High-fat diet can trigger cardiomyopathies that are associated with obesity-induced co-morbidities (e.g. diabetes, hypertension). High-fat diet can also directly trigger cardiac changes without obesity (e.g. altering cardiac metabolism) and has been shown to increase vulnerability to ischemia-reperfusion (I/R) injury. The underlying mechanism for the latter could be due to changes in mitochondria as it has been reported that larger, fused mitochondria are better able to survive I/R. The aim of this work was to investigate the effects of high-fat diet, without obesity and associated co-morbidities, on cardiac mitochondrial morphology and function.
Male C57BL/6 mice were fed either a normal diet (13% kcal from fat) or a high-fat diet (45% kcal from fat) for 20 weeks. Isolated hearts and isolated mitochondria were used to study mitochondrial morphology and function, respectively. Hearts were fixed and sliced in longitudinal plane for electron microscopic imaging. Isolated mitochondria were supplied with either pyruvate and malate (P/M) or palmitoyl-carnitine and malate (Pal-Car) as substrates. Isolated mitochondria were assessed for oxygen consumption and H2O2 production. Data are presented as mean ± SEM and analysed using unpaired student's t-test.
C57BL/6 mice fed a high-fat diet had elevated blood cholesterol and triglycerides but no evidence of hypertrophy or change in body weight and insulin sensitivity.
Following high-fat feeding interfibrillar mitochondria were smaller, shorter and less dense (n = 4 hearts/group and ∼1000 mitochondria/heart from >10 electron micrographs). The mitochondrial size decreased from 0.682 ± 0.06μm2 in the normal diet group to 0.427 ± 0.021μm2 in the high-fat group (P>0.01). The amount of longer mitochondria (>2.1μm in length) in the high-fat group was lower compared to the normal diet (6.7 ± 0.1 vs. 11.8 ± 1.6% of total mitochondria, P>0.05). The density of interfibrillar mitochondria was higher, 31.4 ± 0.8%, in the normal diet group compared to 26.6 ± 1.6% in the high-fat group (P>0.05).
The oxygen consumption rates of isolated mitochondria were not different between normal diet and high-fat diet in state 3, 4 and 3.5. H2O2 production in state 3.5 was not different when using P/M as a substrate. With Pal-Car H2O2 production in state 3.5 was higher in mitochondria isolated from high-fat fed mice compared to normal diet, (46.0 ± 3.4 vs. 30.9 ± 0.9 pmol/min/mg protein, P>0.05, n=3 isolations/group using 2 mice per isolation).
This works shows changes in mitochondrial morphology and increased H2O2 production in high-fat diet hearts, which could contribute to the increased vulnerability during I/R.