Measuring changes in substrate utilization in the myocardium in response to fasting using hyperpolarized [1-13C]butyrate and [1-13C]pyruvate

Cardiac dysfunction is often associated with a shift in substrate preference for ATP production. Hyperpolarized (HP) C-13 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-C-13]pyruvate and [1-C-13]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 [C-13] bicarbonate (-48%), [1-C-13]acetylcarnitine (+113%), and [5-C-13]glutamate (-63%), following fasting. Butyrate uptake was unaffected by fasting, as indicated by [1-C-13]butyrylcarnitine. Mitochondrial pseudoketogenesis facilitated the labeling of the ketone bodies [1-C-13]acetoacetate and [1-C-13]beta-hydroxybutyryate, without evidence of true ketogenesis. HP [1-C-13]acetoacetate was increased in fasting (250%) but decreased during pyruvate co-injection (-82%). Combining HP C-13 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.