Results will be of interest to researchers working on understanding metabolic changes associated with cardiac hypertrophy and to those interested in hyperpolarized magnetic resonance spectroscopy of the heart.Left ventricular hypertrophy is closely associated with hypertension and diabetes which afflicts more than 15% of the general populace while showing a higher incidence rate amongst specific groups.1 It has been shown in the literature that there is an increase in the glucose metabolism and decrease in fatty acid metabolism in the hypertrophic heart. In this work, robust identification of this metabolic shift using sodium propionate as a probe is explored using hyperpolarized magnetic resonance spectroscopy.

C57/BL6 mice were used as models for all experiments. Animals were handled in compliance with the UTSW IACUC regulations.

Non-hyperpolarized (bench) Experiments: Mice of age 8 – 14 weeks old were sacrificed by cervical dislocation. Hearts were excised, cleaned and perfused in langendorff mode with krebs henseleit buffer containing [1,6-¹³C₂] glucose, [U-¹³C] mixed fatty acids and varying concentrations of [1-¹³C] sodium propionate (0 – 6 mM). Hearts were perfused for 30 min and freeze clamped in liquid nitrogen. Metabolites were extracted using perchloric acid (PCA) extraction and analyzed using NMR spectroscopy. NMR Spectra were measured using a spectrometer operating at 600 MHz 1H resonance frequency and equipped with either a 10 mm cryogenically cooled probe (Bruker, MA) or 1.5 mm superconducting probe2.

Hyperpolarized Experiments: Hearts were perfused in langendorff mode with krebs henseleit buffer containing [1,6-¹³C₂] glucose, [U-¹³C] mixed fatty acids and a predetermined concentration of [1-¹³C] sodium propionate (based on the results from bench experiments; see figure caption). At the end of 30 min, 4 mM hyperpolarized [1-¹³C] pyruvic acid was injected directly into the heart and ¹³C NMR spectra were acquired in real time. All experiments were carried out in a NMR spectrometer (14.1 T magnet) equipped with a 10 mm cryogenically cooled probe (Bruker, MA).

Mass spectrometry: Hearts were perfused under identical conditions as described above (except for the usage of natural abundance substrates) in langendorff mode and freeze clamped. Pool sizes of citric acid cycle intermediates and associated metabolites were measured using mass spectrometry.

NADH/NAD⁺ Assay: Small fractions of the heart used for PCA extracts were saved for an assay to determine the NADH/NAD⁺ ratio using optical spectroscopy. The assay was carried out according to the manufacturer’s instructions (Abcam, Massachusetts).

NMR spectra of the PCA extracts (Figure 1) of the hearts illustrate the changes in the metabolite concentrations of the heart when exposed to propionate. It can be see that glutamate peaks almost disappear while aspartate, malate and fumarate peaks increase in intensity. The absolute pool sizes of these metabolites as measured by mass spectrometry are shown in Figure 2 (top). The pool sizes of malate and fumarate go up by 50 and 35 times respectively when compared between propionate absent and 3 mM propionate hearts. However, between 0.25 mM and 3 mM propionate perfused hearts, the change in malate and fumarate is 4X and 2.5X only. The changes in pool sizes with respect to incremental changes in propionate concentration are shown in Figure 2 (bottom). Interestingly, these metabolic changes are accompanied by a change in redox state as shown in Figure 3. NADH and NAD+ concentrations measured using optical spectroscopy suggest an increase in the accumulation of NAD+ in the heart upon the introduction of propionate. These changes, however, do not reflect on myocardial energetics since the oxygen consumption is uniform across all conditions (with or without propionate). A significant change in pool sizes of metabolites in the heart can be triggered using sodium propionate even at relatively low concentrations (e.g., 0.25 mM). Higher concentrations of propionate while accentuating the effect do not cause large changes when compared with the “initial burst” provided by 0.25 mM propionate (Figures 1 & 2). Fitting a mathematical model to the isotopomer data3 reveals that using 3 mM propionate in the perfusate results in maximal activation of both pyruvate dehydrogenase and anaplerotic pathways (Figure 4, bottom).

Using the results from bench perfusions (above), experiments carried out with hyperpolarized [1-13C] pyruvic acid perfused mouse hearts shows a massive increase (~22X increase across 4 repeats) in bicarbonate production in the presence of 3 mM propionate as shown in Figure 5.

Propionate activates PDH flux in the presence of long chain fatty acids while simultaneously serving as an anaplerotic substrate. The increase in PDH flux triggered by the presence of propionate can be exploited to probe ailments such as hypertrophy by increasing biological sensitivity.