Aquaporin-CHIP is a 28 kD channel forming integral membrane protein. It acts as an osmotically driven, water-selective pore. The presence of aquaporin-CHIP has been demonstrated in the proximal tubule in the kidney and in the pleura, as well as in other tissues. During peritoneal dialysis a dissociation between the transport of water and sodium using hyperosmolar solutions has been reported, suggesting the presence of ultrasmall pores. Water channels, like aquaporin-CHIP, could be the morphological equivalent of these pores. We investigated the possible presence of aquaporinCHIP in cryo-sections of peritoneal tissue using affinity purified human anti-CHIP IgG (P. Agre, Baltimore, MD). Peritoneal biopsies (omenta) were obtained at catheter insertion in 2 uremic patients with end-stage renal disease, and at catheter reimplantation of 1 patient treated with continuous ambulatory peritoneal dialysis (CAPD) for two years. Peritoneal tissue obtained at autopsy from 1 patient who had been on CAPD for four years, but in whom CAPD had been discontinued for five months, was also studied. Aquaporin-CHIP antiserum specific staining was found in the endothelial cells of the peritoneal capillaries in all patients. No obvious difference in the intensity of staining was seen between uremic and CAPD patients. This demonstration of aquaporin-CHIP in human peritoneal endothelial cells supports the hypothesis of the existence of ultrasmall pores within the peritoneal membrane. These water channels facilitate the transcellular transport of water, induced by an osmotic gradient, in the absence of sodium transport. It may be the explanation for the dissociation of water and sodium transport that occurs during hyperosmolar solutions. Aquaporin-CHIP is present in human peritoneal endothelial cells in both uremic and CAPD patients. Aquaporin-CHIP may be the morphological equivalent of the ultrasmall pores within the peritoneal membrane.
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