The Damage Effect Of NO₂ Inhalation Exposure On Myocardial Cell Mitochondria In Rats

With the exponential growth of China’s national economy and rapid improvement of people’s living standards, per capital car ownership and the total vehicle exhaust emissions in the city increased dramatically, resulting in the serious nitrogen dioxide (NO2) pollution. A large number of epidemiological studies suggested that heart may be one of the important target organs of NO2 except for respiratory system. Because mitochondria plays an important role in maintaining normal function of myocardial cell, it has important implications to determine the mitochondrial related indicators to explore the correlation between NO2 exposure and cardiovascular disease for effective prevention of NO2 exposure related diseases. However, the relevant information is focused on epidemiological investigation, the use of experimental animal models to determine the occurrences of effects and mechanisms were rarely reported. Thus we put forward this discussion to investigate the injury in mitochondria of myocardial cell in rats and its mechanisms after NO2 exposure.(1) In order to investigate the effects of subacute NO2 exposure on myocardial cell mitochondria, Wistar rats were treated with NO2 at various concentrations (0 mg/m3,5 mg/m3,10 mg/m3 and 20 mg/m3,6 h/d, for 7 days). The histopathological structure of rats cardiac muscles were determined by HE staining; the mitochondrial ultrastructure in rat myocardial cell were measured by electron microscopy; the mitochondrial function membrane potential and activity was detected by JC-1 and MTT respectively; the protein levels of mitochondrial respiratory chain transcription factors PGC-la, NRF1, and TFAM and mitochondria fusion related factors Mfnl and Mfn2 were assayed by Western blot. The results showed that the histopathological structure of cardiac muscles in rats was changed and the number of myocardial cell mitochondria increased after acute and high concentration NO2 stimulation. The mitochondria activity increased 1.52 times and 2.12 times of control after exposure to 10 mg/m3 and 20 mg/m3 NO2, respectively; the membrane potential was increased significantly with different concentration,5,10,20 mg/m3 concentration group was increased 1.50 times,1.62 times and 2.25 times compared with control, respectively. The expression levels of PGC-la, NRF1 and TFAM and mitochondria fusion related factors Mfn2 were significant raised in myocardial cell of rats, they both had significant differences under 20 mg/m3, and it was 1.51 times,1.47 times,1.60 times and 1.53 times compared with control group, respectively. We can inference that subacute high concentration NO2 exposure causes injuries in mitochondria of myocardial cell in rats, however, due to compensate effect, the function of rats myocardial mitochondria was enhanced to adjust to poor environment situation and external stimulation. Nevertheless, NO2 still can induce the occurrence of cardiovascular system diseases as for the damage of myocardial mitochondria.(2) In order to approach the actual exposure condition and to investigate the effects of subchronic NO2 exposure on myocardial cell mitochondria, Wistar rats were treated with NO2 at various concentrations (0 mg/m3,2 mg/m3 and 5 mg/m3,6 h/d, for 28 days). After treatment, the histopathological structure of rats cardiac muscles were determined by HE staining; the mitochondrial ultrastructure in rat myocardial cell were measured by electron microscopy; the mitochondrial function membrane potential and activity was detected by JC-1 and MTT, respectively; the protein levels of mitochondrial respiratory chain transcription factors PGC-la, NRF1, and TFAM and mitochondria fusion related factors Mfnl and Mfn2 were assayed by Western blot. The results showed that after chronic and low concentration NO2 exposure, the histopathological structure of cardiac muscles in rats was changed and the numbers of mitochondria in myocardial cells decreased, and swell of mitochondria were also observed; the mitochondria activity of lower concentration group was significantly declined 0.79 times compare with control and that of higher concentration group was declined 0.56 times, respectively; and compare with control, the membrane potential of lower concentration group was significantly decreased 0.89 times and that of higher concentration group was declined 0.79 times, respectively; meanwhile, the expressions of PGC-1α, NRF1 and TFAM and mitochondria fusion related factors Mfh1 and Mfn2 of higher concentration group were significantly declined to 0.76 times,0.85 times,0.52 times,0.77 times and 0.80 times of control respectively. The results showed that after chronic low concentration NO2 exposure, the mitochondrial ultrastructures in rat myocardial cells have been damaged, mitochondrial membrane potential and activity were declined, oxidative phosphorylation capacity were weakened, mitochondrial synthesis ability were declined, then mitochondrial function were damaged which induced the occurrence of cardiovascular system diseases.Present study illustrates the damage effects and molecular mechanism of subchronic and subacute NO2 exposure on mitochondria in rat myocardial cells and sets up biomarkers for testing and risk assessment. The results suggested that the damaged mitochondrial structure and function might be one of the molecular mechanisms of NO2 pollution induced cardiovascular diseases, and present study provides theoretical foundation for clinic treatment when pollution events occurred.