Abstract: Objective: To investigate the process of neointimal formation, level of platelet activation, expression of angiotensinⅡ AT1 receptor mRNA and effects of losartan on them after vascular balloon injury in rats. Methods: Fortyeight male Wister rats were randomly given aortic balloon injury by 2F balloon catheters (n=24), aortic balloon injury plus losartan (30?mg•kg-1•d-1) from 1 day before injury to 28 days after injury, and compared with the controls (n=24). The process of intimal thickening, number of platelet GMP140 and level of AT1 receptor mRNA were investigated at day 3,7,14 and 28 after injury by histological method, radioimmunological method and RTPCR technique, respectively. Results: ① The expression of AT1 receptor mRNA increased significantly at day 3 after balloon injury and remained high at day 14 (P<0.01). ② The number of platelet GMP140 was high at day 3 and began to decrease at day 7 after injury（P<0.01）. ③ The migration and proliferation of vascular smooth muscle cells(VSMC) had existed at day 3 after balloon injury.The thickening of intima happened at day 7 after injury　(P<0.01) and acted more significant at day 14　(P<0.01). The proliferation of VSMC decreased at day 28, but extracellular matrix increased and intimal thickening continued　(P<0.01).④ In losartan treated group，the number of platelet GMP140，proliferation of VSMC and intimal thickening reduced　(P<0.01), but the level of AT1R mRNA increased　(P<0.01). Conclusion: The expression of AT1R mRNA and the number of platelet GMP140 increase in the process of intimal thickening after balloon injury. Losartan inhibits the activation of platelet and intimal thickening，but upregulates the expression of AT1R mRNA.

Key words: Balloon injury, Receptors, angiotensin, Platelet avtivation, Losartan, Rat, Wistar