Abstract: OBJECTIVE To explore the effect and mechanism of hirudin on calcification of vascular smooth muscle cells (VSMCs) induced by high phosphorus. METHODS Rat VSMCs cultured in vitro were induced by β-glycerophosphoric acid to establish calcification model, and different concentrations of hirudin were added to intervene. The experiment set five groups: control group, calcification group, low concentration hirudin group, medium concentration hirudin group and high concentration hirudin group. The cellular calcification was detected by alizarin red S staining and intracellular calcium content. The activity of alkaline phosphatase(ALP) and the mRNA and protein expression of α-SMA, RUNX2, BMP2 which were assayed by qRT-PCR and Western blotting to detect the transdifferentiation of cells in each group. RESULTS Compared with the control group, alizarin red S staining showed obvious orange calcium nodules in high phosphorus-induced calcification group, the detection of calcium content showed that the calcium content in cells increased significantly(P<0.01), intracellular ALP activity increased(P<0.01), mRNA and protein expression of α-SMA decreased(P<0.01), mRNA and protein expression of RUNX2 and BMP2 increased(P<0.01). Compared with calcification group, the number of orange-red calcium nodules stained by alizarin red S in hirudin group decreased in a concentration-dependent manner, the intracellular calcium content and ALP activity decreased in a concentration-dependent manner(P<0.05 or P<0.01), the mRNA and protein expression of α-SMA increased in a concentration-dependent manner(P<0.05 or P<0.01), the mRNA and protein expression of RUNX2 and BMP2 decreased significantly in a concentration-dependent manner(P<0.05 or P<0.01). CONCLUSION Hirudin attenuates the calcification of VSMCs by inhibiting the transdifferentiation of VSMCs induced by high phosphorus. The mechanism may be related to the up-regulation of α-SMA expression and down-regulation of ALP activity, RUNX2 and BMP2 expression.