Background: Hypoxic exposure contributes to the phenotypic switching of smooth muscle cells (SMCs), while the mechanisms involved in this process is not yet fully elucidated. Myocardin as a co-actor of serum reaction factor plays a crucial role in differentiation of SMCs. This study was aimed to investigate the role of myocardin in hypoxia-induced phenotypic switching of rat pulmonary arterial SMCs (PASMCs). Methods: Primary PASMCs were cultured under normoxia and hypoxia (3%O-2, 48 h) respectively, and then the cell proliferation was assessed and the expression of SM22 alpha, osteopontin (contractile and synthetic marker of SMCs, respectively), myocardin and platelet-derived growth factor-BB (PDGF-BB) were detected. After pGCSIL-GFP-shMYOCD lentviral vector was transduced to the PASMCs, the expression of myocardin and SM22 alpha were examined. Moreover, myocardin expression in PASMCs treated with medium enriched with PDGF-BB and conditional medium (CM) from normoxia- and hypoxia-exposed PASMCs was assessed. Results: Exposing PASMCs to hypoxia led to an increased cell numbers and the up-regulation of proliferating cell nuclear antigen (PCNA), osteopontin and PDGF-BB: moreover, a significant down-regulation of SM22 alpha and myocardin was identified. Further analysis revealed that knock-down of myocardin with pGCSIL-GFP-shMYOCD vector followed by a decreased SM22 alpha in the PASMCs, and treatment of PASMCs with either exogenous PDGF-BB or hypoxic CM led to a marked decrease of myocardin. Conclusions: Our findings suggest that the decrease of myocardin in PASMCs exposed to hypoxia is partly regulated by the increase of PDGF-BB, which contributes to the phonotypic switching of PASMCs in hypoxic condition. (C) 2010 Published by Elsevier Inc.