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Background and Objectives
The Krebs cycle is fundamental to cardiac energy production, and is often implicated in energetic imbalances characteristic of heart disease. To date, Krebs cycle flux has been measured using 13C-magnetic resonance spectroscopy with isotopomer analysis; however, this approach is limited to the study of steady-state metabolism only and has limited in-vivo applications. The aim of this work was to assess the feasibility of using hyperpolarised [2-13C]pyruvate as a metabolic tracer to monitor real-time Krebs cycle metabolism directly in vivo.
[2-13C]Pyruvate was hyperpolarised and dissolved to form an 80 mM solution, 1 ml of which was injected over 10 s via a tail vein catheter into an anaesthetised rat positioned in a 7T magnetic resonance scanner. Spectra were acquired for 1 minute following injection with 1 s temporal resolution. The signal was localised to the heart using a surface coil.
Peaks arising from hyperpolarised [2-13C]pyruvate were identified as citrate, glutamate, acetyl-carnitine, lactate and alanine using phantom experiments and 1H-13C correlation nuclear magnetic resonance spectroscopy of tissue extracts. Identified peaks visible with 1 s resolution were analysed.
This result demonstrates the first example of direct monitoring of instantaneous Krebs cycle metabolism in vivo. The entry of [2-13C]pyruvate into the Krebs cycle has been monitored with 1 s temporal resolution. Future experiments utilising hyperpolarised [2-13C]pyruvate in a variety of pathological and physiological conditions will undoubtedly provide useful insights into the mechanisms driving energetic imbalances often expressed in heart disease.