Background. A strong association exists between obesity and heart failure with preserved ejection fraction (HFpEF). Therefore, diet-induced obesity might be a valuable model for HFpEF. However, the optimal dietary composition and feeding duration required to induce HFpEF are not established. A high fat, high fructose diet may be effective for inducing obesity and an HFpEF phenotype due to its well-known high pathogenicity.

Methods and results. Male Wistar rats were fed standard chow (CO), high fat/fructose (HFD) or low fat (LFD) diets for 16 weeks. Following this, cardiac ventricular myocytes and mitochondria were isolated. We measured sarcomere length, cytosolic Ca²⁺ (Indo1, AM) and mitochondrial redox state (autofluorescence of NAD(P)H and FAD), membrane potential (TMRM), and ROS (DCF) in myocytes using an automatic Ionoptix fluorescence setup. Pacing at 0.3 Hz, followed by β-adrenergic stimulation and increasing stimulation rate at 3 Hz for 3 minutes, was used to subject cardiac myocytes to a physiological stress regimen.

We found an increase in [Ca²⁺]_i and Ca²⁺-transient amplitude in LFD (n=90) vs. CO (n=61) and HFD (n=60), along with increased contractility (n=51/70/79 CO/HFD/LFD). Mitochondrial redox state (n=54/49/41 CO/HFD/LFD) and membrane potential (n=62/60/90 CO/HFD/LFD) were unaltered during the protocol, but ROS production was considerably lower in HFD (n=46) vs. CO (n=30) and LFD (n=57).

In isolated mitochondria, we examined mitochondrial respiration, Ca²⁺-retention capacity using Calcium-Green, mitochondrial membrane potential using TMRM, and NAD(P)H levels. In rats fed a HFD, pyruvate/malate (P/M, Complex I)- and succinate (Complex II)-supported respiration increased dramatically, as did uncoupled respiration. LFD induced an increase in P/M supported respiration, succinate respiration, and when uncoupling using 2,4-dinitrophenol at supraphysiological ADP levels only. However, fatty acid supported respiration did not differ between HFD, LFD and controls. Comparable quantities of Ca²⁺-retention, mitochondrial membrane potential, and NAD(P)H levels were also observed.

Conclusion. A high-fat, high fructose diet for 16 weeks boosted mitochondrial complex I and II-driven respiration without pathological alterations such as elevated ROS emission. Furthermore, cardiac myocyte function is preserved, indicating that obesity alone does not cause cardiac myocyte and mitochondrial pathology. Further studies will elucidate how the combination of obesity with hypertension will affect the mechano-energetic coupling phenotype.