PCr/ATP ratios and mitochondrial function in the heart. A comparative study in humans

Abstract

Cardiac energy status, measured as phosphocreatine (PCr)/adenosine triphosphate (ATP) ratio with 31P-Magnetic Resonance Spectroscopy (31P-MRS) in vivo, is a prognostic factor in heart failure and is lowered in cardiometabolic disease. It has been suggested that, as oxidative phosphorylation is the major contributor to ATP synthesis, PCr/ATP ratio might be a reflection of cardiac mitochondrial function. The objective of the study was to investigate whether PCr/ATP ratios can be used as in vivo marker for cardiac mitochondrial function. We enrolled thirty-eight patients scheduled for open-heart surgery in this study. Cardiac 31P-MRS was performed before surgery. Tissue from the right atrial appendage was obtained during surgery for high-resolution respirometry for the assessment of mitochondrial function. There was no correlation between the PCr/ATP ratio and ADP-stimulated respiration rates (octanoylcarnitine R² < 0.005, p = 0.74; pyruvate R² < 0.025, p = 0.41) nor with maximally uncoupled respiration (octanoylcarnitine R² = 0.005, p = 0.71; pyruvate R² = 0.040, p = 0.26). PCr/ATP ratio did correlate with indexed LV end systolic mass. As no direct correlation between cardiac energy status (PCr/ATP) and mitochondrial function in the heart was found, the study suggests that mitochondrial function might not the only determinant of cardiac energy status. Interpretation should be done in the right context in cardiac metabolic studies.

Figure 1

Volume of interest to acquire a cardiac ³¹P-MRS spectrum. The volume of interest (yellow) is carefully placed in maximum systole to ensure the myocardium of the LV is taken into account, without muscle mass of the pectoralis and diaphragm, and minimizing the volume that is placed in the lungs. (A) sagittal view, (B) coronal view, (C) transversal view.

Figure 2

Cardiac ³¹P-MRS spectrum. Participant with PCr/ATP ratio = 1.239. In black the original cardiac 31P spectrum with first order phase adjustment and in red the fit of the high energy phosphorus metabolite peaks Pi, PDE, 2,3DPG, PCr, γ-ATP, and α-ATP, at their typical resonance frequency. The resonances of PCr and γ-ATP were considered for calculation of PCr/ATP.

Figure 3

No correlation between PCr/ATP and mitochondrial respirometry. PCr/ATP did not correlate with ADP-stimulated respiration, neither upon (A) malate, octanoylcarnitine, glutamate, succinate nor upon (B) malate, pyruvate, glutamate, succinate. PCr/ATP did also not correlate with maximally uncoupled respiration, neither upon (C) malate, octanoylcarnitine, glutamate, succinate nor upon (D) malate, pyruvate, glutamate, succinate.

Figure 4

Mitochondrial oxidative capacity and mitochondrial respiratory chain proteins in permeabilized muscle fibers of the right atrial appendage tissue. Respiration measured after addition of malate (state 2, M), (A) octanoyl (state 2, MO) or (B) pyruvate (state 2, MP), ADP (state 3, 3MO), glutamate (state 3, 3MOG), succinate (state 3, 3MOGS), and FCCP (maximal uncoupled respiration, state U). Data depicts oxygen consumption per mg wet muscle weight per second and is presented as mean ± SEM (standard error of the mean). (C) Relative expression of the protein levels of the OXPHOS complexes. (D) Representative western blot from 8 participants depicting the oxidative phosphorylation complexes.

Figure 5

PCr/ATP is correlated with indexed LV end systolic mass. PCr/ATP did not correlate with indexed LV end diastolic (A) nor systolic (B) volume. There is also no correlation between PCr/ATP and LV Stroke volume (C), LV ejection fraction (D) nor Cardiac output (E). PCr/ATP tended to correlate with End diastolic mass (F) and correlated statistically significant with indexed LV end systolic mass (G).