Decreased ATP production and myocardial contractile reserve in metabolic heart disease

Abstract

Metabolic syndrome is a cluster of obesity-related metabolic abnormalities that lead to metabolic heart disease (MHD) with left ventricular pump dysfunction. Although MHD is thought to be associated with myocardial energetic deficiency, two key questions have not been answered. First, it is not known whether there is a sufficient energy deficit to contribute to pump dysfunction. Second, the basis for the energy deficit is not clear. To address these questions, mice were fed a high fat, high sucrose (HFHS) 'Western' diet to recapitulate the MHD phenotype. In isolated beating hearts, we used ³¹P NMR spectroscopy with magnetization transfer to determine a) the concentrations of high energy phosphates ([ATP], [ADP], [PCr]), b) the free energy of ATP hydrolysis (∆G_~ATP), c) the rate of ATP production and d) flux through the creatine kinase (CK) reaction. At the lowest workload, the diastolic pressure-volume relationship was shifted upward in HFHS hearts, indicative of diastolic dysfunction, whereas systolic function was preserved. At this workload, the rate of ATP synthesis was decreased in HFHS hearts, and was associated with decreases in both [PCr] and ∆G_~ATP. Higher work demands unmasked the inability of HFHS hearts to increase systolic function and led to a further decrease in ∆G_~ATP to a level that is not sufficient to maintain normal function of sarcoplasmic Ca²⁺-ATPase (SERCA). While [ATP] was preserved at all work demands in HFHS hearts, the progressive increase in [ADP] led to a decrease in ∆G_~ATP with increased work demands. Surprisingly, CK flux, CK activity and total creatine were normal in HFHS hearts. These findings differ from dilated cardiomyopathy, in which the energetic deficiency is associated with decreases in CK flux, CK activity and total creatine. Thus, in HFHS-fed mice with MHD there is a distinct metabolic phenotype of the heart characterized by a decrease in ATP production that leads to a functionally-important energetic deficiency and an elevation of [ADP], with preservation of CK flux.

Keywords: Contractile function; Heart failure; Metabolic syndrome; Metabolism; Obesity.

Fig. 1

Diastolic function is impaired while contractile function is preserved in HFHS- vs. CD-fed hearts under low workload conditions. LV function was assessed by the isovolumic Langendorff method over a range of LV volumes while hearts were paced at 450 bpm. For any given LV volume, end-diastolic pressure (EDP) was higher in HFHS-vs. CD-fed mice, indicating diastolic dysfunction (Panel A). For HFHS-fed hearts the end-systolic pressure (ESP) - volume relationship was steeper (Panel B), but the maximal developed pressures (Panel C) and the relationship between developed pressure (DevP) and end-diastolic pressure (EDP) (Panel D) were similar to those in CD-fed hearts. For all panels data are shown as mean ± SEM; open circles = CD-fed mice and filled circles = HFHS-fed mice; n = 8 in each group; * = p < 0.05.

Fig. 2

Contractile reserve is decreased in isolated perfused hearts from HFHS-fed mice. LV contractile function was assessed at 4 levels of progressively increasing myocardial work demand. Hearts were paced at heart rate (HR) 450 bpm at baseline, and paced at 600 bpm and/or Ca²⁺ was raised (4 mM) to increase work demand and in competent hearts, contractile performance. With increased work demand, the LV end-diastolic pressure (EDP) increased more in hearts from HFHS-fed (vs. CD-fed) mice (Panel A). At baseline demand (2 mM Ca2+, 450 bpm), end-systolic pressure (ESP) (Panel B), developed pressure (DevP) (Panel C) and the rate x pressure product (RPP) (Panel D) were all similar in HFHS- and CD-fed hearts. However, with increased work demand all three measures failed to increase appropriately. For all panels data are shown as mean ± SEM; open circles = CD-fed mice and filled circles = HFHS-fed mice; n = 8 in each group; * = p < 0.05.

Fig. 3

Energetic function is impaired in isolated perfused hearts from HFHS-fed mice. Concurrent with hemodynamic measurements at progressively increasing levels of work demand (Fig. 2), myocardial high energy phosphates were measured using ³¹P NMR spectroscopy. Stack of representative ³¹P NMR spectra at each stage of the protocol is shown in Panel A (CD) and Panel D (HFHS). ATP concentrations ([ATP]) were similar in HFHS- and CD-fed hearts at all work demands (Panel B). However, in HFHS hearts the concentration of phosphocreatine (PCr; Panel C) and the PCr/ATP ratio (Panel E) were lower at baseline and at all workloads, reflecting an impairment in energy reserve. Calculated cytosolic free [ADP] was higher in HFHS- vs. CD-fed hearts at baseline and all increased workloads (Panel F). The energy available for chemical work in ATP is expressed by the free energy of ATP hydrolysis (ΔG_~ATP). As ΔG_ATP is a negative number due to the exergonic nature of ATP hydrolysis, the absolute value is depicted for the sake of clarity. |ΔG_~ATP| was lower in HFHS-fed hearts at baseline and all stages of work demand (Panel G). The lack of energy reserve in HFHS group is even better visualized when |ΔG_~ATP| is plotted against actual work performed, RPP (Panel H). In panels G and H, the dotted horizontal line at a |ΔG_~ATP| of 52 kJ/mol is the value required for normal function of sarcoplasmic reticulum calcium ATP-ase. For all panels data are shown as mean ± SEM; open circles = CD-fed mice and filled circles = HFHS-fed mice; n = 8 in each group; * = p < 0.05.

Fig. 4

Representative ³¹P NMR magnetization transfer spectra. Spectra were collected from CD and HFHS hearts at 0 s (M₀) and 4.8 s (M∞) after a saturating pulse was applied to γ-P of the ATP. The peaks are assigned (from left to right) as Pi, PCr, γ-ATP, α-ATP and β-ATP. Note the disappearance of the γ-ATP peak (saturated) at M∞. The exchange of the saturated [γ-P] between ATP and Pi resulted in reduced Pi peak area (decrease is larger in CD heart than in HFHS heart reflecting higher rate of ATP synthesis). Similarly, the decrease in PCr peak area reflects the CK forward flux. These measurements were performed at baseline workload with 2 mM [Ca²⁺] in the perfusate and paced at 450 bpm.