Metabolic responsiveness to insulin in the diabetic heart

Proteomic changes in the heart of diet-induced pre-diabetic mice

The development of type 2 diabetes (T2D) is strongly associated with obesity. In humans, T2D increases the risk for end organ complications. Among these, heart disease has been ranked as the leading cause of death. We used a proteomic methodology to test the hypothesis that a pre-diabetic state generated by high-fat diet leads to changes in proteins related to heart function and structure. Over 300 protein spots were resolved by two-dimensional gel electrophoresis (2-DE). Fifteen protein spots were found to be altered (7 decreased and 8 increased) in pre-diabetic hearts. The protein spots were then identified by mass spectrometry and immunoblots. Among the decreased proteins, 3 are involved in heart structure (one isoform of desmin, troponin T2 and α-cardiac actin), 3 are involved in energy metabolism (mitochondrial ATP synthase β subunit, adenylate kinase and creatine kinase) and one is a component of the citric acid cycle (isocitrate dehydrogenase 3). In contrast, proteins involved in fatty acid oxidation (two isoforms of peroxisomal enoyl-CoA hydratase) and the citric acid cycle (three isoforms of malate dehydrogenase) were increased in pre-diabetic hearts. The results suggest that changes in the levels of several heart proteins may have implications in the development of the cardiac phenotype associated to T2D.

Myocardial fatty acid metabolism in health and disease

There is a constant high demand for energy to sustain the continuous contractile activity of the heart, which is met primarily by the beta-oxidation of long-chain fatty acids. The control of fatty acid beta-oxidation is complex and is aimed at ensuring that the supply and oxidation of the fatty acids is sufficient to meet the energy demands of the heart. The metabolism of fatty acids via beta-oxidation is not regulated in isolation; rather, it occurs in response to alterations in contractile work, the presence of competing substrates (i.e., glucose, lactate, ketones, amino acids), changes in hormonal milieu, and limitations in oxygen supply. Alterations in fatty acid metabolism can contribute to cardiac pathology. For instance, the excessive uptake and beta-oxidation of fatty acids in obesity and diabetes can compromise cardiac function. Furthermore, alterations in fatty acid beta-oxidation both during and after ischemia and in the failing heart can also contribute to cardiac pathology. This paper reviews the regulation of myocardial fatty acid beta-oxidation and how alterations in fatty acid beta-oxidation can contribute to heart disease. The implications of inhibiting fatty acid beta-oxidation as a potential novel therapeutic approach for the treatment of various forms of heart disease are also discussed.

Effects of coronary artery disease and percutaneous intervention on the cardiac metabolism of nonesterified fatty acids and insulin: Implications of diabetes mellitus

BACKGROUND

Nonesterified fatty acids (NEFA) and insulin have been implicated in the pathogenesis of diabetes mellitus (Type 2 diabetes) and coronary artery disease (CAD). We hypothesized that intracardiac levels of insulin and NEFA within the aortic root, coronary sinus and systemic venous levels would be different in patients with coronary atherosclerosis and/or diabetes. We also studied the metabolic cardiac response following percutaneous coronary intervention (PCI).

METHODS

A total of 67 subjects (42 males; mean age 60 +/- 11 years) were recruited, of which three groups were identified: Group I - those with no CAD or Type 2 diabetes (n = 17); Group II - those with CAD but no Type 2 diabetes (n = 40); and Group III - patients with Type 2 diabetes and CAD (n = 10). Of the whole cohort, 34 patients (51%) proceeded to PCI. NEFA and insulin levels were analysed using enzymatic colorimetric and a monoclonal immuno-autoanalyser techniques, respectively. Subsequently, fractional extraction (FFE) of both variables was calculated.

RESULTS

Nonesterified fatty acids and insulin concentrations were lower in the aortic root versus coronary sinus (both P < 0.05). FFE of NEFA was 2x higher in Group I (P < 0.01) with a sevenfold reduction in insulin FFE in Group III. Following PCI, systemic NEFA levels increased significantly (P < 0.05) with no significant change seen within the coronary sinus (P = NS), whilst a reduction in insulin concentrations at all three sites was observed (all P < 0.01). No significant difference in FFE of NEFA was seen after PCI when comparing Groups II and III. There was a drop in insulin extraction in Group II (nondiabetic subjects, from 12% to -4%, P = 0.04), compared with an increase seen in Group III (Type 2 diabetes patients, from -4% to 3%, P = 0.03).

CONCLUSION

There is an intracardiac gradient of NEFA and insulin in Groups I-III. Cardiac NEFA metabolism was higher in those with mild CAD compared with those with obstructive CAD whereas intracardiac insulin extraction was lower in Group III (diabetic) patients. PCI was associated with a systemic rise in NEFA, with a reduction in insulin levels and cardiac utilization, but these effects were blunted in diabetic patients.

Glucose and insulin improve cardiac efficiency and postischemic functional recovery in perfused hearts from type 2 diabetic (db/db) mice

Hearts from type 2 diabetic (db/db) mice demonstrate altered substrate utilization with high rates of fatty acid oxidation, decreased functional recovery following ischemia, and reduced cardiac efficiency. Although db/db mice show overall insulin resistance in vivo, we recently reported that insulin induces a marked shift toward glucose oxidation in isolated perfused db/db hearts. We hypothesize that such a shift in metabolism should improve cardiac efficiency and consequently increase functional recovery following low-flow ischemia. Hearts from db/db and nondiabetic (db/+) mice were perfused with 0.7 mM palmitate plus either 5 mM glucose (G), 5 mM glucose and 300 microU/ml insulin (GI), or 33 mM glucose and 900 microU/ml insulin (HGHI). Substrate oxidation and postischemic recovery were only moderately affected by GI and HGHI in db/+ hearts. In contrast, GI and particularly HGHI markedly increased glucose oxidation and improved postischemic functional recovery in db/db hearts. Cardiac efficiency was significantly improved in db/db, but not in db/+ hearts, in the presence of HGHI. In conclusion, insulin and glucose normalize cardiac metabolism, restore efficiency, and improve postischemic recovery in type 2 diabetic mouse hearts. These findings may in part explain the beneficial effect of glucose-insulin-potassium therapy in diabetic patients with cardiac complications.