OBJECTIVE: To determine the influence of thrombin, which is generated intraperitoneally during peritoneal dialysis, on the synthesis of fibrinolytic system components in human peritoneal mesothelial cells (HMC). METHODS: Confluently grown HMC, isolated from the omental tissue, were used in the experiments. Conditioned media were obtained by incubating cells with serum-free M199 containing the appropriate concentration of the test compound. Tissue type plasminogen activator (tPA) and plasminogen activator inhibitor type 1 (PAI-1) antigen concentrations were measured by ELISA. Northern blot analysis was conducted for mRNA expression experiments. To test thrombin specificity, we used the thrombin inhibitor hirudin. The protein kinase C (PKC) inhibitor Ro 31-8220 was inserted to examine whether the effect of thrombin depends on PKC activity. RESULTS: Thrombin increased PAI-1 antigen in the conditioned media of HMC in a time- and concentration-dependent manner. After 24 hours incubation, PAI-1 levels increased from 350+/-30 ng/10(5) cells in control conditions to 620+/-30 ng/10(5) cells in HMC exposed to 5 U/mL thrombin (n = 8, p < 0.05). In contrast, there was no effect of thrombin on tPA antigen levels. An increase of PAI-1 mRNA expression was also observed by Northern blot hybridization. Hirudin (10 U/mL) inhibited the thrombin-induced increase in PAI-1 synthesis. In addition, a complete inhibition of the stimulating effect of thrombin on PAI-1 synthesis was obtained by blocking PKC activity with Ro 31-8220 (3 micromol/L). CONCLUSIONS: Thrombin increases PAI-1 synthesis in HMC via a PKC-dependent mechanism.Thereby the synthesis of tPA is not affected. Thus, thrombin may not only promote fibrin formation in the peritoneal cavity, but may also inhibit fibrin degradation by release of free PAI-1 from HMC.

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