In vivo metabolic studies in skeletal and cardiac muscle using ¹³C magnetic resonance spectroscopy

Cardiac and skeletal muscle function relies on a continuous energy production via fatty acid metabolism and mitochondrial oxidation of pyruvate, with a fine balance between substrate delivery and utilization. Changes in metabolism are increasingly being implicated as playing an intrinsic role in many diseases, such as diabetes, cancer, and heart failure. Investigating and identifying fundamental metabolic processes are paramount to understanding pathologies. Beyond morphology and functional information, magnetic resonance can provide insights at a metabolic level using spectroscopic techniques such as 13C NMR. The low natural abundance and sensitivity of the 13C nucleus makes 13C NMR in biological systems challenging. In addressing this issue, hyperpolarized methods have emerged as a very promising tool, obtaining signal enhancements up to 10,000 fold. Spectra of hyperpolarized 13C labeled substrates and their downstream metabolic products offer insight into metabolic processes occurring in vivo within seconds after the injection. This thesis focused on the development of MR hyperpolarization methods and applications to study energy metabolism in cardiac and skeletal muscle in vivo. This ranged from development of the experimental frame work to mathematical tools to characterize the observed metabolic processes. Methods were developed to visualize 13C labeling kinetics of acetylcarnitine in vivo in resting skeletal muscle following the administration of hyperpolarized [1-13C]acetate. Two different, novel mathematical models were constructed to quantify the kinetic rate constants. Although separated by two enzymatic reactions, the conversion of acetate to acetylcarnitine was uniquely defined by the enzymatic activity of acetylCoA synthetase (ACS). A 13C MRS protocol was developed and implemented for hyperpolarized studies in the heart, which included selecting appropriate cardiac triggers to align the measurements with the cardiac phase. The 13C label propagation into acetylcarnitine and citrate could be measured in real time in the beating rat heart following the infusion of hyperpolarized [1-13C]acetate, using a newly constructed 13C RF coil which improved the detection sensitivity. The substantial spectral resolution at 9.4T and a triggered shimming and MR acquisition protocol allowed for the detection of citrate for the first time in vivo after injection of hyperpolarized [1-13C]acetate. Mathematical models were successfully extended to include mitochondrial oxidation and analytical expressions were derived to interpret the dynamic 13C labeling of citrate. Cardiac dysfunction is often associated with a shift in substrate preference, while diagnostic methods such as PET provide only information on substrate uptake. The potential of hyperpolarized 13C MRS to measure simultaneously lipid and carbohydrate oxidation was demonstrated in vivo, and the sensitivity of the method to a metabolic perturbation was assessed. Hyperpolarized [1-13C]butyrate a