〔Abstract〕 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)(1×10-3mol/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 calciumion 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.