PRESENCE IN FROG URINARY-BLADDER OF PROTEINS IMMUNOLOGICALLY RELATED TO THE AQUAPORIN-CHIP

Aquaporin-CHIP, a 28 kDa channel forming protein already referred to a s CHIP28, has been identified as the water channel in red blood cells as well as in mammalian renal tubule cells. Another member of the aqua porin family WCH-CD, has been found in the apical membrane of collecti ng duct principal cells and may represent the ADH-sensitive water chan nel. The present study investigates the possible presence of CHIP28-li ke proteins in amphibian urinary bladder, where the presence of water channels has been postulated. For this purpose,,ve raised polyclonal a ntibodies against human erythrocyte CHIP28. Immune serum precipitated a protein of about 30 kDa from the whole homogenate of urinary epithel ial cells. By Western blotting, in addition to the reaction with the 3 0 kDa component, the immune serum reacted with higher molecular weight components from the bladder homogenate. The 30 kDa band was detected by Western blot only in bladders having a high water permeability. Mor eover, a 30 kDa protein was also recognized in frog red blood cell mem branes by the anti-CHIP28 antibodies. In line with the immunoblotting studies, in immunohistofluorescence anti-CHIP28 antibodies stained fro g red blood cells and urinary bladder epithelial cells. However, in wh ole tissue water permeability studies apical treatment with the anti-C HIP28 antibodies had no effect on either the hydrosmotic response to A DH or on the basal net water flow of the bladder. All together, these results indicate the presence in the frog red blood cells and urinary epithelium of proteins sharing immunological analogies with aquaporin- CHIP.