Enhancing the [ 13 C]bicarbonate signal in cardiac hyperpolarized [1- 13 C]pyruvate MRS studies by infusion of glucose, insulin and potassium Mette Hauge Lauritzen a *, Christoffer Laustsen a,b , Sadia Asghar Butt a , Peter Magnusson a , Lise Vejby Søgaard a , Jan Henrik Ardenkjær-Larsen a,c,d and Per Åkeson a A change in myocardial metabolism is a known effect of several diseases. MRS with hyperpolarized 13 C-labelled py- ruvate is a technique capable of detecting changes in myocardial pyruvate metabolism, and has proven to be useful for the evaluation of myocardial ischaemia in vivo. However, during fasting, the myocardial glucose oxidation is low and the fatty acid oxidation (b-oxidation) is high, which complicates the interpretation of pyruvate metabolism with the technique. The aim of this study was to investigate whether the infusion of glucose, insulin and potassium (GIK) could increase the myocardial glucose oxidation in the citric acid cycle, reflected as an increase in the [ 13 C]bicarbon- ate signal in cardiac hyperpolarized [1- 13 C]pyruvate MRS measurements in fasted rats. Two groups of rats were in- fused with two different doses of GIK and investigated by MRS after injection of hyperpolarized [1- 13 C]pyruvate. No [ 13 C]bicarbonate signal could be detected in the fasted state. However, a significant increase in the [ 13 C]bicarbon- ate signal was observed by the infusion of a high dose of GIK. This study demonstrates that a high [ 13 C]bicarbonate signal can be achieved by GIK infusion in fasted rats. The increased [ 13 C]bicarbonate signal indicates an increased flux of pyruvate through the pyruvate dehydrogenase enzyme complex and an increase in myocardial glucose oxi- dation through the citric acid cycle. Copyright © 2013 John Wiley & Sons, Ltd. Keywords: hyperpolarization; 13 C; MRS; cardiac metabolism; pyruvate dehydrogenase; glucose; insulin INTRODUCTION The heart has the capability to utilise a variety of different substrates for energy production, such as glucose, fatty acids (FAs), lactate and ketone bodies. Under normal conditions, FAs, catabolised by b-oxidation, account for 60–90% of the total myocardial energy production, whereas glucose metabolised through glycolysis accounts for 10–40% of total energy production (1–3). Several diseases can be characterised by alterations in myocardial substrate metabolism, such as diabetes, hyperthyroidism, ischaemic heart disease and heart failure (4–7). Thus, there is great interest in developing new methods for the evaluation of these metabolic changes. MRS using hyperpolarized 13 C-labelled pyruvate is a technique capable of detecting changes in myocardial metabolism (8,9). After injection, hyperpolarized [1- 13 C]pyruvate is taken up by the cells and the metabolic conversion of [1- 13 C]pyruvate to [1- 13 C]lactate, [1- 13 C]alanine and [ 13 C]bicarbonate through spe- cific enzymes can be detected in real time (10–12). The pyruvate dehydrogenase (PDH) enzyme complex catalyses the irreversible oxidation of pyruvate into acetyl-CoA and CO 2 (in equilibrium with bicarbonate) in the mitochondria. Acetyl-CoA enters the citric acid cycle for the generation of ATP. Thus, the production of [ 13 C]bicarbonate in hyperpolarized 13 C MRS studies represents the flux of pyruvate through PDH and the supply of acetyl-CoA from the glycolytic pathway to the citric acid cycle (11). The activity of PDH is tightly regulated. PDH is inactivated by PDH kinase (PDK) and reactivated by PDH phosphatase (PDP). High intramitochondrial concentration of acetyl-CoA and nicotinamide * Correspondence to: M. H. Lauritzen, Copenhagen University Hospital Hvidovre, Kettegårds Allé 30, 2650 Hvidovre, Denmark E-mail: metteh@drcmr.dk a M. H. Lauritzen, C. Laustsen, S. A. Butt, P. Magnusson, L. V. Søgaard, J. H. Ardenkjær-Larsen, P. Åkeson Danish Research Centre for Magnetic Resonance (DRCMR), Copenhagen Uni- versity Hospital Hvidovre, Hvidovre, Denmark b C. Laustsen MR Research Centre, Institute of Clinical Medicine, Aarhus University Hospital, Aarhus, Denmark c J. H. Ardenkjær-Larsen GE Healthcare, Brøndby, Denmark d J. H. Ardenkjær-Larsen Technical University of Denmark, Department of Electrical Engineering, Kgs Lyngby, Denmark Abbreviations used: ANOVA, analysis of variance; CI, confidence interval; CSI, chemical shift imaging; DCA, dichloroacetic acid; FA, fatty acid; GIK, glucose, insulin and potassium; NADH, nicotinamide adenine dinucleotide hydride; PDH, pyruvate dehydrogenase; PDK, pyruvate dehydrogenase kinase; PDP, pyruvate dehydrogenase phosphatase; PET, positron emission tomography. Research article Received: 17 July 2012, Revised: 7 May 2013, Accepted: 8 May 2013, Published online in Wiley Online Library: 24 June 2013 (wileyonlinelibrary.com) DOI: 10.1002/nbm.2982 NMR Biomed. 2013; 26: 1496–1500 Copyright © 2013 John Wiley & Sons, Ltd. 1496