Hyperpolarized (13)C magnetic resonance reveals early- and late-onset changes to in vivo pyruvate metabolism in the failing heart

Abstract

Aims: Impaired energy metabolism has been implicated in the pathogenesis of heart failure. Hyperpolarized (13)C magnetic resonance (MR), in which (13)C-labelled metabolites are followed using MR imaging (MRI) or spectroscopy (MRS), has enabled non-invasive assessment of pyruvate metabolism. We investigated the hypothesis that if we serially examined a model of heart failure using non-invasive hyperpolarized [(13)C]pyruvate with MR, the profile of in vivo pyruvate oxidation would change throughout the course of the disease.

Methods and results: Dilated cardiomyopathy (DCM) was induced in pigs (n = 5) by rapid pacing. Pigs were examined using MR at weekly time points: cine-MRI assessed cardiac structure and function; hyperpolarized [2-(13)C]pyruvate was administered intravenously, and (13)C MRS monitored [(13)C]glutamate production; (31)P MRS assessed cardiac energetics [phosphocreatine (PCr)/ATP]; and hyperpolarized [1-(13)C]pyruvate was administered for MRI of pyruvate dehydrogenase complex (PDC)-mediated pyruvate oxidation via [(13)C]bicarbonate production. Early in pacing, the cardiac index decreased by 25%, PCr/ATP decreased by 26%, and [(13)C]glutamate production decreased by 51%. After clinical features of DCM appeared, end-diastolic volume increased by 40% and [(13)C]bicarbonate production decreased by 67%. Pyruvate dehydrogenase kinase 4 protein increased by two-fold, and phosphorylated Akt decreased by half. Peroxisome proliferator-activated receptor-α and carnitine palmitoyltransferase-1 gene expression decreased by a half and a third, respectively.

Conclusion: Despite early changes associated with cardiac energetics and (13)C incorporation into the Krebs cycle, pyruvate oxidation was maintained until DCM developed, when the heart's capacity to oxidize both pyruvate and fats was reduced. Hyperpolarized (13)C MR may be important to characterize metabolic changes that occur during heart failure progression.

Figure 1

An overview of the experimental protocol, applied to each of the pigs (n = 5). The top time line (A) shows that each pig was examined at weekly intervals, whereas the bottom time line (B) shows the magnetic resonance (MR) scans performed at each examination point. (A) After pacemaker implantation, the pig was examined at baseline, before the pacemaker was programmed to 188 b.p.m. Pigs were examined using MR imaging (MRI) and MR spectroscopy (MRS) at weekly intervals, until heart failure developed (∼5 weeks of pacing). After the pig began to display clinical signs of heart failure, a terminal examination was performed, the pig was sacrificed, and myocardial tissue was harvested. (B) Each MR examination consisted of: (i) metabolic preparation, which consisted of a 5 h fast and an oral glucose load given 1 h prior to MR examination; (ii) anaesthesia, cessation of pacing, and positioning in the MR scanner; (iii) Cine-MRI; (iv) infusion of hyperpolarized [2-¹³C]pyrvuate and MRS; (v) ³¹P MRS; and (vi) infusion of hyperpolarized [1-¹³C]pyrvuate and ¹³C MRI.

Figure 2

Changes in cardiac structure and function during the development of dilated cardiomyopathy (DCM), measured using cine-magnetic resonance imaging (MRI). Representative images of a mid-papillary slice in diastole from the same pig, acquired at baseline (A), and in overt DCM (B). An artefact from the right ventricular (RV) pacing lead is visible in both images as indicated by the arrows. Changes in cardiac EF and cardiac index (CI) (C), volumes (D), and diastolic wall thickness (E), in all five pigs. *P < 0.05 compared with baseline; §P < 0.05 compared with the early time point. EDVi, end-diastolic volume index; EDSi, end-systolic volume index; LVMi, LV mass index.

Figure 3

Hyperpolarized ¹³C magnetic resonance spectroscopy (MRS) showing altered [5-¹³C]glutamate production during development of dilated cardiomyopathy (DCM), following infusion of hyperpolarized [2-¹³C]pyruvate. (A) Representative spectra taken from a healthy pig (left), and from the same pig after 3 weeks of pacing, when it had moderate cardiac dysfunction. (B) Representative time courses of the infused [2-¹³C]pyruvate and its conversion into [5-¹³C]glutamate, following quantification of the spectra shown in A. (C) The [5-¹³C]glutamate/[2-¹³C]pyruvate ratio, measured for all pigs during development of DCM.

Figure 4

Hyperpolarized ¹³C magnetic resonance imaging (MRI) results describing alterations to pyruvate dehydrogenase complex (PDC) flux and [¹³C]lactate production with the pathogenesis of dilated cardiomyopathy (DCM), following infusion of hyperpolarized [1-¹³C]pyruvate. (A) Representative pyruvate (Pyr, top), bicarbonate (Bic, middle), and lactate (Lac, bottom) ¹³C MR images taken from the same pig and at weekly intervals during the pacing protocol, until DCM developed. The images displayed for each metabolite were selected from the same, mid-papillary slice and in the same respiratory cycle. Signal intensity in the pyruvate image was scaled based on 15–100% of the maximum pyruvate signal at week 0, whereas the bicarbonate and lactate signal intensities were scaled based on 15–100% of the maximum bicarbonate signal intensity at week 0. (B) Relative changes to PDC flux with DCM in five pigs.

Figure 5

(A and B) Alterations to myocardial proteins involved in carbohydrate metabolism, in control pigs and in pigs with dilated cardiomyopathy (DCM). The blot for p-Akt was normalized to the total Akt-1 protein content. The blot for pyruvate dehydrogenase kinase 4 (PDK4) was normalized to glyceraldehyde 3-phosphate dehydrogenase (GAPDH) protein, run on the same membrane. (C) Alterations to myocardial genes involved in fatty acid oxidation, normalized to GAPDH mRNA, in pigs with DCM. MCT1, monocarboxylate transporter 1; PPARα, peroxisome proliferator-activated receptor-α; CPT1, carnitine palmitoyltransferase-1. *P < 0.05; **P < 0.005.

Figure 6

An overview of LV metabolic, energetic, structural, and functional remodelling, measured non-invasively using magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS), throughout the development of tachycardia-induced dilated cardiomyopathy (DCM). *P < 0.05 compared with baseline; §P < 0.05 compared with the early time point. CI, cardiac index; EDVi, end-diastolic volume index; PCr, phosphocreatine; PDC, pyruvate dehydrogenase complex.