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Abstract
Cardiac dysfunction is often associated with a shift in substrate preference for ATP production. Hyperpolarized (HP) 13C magnetic resonance spectroscopy (MRS) has the unique ability to detect real-time metabolic changes in vivo due to its high sensitivity and specificity. Here a protocol using HP [1-13C]pyruvate and [1-13C]butyrate is used to measure carbohydrate versus fatty acid metabolism in vivo. Metabolic changes in fed and fasted Sprague Dawley rats (n = 36) were studied at 9.4 T after tail vein injections. Pyruvate and butyrate competed for acetyl-CoA production, as evidenced by significant changes in [13C]bicarbonate (−48%), [1-13C]acetylcarnitine (+113%), and [5-13C]glutamate (−63%), following fasting. Butyrate uptake was unaffected by fasting, as indicated by [1-13C]butyrylcarnitine. Mitochondrial pseudoketogenesis facilitated the labeling of the ketone bodies [1-13C]acetoacetate and [1-13C]β-hydroxybutyryate, without evidence of true ketogenesis. HP [1-13C]acetoacetate was increased in fasting (250%) but decreased during pyruvate co-injection (−82%). Combining HP 13C technology and co-administration of separate imaging agents enables noninvasive and simultaneous monitoring of both fatty acid and carbohydrate oxidation. This protocol illustrates a novel method for assessing metabolic flux through different enzymatic pathways simultaneously and enables mechanistic studies of the changing myocardial energetics often associated with disease.
Highlights
New techniques that can measure substrate competition will serve an important purpose in establishing which model of myocardial metabolism is most accurate
Acetyl-CoA is the metabolite formed at the crossroads between lipid and carbohydrate metabolism and it is located at the entry of the tricarboxylic acid (TCA) cycle (Fig. 1)
Metabolism of hyperpolarized [1-13C] butyrate provides an abundance of metabolic information about its exchange with butyrylCoA via the formation of [1-13C]butyrylcarnitine, TCA cycle turnover via the formation of [5-13C]citrate, the 13C transfer rate between α -ketoglutarate and glutamate through the evolution of the [5-13C]glutamate signal, the enzymatic activity of carnitine acetyl transferase (CAT) via the formation of [1-13C]acetylcarnitine, and a process referred to as pseudoketogenenesis[24,25], via the formation of [1-13C]β -hydroxybutyrate (BHB) and [1-13C]acetoacetate (AcAc)
Summary
New techniques that can measure substrate competition will serve an important purpose in establishing which model of myocardial metabolism is most accurate. Acetyl-CoA is the metabolite formed at the crossroads between lipid and carbohydrate metabolism and it is located at the entry of the tricarboxylic acid (TCA) cycle (Fig. 1). It is produced from fatty acids via β -oxidation and from carbohydrates through the glycolytic pathway, via the intramitochondrial pyruvate dehydrogenase (PDH) complex. The considerable advantage of the hyperpolarized 13C MR method is that substrate selection is monitored by the appearance of specific metabolites produced by pathways leading to acetyl-CoA production and its subsequent entry into the TCA cycle proper. The aim of this study was to measure the change in substrate utilization, in vivo and in real-time, in response to an induced shift in metabolism using the co-administration of hyperpolarized [1-13C]pyruvate and [1-13C]butyrate
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