Assessment of hepatic pyruvate carboxylase activity using hyperpolarized [1-13 C]-l-lactate

Abstract

Purpose: To evaluate the utility of hyperpolarized [1-¹³ C]-l-lactate to detect hepatic pyruvate carboxylase activity in vivo under fed and fasted conditions.

Methods: [1-¹³ C]-labeled sodium L-lactate was polarized using a dynamic nuclear polarizer. Polarization level and the T₁ were measured in vitro in a 3 Telsa MR scanner. Two groups of healthy rats (fasted vs. fed) were prepared for in vivo studies. Each rat was anesthetized and intravenously injected with 60-mM hyperpolarized [1-¹³ C]-l-lactate, immediately followed by dynamic acquisition of ¹³ C (carbon-13) MR spectra from the liver at 3 Tesla. The dosage-dependence of the ¹³ C-products was also investigated by performing another injection of an equal volume of 30-mM hyperpolarized [1-¹³ C]-l-lactate.

Results: T₁ and liquid polarization level of [1-¹³ C]-l-lactate were estimated as 67.8 s and 40.0%, respectively. [1-¹³ C]pyruvate and [1-¹³ C]alanine, [¹³ C]bicarbonate ( ${\text{HCO}}_3^{-}$ ) and [1-¹³ C]aspartate were produced from hyperpolarized [1-¹³ C]-l-lactate in rat liver. Smaller ${\text{HCO}}_3^{-}$ and larger aspartate were measured in the fed group compared to the fasted group. Pyruvate and alanine production were increased in proportion to the lactate concentration, whereas the amount of ${\text{HCO}}_3^{-}$ and aspartate production was consistent between 30-mM and 60-mM lactate injections.

Conclusion: This study demonstrates that a unique biomarker of pyruvate carboxylase flux, the appearance of [1-¹³ C]aspartate from [1-¹³ C]-l-lactate, is sensitive to nutritional state and may be monitored in vivo at 3 Tesla. Because [¹³ C] ${\text{HCO}}_3^{-}$ is largely produced by pyruvate dehydrogenase flux, these results suggest that the ratio of [1-¹³ C]aspartate and [¹³ C] ${\text{HCO}}_3^{-}$ (aspartate/ ${\text{HCO}}_3^{-}$ ) reflects the saturable pyruvate carboxylase/pyruvate dehydrogenase enzyme activities.

Keywords: aspartate; bicarbonate; hyperpolarized 13C; lactate; liver; pyruvate carboxylase; pyruvate dehydrogenase.