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 submesothelial fibrous layer of the peritoneum greater than 400 microm compared with less than 400 microm. Free-water transport through AQP-1 was 42% +/- 12% from total UF after 1 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.

Dialysis with icodextrin interferes with measurement of serum alpha-amylase activity.

BACKGROUND: The glucose polymer icodextrin has gained a widespread use in peritoneal dialysis especially in patients with low ultrafiltration and high peritoneal transport properties. In patients using a once-daily exchange with icodextrin, a decreased serum amylase activity has been reported. We explored the potential underlying mechanisms of this effect. METHODS: Using standard chromolytic methods, serum amylase activity was measured in blood samples from 11 patients on icodextrin treatment and from 11 patients on conventional glucose treatment. Samples were additionally supplemented with alpha-amylase and unused icodextrin dialysis fluid. Potential complex formation between icodextrin and alpha-amylase was studied by SDS-gel electrophoresis with protein silver staining and fluorophore-assisted carbohydrate staining with the AMAC (FACE) method, which provides oligosaccharide labelling. Lipase activity was measured in parallel in all samples by two standard methods. RESULTS: Amylase activity was reduced by 90% in serum from patients using icodextrin for the long dwell (15.9+/-10.9 U/l) vs patients using standard glucose (157.1+/-23.7 U/l; P<0.001). Addition of icodextrin to serum samples from patients using conventional glucose solutions induced a dose-dependent decrease in amylase activity. The assay results indicated a substrate competition between ET7-G7PNP and icodextrin. AMAC fluorophore staining of icodextrin and subsequent gel electrophoresis failed to demonstrate complex formation between icodextrin and alpha-amylase. Unlike the amylase findings, icodextrin did not affect lipase activity. CONCLUSIONS: The present findings indicate that icodextrin competitively interacts as a substrate in the amylase assay. In support of this, fluorophore-assisted oligosaccharide electrophoresis on SDS gel failed to reveal the formation of an 'amylase/icodextrin complex'. Lipase measurement should provide an alternative and unconfounded method for diagnosing pancreatitis in icodextrin patients.