Expression of multiple water channel activities in Xenopus oocytes injected with mRNA from rat kidney

To test the hypothesis that renal tissue contains multiple distinct water channels, mRNA prepared from either cortex, medulla, or papilla of rat kidney was injected into Xenopus oocytes. The osmotic water permeability (P_f) of oocytes injected with either 50 nl of water or 50 nl of renal mRNA (1 μg/μl) was measured 4 d after the injection. P_f was calculated from the rate of volume increase on exposure to hyposmotic medium. Injection of each renal mRNA preparation increased the oocyte P_f. This expressed water permeability was inhibited by p-chloromercuriphenylsulfonate and had a low energy of activation, consistent with the expression of water channels. The coinjection of an antisense oligonucleotide for CHIP28 protein, at an assumed > 100-fold molar excess, with either cortex, medulla, or papilla mRNA reduced the expression of the water permeability by ∼70, 100, and 30%, respectively. Exposure of the oocyte to cAMP for 1 h resulted in a further increase in P_f only in oocytes injected with medulla mRNA. This cAMP activation was not altered by the CHIP28 antisense oligonucleotide. These results suggest that multiple distinct water channels were expressed in oocytes injected with mRNA obtained from sections of rat kidney: (a) CHIP28 water channels in cortex and medulla, (b) cAMP-activated water channels in medulla, and (c) cAMP-insensitive water channels in papilla.