In vivo assessment of β-hydroxybutyrate metabolism in mouse brain using deuterium (2 H) MRS

Abstract

Purpose: To monitor the metabolic turnover of β-hydroxybutyrate (BHB) oxidation using ² H-MRS in conjunction with intravenous administration of ² H labeled BHB.

Methods: Nine-month-old mice were infused with [3,4,4,4]-² H₄ -BHB (d₄ -BHB; 3.11 g/kg) through the tail vein using a bolus variable infusion rate for a period of 90 min. The labeling of downstream cerebral metabolites from the oxidative metabolism of d₄ -BHB was monitored using ² H-MRS spectra acquired with a home-built ² H surface coil on a 9.4T preclinical MR scanner with a temporal resolution of 6.25 min. An exponential model was fit to the BHB and glutamate/glutamine (Glx) turnover curves to determine rate constants of metabolite turnover and to aid in the visualization of metabolite time courses.

Results: Deuterium label was incorporated into Glx from BHB metabolism through the tricarboxylic acid (TCA) cycle, with an increase in the level of [4,4]-² H₂ -Glx (d₂ -Glx) over time and reaching a quasi-steady state concentration of ∼0.6 ± 0.1 mM following 30 min of infusion. Complete oxidative metabolic breakdown of d₄ -BHB also resulted in the formation of semi-heavy water (HDO), with a four-fold (10.1 to ∼42.1 ± 7.3 mM) linear (R² = 0.998) increase in its concentration by the end of infusion. The rate constant of Glx turnover from d₄ -BHB metabolism was determined to be 0.034 ± 0.004 min^-1 .

Conclusion: ² H-MRS can be used to monitor the cerebral metabolism of BHB with its deuterated form by measuring the downstream labeling of Glx. The integration of ² H-MRS with deuterated BHB substrate provides an alternative and clinically promising MRS tool to detect neurometabolic fluxes in healthy and disease conditions.

Keywords: 2H-MRS; TCA; brain; d4-BHB; ketone bodies; metabolism.