Cardiac energetics in the isolated heart by NMR spectroscopy and mathematical modeling

The heart is the most metabolically active organ. Several prevalent cardiac diseases, such as heart failure, ischemic and diabetic cardiomyopathy, are associated with abnormal energy metabolism. However, due to the lack of nondestructive methods to quantify metabolic activities and mitochondrial function, an integrative understanding of the mechanisms underlying metabolic dysfunction in diseased heart has been incomplete. Therefore, in the current thesis, we aimed at developing a novel approach that combines magnetic resonance spectroscopy (MRS) technique with system biology for quantitative understanding of metabolic communication between subcellular compartments and mitochondrial function in intact hearts.;A comprehensive multi-domain model of cardiac metabolism that encompasses the malate-aspartate (M-A) shuttle was developed to investigate the metabolic responses of cardiomyocytes to ischemia (Chapter 2). By functionally localizing the M-A shuttle in a subdomain within the cardiomyocytes, model simulations suggested that the decreased shuttle flux during ischemia was due to the redistribution of shuttle-associated metabolites across the mitochondrial membrane. Chapter 3 focused on developing a kinetic analysis method for evaluation of fluxes through major metabolic pathways from dynamic 13C MRS data in intact hearts. Based on the model of Chapter 2, we developed a novel comprehensive model of cardiac metabolism that incorporates the dynamic labeling of major metabolite pools with 13C. By least-square fitting of this model to NMR-measured dynamic 13C-enrichment of glutamate from isolated perfused hearts, the responses of TCA cycle flux and M-A shuttle activity to altered cytosolic redox states were examined. Finally, a dynamic 17O MRS method was developed and then applied to interrogate mitochondrial respiration in isolated perfused hearts in Chapter 4. By using a specially designed closed-loop perfusion system, feasibility and sensitivity of our 17O MRS approach in detecting altered metabolic rate associated with changes in cardiac workload were demonstrated. In combination with kinetic modeling of metabolic H217O production, this high temporal resolution 17O MRS approach has the potential to quantify mitochondrial respiration rate in functional, beating hearts.;In conclusion, the multi-nuclear MRS kinetic analysis methodologies developed in the current thesis provided the opportunity to comprehensively evaluate cardiac energetics under both normal and pathophysiological conditions.