Exercise training reduces ventricular arrhythmias through restoring calcium handling and sympathetic tone in myocardial infarction mice

Figure 1. Cross‐sectional findings of the left ventricles at the papillary muscle level and cardiopulmonary function. (A) In Masson's trichrome staining images of the left ventricle (LV), the blue thinning region indicates the infarct area. (B) There was no difference in the infarct area between the three MI groups (MI‐Sed group: 40.1 ± 1.8% vs. MI‐Ex group: 39.4 ± 1.4% vs. MI‐βb: 39.3 ± 2.7%; n = 8/group). *P < 0.05, versus Sham; #P < 0.05, versus MI‐Sed. (C) Oxygen consumption at peak exercise (Peak‐VO2) was decreased in MI‐Sed and MI‐βb groups, and was restored in MI‐Ex, compared with Sham (Sham, 97.2 ± 2.3; MI‐Sed, 83.8 ± 3.7; MI‐Ex, 95.7 ± 2.4; MI‐βb, 86.3 ± 1.0, NmL/min/kg) (n = 5/group in Sham, MI‐Sed, and MI‐βb; n = 6 in MI‐Ex).

Figure 2. The fibrosis in non‐infarcted area of the left ventricles. (A, B and C) There were no significant differences in the gene expression levels of Col1a1, Col3a1, and TGF‐β1 between all groups. (D) In Masson's trichrome staining images of non‐infarcted area (at 20 × magnification), the regions stained blue indicate collagen deposition. (E) There was no obvious difference in the collagen deposition area between groups in non‐infarcted area (Sham, 2.86 ± 0.21%; MI‐Sed, 4.53 ± 0.40%; MI‐Ex, 3.75 ± 0.67%; MI‐βb, 3.62 ± 0.40%; n = 6/group).

Figure 3. Representative images of echocardiographic analysis. The representative two‐dimensional M‐mode images of the left ventricular short‐axis view show thinning of the anteroseptal wall, enlargement of the left ventricle, and reduced left ventricular systolic function in all MI groups compared with Sham.

Figure 4. Heart rate variability (HRV) analysis in the time and frequency domains. (A) Mean heart rate was significantly lower in MI‐βb compared with the other groups (Sham, 550 ± 17 beats per min (bpm); MI‐Sed, 563 ± 51 bpm; MI‐Ex, 554 ± 29 bpm; MI‐βb, 476 ± 35 bpm). (B, C) In time‐domain parameters of HRV analysis, MI‐βb demonstrated prolonged mean R–R intervals (Sham, 114.1 ± 4.2 ms; MI‐Sed, 112.2 ± 3.8 ms; MI‐Ex, 115.4 ± 4.1 ms; MI‐βb, 132.9 ± 3.6 ms) and standard deviation of R–R intervals (SDNN; Sham, 32.6 ± 2.0 ms; MI‐Sed, 28.0 ± 3.0 ms; MI‐Ex, 29.8 ± 5.4 ms; MI‐βb, 46.7 ± 5.2 ms). (D) The high frequency power (HF) in frequency domain analysis showed a trend towards a decrease in MI groups (Sham, 0.068 ± 0.0059 s²/Hz; MI‐Sed, 0.057 ± 0.0096 s²/Hz; MI‐Ex, 0.057 ± 0.0021 s²/Hz; MI‐βb, 0.050 ± 0.0035 s²/Hz); (E) The low frequency power (LF) has no obvious difference (Sham, 0.19 ± 0.07 s²/Hz; MI‐Sed, 0.16 ± 0.01 s²/Hz; MI‐Ex, 0.15 ± 0.002 s²/Hz; MI‐βb, 0.17 ± 0.01 s²/Hz). (F) The LF/HF ratio, indicating the sympathetic tone activity, was significantly increased in MI‐Sed compared with Sham, and it was restored in MI‐Ex and MI‐βb (Sham, 2.60 ± 0.90 s²/Hz; MI‐Sed, 3.92 ± 0.38 s²/Hz;…

Figure 5. Representative images of the spontaneous ventricular arrhythmic events including premature ventricular contractions (PVC) and ventricular tachycardias (VT) in every group (The arrow indicates PVC, and the star indicates VT). (A) in Sham; (B) in MI‐Sed; (C) in MI‐Ex; (D) in MI‐βb. (E) Mice in MI‐Sed showed an increased incidence of PVCs compared with Sham, while exercise training and β‐blocker treatment effectively reduced the occurrence (Sham, 148 ± 143 counts/12 h; MI‐Sed, 739 ± 377 counts/12 h; MI‐Ex, 109 ± 44 counts/12 h; MI‐βb, 172 ± 102 counts/12 h, n = 6/group). (F) VTs were frequently observed in 3 of 6 mice (50%) in MI‐Sed and 1 of 6 mice (17%) in MI‐Ex. The longest duration of VT was 22 seconds in a MI mouse. However, no VT was observed in Sham and MI‐βb (n = 6/group). *P < 0.05, versus Sham; #P < 0.05, versus MI‐Sed.

Figure 6. Gene expression levels analyzed by real‐time PCR. (A) The mRNA expression of Nppb (BNP) was significantly higher in MI‐Sed than Sham, and was significantly lower in MI‐Ex and MI‐βb than MI‐Sed. (B–F) The expression levels of MHC α/β, β1‐AR, Grk5, β2‐AR, and Chrm2 showed no obvious changes. (G–I) In calcium handling related genes, there were no obvious differences in the expressions of Ryr2 and PLN, while Atp2a2 (SERCA2a) was significantly lower in MI‐Sed than Sham, and was restored in MI‐Ex and MI‐βb (n = 8/group). *P < 0.05, versus Sham; #P < 0.05, versus MI‐Sed.

Figure 7. Western blotting analysis. (A) The phosphorylated RyR2 at Serine 2814 site was significantly increased in MI‐Sed compared with Sham and was decreased in MI‐Ex and MI‐βb. There was no change in the expression of phosphorylated RyR2 at Serine 2808 site or total RyR2. (B) The protein expression of SERCA2a in MI‐Sed was significantly decreased compared with Sham and was recovered in MI‐Ex and MI‐βb. (C) The expression of p‐PLN/PLN showed slightly increased in MI‐ Ex, but the differences were not significant between all groups. (D) Although both the expressions of TNI and p‐TNI were decreased in MI groups compared with Sham, the ratio of p‐TNI to TNI showed no obvious difference. (E) The expression level of p‐CaMKII was obviously increased in MI‐Sed, and it was decreased in MI‐Ex and MI‐βb without significant alterations of ox‐CaMKII and total CaMKII expressions. (F) There were no significant differences in PKA and phosphorylated PKA (T197) expressions between all groups (n = 6/group). *P < 0.05, versus Sham; #P < 0.05, versus MI‐Sed.

Figure 8. (A) The expression of microRNA‐1 (miR‐1) in the LV was elevated in MI‐Sed, and reduced by exercise and β‐blocker treatment. (B) There was no difference in the expression of miR‐133a between the groups (n = 5/group). (C) There was no difference in expression of protein phosphatase 1 (PP1), while PP2A‐B56‐α (F‐10) expression was significantly decreased in MI‐Sed compared to that in Sham and reversed in MI‐Ex and MI‐βb (D) (n = 6/group) *P < 0.05, versus Sham; #P < 0.05, versus MI‐Sed.