Main Reagents. Butorphanol was purchased from Hengrui (Jiangsu, China). Lipopolysaccharide was obtained from Sigma (St. Louis, USA). Oleic acid was purchased from XiLong Scientific (China). TNF-α, IL-1β, and IL-6 ELISA kits were obtained from Abcam (UK). The NF-κB p65 transcription factor assay kits were obtained from Cayman Chemical. Antibodies against Bax, Bcl-2 and cleaved Caspase-3 were obtained from Cell Signaling Technology, Inc. (Danvers, MA, USA). β-actin antibody, HRP-conjugated anti-rabbit and anti-mouse were purchased from Santa Cruz Biotechnology, Inc. (Dallas, TX, USA). The Annexin Vfluorescein isothiocyanate (FITC) kit was acquired from BestBio Co. (Shanghai, China).
Ethics statement. The study was carried out in compliance with the ARRIVE guidelines. The animal experiments involved in this study have been approved by the Ethics Committee of the Huazhong University of Science and Technology. All experiments were performed in accordance with relevant guidelines and regulations.
Rat “two-hit” modeling and grouping. A total of 40 Sprague‒Dawley (SD) male rats (200–220 g body weight, 6–8 weeks old) purchased from Tongji Medical College, Huazhong University of Science and Technology Animal Center (Wuhan China). Rats were maintained in a stable temperature of 22–24℃,60% humidity, under a 12-h dark/light cycle with free access to food and water for 7 days. All the rats were injected by the tail vein, the day of the first injection was considered as day 0 and observed for a week. Rats were randomly divided into four groups: control group [2.5 ml/kg normal saline (NS) i.v. given at 0 min, 60 min and 90 min); OA + LPS group [2.5 ml/kg NS i.v. given at 0 min, 0.5 ml/kg OA i.v. given at 60 min, and 2.5 mg/kg LPS, i.v. administered at 90 min]; B1 group and B2 group [4 mg/kg or 8 mg/kg butorphanol i.v. given at 0 min, 0.5 ml/kg OA i.v. given at 60 min, and 2.5 mg/kg LPS, i.v. given at 90 min] (Fig. 1B). For the research, all rats were housed at a constant temperature of 23–25°C and monitored for 48 h8,26. All the rats’ activity, appetite, breathing and heart rate were recorded daily for a week, and rats’ death was judged by heartbeat, breathing and body temperature.
Rat pulmonary microvascular endothelial cell (RPMVEC) culture and “two-hit” modeling. RPMVECs were purchased from iCell Bioscience Inc, Shanghai. Cells were cultured in DMEM (Gibco, Fisher Scientific, Inc.) with 10% FBS, 1.5 g/l glucose and 1% penicillin‒streptomycin at 37°C in a humidified atmosphere containing 5% CO2. Cells were divided into the following four groups: control group [PBS given at 0 min,2 h and 2.5 h]; OA + LPS group [PBS given at 0 min,5×10− 4mol/L OA given at 2 h, and 5µg/ml LPS given at 2.5 h];4µM group and8µM group[4µM,8µM Butorphanol given at 0 min and 5×10− 4mol/L OA given at 2 h and 5µg/ml LPS given at 2.5 h] (Fig. 1B); The cells were used for further experiment after 12 h cultivation at 37℃12,27.
CCK-8 cell viability assay. RPMVECs viability was detected by the Cell Counting Kit-8 (MedChem Express, New Jersey, USA) following the manufacturer’s guidelines. The absorbance was measured at a wavelength of 450 nm by an automatic porous spectrophotometer (Molecular Devices, USA).
Histological examination. The upper lobe of the right lung was isolated, perfused, and fixed with 4% formaldehyde, embedded and cut into 5-µm sections. The samples were stained with hematoxylin and eosin (HE) for the analysis of lung injury histopathological changes and assessment of injury score. A scoring system was used to assess the degree of lung injury, and the criteria were as follows: 0: normal; 1: mild pulmonary interstitial hyperemia and leukocyte infiltration; 2: alveolar edema and moderate lung structural damage; 3: moderate alveolar structure collapse and massive inflammatory cell infiltration; and 4: severe alveolar structure destruction and substantial leukocyte infiltration28.
Immunofluorescence analysis and flow cytometry. RPMVECs were digested with trypsin and resuspended to 5×107-1×108/L in each group. These cells were centrifuged at 500 r/min for 5 min at 4°C, washed three times with cold PBS and resuspended in 400 µl binding buffer. Annexin V-FITC staining solution (5 µl) was added and incubated at room temperature for 15 min in darkness, and then 10 µl propidium iodide was added and incubated at room temperature without light for 5 min. Apoptosis was observed using fluorescence microscopy and analyzed by flow cytometry. The results were processed by CellQuest software version 3.3 (BD Biosciences, San Jose, CA, USA).
Detection of ALI inflammatory factors, activity of NF-κB and arterial blood gas analysis. The levels of cytokines (TNFα, IL-1β and IL-6) and activity of NF-κB in serum and RPMVECs were measured according to ELISA kits (R&D Systems, Inc., USA; NF-κB p65 Transcription Factor Assay Kit) following the manufacturer’s instructions. The rat common carotid arterial blood samples were collected 0.4ml into an ABL700 Radiometer (Radiometer America, USA) to measure the pH value, PaO2 and PaCO2.
Lung wet/dry weight ratio. The lower lobe of the right lung was removed, the surface was rinsed, and then the wet weight was measured. The lung tissue was then dried at 80°C in an oven for 48 h and reweighed to obtain the dry weight (the difference between the two measurements was less than 0.05 mg). The lung tissue W/D ratios were calculated.
Western blotting analysis. Total protein was extracted from lung tissue and RPMVECs. A total of 10 µg protein per lane was resolved by 8%-15% SDS‒PAGE and transferred using electroblotting to PVDF membranes. The membranes were blocked with 5% nonfat milk and incubated with primary antibodies against cleaved caspase3 (1:1000 dilution), Bcl-2 (1:1000 dilution), Bax (1:1000 dilution) and β-actin (1:1000 dilution) overnight. Next, the membranes were washed with PBS solution three times, incubated with HRP-conjugated anti-rabbit or anti-mouse secondary antibodies (1:4000 dilution) at room temperature for 1 h and developed using an enhanced chemiluminescence detection kit (EMD Millipore). The band intensity was analyzed using ImageJ software (National Institutes of Health, Bethesda, MA, USA). β-actin was used for normalization. Images of blots with adequate length and membrane edges could not be provided because the blots have been cut prior to hybridization with antibodies and developed by traditional film. The experiment was repeated in triplicate with 3 replicates.
Statistical analysis. All results are expressed as the mean ± standard deviation. All data were obtained by repeated three times. The data were analyzed by using SPSS 20.0 (IBM, Chicago, USA) and GraphPad Prism 5.0 Statistical differences between multiple groups were determined using one-way ANOVA followed by Tukey’s post hoc test. Comparisons between two groups were analyzed using the unpaired Student’s t-test. P < 0.05 was considered to indicate a statistically significant difference.