Aim To explore the effect of endothelial progenitor cells(EPC) on the phenotype transformation of cultured rat vascular smooth muscle cells(VSMC) stimulated by angiotensinⅡ.Methods Mononuclear cells were isolated from fresh cord blood by 3%Gelatin and density gradient centrifugation.Isolated cells were cultured in EGM-2 medium supplemented with 20%FBS,VEGF,bFGF and other growth factors.Biological features of EPC were observed at different time points,and EPC was identified by morphology,fluorescence double-staining and flow cytometry.Indirect immunofluorescence was performed to analyze expression ofα-SM-actin,calponin of VSMC special antigen.RT-PCR and Western blotting were performed to analyze the effect of endothelial progenitor cell conditional medium(E-EPC-CM,LEPC -CM) and human umbilicus vein endothelial cell conditional medium(HUVEC-CM) on AngⅡ-induced expression ofα-SM-actin and osteopontin in VSMC.VSMC was divided into control group,AngⅡgroup,AngⅡ+EGM-2 group,AngⅡ+E-EPC-CM group,AngⅡ+L-EPC-CM group and AngⅡ+HUVEC-CM group.Results After stimulation with angiotensinⅡfor 48 h,the expression of theα-SM-actin mRNA and protein significantly decreased and the osteopontin mRNA and protein markerly increased compared with control group,suggesting that VSMC was changed from contractile to synthesize type by angiotensinⅡ.Treatments with E-EPC-CM,L-EPC-CM,HUVEC-CM,especially with E-FPC-CM, could inhibit significantly the down-regulation ofα-SM-actin expression and the up-regulation of osteopontin expression by AngⅡ.Conclusion EPC could inhibit the phenotype transformation of VSMC from contractile to synthesize type induced by angiotensinⅡ.