Coupling of Transmural Flows of NaCl and Water in the Intestine of the EEL (Anguilla Anguilla)

ABSTRACT An in vivo perfusion of the intestine of the yellow European eel (Anguilla anguilla) was used to measure the net absorption of NaCl and water, the osmotic permeability coefficient, the solute-linked water flow, and the osmolality difference against which the intestine could transport water as functions of the salinity of the surrounding water. The eels were adapted to fresh water, to sea water, and to strength sea water. The osmolality difference against which the intestine could transport water was observed to be linearly related to the net transmural flow of NaCl ; the solute-linked water flow had a constant hypertonicity in spite of differing net flows of NaCl. The findings are in agreement with the hypothesis of uphill water movement being caused by local osmosis due to the salt flow and with a shunt leak proportional to the transmural osmotic difference. An important part of adaptation to waters of higher salinity is a pronounced increase in the intestinal absorption of NaCl. The osmotic permeability coefficient varied from experiment to experiment without relation to the state of adaptation. An explanation for this finding may be that the osmotic permeability of the intestinal epithelium is of little importance for the total intestinal transfer of water.