The role of the intestine in the osmoregulation of the European eel

Abstract

European silver eels 378g ± 11 (SEM) were transferred freshwater - freshwater and freshwater - seawater, their blood sampled and plasma ions, osmolality and circulating concentrations of Cortisol and ANG-II measured to determine the effects of freshwater - seawater transfer. • Sodium and water uptake occurs via transporters and channels present at different ratios in different intestinal regions and showing regional inhibition to specific drugs. Na+,K+-ATPase and NKCC-2 are present in all sections of the intestine, but have greater activity in the mid and posterior intestinal regions. NKCC-2 (inhibited by bumetanide) has greatest activity in the mid intestinal section. CCC-3 (inhibited by thiazide) is functionally present in the mid intestinal region alone. • HgCl₂sensitive aquaporins are present in the mid and posterior regions of the eel intestine and significantly increased in yellow eels on seawater transfer. Inhibition of aquaporins did not alter sodium uptake. • Freshwater silver eel intestines showed greater inhibition of Na+ and H2O uptake with ouabain, bumetanide and HgCl₂ compared with freshwater yellow eels, potentially indicating pre-acclimation for seawater migration. • Infusion of single osmoregulatory hormones (Cortisol, ANG-II and AVT) via intra¬ peritoneal implantation in freshwater yellow and silver eels were used to determine hormonal effects on sodium and water uptake inhibited by ouabain, bumetanide and HgCl₂ • Cortisol infusion reduced basal sodium and water transport across the intestine, in both yellow and silver eels. ANG-II infusion reduces sodium and water transport in yellow but not the silver eel. Cortisol and ANG-II infusion increased HgCl₂ inhibition of AQP water channels confirming their presence and also their importance in water transport. • AVT infusion increases basal sodium and water transport, and HgCl₂ sensitive water transport was dramatically increased, indicating AQPs in the intestine under hormonal regulation and identifying the eel as a potential tool for medical research.