Relationship between coupled Na⁺−K⁺−Cl⁻ transport and water absorption across the seawater eel intestine

Summary

Simultaneous measurements of net ion and water fluxes were made in the stripped intestine of the seawater eel, and the relationship between Na⁺, K⁺, Cl⁻ and water transport were examined in the presence of mucosal KCl and serosal NaCl Ringer (standard condition). When Cl⁻ was removed from both sides of the intestine, net K⁺ flux from mucosa to serosa was reduced, accompanied by complete blockage of water absorption. Since it has been shown that net Cl⁻ and water fluxes depend on K⁺ transport under the standard condition (Ando 1983), the interdependence of K⁺ and Cl⁻ transport suggests the existence of a coupled KCl transport system, while the parallelism between the net Cl⁻ and water fluxes suggests that water absorption is linked to the coupled KCl transport. The coupled KCl and water transport were inhibited by treatment with ouabain or with Na⁺-free Ringer solutions, suggesting the existence of a Na⁺-dependent KCl transport system and linkage of water absorption to the coupled Na⁺−K⁺−Cl⁻ transport. Since ouabain blocked the active Na⁺−K⁺−Cl⁻ transport almost completely, the permeability coefficients for K⁺ and Na⁺ through the paracellular shunt pathway were estimated as P_K=0.076 and P_Na=0.058 cm/h, and P_Cl was calculated as 0.005 cm/h. Although Na⁺-independent K⁺ and Cl^tt- fluxes were observed again in the present study, these fluxes were not inhibited by CN⁻, ouabain or diuretics, and evoked even after blocking the Na⁺−K⁺−Cl⁻ transport completely with ouabain. These results indicate that the Na⁺-independent K⁺ and Cl⁻ fluxes are distinct from the active Na⁺−K⁺−Cl⁻ transport and are not themselves active.