Objective : To investigate the expression characteristics of mu opioid receptor ( MOR) in rat colon smooth muscle cells by cultured rat primary colonic smooth muscle cells . Methods : colonic smooth muscle cells were isolated , cultured and identified ; immunofluorescence double labeling method was used to observe the distribution characteristics of MOR , Endomorphin⁃2 (EM2) , and calmodulin (CaM) in colonic smooth muscle cells ; Western Blot method was used to detect the expression of MOR and EM2 in smooth muscle cells of the colon . After the intervention measures Acetylcholine (ACh) ( 1 × 10 - 3 mol/L) and EM2 (2 μmol/L) were applied , the changes of CaM protein expression were observed ; The calciumion imaging method was used to detect the changes of calciumi on concentration in smooth muscle cells. Results: The colonic smooth muscle cells were cultured and identified . The positive cells labeled with α ⁃smooth muscle actin ( α ⁃SMA) accounted for more than 95% of the total number of cells . Immunofluorescence double labeling showed that there were MOR and EM2 distributions in colonic smooth muscle cells , and all MOR and EM2 positive cells coexisted with α ⁃SMA . Western Blot results showed that there were MOR and EM2 expressions in colonic smooth muscle cells . CaM in the ACh group significantly increased at 10 minutes (P < 0. 05) , CaM in the EM2 group significantly decreased ( P < 0. 05) ; The calciumion imaging results showed that alone applied ACh , the calciumion concentration in smooth muscle cells significantly increased ( P <0. 05) ; Alone applied EM2 , the calciumion concentration in colonic smooth muscle cells was down⁃regulated (P <0. 05) ; Applied ACh and EM2 sequentially , EM2 significantly reduced the increase of the calciumion concentration in smooth muscle cells induced by ACh (P < 0. 05) . Conclusion MOR and EM2 are expressed in colonic smooth muscle cells , and EM2 may inhibit the expression of CaM and reduce the concentration of calcium ions through MOR .