Histochemical distribution and expression of aquaporin 1 in the peritoneum of patients undergoing peritoneal dialysis: Relation to peritoneal transport

Background. Aquaporin 1 (AQP-1) channels have been claimed to be responsible for osmotically driven free-water movement across the peritoneal membrane. Data about AQP-1 expression and its location in the human peritoneum related to clinical findings concerning ultrafiltration (UF) and free-water transport are still lacking. Methods: Fifty-seven peritoneal biopsy specimens obtained from peritoneal dialysis (PD) patients were investigated. AQP-1 expression was detected by means of immunohistochemistry and a semiquantitative scoring system. Histological findings were related to peritoneal transport properties measured by means of an extended peritoneal equilibration test (PET) using dextran 70 as a volume marker. Results: AQP-1 expression in the peritoneum was detected in both vascular endothelial cells (capillaries and small venules; score, 2.96 +/- 0.92) and the mesothelial cell layer (score, 2.31 +/- 1.54). There was significantly greater AQP-1 expression in vascular endothelial cells of patients showing increased thickness of the submesothellal fibrous layer of the peritoneum greater than 400 mum compared with less than 400 mum. Free-water transport through AQP-1 was 42% +/- 12% from total UF after I hour. There was a significant correlation between AQP-1 expression and free-water transport after 1 hour of equilibration with 3.86% glucose in the PET (r = 0.753; P < 0.001). Conclusion: Our data indicate that AQP-1 is located not only in the endothelial cell layer of capillaries and small vessels in the peritoneum of PD patients, but also in the mesothelial cell layer. AQP-1 expression correlated with free-water transport after 1 hour of equilibration, reaching a significant part from total UF at this time.