The mechanism of salt and water absorption in the intestine of the eel (Anguilla anguilla) adapted to waters of various salinities

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Abstract
1. The absorption of NaCl and water was studied by intraluminal in vivo perfusion of the intestine of the yellow European eel (Anguilla anguilla) adapted to fresh water (FW), to sea water (SW), and to double strength SW (DSW).2. The net lumen to plasma NaCl transport from diluted SW perfusion fluids was independent of the NaCl concentration in the Na(+) concentration range tested. The NaCl absorption (expressed as mu-equiv/100 g.hr.) increased from FW (mean +/- S.E.): Na(+) 166 +/- 17, Cl(-) 205 +/- 24 to SW: Na(+) 363 +/- 33, Cl(-) 423 +/- 37, and again in DSW: Na(+) 640 +/- 110, Cl(-) 676 +/- 149.3. The osmolality of the perfusion fluid which resulted in zero net water transport was higher than plasma osmolality by 73 +/- 3 m-osmole in FW, 126 +/- 5 m-osmole in SW, and 244 +/- 32 m-osmole in DSW (mean +/- S.E.). A fairly constant ratio between net NaCl transport and this osmolality difference prevailed.4. The general osmotic permeability to water in the serosa-mucosa direction (expressed as mul./100 g. hr. m-osmole) measured from experiments with impermeant solute increased from FW: 3.7 +/- 0.5 to SW: 7.2 +/- 1.0 (mean +/- S.E.).5. These results are compatible with the interpretation that the water flow occurring in the absence of a general transmural osmotic gradient, the ;solute-linked water flow', is linearly related both to net NaCl transport and to the osmotic permeability to water. The findings support the view that the ;solute-linked water flow' is, indeed, secondary to the salt movement and is due to osmotic force.6. The amount of water absorbed from dilute SW perfusion fluids isosmotic with plasma was larger than in most other intestinal epithelia. FW: 650, SW: 1620 mul./100 g. hr. The NaCl concentration of the absorbate was hypertonic to plasma.7. The passive permeability of the intestine to NaCl was very low, and the reflexion coefficient was close to unity. Therefore metabolic energy will be used to absorb NaCl, even when the NaCl concentration in the gut is higher than that of plasma due to ingestion of SW. There appears to be a limited interaction in the intestinal wall between passive salt and water flow.8. In DSW the Na(+) ingestion with the oral intake of the surrounding fluid matched the gut absorption capacity. Since DSW is close to the tolerance limit, it is concluded that the gut NaCl transport capacity may be one of the factors limiting the tolerance to water or higher salinity.

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