Exercise training after myocardial infarction increases survival but does not prevent adverse left ventricle remodeling and dysfunction in high-fat diet fed mice

Abstract

Aims: Aerobic exercise is an important component of rehabilitation after cardiovascular injuries including myocardial infarction (MI). In human studies, the beneficial effects of exercise after an MI are blunted in patients who are obese or glucose intolerant. Here, we investigated the effects of exercise on MI-induced cardiac dysfunction and remodeling in mice chronically fed a high-fat diet (HFD).

Main methods: C57Bl/6 male mice were fed either a standard (Chow; 21% kcal/fat) or HFD (60% kcal/fat) for 36 weeks. After 24 weeks of diet, the HFD mice were randomly subjected to an MI (MI) or a sham surgery (Sham). Following the MI or sham surgery, a subset of mice were subjected to treadmill exercise.

Key findings: HFD resulted in obesity and glucose intolerance, and this was not altered by exercise or MI. MI resulted in decreased ejection fraction, increased left ventricle mass, increased end systolic and diastolic diameters, increased cardiac fibrosis, and increased expression of genes involved in cardiac hypertrophy and heart failure in the MI-Sed and MI-Exe mice. Exercise prevented HFD-induced cardiac fibrosis in Sham mice (Sham-Exe) but not in MI-Exe mice. Exercise did, however, reduce post-MI mortality.

Significance: These data indicate that exercise significantly increased survival after MI in a model of diet-induced obesity independent of effects on cardiac function. These data have important translational ramifications because they demonstrate that environmental interventions, including diet, need to be carefully evaluated and taken into consideration to support the effects of exercise in the cardiac rehabilitation of patients who are obese.

Keywords: Exercise; Glucose intolerance; High-fat diet; Myocardial infarction; Obesity.

Figure 1.. Experimental design.

Six-week-old C57BL/6 mice were placed on a HFD for 24 weeks then subjected to Myocardial Infarction. Four weeks after MI, mice from the Exe groups were subjected to treadmill exercise training (EXE) for 8 weeks. Body composition assessment was performed 2 weeks before MI, 4 and 12 weeks post-MI. Glucose tolerance tests (GTT), and Insulin tolerance tests (ITT) were performed 4 and 12 weeks post-MI. Echocardiography was performed 2 weeks before MI, and at 12 weeks post MI. At 12 weeks post MI, mice were euthanized, and tissues were collected for assessment of gene expression.

Figure 2.. Exercise increased survival and did not affect body composition or glucose metabolism post-MI.

(A) Survival curve after the start of exercise training, at 4 weeks post MI with initial and final n in Chow-fed (n=11, n=11), Sham-Sed (n=9, n=8), MI-Sed (n=10, n=5), Sham-Exe (n=4, n=4), and MI-Exe (n=13, n=11). The body composition was assessed by Echo MRI. (B) Body weight, (C) % fat mass, and (D) % lean mass at 4 and 12 weeks post MI in Chow-fed (n=10), Sham-Sed (n=8), MI-Sed (n=5), Sham-Exe (n=4), and MI-Exe (n=10). Whole-body glucose homeostasis was assessed by insulin tolerance tests (ITT), and glucose tolerance test (GTT). (E) GTT excursion curve at 11 weeks post MI. (F) Glucose tolerance test (GTT) area under curve (AUC) at 4 and 11-weeks post MI in Chow-fed (n=5), Sham-Sed (n=8), MI-Sed (n=5), Sham-Exe (n=4, n=4), and MI-Exe (n=13, n=11). (G) ITT excursion curve at 12 weeks post MI. (H) AUC of Insulin tolerance test (ITT) at 4, and 12 weeks post MI. (I) Overnight fasting glucose at 4, and 12 weeks post MI. Data are presented as mean ± S.E.M. Survival curves were compared using Logrank test, and $ symbols represent significant difference vs all other groups. One-way ANOVA was used with Tukey’s multiple comparisons tests. Values of p<0.05 were considered statistically significant.; (a) Symbols represent difference vs. Chow-fed mice, (c) Symbols represent difference vs MI-Sed, and † symbols represent difference vs 4 weeks post MI.

Figure 3.. Exercise training did not improve MI-induced cardiac dysfunction and remodeling in obese mice.

Cardiac function and structure were measured by (A) ejection fraction, (B) fraction shortening, (C) end systolic diameter, (D) end diastolic diameter, (E) end systolic volume, (F) end diastolic volume, and (G) left ventricle mass before MI in Chow-fed (n=11) and HFD (n=28), and 12 weeks post MI in Sham-Sed (n=8), MI-Sed (n=5), Sham-Exe (n=4), and MI-Exe (n=11). Data are presented as mean ± S.E.M. One-way ANOVA was used with Tukey’s multiple comparisons tests. Kaplan–Meyer survival curve was plotted to identify the survival rate post-MI. Values of p<0.05 were considered statistically significant, and *represent difference vs Chow-fed pre-MI; # represent difference vs HFD pre-MI; Symbols represent difference vs. Chow-fed mice (a), difference vs Sham-Sed (b), and difference vs Sham-Exe (d).

Figure 4.. Exercise training did not protect against the development of MI-induced cardiac fibrosis.

Masson’s Trichrome staining was used to measure fibrosis. (A) The % of fibrosis is represented in Chow-fed (n=10), Sham-Sed (n=3), MI (n=6), Sham-Exe (n=4), and MI-Exe (n=6). Representative heart images are shown below the bar graph. Data are presented as mean ± S.E.M. One-way ANOVA was used with Tukey’s multiple comparisons tests. Values of p<0.05 were considered statistically significant. Symbols represent difference vs. Chow-fed mice (a), difference vs Sham-Sed (b), and difference vs Sham-Exe (d).

Figure 5.. Exercise did not prevent MI-induced increase in cardiac remodeling gene expression.

Quantitative PCR (qPCR) was performed on heart tissue isolated from mice that were sacrificed at 42 weeks of age (12 weeks post-MI). Genes (expressed as arbitrary units; A.U.) related to (A) cardiac function, and (B) remodeling were accessed in Chow-fed (n=5–11), Sham-Sed (n=8), MI-Sed (n=5), Sham-Exe (n=3), and MI-Exe (n=8). Data are presented as mean ± S.E.M. One-way ANOVA was used with Tukey’s multiple comparisons tests. Values of p<0.05 were considered statistically significant. Symbols represent difference vs. Chow-fed mice (a), difference vs Sham-Sed (b), difference vs MI-Sed (c), difference vs Sham-Exe (d), and difference vs. MI-Exe mice (e).

Figure 6.. Exercise did not prevent MI-induced increase in cardiac inflammation gene expression and increased ER stress genes.

Quantitative PCR (qPCR) was performed on heart tissue isolated from mice that were sacrificed at 42 weeks of age (12 weeks post-MI). Genes (expressed as arbitrary units; A.U.) related to (A) fibrosis, (B) inflammation were accessed in Chow-fed (n=5–11), Sham-Sed (n=8), MI-Sed (n=5), Sham-Exe (n=4), and MI-Exe (n=11). Data are presented as mean ± S.E.M. One-way ANOVA was used with Tukey’s multiple comparisons tests. Values of p<0.05 were considered statistically significant. Symbols represent difference vs. Chow-fed mice (a), difference vs Sham-Sed (b), and difference vs Sham-Exe (d).