BACKGROUND: To examine the changes of peritoneal aquaporin-1 (AQP-1) expression and peritoneal transport in rats in response to continuous exposure of same osmotic stimuli which was induced by either mannitol or glucose in dialysis solution or glucocorticoids co-reatment. METHODS: Twenty-eight male Sprague-awley rats were divided into three groups:group I (n=10) rats were dialyzed with 5% mannitol solution (0.27 M); group II (n=9) rats were dialyzed with 5% glucose solution (0.28 M): and group III (n=9) rats were dialyzed with 5% glucose solution (0.28 M) accompanied by daily injection of dexamethasone (2 mg/kg, im) for the last 7 days of the 1 month dialysis period. Dialysis exchanges were performed 2 times a day for 1 month with 25 mL per exchange. Immunohistochemistry was performed using a polyclonal anti AQP-1 antibody. The peritoneal membrane (PM) function was assessed by one-hour PET for comparing transport characteristics. PM transport rates were assessed by D/P of urea nitrogen and D/Do of glucose. Water transport of peritoneal membrane was assessed by D/P of sodium at 1 month. RESULTS: The immunolabeling intensity of peritoneal AQP-1 was markedly increased and the most prominent in rats dialyzed with 5% glucose solution accompanied by injection of dexamethasone (group III). Consistent with this, D/P of sodium in rats of group III was significantly decreased (p<0.05), indicating an increased water permeability and ultrafiltration. In contrast, peritoneal AQP-1 labeling intensity was weaker in PD groups dialyzed with mannitol (group I) and glucose (group II) solution having an identical osmolality and was not different between the groups. Rats in group I exhibited significantly lower D/Do of glucose and increased D/P of urea nitrogen, suggesting high PM transport. CONCLUSION: These results indicate that the change of peritoneal AQP-1 expression is not specific to the glucose itself, but rather it could be altered in response to the changes of osmolality per se in PD solution. Moreover, corticosteroid up-egulates peritoneal AQP-expression, which is paralleled by an increased water permeability.