Mechanisms underlying β2-adrenoreceptor (β2AR) inverse agonist mediated bronchoprotectiveness remain unknown. We incubated ICI118,551, formoterol, budesonide, and formoterol plus budesonide, as well as ICI118,551 or pindolol plus formoterol, ICI118,551 plus forskolin, SQ22,536 or H89 plus formoterol in ASMCs to detect expressions of M3R, PLCβ1 and IP3. The level of M3R in the presence of 10-5 mmol/L ICI118,551 were significantly decreased at 12 h, 24 h and 48 h (P < 0.05), and at 24 h were significantly reduced in ICI118,551 with concentration of 10-5 mmol/L, 10-6 mmol/L, 10-7 mmol/L, and 10-8 mmol/L (P < 0.05). The level of IP3 in 10-5 mmol/L ICI118,551 was significantly diminished at 24 h (P < 0.01), except for that at 1 h, neither was in the level of PLCβ1. A concentration of 10-5 mmol/L ICI118,551 at 24 h showed a significant reduction of M3R level compared to formoterol (P < 0.01), budesonide (P < 0.01), and formoterol + budesonide (P < 0.05), but significant reduction of PLCβ1 and IP3 was only found between 10-5 mmol/L ICI118,551 and formoterol at 24 h, but not in the comparison of budesonide or formoterol + budesonide. Pindolol and H89 could not inhibit the formoterol-induced expression of M3R (P > 0.05), but SQ22,536 significantly antagonized the formoterol-induced M3R expression (P < 0.05). In conclusions, β2AR inverse agonist, ICI118,551, exerts similar bronchoprotective effects to corticosteroids via decreasing the expression of M3R and inhibiting the production of IP3.

The persistent administration of β2‑adrenergic (β2AR) agonists has been demonstrated to increase the risk of severe asthma, partly due to the induction of tolerance to bronchoprotection via undefined mechanisms. The present study investigated the potential effect of the long‑acting β2‑adrenergic agonist, formoterol, on the expression of muscarinic M3 receptor (M3R) in rat airway smooth muscle cells (ASMCs). Primary rat ASMCs were isolated and characterized following immunostaining with anti‑α‑smooth muscle actin antibodies. The protein expression levels of M3R and phospholipase C‑β1 (PLCβ1) were characterized by western blot analysis and the production of inositol 1,4,5‑trisphosphate (IP3) was determined using an enzyme‑linked immunosorbent assay. Formoterol increased the protein expression of M3R in rat ASMCs in a time‑ and dose‑dependent manner, which was significantly inhibited by the β2AR antagonist, ICI118,551 and the cyclic adenosine monophosphate (cAMP) inhibitor, SQ22,536. The increased protein expression of M3R was positively correlated with increased production of PLCβ1 and IP3. Furthermore, treatment with the glucocorticoid, budesonide, and the PLC inhibitor, U73,122, significantly suppressed the formoterol‑induced upregulated protein expression levels of M3R and PLCβ1 and production of IP3. The present study demonstrated that formoterol mediated the upregulation of M3R in the rat ASMCs by activating the β2AR‑cAMP signaling pathway, resulting in increased expression levels of PLCβ1 and IP3, which are key to inducing bronchoprotection tolerance. Administration of glucocorticoids or a PLC antagonist prevented formoterol‑induced bronchoprotection tolerance by suppressing the protein expression of M3R.