Evidence of Glycolysis Up-Regulation and Pyruvate Mitochondrial Oxidation Mismatch During Mechanical Unloading of the Failing Human Heart: Implications for Cardiac Reloading and Conditioning

Abstract

This study sought to investigate the effects of mechanical unloading on myocardial energetics and the metabolic perturbation of heart failure (HF) in an effort to identify potential new therapeutic targets that could enhance the unloading-induced cardiac recovery. The authors prospectively examined paired human myocardial tissue procured from 31 advanced HF patients at left ventricular assist device (LVAD) implant and at heart transplant plus tissue from 11 normal donors. They identified increased post-LVAD glycolytic metabolites without a coordinate increase in early, tricarboxylic acid (TCA) cycle intermediates. The increased pyruvate was not directed toward the mitochondria and the TCA cycle for complete oxidation, but instead, was mainly converted to cytosolic lactate. Increased nucleotide concentrations were present, potentially indicating increased flux through the pentose phosphate pathway. Evaluation of mitochondrial function and structure revealed a lack of post-LVAD improvement in mitochondrial oxidative functional capacity, mitochondrial volume density, and deoxyribonucleic acid content. Finally, post-LVAD unloading, amino acid levels were found to be increased and could represent a compensatory mechanism and an alternative energy source that could fuel the TCA cycle by anaplerosis. In summary, the authors report evidence that LVAD unloading induces glycolysis in concert with pyruvate mitochondrial oxidation mismatch, most likely as a result of persistent mitochondrial dysfunction. These findings suggest that interventions known to improve mitochondrial biogenesis, structure, and function, such as controlled cardiac reloading and conditioning, warrant further investigation to enhance unloading-induced reverse remodeling and cardiac recovery.

Keywords: cardiac conditioning; cardiac metabolism; left ventricular assist device.

Graphical abstract

Figure 1

Gas Chromatography–Mass Spectroscopy of a Representative Myocardial Sample Before and After LVAD Unloading (Top) Before left ventricular assist device (LVAD) unloading (red); (bottom) after LVAD unloading (green). The insets show the area under the curve ratio of metabolite signals at pre- (pink) and post-LVAD (green) time points. The glycolysis intermediates glucose-6-phosphate (G6P), pyruvate, and lactate increased significantly post-LVAD. Similarly, the branched chain amino acids leucine (leu) and isoleucine (ile) also increased in the post-LVAD unloading sample.

Figure 2

Glycolytic Metabolites in the Nonfailing Donor Heart and the Failing Heart Before and After LVAD Unloading The diagram depicts glycolytic metabolism of glucose derived from glycogen or extracellular glucose. Glycolytic intermediates are increased following left ventricular assist device (LVAD) unloading as exemplified by glucose 1 phosphate, glucose 6 phosphate, pyruvate, and also increased levels of lactate. CoA = coenzyme A; ND = nonfailing donor heart; TCA = tricarboxylic acid.

Figure 3

Krebs (TCA) Cycle Intermediates The center diagram depicts the TCA cycle following pyruvate oxidation, and the graphs show the quantitative comparisons of TCA intermediates in cardiac tissue of NDs and at the pre- and post-LVAD time points. Despite the post-LVAD increase of pyruvate, early TCA cycle intermediates, that is, citrate and α-ketoglutarate, remained unaltered in the post-LVAD unloaded human heart, whereas levels of succinate, fumarate and malate are increased. Abbreviations as in Figure 2.

Figure 4

Amino Acids Were Decreased in the Failing Human Heart Compared With the Normal Heart Left ventricular assist device (LVAD) unloading restored amino acid levels. #Significant difference in comparison to the normal donor heart; p < 0.05. *Significant difference in comparison to the pre-LVAD time point; p < 0.05.

Figure 5

Oxidative Function of Mitochondria in the Nonfailing Donor Heart and in the Advanced Failing Heart at Pre- and Post-LVAD Unloading Time Points Complex I, complex I + II, and complex IV respiration of isolated permeabilized myofibers from failing hearts was significantly lower compared with the nonfailing heart. Mitochondrial oxidative capacity remained significantly reduced after left ventricular assist device (LVAD) unloading. *p < 0.05; **p < 0.01; ***p < 0.001.

Figure 6

Ultrastructural Morphology, Mitochondrial Volume Density, and Mitochondrial DNA Content in the Normal Donor Heart and in the Failing Human Heart Before and After LVAD Unloading (A) Transmission electron microscopy photomicrographs depicting longitudinal rows of mitochondria located in parallel with the sarcomeres/contractile apparatus. The magnification is 15,000×, and the scale bar indicates the length of 2 μm. (B) Higher magnification of mitochondrial ultrastructure at 50,000×. Compared with the normal donor heart, the mitochondria at the pre-LVAD time point were smaller, had disorganized cristae, and had decreased matrix density, whereas the mitochondria after LVAD unloading consistently showed improvement in these specific morphological characteristics. The scale bar indicates the length of 1 μm. Mitochondrial volume density (C) and mitochondrial DNA (mtDNA) content (D) were lower in the failing human heart compared with the nonfailing donor heart. mtDNA levels and volume density remained unchanged after LVAD unloading. The number of large mitochondria in the normal control group were higher than the failing human heart (E). The average size of mitochondria improved slightly after LVAD unloading. *Significant difference in comparison to the normal donor heart; p < 0.05. +Significant difference in comparison to the pre-LVAD time point. Abbreviations as in Figure 2.

Figure 7

Nucleotide Levels in Normal Donor Heart and Advanced Failing Heart Before and After LVAD Unloading The myocardial tissue post-LVAD unloading showed a significant increase in nucleotide levels. #Significant difference in comparison to the normal donor heart; p < 0.05. *Significant difference in comparison to the pre-LVAD time point; p < 0.05.