In vivo assessment of intracellular redox state in rat liver using hyperpolarized [1-¹³C]Alanine

Purpose: The intracellular lactate to pyruvate concentration ratio is a commonly used tissue assay biomarker of redox, being proportional to free cytosolic [NADH]/[NAD⁺]. In this study, we assessed the use of hyperpolarized [1-¹³C]alanine and the subsequent detection of the intracellular products of [1-¹³C]pyruvate and [1-¹³C]lactate as a useful substrate for assessing redox levels in the liver in vivo. Methods: Animal experiments were conducted to measure in vivo metabolism at baseline and after ethanol infusion. A solution of 80-mM hyperpolarized [1-¹³C]alanine was injected intravenously at baseline (n = 8) and 45 min after ethanol infusion (n = 4), immediately followed by the dynamic acquisition of ¹³C MRS spectra. Results: In vivo rat liver spectra showed peaks from [1-¹³C] alanine and the products of [1-¹³C]lactate, [1-¹³C]pyruvate, and ¹³C-bicarbonate. A significantly increased ¹³C-lactate/¹³C-pyruvate ratio was observed after ethanol infusion (8.46 ± 0.58 at baseline versus 13.58 ± 0.69 after ethanol infusion; P < 0.001) consistent with the increased NADH produced by liver metabolism of ethanol to acetaldehyde and then acetate. A decrease in ¹³C-bicarbonate production was also noted, potentially reflecting ethanol-induced mitochondrial redox changes. Conclusion: A method to measure in vivo tissue redox using hyperpolarized [1-¹³C]alanine is presented, with the validity of the proposed ¹³C-pyruvate/¹³C-lactate metric tested using an ethanol challenge to alter liver redox state. Magn Reson Med 77:1741–1748, 2017.