LAUSR

Regarding the human health effects, airborne fine particulate matter 2.5 (PM2.5) is an important environmental risk factor. However, the underlying molecular mechanisms are largely unknown. The present study examined the hypothesis that PM2.5 causes bronchial hyperreactivity by upregulated muscarinic receptors via the mitogen‐activated protein kinase (MAPK) pathway. The isolated rat bronchi segments were cultured with different concentration of PM2.5 for different time. The contractile response of the bronchi segments were recorded by a sensitive myograph. The mRNA and protein expression levels of M3 muscarinic receptors were studied by quantitative real‐time PCR and immunohistochemistry, respectively. The muscarinic receptors agonist, carbachol induced a remarkable contractile response on fresh and DMSO cultured bronchial segments. Compared with the fresh or DMSO culture groups, 1.0 µg/mL of PM2.5 cultured for 24 h significantly enhanced muscarinic receptor‐mediated contractile responses in bronchi with a markedly increased maximal contraction. In addition, the expression levels of mRNA and protein for M3 muscarinic receptors in bronchi of PM2.5 group were higher than that of fresh or DMSO culture groups. SB203580 (p38 inhibitor) and U0126 (MEK1/2 inhibitor) significantly inhibited the PM2.5‐induced enhanced contraction and increased mRNA and protein expression of muscarinic receptors. However, JNK inhibitor SP600125 had no effect on PM2.5‐induced muscarinic receptor upregulation and bronchial hyperreactivity. In conclusion, airborne PM2.5 upregulates muscarinic receptors, which causes subsequently bronchial hyperreactivity shown as enhanced contractility in bronchi. This process may be mediated by p38 and MEK1/2 MAPK pathways. © 2016 Wiley Periodicals, Inc. Environ Toxicol 32: 371–381, 2017.