4303

In-Vivo Metabolism Differences of Multiple Body Parts using Hyperpolarized [2-13C]pyruvate

Jian-Xiong Wang¹

¹UT Southwestern Meidcal Center, Dallas, TX, United States

Synopsis

Hyperpolarized [2-¹³C]-pyruvate can assess multiple metabolic pathways within mitochondria to follow the ¹³C label beyond flux through PDH complex and investigate the incorporation of acetyl-coenzyme into different metabolic pathways. This study investigated metabolic products from [2-¹³C]-pyruvate and the differences of LDH and ATL process in heart, liver and kidney.

Introduction

In principle pyruvate can monitor in-vivo mitochondrial metabolism via pyruvate dehydrogenase (PDH), lactate dehydrogenase (LDH), and alanine aminotransferase (ALT). In the case of [1-¹³C]pyruvate, the ¹³C labeling is lost in the conversion of pyruvate to acetyl-CoA. Using hyperpolarized [2-¹³C]pyruvate it’s possible to assess multiple metabolic pathways within mitochondria to follow the ¹³C label beyond flux through PDH complex and investigate the incorporation of acetyl-coenzyme into different metabolic pathways as shown in Figure 1. Hyperpolarized [2-¹³C]pyruvate in-vivo heart and brain metabolism studies was summarized in reference [1]. The purpose of is study was to study these pathways simultaneously in heart, liver and kidney in order to investigate metabolic differences between body parts and provide guidance in protocols design for in-vivo metabolic studies.

Methods

The experiments were performed on a clinical 3T MR750 scanner and the polarization is performed in a SpinLab (both GE Healthcare, Waukesha, WI). After dissolution, the final [2-¹³C]pyruvate solution forms a concentration of 80mM and pH of 7.4. About 3.0-3.5ml were injected manually through a tail vein catheter at a rate of 0.2-0.3ml/s. The MRS was acquired which started immediately following the completion of the injection. 3 axial slices were prescribed to cover the rat heart, liver and kidneys. Wide band RF pulse was used to cover chemical-shift from [2-¹³C]alanine to [2-¹³C]pyruvate. The excitation is repeated every 1.5s which formed an effective TR of 4.5s for each slice. A dual tune rat size volume coil [2, 3] was used for the experiment. Animal was anesthetized with isoflurane and warmed with heated air at 37°. DCA was injected into the tail vein 5 minutes before the hyperpolarized [2-¹³C]pyruvate injection with a dose of 200mg/kg of rat mass to enhance the PDH pathway. To quantitative analysis of chemical components, spectra were decomposed in time domain using Hankel singular value decomposition (hsvd) method. Liver was freeze-clammed for tissue extracted and stable ¹³C and 1H MRS were performed afterwards.

Results

In the result MRS, expected metabolites from different pathways were identified. Figure 2 shows a typical spectrum with two zoomed-in regions. Beside [2-¹³C]lactate, [2-¹³C]-alanine, [1-¹³C]acetylcarnitine, [5-¹³C]glutamate and [1-¹³C]glycerol, a component around 148.5ppm appeared consistently in all experiment, which was also reported in reference [1] and was unidentified. [2-¹³C]alanine and [2-¹³C]lactate appeared in doublet due to C-H coupling and the [1-¹³C]pyruvate doublet is caused by C-C coupling. The results showed that in heart [2-¹³C]alanine concentration is very low. It increases in liver and becomes strongest in kidney. It was also consistent in every rat, that the [2-¹³C]lactate was stronger in kidney, weakest in heart. Figure 3 showed the ratio of metabolite over [2-¹³C]pyruvate and it clearly demonstrated that LDH and ATL are different in different body parts, especially low in heart.

Conclusion

This work successfully observed the metabolic activities in the TCA cycle by detecting [5-¹³C]glutamate and other metabolic pathways in ketonbodies and carnitine through PDH by detecting [1-¹³C]acetylcarnitine and [1-¹³C]acetoacetate. The observation of the differences in different body parts will provide guidance for metabolic research protocol design.

Acknowledgements

The study was supported by grants from the National Institutes of Health (P41-EB015908).

References

[1] Rider OJ, Tyler DJ. Clinical implications of cardiac hyperpolarized magnetic resonance imaging. J Cardiovasc Magn Reson. 2013 Oct 8;15:93. doi: 10.1186/1532-429X-15-93. Review. PubMed PMID: 24103786; PubMed Central PMCID: PMC3819516.

[2] Wang JX, Merritt ME, Sherry D, Malloy CR. A general chemical shift decomposition method for hyperpolarized (13) C metabolite magnetic resonance imaging. Magn Reson Chem. 2016 Aug;54(8):665-73. doi: 10.1002/mrc.4435. PubMed PMID: 27060361; PubMed Central PMCID: PMC5022286.

[3] Wang JX, Merritt ME, Sherry AD, Malloy CR. Accelerated chemical shift imaging of hyperpolarized (13) C metabolites. Magn Reson Med. 2016 Oct;76(4):1033-8. doi: 10.1002/mrm.26286. PubMed PMID: 27373705.

Figures

Figure 1. Pathway of possible metabolic products following the injection of hyperpolarized [2-¹³C]pyruvate.

Figure 2. A typical hyperpolarized [2-¹³C]pyruvate spectrum from rat with two zoomed-in regions. Blue dots represent the data, solid lines show the HSVD decomposed components, and the red dots are the sum of the components with phase. This demonstrate that it fits very well with the experiment data. [2-¹³C]alanine and [2-¹³C]lactate appeared as doublet due to C-H coupling and the [1-¹³C]pyruvate doublet by C-C coupling.

Figure 3. Ratio of metabolites over [2-¹³C]pyruvate. The result shows that the product metabolites [5-¹³C]glutamate and [1-¹³C]acetoacetate have similar values in all organs. In the heart the [1-¹³C]acetylcarnitine has higher concentration followed by liver and kidney. This is reversed for [2-¹³C]lactate and [2-¹³C]alanine which demonstrate lower LDH and ATL than in liver and kidney.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)

4303