Bastiaansen, Jessica

Berthonneche, Corinne

Comment, Arnaud

Schwitter, Juerg

Yoshihara, Hikari Ananda Infinity

Infoscience - École polytechnique fédérale de Lausanne

5706 Probing energy metabolism in the ischemic rat heart with hyperpolarized 13C MRS Hikari A. I. Yoshihara1,2, Jessica A. M. Bastiaansen2,3, Corinne Berthonneche4, Arnaud Comment2,3, and Juerg Schwitter5 1Division of Cardiology, Lausanne University Hospital, Lausanne, Switzerland, 2Center for Biomedical Imaging (CIBM), Lausanne, Switzerland, 3Institute of Physics of Biological Systems, Ecole Polytechnique Fédérale de Lausanne, Switzerland, 4Cardiovascular Assessment Facility, Lausanne University Hospital, Switzerland, 5Division of Cardiology and Cardiac MR Center, Lausanne University Hospital, Switzerland  Background The application of MR methods to follow the biological conversion of hyperpolarized 13C -labelled metabolites provides a non-invasive means to study metabolism in real time.   The high metabolic activity of the heart makes this technique particularly suited to detect the changes in cardiac metabolism known to result from heart failure or ischemic injury [1,2].  Energy metabolism in ischemic rat hearts has been interrogated with hyperpolarized 13C-labelled pyruvate ex vivo [3,4], and its hyperpolarized metabolites have been imaged in the ischemic pig heart in vivo [5]. In both, cases an increase in the conversion to labelled lactate vs. bicarbonate is observed, reflecting the expected decrease in pyruvate oxidation.  As a widely-used laboratory animal, the rat provides an attractive platform for cardiac metabolic studies, benefiting from well-established disease models and an extensive body of work on its physiology and metabolism.    Methods Animal Model.  Male Wistar rats (n=8, 262 ±14g) were anesthetized with isoflurane and intubated.  Catheters were installed in the femoral arteries for blood sampling and invasive blood pressure measurement and in a femoral vein for infusion. Myocardial ischemia was effected via occlusion with a snare installed by passing a suture underneath the left coronary artery.  The two ends of the suture were then threaded through a small piece of tubing, to form the snare around the left coronary artery.  The reversible occlusion was created by tightening the snare and holding it in place with a clip for 15 minutes (no occlusion in control experiments) and hyperpolarized sodium [1-13C]pyruvate was infused as soon as possible afterward.  At the end of the experiment, the heart was stained with Evans blue to assess the extent of the ischemic area.    Polarization. 12 x 10μl glassy beads of a frozen 2.7M sodium [1-13C]pyruvate and 50mM TEMPOL in HOD and glycerol (18% v/v) were polarized with microwave irradiation (197.25 GHz, 55mW) for ~2hr at 1.0K in a custom-built 7T polarizer.  The sample was rapidly dissolved in 6mL of preheated D2O and transferred automatically to a phase separator / infusion pump in the magnet bore [6], where its polarization was ~20%, and 1.7mL was infused IV by remote control.   MR acquisition.  Scanning was formed in a 9.4T 31cm horizontal bore magnet with a VNMRS console (Varian).  A surface coil with a single loop for 1H and two overlapping loops in quadrature for 13C was positioned over the heart.  Breathing motion and blood pressure signals were used for respiratory and cardiac gating.  Coil placement was confirmed with 1H anatomical images.  Following the hyperpolarized 13C infusion, dynamic series of 40 single pulse 13C spectra were acquired (gated, TR≈3s) using a 30º BIR4 pulse centered at ~175 ppm.    Data analysis. Spectral peaks of metabolites were quantified by fitting using Bayes (Washington University).  Areas under curve (AUC) for metabolite hyperpolarized signal time courses were calculated using the spectral signal amplitude and the variable interval between the gated acquisitions.  Ratios of 13C-bicarbonate-to-[1-13C]lactate AUCs acquired before and after the ischemic occlusion were compared.  Student’s t-test was used to assess statistical significance.   Surgical prep & load into scannerShimming & anatomical imaging2 hr 1 hr 15minPolarization Buildup Polarization BuildupBaselinehyperpolarized infusionPost-ischemichyperpolarized infusionOcclusion  Figure 1.  Timeline of hyperpolarized myocardial ischemia experiment  Results and Discussion The conversion of [1-13C]pyruvate to its metabolites [1-13C]lactate, [1-13C]alanine and 13C -bicarbonate was detected before and after myocardial ischemia.  Quantitation of the peak areas revealed a significant reduction in the bicarbonate-to-lactate ratio, 1.49 ± 0.10 fold lower, compared to 0.96 ± 0.23 in control experiments (p < 0.01), consistent with a shift away from oxidative metabolism and to anaerobic, glycolytic metabolism. While the bicarbonate-to-lactate ratio varied between animals, its variation between consecutive infusions of hyperpolarized [1-13C]pyruvate was smaller and more consistent. The mean delay between the removal of the occlusion and the start of the infusion was 105 s (range: 80-120 s); greater differences may be observed in earlier reperfusion. Conclusion This study demonstrates the feasibility of using hyperpolarized 13C MRS to detect metabolic changes in rat myocardial metabolism in vivo after a brief ischemic episode, which provides the opportunity to investigate changes in energy metabolism with other probes.  Additionally, the functional effects of other interventions, such as metabolism-modulating drugs, can be investigated. References [1] J.R. Ussher et al., Heart Metab, 32, 9 (2006).  [2] H. Ashrafian et al., Circulation, 116, 434 (2007).  [3] M.E. Merritt et al., Magn Reson Med, 60, 1029 (2008).  [4] D.R. Ball et al., NMR Biomed, (2013).  [5] A.Z. Lau et al., Magn Reson Med, 69, 1063 (2013).  [6] A. Comment et al., Concept Magn Reson B, 31B, 255 (2007).    Figure 2.  Change in the hyperpolarized bicarbonate-to-lactate ratio before and after 15 min occlusion of the left coronary artery.    BL: baseline, RP: reperfusion    Figure 3.  Evans blue-stained heart slices showing extent of unstained ischemic region Proc. Intl. Soc. Mag. Reson. Med. 22 (2014) 2547.

Probing energy metabolism in the ischemic rat heart with hyperpolarized 13C MRS

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Probing energy metabolism in the ischemic rat heart with hyperpolarized 13C MRS

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