Distribution of mRNA encoding the FA-CHIP water channel in amphibian tissues: effects of salt adaptation.

A water channel, the frog aquaporin-CHIP (FA-CHIP) was recently cloned from Rana esculenta urinary bladder. The 28.9 kDa encoded protein shows 78.8%, 77.4%, 42.4% and 35.6% identity with rat CHIP28, human CHIP28, rat WCH-CD and gamma-TIP, other members of the new transmembrane water channel family (Aquaporin-CHIP). We have now studied membranes from different frog (R. esculenta) organs employing semiquantitative PCR using FA-CHIP specific primers and an internal standard to quantify the PCR products. The FA-CHIP mRNA was abundantly expressed in the frog urinary bladder, skin, lung and gall bladder, while a lower expression was detected in the colon, liver and oviduct. FA-CHIP mRNA was not detected in the frog kidney, erythrocytes and brain but its expression was observed in the toad (Bufo arenarum) urinary bladder and skin, showing that FA-CHIP is probably a general amphibian water channel. Salt acclimation is known to increase the water permeability of frog and toad epithelia. We have now observed that salt acclimation for 1, 3, 4 or 5 days markedly increased skin and urinary bladder FA-CHIP mRNA expression. It is generally accepted that water permeability is controlled in these tissues by the rate of water channel transfer from subapical vesicles (aggrephores) to the apical membrane. Our results indicate that water permeability is also regulated at the level of the FA-CHIP transcription.