The Effect of Mucosal and Serosal Solution Cations on Bioelectric Properties of the Isolated Toad Bladder

The spontaneous transtissue potential and the DC conductance of the isolated toad bladder were measured when the tissue was exposed to sulfate Ringer's solutions of modified ionic composition. Na⁺ was replaced to varying extents by (C₂H₅)₃NH⁺, (C₂H₅)₄N⁺, Li⁺, Cs⁺, K⁺, or Rb⁺. Reversible and irreversible changes were observed. The reversible changes were consistent with equations derived from the Nernst-Planck diffusion equation, and gave the following functional description of the bladder: (a) the potential measurements were compatible with two membranes in series; (b) the mucosal surface was more permeable to Na⁺ than to other monovalent cations; (c) the serosal surface was permeable to both K⁺ and Na⁺ but preferentially to K⁺; (d) the rate of Na⁺ diffusion across the mucosal membrane appeared to approach a maximum but two alternative interpretations are discussed; (e) the conductance data were consistent with the assumption of a constant concentration gradient for the penetrating ions within the membrane (Henderson's assumption) provided suitable hypotheses are made concerning the Na⁺ distribution between the membrane surfaces and the bulk phases of the adjacent solutions; (f) the conductance and spontaneous potential data suggested that the mucosal membranes of a small fraction of the epithelial cells were more permeable than the mucosal membranes of the majority of these cells. The irreversible changes were almost entirely associated with cation substitution in the serosal solution. However, Li⁺ produced an irreversible fall in voltage when added to either side of the tissue.