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First published online August 25, 2003
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Intestinal water absorption through aquaporin 1 expressed in the apical membrane of mucosal epithelial cells in seawater-adapted Japanese eel

Mayumi Aoki1, Toyoji Kaneko1,2,*, Fumi Katoh1,2, Sanae Hasegawa2, Naoaki Tsutsui1,3 and Katsumi Aida1

1 Department of Aquatic Bioscience, Graduate School of Agricultural and Life Sciences, University of Tokyo, Bunkyo, Tokyo 113-8657, Japan
2 Ocean Research Institute, University of Tokyo, Nakano, Tokyo 164-8639, Japan
3 Japan International Research Center for Agricultural Sciences, Tsukuba, Ibaraki 305-8686, Japan



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  		Fig. 1. Changes in the amount of water ingested by freshwater- and seawater-adapted eels with time. Drinking rates were estimated to be 0.02 ml kg-1 h-1 and 0.82 ml kg-1 h-1 in freshwater and seawater eels, respectively.

 


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  		Fig. 2. Water absorption in intestinal sacs prepared from freshwater- and seawater-adapted eels. The water absorption increased with time and pressure. Water absorption rates were 2.7-3.7 times greater in seawater sacs than in freshwater sacs when compared at the same pressure.

 


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  		Fig. 3. Water absorption rates in the anterior and posterior intestine of freshwater- and seawater-adapted eels. The posterior intestine of seawater eel showed the highest water absorption rate. *Significantly different from the values of freshwater-adapted eels; {dagger}significantly different from the values of the anterior intestine at P<0.01.

 


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  		Fig. 4. Deduced amino acid sequences of eel homologues of AQP1, S-AQP and L-AQP, in comparison to the sequence of mouse AQP1. Periods and hyphens indicate identical residues and gaps introduced for alignment, respectively. Two NPA motifs are shaded. Asterisks indicate differences between S-AQP and L-AQP. The antibody was raised against a synthetic peptide corresponding to the underlined sequence.

 


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  		Fig. 5. Tissue distribution of eel AQP1 (S-AQP and L-AQP) mRNA determined by northern hybridization in freshwater (FW)- and seawater (SW)-adapted eels. Hybridization signals were detected as a single band of approximately 1.3 kb (arrows) in the heart, intestine, spleen and swim bladder, but not in the other tissues examined.

 


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  		Fig. 6. Quantitative analysis of eel AQP1 (S-AQP and L-AQP) mRNA levels in the anterior and posterior intestines of freshwater- and seawater-adapted eels. *Significantly different from the freshwater value at P<0.01.

 


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  		Fig. 7. Western blot analysis for eel AQP1 (S-AQP and L-AQP) expressed in the intestine of seawater-adapted eel. The membranes were incubated with anti-eel AQP1 (lane A) and normal rabbit serum (lane B). The antibody recognized one specific protein band of approximately 30 kDa (arrow), agreeing with the expected molecular mass of eel AQP1. Positions of molecular mass markers (kDa) are indicated on the left.

 


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  		Fig. 8. Cross sections of the anterior (Ante: A,B) and posterior (Post: C,D) intestines from freshwater (FW: A,C)- and seawater (SW: B,D)-adapted eels, stained with Hematoxylin and Eosin. In the posterior intestine, both inner circular (c) and outer longitudinal (l) muscle layers were thinner in seawater eel than in freshwater eel, whereas there was no apparent structural difference in the anterior intestine of freshwater and seawater eels. Scale bar, 100 µm.

 


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  		Fig. 9. Cross sections of the anterior (Ante: B,C) and posterior (Post: A,D,E) intestines from freshwater (FW: B,D)- and seawater (SW: A,C,E)-adapted eels, stained immunocytochemically with anti-eel AQP1. In seawater eel, intense immunoreaction for AQP1 was observed in the apical surface of columnar epithelial cells (arrows) in the mucosa of the posterior intestine. In addition, AQP1 immunoreaction was detected in endothelial cells of the blood vessels (arrowheads) distributed in the muscularis, submucosa and mucosa. Asterisks indicate mucous cells. Scale bars: 100 µm (A); 50 µm (B-E).

 

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© The Company of Biologists Ltd 2003