Abstract 3560: Use of Hyperpolarized Magnetic Resonance for Non-Invasive Observation of Metabolic Regulation

Abstract

Hyperpolarized magnetic resonance (HP MR) has enabled real time visualization of in vivo metabolism. In this study, we postulated that HP MR could also non-invasively provide a measure of metabolic regulation. We focused on regulation of pyruvate dehydrogenase (PDH), a highly controlled enzyme that catalyzes the oxidation of pyruvate to acetyl CoA and CO2/HCO3-. We compared PDH flux in conditions of normal and attenuated enzyme activity, and in the presence of normal and augmented Krebs cycle flux, to determine the contributions of PDH activity and end product inhibition to enzyme regulation. Six rats were examined in the fed and fasted states (to modulate PDH activity), with 40 μmol HP 13C1-pyruvate alone and 40 μmol HP pyruvate co-infused with 40 μmol malate (to manipulate Krebs cycle flux/acetyl CoA uptake). HP tracer was infused into the rats in an MR scanner and cardiac spectra were acquired every second for 1 min. Conversion of pyruvate to 13HCO3-was monitored and the 13HCO3-/pyruvate ratio was used as a marker of PDH flux. Infusion of malate increased PDH flux by 31% compared with pyruvate alone, indicating that removal of acetyl CoA by incorporation into the Krebs cycle increased PDH flux. PDH flux was 57% lower in fasted rats injected with pyruvate alone compared with fed rats, and did not change with malate co-infusion. Here, low PDH activity prevented additional enzyme flux. These results suggest that end product inhibition limits fed state PDH flux, whereas PDH activity regulates pyruvate oxidation in the fasted state. In conclusion, this study has provided evidence that HP MR may be useful to obtain details of metabolic regulation, rather than just reflecting metabolic state. Figure 1 Bicarbonate/pyruvate ratio in fed and fasted rats, following an injection of pyruate or pyruate plus malate. In fed rats, co-infusion of malate increased PDH flux by 31% compared with injection of pyruvate alone (*p=0.02). Fasting reduced PDH flux by 57% (**p=0.002) following injection of pyruvate alone. Co-infusion with malate did not affect PDH flux in fasted rats.