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The role of pyruvate dehydrogenase in mediating lipid-induced insulin resistance stands as a central question in the pathogenesis of type 2 diabetes mellitus. Many researchers have invoked the Randle hypothesis to explain the reduced glucose disposal in skeletal muscle by envisioning an elevated acetyl CoA pool arising from increased oxidation of fatty acids. Over the years, in vivo NMR studies have challenged that monolithic view. The advent of the dissolution dynamic nuclear polarization NMR technique and a unique type 2 diabetic rat model provides an opportunity to clarify. Dynamic nuclear polarization enhances dramatically the NMR signal sensitivity and allows the measurement of metabolic kinetics in vivo. Diabetic muscle has much lower pyruvate dehydrogenase activity than control muscle, as evidenced in the conversion of [1-13C]lactate and [2-13C]pyruvate to HCO3- and acetyl carnitine. The pyruvate dehydrogenase kinase inhibitor, dichloroacetate, restores rapidly the diabetic pyruvate dehydrogenase activity to control level. However, diabetic muscle has a much larger dynamic change in pyruvate dehydrogenase flux than control. The dichloroacetate-induced surge in pyruvate dehydrogenase activity produces a differential amount of acetyl carnitine but does not affect the tricarboxylic acid flux. Further studies can now proceed with the dynamic nuclear polarization approach and a unique rat model to interrogate closely the biochemical mechanism interfacing oxidative metabolism with insulin resistance and metabolic inflexibility.

Background Pyruvate dehydrogenase (PDH) and lactate dehydrogenase are essential for adenosine triphosphate production in skeletal muscle. At the onset of exercise, oxidation of glucose and glycogen is quickly enabled by dephosphorylation of PDH. However, direct measurement of PDH flux in exercising human muscle is daunting, and the net effect of covalent modification and other control mechanisms on PDH flux has not been assessed. Purpose To demonstrate the feasibility of assessing PDH activation and changes in pyruvate metabolism in human skeletal muscle after the onset of exercise using carbon 13 (13C) MRI with hyperpolarized (HP) [1-13C]-pyruvate. Materials and Methods For this prospective study, sedentary adults in good general health (mean age, 42 years ± 18 [standard deviation]; six men) were recruited from August 2019 to September 2020. Subgroups of the participants were injected with HP [1-13C]-pyruvate at resting, during plantar flexion exercise, or 5 minutes after exercise during recovery. In parallel, hydrogen 1 arterial spin labeling MRI was performed to estimate muscle tissue perfusion. An unpaired t test was used for comparing 13C data among the states. Results At rest, HP [1-13C]-lactate and [1-13C]-alanine were detected in calf muscle, but [13C]-bicarbonate was negligible. During moderate flexion-extension exercise, total HP 13C signals (tC) increased 2.8-fold because of increased muscle perfusion (P = .005), and HP [1-13C]-lactate-to-tC ratio increased 1.7-fold (P = .04). HP [13C]-bicarbonate-to-tC ratio increased 8.4-fold (P = .002) and returned to the resting level 5 minutes after exercise, whereas the lactate-to-tC ratio continued to increase to 2.3-fold as compared with resting (P = .008). Conclusion Lactate and bicarbonate production from hyperpolarized (HP) [1-carbon 13 {13C}]-pyruvate in skeletal muscle rapidly reflected the onset and the termination of exercise. These results demonstrate the feasibility of imaging skeletal muscle metabolism using HP [1-13C]-pyruvate MRI and the sensitivity of in vivo pyruvate metabolism to exercise states. © RSNA, 2021 Online supplemental material is available for this article.

Purpose: Pyruvate, produced from either glucose, glycogen, or lactate, is the dominant precursor of cerebral oxidative metabolism. Pyruvate dehydrogenase (PDH) flux is a direct measure of cerebral mitochondrial function and metabolism. Detection of [13C]bicarbonate in the brain from hyperpolarized [1‐13C]pyruvate using carbon‐13 (13C) MRI provides a unique opportunity for assessing PDH flux in vivo. This study is to assess changes in cerebral PDH flux in response to visual stimuli using in vivo 13C MRS with hyperpolarized [1‐13C]pyruvate. Methods: From seven sedentary adults in good general health, time‐resolved [13C]bicarbonate production was measured in the brain using 90° flip angles with minimal perturbation of its precursors, [1‐13C]pyruvate and [1‐13C]lactate, to test the hypothesis that the appearance of [13C]bicarbonate signals in the brain reflects the metabolic changes associated with neuronal activation. With a separate group of healthy participants (n = 3), the likelihood of the bolus‐injected [1‐13C]pyruvate being converted to [1‐13C]lactate prior to decarboxylation was investigated by measuring [13C]bicarbonate production with and without [1‐13C]lactate saturation. Results: In the course of visual stimulation, the measured [13C]bicarbonate signal normalized to the total 13C signal in the visual cortex increased by 17.1% ± 15.9% (p = 0.017), whereas no significant change was detected in [1‐13C]lactate. Proton BOLD fMRI confirmed the regional activation in the visual cortex with the stimuli. Lactate saturation decreased bicarbonate‐to‐pyruvate ratio by 44.4% ± 9.3% (p < 0.01). Conclusion: We demonstrated the utility of 13C MRS with hyperpolarized [1‐13C]pyruvate for assessing the activation of cerebral PDH flux via the detection of [13C]bicarbonate production. [ABSTRACT FROM AUTHOR]

The goal of this study was to investigate the origin of brain lactate (Lac) signal in the healthy anesthetized rat after injection of hyperpolarized (HP) [1‐13C]pyruvate (Pyr). Dynamic two‐dimensional spiral chemical shift imaging with flow‐sensitizing gradients revealed reduction in both vascular and brain Pyr, while no significant dependence on the level of flow suppression was detected for Lac. These results support the hypothesis that the HP metabolites predominantly reside in different compartments in the brain (i.e., Pyr in the blood and Lac in the parenchyma). Data from high‐resolution metabolic imaging of [1‐13C]Pyr further demonstrated that Lac detected in the brain was not from contributions of vascular signal attributable to partial volume effects. Additionally, metabolite distributions and kinetics measured with dynamic imaging after injection of HP [1‐13C]Lac were similar to Pyr data when Pyr was used as the substrate. These data do not support the hypothesis that Lac observed in the brain after Pyr injection was generated in other organs and then transported across the blood–brain barrier (BBB). Together, the presented results provide further evidence that even in healthy anesthetized rats, the transport of HP Pyr across the BBB is sufficiently fast to permit detection of its metabolic conversion to Lac within the brain. [ABSTRACT FROM AUTHOR]

Purpose: [13C]Bicarbonate formation from hyperpolarized [1‐13C]pyruvate via pyruvate dehydrogenase, a key regulatory enzyme, represents the cerebral oxidation of pyruvate and the integrity of mitochondrial function. The present study is to characterize the chronology of cerebral mitochondrial metabolism during secondary injury associated with acute traumatic brain injury (TBI) by longitudinally monitoring [13C]bicarbonate production from hyperpolarized [1‐13C]pyruvate in rodents. Methods: Male Wistar rats were randomly assigned to undergo a controlled‐cortical impact (CCI, n = 31) or sham surgery (n = 22). Seventeen of the CCI and 9 of the sham rats longitudinally underwent a 1H/13C‐integrated MR protocol that includes a bolus injection of hyperpolarized [1‐13C]pyruvate at 0 (2 h), 1, 2, 5, and 10 days post‐surgery. Separate CCI and sham rats were used for histological validation and enzyme assays. Results: In addition to elevated lactate, we observed significantly reduced bicarbonate production in the injured site. Unlike the immediate appearance of hyperintensity on T2‐weighted MRI, the contrast of bicarbonate signals between the injured region and the contralateral brain peaked at 24 h post‐injury, then fully recovered to the normal level at day 10. A subset of TBI rats demonstrated markedly increased bicarbonate in normal‐appearing contralateral brain regions post‐injury. Conclusion: This study demonstrates that aberrant mitochondrial metabolism occurring in acute TBI can be monitored by detecting [13C]bicarbonate production from hyperpolarized [1‐13C]pyruvate, suggesting that [13C]bicarbonate is a sensitive in‐vivo biomarker of the secondary injury processes. [ABSTRACT FROM AUTHOR]

Purpose: This study is to investigate time‐resolved 13C MR spectroscopy (MRS) as an alternative to imaging for assessing pyruvate metabolism using hyperpolarized (HP) [1‐13C]pyruvate in the human brain. Methods: Time‐resolved 13C spectra were acquired from four axial brain slices of healthy human participants (n = 4) after a bolus injection of HP [1‐13C]pyruvate. 13C MRS with low flip‐angle excitations and a multichannel 13C/1H dual‐frequency radiofrequency (RF) coil were exploited for reliable and unperturbed assessment of HP pyruvate metabolism. Slice‐wise areas under the curve (AUCs) of 13C‐metabolites were measured and kinetic analysis was performed to estimate the production rates of lactate and HCO3‐. Linear regression analysis between brain volumes and HP signals was performed. Region‐focused pyruvate metabolism was estimated using coil‐wise 13C reconstruction. Reproducibility of HP pyruvate exams was presented by performing two consecutive injections with a 45‐minutes interval. Results: [1‐13C]Lactate relative to the total 13C signal (tC) was 0.21–0.24 in all slices. [13C]HCO3‐/tC was 0.065–0.091. Apparent conversion rate constants from pyruvate to lactate and HCO3‐ were calculated as 0.014–0.018 s−1 and 0.0043–0.0056 s−1, respectively. Pyruvate/tC and lactate/tC were in moderate linear relationships with fractional gray matter volume within each slice. White matter presented poor linear regression fit with HP signals, and moderate correlations of the fractional cerebrospinal fluid volume with pyruvate/tC and lactate/tC were measured. Measured HP signals were comparable between two consecutive exams with HP [1‐13C]pyruvate. Conclusions: Dynamic MRS in combination with multichannel RF coils is an affordable and reliable alternative to imaging methods in investigating cerebral metabolism using HP [1‐13C]pyruvate. [ABSTRACT FROM AUTHOR]

Purpose: Previous cardiac imaging studies using hyperpolarized (HP) [1‐13C]pyruvate were acquired at end‐diastole (ED). Little is known about the interaction between cardiac cycle and metabolite content in the myocardium. In this study, we compared images of HP pyruvate and products at end‐systole (ES) and ED. Methods: A dual‐phase 13C MRI sequence was implemented to acquire two sequential HP images within a single cardiac cycle at ES and ED during successive R‐R intervals in an interleaved manner. Each healthy volunteer (N = 3) received two injections of HP [1‐13C]pyruvate for the dual‐phase imaging on the short‐axis and the vertical long‐axis planes. Spatial distribution of HP 13C metabolites at each cardiac phase was correlated to multiphase 1H MRI to confirm the mechanical changes. Ratios of myocardial HP metabolites were compared between ES and ED. Segmental analysis was performed on the midcavity short‐axis plane. Results: In addition to mechanical changes, metabolic profiles of the heart detected by HP [1‐13C]pyruvate differed between ES and ED. The myocardial signal of [13C]bicarbonate relative to [1‐13C]lactate was significantly smaller at ED than the ratio at ES (p <.05), particularly in mid‐anterior and mid‐inferoseptal segments. The distinct metabolic profiles in the myocardium likely reflect the technical aspects of the imaging approach such as the coronary flow in addition to the cyclical changes in metabolism. Conclusion: The study demonstrates that metabolic profiles of the heart, measured by HP [1‐13C]pyruvate, are affected by the cardiac cycle in which that the data are acquired. [ABSTRACT FROM AUTHOR]