2.1. Animal model and experiment protocol
56 male Sprague-Dawley rats, purchased from animal experiment center of China Three Gorges University, were raised in Wuhan No.3 Hospital animal administration. All experiments were officially permitted and performed in Hubei Key Laboratory of Cardiology, complied with the Guide for the Care and Use of Laboratory Animals published by the US National Institutes of Health (NIH Publication No. 82 − 23, revised in 1996).
The rats initially received left anterior descending coronary artery (LAD) ligation or sham surgery. Briefly, rats were anesthetized by 3% pentobarbital (2 mL/kg; Sigma-Aldridge, St. Louis, USA, intraperitoneal), followed by tracheal intubation and respiratory support (tidal volume: 5–10 mL, rate: 70 times/minute). Subsequent heart exposure was performed and the LAD was ligated by 7 − 0 monofilament nylon sutures. Occurrence of paleness of the apex cordis or ST segment elevation on Ⅱ lead of electrocardiogram indicated the induction of MI. The sham operation underwent the identical procedures except the LAD ligation. Penicillin (200000 IU) was injected intramuscularly once a day for a week after operation.
Afterwards, the rats were carefully raised for 6 weeks to produce PIHF. Rats were then randomly split into 4 groups: i. sham + saline (sham group); ii. sham + Pino group; iii. LAD ligation + saline (HF group); iv. LAD ligation + Pino (HF + Pino group). Pinocembrin (5 mg/kg, purity > 98%; Sigma-Aldridge) or same volume of saline was injected intravenously through the tail vein every other day for 2 weeks. The dosage referred to previous study(21). The protocol was illustrated in Fig. 1B.
2.2. Echocardiography Measurements
The cardiac functions were evaluated by transthoracic echocardiography utilizing an ultrasound imaging system (Vevo 2100, Visual Sonics Inc., Toronto, Canada), as previously reported in our group(22). Left ventricular inner dimension at end-diastolic stage (LVIDd) and end-systolic stage (LVIDs), left ventricular end-systolic and end-diastolic volume (LVESV and LVEDV) were recorded by two-dimensional M-mode echocardiograms for at least 3 consecutive cycles for each rat. Left ventricular ejection fraction (LVEF) and fractional shortening (FS), which reflected the systolic function, were subsequently calculated.
2.3. Sample Preparation
After administration of Pino and echocardiography tests, rats were anesthetized again. More than 2 mL venous blood was captured from tail vein into ethylenediamine tetraacetic acid (EDTA)-containing vacuum blood-collecting tubes with successive centrifugation at speed of 3000 r/min for 15 min and at a temperature of 4°C, followed by isolation of serum supernatant and preservation at -80°C. Then the hearts were apace harvested. Part of infarct border zone of each heart was cut off and frozen in liquid nitrogen for western blot analysis, with the other heart tissue fixed in 4% paraformaldehyde for more than 24h.
2.4. Masson’s staining
We performed Masson’s staining to detect the collagen deposition in the infarct border zone. Paraformaldehyde-fixed ventricles were embedded by paraffin and then cut into 5-µm sections. The sections were subsequently stained with Masson trichrome where the collagen was dyed in blue, the myocardium in pink and the nucleus in black. The collagen deposition proportion was calculated by Image J software (NIH, Bethesda, MD) at ×200 magnification.
2.5. Immunohistochemical analysis
Immunohistochemical analysis was performed to evaluate the density of vascular endothelial growth factor receptor 2 (VEGFR2) whose upregulation promoted angiogenesis that was salutary and important in attenuation of remodeling(3). The ventricular sections were manufactured like those conducted in Masson staining, followed by deparaffinized and rehydrated. Then the sections received heat-mediated antigen retrieval and were treated with 3% H2O2 to block endogenous peroxidase, with incubation of 1 µg/mL VEGFR2 antibody (Abcam, ab9530) for 12h at 4 ℃ fulfilled. Whereafter, the sections were incubated by horseradish peroxidase (HRP)-labeled secondary antibody (1:200, Servicebio, G1213) for 50 min at 37℃. In addition, 3,3’-diaminobenzidine was utilized to develop positive staining which was shown in tawny. Finally, the average optical density of VEGFR2 was analyzed and calculated by Image Pro Plus 6.0 software (Media Cybernetics, USA).
2.6. Western blot
We implement western blot to elucidate the underlying mechanisms by which Pino ameliorated PIHF. The procedures of protein sample extraction and preparation resembled those previously reported in our study (23). The membranes were probed with antibodies against the nuclear factor erythroid 2-related factor 2 (Nrf2, ), heme oxygenase-1 (HO-1; 1:2000, abcam, ab189491), p53 (1:1000, cell signaling technology, 2524S), bcl-2 (1:1000, cell signaling technology, 2870P), bax (1:1000, abcam, ab32503), cleaved caspase-3 (1:1000, cell signaling technology, 9664T), collagen Ⅰ (1:1000, Affinity, AF7001), glyceraldehyde-3-phosphate dehydrogenase (GAPDH, Servicebio, GB12002), with subsequent visualization by HRP-labeled secondary antibodies (Servicebio, G1213 and G1214) and chemiluminescence detection reagents. The blot images were finally analyzed and quantified by Image J software.
2.7. H9c2 cardiomyocytes culture and disposal in vitro
The rat heart ventricular derived H9c2 cardiomyocytes, which were purchased from the American Type Culture Collection (CRL-1446), were cultured to further verify the effects exerted by Pino in vitro. H9c2 cardiomyocytes were cultured in e Dulbecco's modified Eagle's medium (DMEM)/ F12 medium that contained 10% fetal bovine serum (Sijiqing, Hangzhou, China) under a temperature at 37 ℃ and an atmosphere of 95% air and 5% CO2. The cells were seeded to 6-well plates after growing to 90%, in addition, serum medium containing 25 µM isoprenaline (ISO) subsequently substituted for normal medium in 3 wells to incubate the cells for 48h to mimic HF state since HF is also characterized by an overactivation of sympathetic tone and stimulation of β1-adrenergic receptor generates ROS(24). Then the cells were split into 6 groups: i. control group; ii. control medium added with 25 µM Pino for 4h (control + Pino group); iii. control medium co-incubated by 25 µM Pino and 5 µM ML385 for 4h (Nrf2 inhibitor; MCE, New Jersey, USA) (control + Pino + ML385 group); iv. ISO group; v. ISO medium with 4-hour incubation by 25 µM Pino (ISO + Pino group); vi. ISO + 25 µM Pino + 5 µM ML385 group. The concentration and incubating time of drugs referred to previous study or the instructions(25, 26).
2.8. ROS Detection
The ability of Pino to scavenge ROS was measured by fluorescent probe dihydroethidium (DHE; Beyotime, Shanghai, China). DHE, which was dissolved in DMSO, was diluted to 5 µM for frozen sections or added to the serum medium to be diluted to the same concentration in accordance with the instructions. The paraformaldehyde-fixed hearts were frozen followed by being sliced up to 4-µm sections. The sections or H9c2 myocytes were co-incubated with DHE solution for 30 minutes at a temperature of 37 ℃, imaged afterwards by an inverted fluorescent microscope (IX70; Olympus, Tokyo, Japan). Briefly, the nucleus which were filled with DNA and RNA were dyed in distinct red due to an affinity between nucleic acid and DHE. In addition, a stronger fluorescence intensity indicated more accumulation of ROS.
2.9. Biochemical Examinations
Biochemical examinations were performed to probe the alterations of serum BNP, SOD and malondialdehyde (MDA) in serum and cellular supernatant which indicated HF phenotype and the oxidant defense, respectively. The concentration was measured by using corresponding biochemical assay kits according to manufacturer’s instructions (Jiancheng, Nanjing, China).
2.10. Statistical Analysis
The continuous variables were represented as mean ± standard error while the proportional variables were shown as percentage. Comparisons in all groups were performed by using one-way analysis of variance (ANOVA) corrected by a Tukey post hoc test except those in echocardiography data which was analyzed by two-way ANOVA corrected by the Fisher least significant difference test. A p value < 0.5 was defined as statistically significance.