The Influence of Mucus on the Diffusion of Ions across the Esophagus of Fish

Abstract

Microelectrodes sensitive to Na⁺, K⁺, and Ca²⁺ have been used to measure the gradients of ion activity existing within the mucous layer on an isolated preparation of the esophagus of Rhombosolea retiaria. These measurements demonstrate that there exists above the mucosal surface a variable unstirred layer. This layer varies in thickness from 50μm to 1,200 μm, as revealed by the response of the ion-sensitive electrodes upon entering the layer. The layer supports a gradient of Na⁺, K⁺, and Ca²⁺ activity with either freshwater or seawater as the mucosal solution. This gradient may be as little as 1% or as great as 51% of the overall activity gradient between mucosal and serosal solutions. The gradients for all three cations are normally linear. It is likely that mucus supports this unstirred layer on the mucosal surface; ion movement through the layer is by diffusion only and results in the establishment of stable, linear, diffusion gradients of ions. In the conditions of these experiments these gradients can reach considerable proportions of the overall gradients. An attempt is made to interpret these findings with respect to the in vivo conditions of, and functions of, the mucous layer.