptozotocin (STZ) after fourteen days at 15, 20, 25 or 30 mg/kg body weight. Compared with chow-fed or high-fat diet fed control rats, a high-fat diet in combination with doses of 15–25 mg/kg STZ did not change insulin concentrations and rats maintained body weight. In contrast, 30 mg/kg STZ induced hypoinsulinaemia, hyperketonaemia and weight loss. There was a dose-dependent increase in blood glucose and plasma lipids with increasing concentrations of STZ. Cardiac and hepatic triglycerides were increased by all doses of STZ, in contrast, cardiac glycogen concentrations increased in a dose-dependent manner with increasing STZ concentrations. Cardiac glucose transporter 4 protein levels were decreased, whereas fatty acid metabolism-regulated proteins, including uncoupling protein 3 and pyruvate dehydrogenase (PDH) kinase 4, were increased with increasing doses of STZ. Cardiac PDH activity displayed a dose-dependent relationship between enzyme activity and STZ concentration. Cardiac insulin-stimulated glycolytic rates were decreased by 17% in 15 mg/kg STZ high-fat fed diabetic rats compared with control rats, with no effect on cardiac contractile function.ConclusionsHigh-fat feeding in combination with a low dose of STZ induced cardiac metabolic changes that mirror the decrease in glucose metabolism and increase in fat metabolism in diabetic patients. While low doses of 15–25 mg/kg STZ induced a type 2 diabetic phenotype, higher doses more closely recapitulated type 1 diabetes, demonstrating that the severity of diabetes can be modified according to the requirements of the study.