Beyond glucose: metabolic substrate provision enhances cardiac energetics, function and stability ex vivo: a simple recipe for healthier hearts?

AIMS: In 1932, Hans Krebs and Kurt Henseleit introduced a bicarbonate-based solution for liver perfusion containing only 5.55 mM glucose. This bicarbonate-based Krebs-Henseleit buffer (KHB) has been frequently modified over many years but typically includes only glucose as a substrate, often to hyperglycaemic levels. However, despite its popularity, hearts perfused under these conditions are close to the limits of substrate and oxygen availability and unable to sustainably respond to increased demand. The aim of the present study was to compare the function, stability and energetics of hearts perfused with standard glucose-containing KHB with those perfused with a Full Metabolic Buffer (FMB) containing a physiologically appropriate range of substrates, including (in mM): fatty acids (0.4), lactate (1), pyruvate (0.1), glucose (5), glutamate (0.5), β-hydroxybutyrate (4), and insulin (5 mIU/L). METHODS AND RESULTS: Isolated rat hearts were perfused with either KHB or FMB, followed by isoprenaline (1 μM for 20 mins) to induce stress. Cardiac function was measured via an intraventricular balloon, while energetics and metabolism were assessed with 31P and 1H NMR spectroscopy while action potential/calcium transients were assessed with cardiac optical mapping. An adapted FMB was also developed for culturing isolated cardiomyocytes, and glycolytic flux in these cells was measured with 2H NMR spectroscopy. In hearts perfused with FMB, basal left ventricular developed pressure was higher (160 ± 7 mmHg vs. 126 ± 6 mmHg in KHB), PCr/ATP ratio was elevated (1.67 ± 0.1 vs. 1.28 ± 0.05), Gibbs free energy of ATP hydrolysis was increased -2.9 ± 0.3 kJ/mol more negative, and mitochondrial membrane potential was more polarized by -19 ± 7 mV. Under stress, Langendorff-perfused FMB hearts maintained contractile function with fewer arrhythmias and maintained higher PCr/ATP ratio. The metabolic profile was significantly different between hearts perfused with either buffer, with higher levels of key mitochondrial metabolites such as succinate and fumarate in FMB hearts. FMB supressed pacing induced calcium transient duration alternans compared to KHB perfused hearts. CONCLUSION: Compared to KHB, FMB provides substantial advantages for cardiac energetics, metabolism, and function, and is therefore a more appropriate buffer to use in cardiovascular metabolism studies.