Peritoneal morphologic changes in a peritoneal dialysis rat model correlate with angiopoietin/Tie-2

The angiopoietin/Tie-2 system plays an important role in the initiation of angiogenesis. However, the role of angiopoietin/Tie-2 in peritoneal angiogenesis and fibrosis is unclear. In our study we investigated the peritoneal morphologic changes in a uremic peritoneal dialysis (PD) rat model, focusing on the relationship between angiopoietin/Tie-2 and peritoneal angiogenesis. We subjected uremic (subtotal nephrectomy) rats to dialysis, using a standard PD solution, for 10 days, 28 days, or 56 days, and compared them with uremic rats that had not undergone dialysis and control rats. Functional [dialysate-to-plasma (D/P) creatinine; ultrafiltration (UF)] and structural (vessel density and thickness of the submesothelial extracellular matrix) changes of the peritoneum were quantified. Levels of angiopoietin (Ang)-1, Ang-2, Tie-2 and vascular endothelial growth factor (VEGF) were examined in the peritoneum by real-time quantitative polymerase chain reaction (PCR) and related to angiogenesis. The uremic group that had not undergone dialysis was characterized by increased vessel density in the peritoneum compared with that of the control, which correlated with decreased UF and increased D/P creatinine. Progressive angiogenesis and fibrosis were found in the PD groups when compared with the uremic non-dialyzed or control group, accompanied by an increased D/P creatinine that occurred in the PD group after 56 days, while UF decreased. Furthermore, Ang-2 and VEGF levels increased, while Tie-2 level decreased significantly in the uremic non-dialyzed group compare with the control. This tendency was more obvious in the PD groups than in the uremic non-dialyzed or control group, but no difference was found among the PD groups. Both VEGF and Ang-2 correlated positively with vessel density, while Tie-2 correlated negatively. We confirmed angiogenesis and fibrosis changes of the peritoneum as a result of uremic status and PD therapy in the uremic PD rat model. An increased level of Ang-2 and a reduced level of Tie-2 in conditions of uremia and PD therapy correlated with peritoneal angiogenesis and functional deterioration.