Potassium-dependent chloride and water transport across the seawater eel intestine

Simultaneous measurements of net ion and water fluxes and transepithelial potential difference (PD) were made in the stripped intestine of the seawater eel, and it was examined whether Cl- was driven following electrochemical gradient for Na+ across the brush border membrane of the epithelium or not. When mucosal Na+ was completely replaced with K+, while the serosa was being bathed with normal Ringer's solution, net Cl- and water fluxes were maintained as high as those in normal Ringer's solution. After serosal Na+ was completely replaced with choline+ while the mucosa was being bathed with Na+-free KCl Ringer's solution, 80% of the original Cl- and water fluxes still persisted, indicating significant Na+-independent Cl- and water transport. These results are against a hypothesis that Cl- is driven by electrochemical gradient of Na+ across the brush border membrane. The Na+-independent Cl- and water fluxes were a saturable function of mucosal K+ concentration, suggesting K+-dependent Cl- and water transport. A possible mechanism of Cl- transport is discussed in relation to K+ transport. On the other hand, a good correlation was observed between the net Cl- and water fluxes. This suggests that water transport depends on Cl- transport system; NaCl and/or KCl cotransport.