Materials
The main materials used in the study were as follows; Foetal bovine serum (FBS), bupivacaine, lipid emulsions, puerarin injections, pentobarbital sodium, RNeasy Mini Kit (QIAGEN), QuantiNova SYBR Green PCR Kit (QIAGEN), QuantiNova Reverse Transcription Kit (QIAGEN), Ant-UCP2 (GeneTex), Ant-NRF1 (GeneTex), Ant-SLC25A6 (GeneTex), Ant-mtTFA (GeneTex), Ant-VDAC1, Ant-PGC-1 (Abcam), Ant-Bcl-2 (Abcam) and ATP assay kit (Abcam, ab83355).
General animal and animal group treatment conditions
Animal experimentation in this study was carried out in accordance with the requirements of Committee of the Animal Protection and Utilization Institute. This study was approved by animal experiment ethics of Gansu University of Traditional Chinese Medicine (No. 2018-018) and complied with the Declaration of Helsinki. Male specific-pathogen-free Wistar rats (8-week-old, weighing 300±10 g) were provided by Shanghai Experimental Animal Co., Ltd. (Shanghai, China). These animals were anaesthetised and placed in the supine position on the surgical platform, and three needle electrodes were placed under the skin to monitor the 12-lead electrocardiograms (ECGs) and other basic conditions, such as blood pressure and heart rate.
Those experimental rats were divided into 5 groups as follows: Control group rats intravenously infused with 3 ml of 0.9% physiological saline solution (kg/min) and sacrificed 30 min later; bupivacaine group rats intravenously infused with physiological saline solution for 30 min and infused with 0.5% bupivacaine to induce cardiac arrhythmia or death (standard criteria for cardiac arrhythmia); lipid emulsion group rats subject to continuous intravenous infusion with 3 ml of 20% lipid emulsions for 30 min, followed by infusion with bupivacaine to induce cardiac arrhythmia or death; puerarin group rats infused with bupivacaine after being injected with 0.1 ml of puerarin at PC6 of both forelimbs; and electroacupuncture group rats infused with bupivacaine after 30 min of electroacupuncture stimulation (Longitudinal wave: 2/10 Hz; current intensity: 2 mA; pulse width: 0.2 ms) at PC6 of both forelimbs. The rate and duration of infusion, as well as lipid emulsion administration were performed according to a study reported by Weinberg [20]. Cardiac arrhythmia was assessed by an electrocardiographer based on premature ventricular contractions (PVC) or ventricular tachycardia (VT), with the duration of QRS complex prolonged to 90 ms. At the conclusion of the experiment, the rats were euthanasia and subjected to retrograde perfusion, followed by immediate resection of their hearts for subsequent analyses.
Isolation of mitochondria
Myocardial mitochondria were isolated according to a previously published method with some modifications [21]. The rats were euthanised with pentobarbital sodium, their hearts harvested, excised into small pieces and homogenised, followed by the removal of cell debris. The supernatant was centrifuged at 13,000 g for 10 min to isolate the mitochondria.
Mitochondrial membrane potentials assay
Mitochondrial membrane potential were measured with JC-1 kit (Beyotime, C2006) according the manufacturers’ instructions. Namely, the 1x JC-1 working solution was added into the purified mitochondria with an appropriated proportion. The results were analysed using fluorescence spectrophotometer. The excitation wavelength was 458nm and the emission wavelength was 590nm.
Determination of calcium ion levels in mitochondria
Calcium ion concentration of mitochondrial was measured using Fluo-3 AM according the previous study[5]. Fluo-3 AM was added in resuspended Mitochondria and incubated for 1h. Then, the Fluo-3 AM was removed. Fluorescence intensity was analyzed by flow cytometry after the mitochondria were subjected to Triton X100 and calcium chloride and EDTA, respectively.
Reactive oxygen species measurement
Cardiomyocytes was isolated from the SD rats. ROS levels were evaluated using DCFH-DA. The cells were incubated with DCFH-DA. After incubated for 30min, cells were subjected to washed and resuspended. Subsequently, the results were analyzed using microplate reader at an excitation wavelength of 488 nm and at an emission wavelength of 525 nm.
ATP content
Myocardial ATP levels were measured using a commercial assay kit (Beyotime). Briefly, tissues and cells were lysed and centrifuged at 12,000 g and 4°C for 5 min. The resulting supernatant was harvested for subsequent assays. Mitochondrial ATP content was analyzed by colorimetry using phosphomolybdic acid.
Enzyme linked immunosorbent assay (ELISA)-
After being homogenised in phosphate-buffered saline, tissue samples were centrifuged to obtain the supernatant for subsequent measurements. All experimental procedures were carried out in accordance with manufacturers’ instructions provided with the commercial kits. Standard samples and samples to be tested were placed in wells intended for blank, standard, and samples to be tested, respectively. After being incubated at 37°C for 30 min, each well was washed and incubated with the enzyme-labelling reagent, following which, the wells were washed again and subjected to colour development. After the reaction was terminated, the absorbance value of each well was measured to estimate sample concentration. The experiment was repeated three times.
qPCR
Cardiac muscle tissues were rapidly harvested and immersed in liquid nitrogen to extract RNA for subsequent analyses. Experimental procedures were carried out in accordance with instructions provided with the commercial kits. The purity and quality of the resulting RNA samples were determined in addition to the removal of DNA, followed by a reverse transcription PCR (RT-PCR) assay. Primer sequences for target genes are listed (Table 1).
Western blot
After the tissue samples were homogenised, the supernatant was obtained for the purpose of isolating total proteins using a protein extraction kit. After measuring the protein concentration, total proteins were loaded in equal amounts and separated via sodium dodecylsulphate polyacrylamide gel electrophoresis. Subsequently, the proteins were transferred onto a polyvinylidene fluoride membrane, which was then and incubated with special primary antibodies, followed with secondary antibodies 1 h at 37℃. Western blot images were obtained via the enhanced chemiluminescence method. Grayscale analysis was performed on target protein bands using Image J software, and the results were analysed statistically.
Transmission electron microscopy
Myocardial tissues were sequentially fixed with 2.5% glutaraldehyde and 1% citric acid, followed by dehydration in an acetone gradient and resin embedding. Then the embedded tissues were dried rapidly prior to sectioning. Next, tissue sections were stained and imaged under an electron microscope. Mitochondrial injuries were assessed using Flameng score [22], whereby 5 microscopic fields were randomly selected to obtain the mean Flameng score for each group. The mitochondria were graded according to the following criteria: Grade 0 (score 0) - mitochondria with normal ultramicrostructure and intact granules; Grade I (score 1) - mitochondria with basically normal ultramicrostructure and partial loss of granules; Grade II (score 2) - swollen mitochondria with transparent matrix; Grade III (score 3) - mitochondria with transparent matrix and fragmented cristae or formation of flocculent densities in the mitochondrial matrix; and Grade IV (score 4) - mitochondria lacking matrix with fragmented cristae and disrupted outer membrane.
Statistical analysis
Statistical analyses in this study were performed using SPSS 20.0 software, and the data were expressed as means of triplicate experiments. Multiple comparisons between groups were carried out using one-way analysis of variance. P-values of <0.05 were considered to indicate a statistically significant result.