The development and application of novel magnetic resonance spectroscopic and imaging techniques to assess cardiac energetics and substrate handling in the human heart
Keywords: Hyperpolarised magnetic resonance
Creatine kinase
Cardiac metabolism
Phosphotransfer

Myocardial energy homeostasis requires an interplay between three stages of energy transduction; the choice of substrate oxidised, the rate of mitochondrial oxidative phosphorylation, and finally transfer of adenosine triphosphate (ATP) to sites of disposal via the intermediate phosphocreatine (PCr). Whilst all three elements are fundamental, until recently, non-invasive assessment was limited to 31Phosphorus magnetic resonance spectroscopy (31P-MRS) assessment of the PCr/ATP ratio. This thesis aims to gain a deeper understanding of pathologic energy metabolism by using two novel MRS techniques. Firstly 31P-MRS at the ultra-high field strength of 7T to assess myocardial Pi, and secondly the first in man clinical translation of hyperpolarised 13C MRS, which offers the ability to quantify in real time the metabolism of an administered hyperpolarised 13C containing substrate into downstream 13C containing products. Using these techniques, a more comprehensive assessment of energy metabolism either side of the mitochondria was made in both the diabetic and failing heart.
The pyruvate dehydrogenase complex (PDH) catalyses the irreversible decarboxylation of glycolytically derived pyruvate to Acetyl CoA (Coenzyme A). Flux through this enzyme determines the prevailing rate of glucose oxidation and is highly regulated. Using a hyperpolarised and 13C labelled version of its substrate ([1-13C]pyruvate), PDH flux can be recorded using MRS by monitoring production of [13C]bicarbonate. Chapter 3 presents preclinical work to define the optimal preparation strategy prior to recording [1-13C]pyruvate MRS. This was essential prior to translation into human hyperpolarised imaging studies, to both minimise variability and maximise baseline flux through the PDH complex. Chapter 4 is a human multi-nuclear 31P, 13C and 1H study of cardiac metabolism in diabetes. I present the first human cardiac hyperpolarised 13C MRS study, and show, along with elevated cardiac triglyceride and reduced PCr/ATP, a significant reduction in flux though PDH in diabetes. The switch from fatty acids to glucose oxidation occurring after feeding is also clearly shown in both controls and diabetics, providing the first non-invasive visualisation of the Randle cycle in vivo. The effect size of diabetes on PDH flux is greater than that on PCr/ATP, with only a small group of patients with diabetes needed to show a significant difference, demonstrating the power of the technique, and the magnitude of this physiological metabolic change.
As all cellular energy requiring processes have a unique value for the free energy of ATP hydrolysis (ΔGATP) above which they halt, measurement of ΔGATP would be a powerful tool to understand the thermodynamic consequences of pathological substrate metabolism. However, whilst this is routinely performed in animal experiments, the measurement of myocardial inorganic phosphate (Pi) is necessary, which has previously not been achieved due to the presence of blood pool 2,3-diphoshphoglycerate (2,3-DPG) obscuring the Pi resonance within the 31P spectrum. In Chapter 5, I report results from a novel 31P MRS STimulated Echo Acquisition Mode (STEAM) sequence, which allowed suppression of the blood pool signal, and demonstration of Pi free from contamination. Firstly, the robustness of such an approach is proven, following which, gross stability of Pi/PCr, and myocardial pH (which can be computed from the chemical shift of Pi) in a healthy cohort during dobutamine stress is shown. In chapter 6, pathological energy metabolism in the diabetic heart is further investigated; a rise in resting Pi/PCr is shown, but in contrast to decreases in PCr/ATP, no change is seen during dobutamine stress. Finally, focus turns to dilated cardiomyopathy (DCM). Here a significant rise in Pi/PCr during stress is reported, independent of the degree of underlying systolic dysfunction, which has implications for a change in ΔGATP during physical exertion.
My thesis data chapters conclude with chapter 7, presenting a small amount of metabolic imaging data obtained using a novel hybrid spiral shot 13C sequence and hyperpolarised [1-13C]pyruvate, in which regional variations of PDH flux in the hearts of those with coronary artery disease is shown. This preliminary work confirms the potential for hyperpolarised [13C]bicarbonate production, to be a novel marker of myocardial viability. Collectively, this work uses new MR techniques to study pathological energy metabolism on both sides of mitochondrial oxidative phosphorylation; upstream at the level of the substrate, and downstream in the high energy phosphate pool.