Effect of mucosal H+ and chemical modification on transcellular K+ current in frog skin

Abstract A transcellular K + current ( I K ) was established across the skin of the frog Rana temporaria , whose apical K + permeability had been previously stimulated by exposure to K + -rich media. Short-term (≤ 15 s) mucosal pH-titration of I K indicated two titrated groups (A and B), with apparent p K A of 6 and p K B of 3. The height of the titration steps, A and B, varied from skin to skin. Intracellular (i) H + -sensitive microelectrode studies on Rana esculenta skin (which lacks apical P K ) were conducted in order to assess possible changes in pH i and basolateral K + conductance as a consequence of the rise in mucosal [H + ]. Cell pH decreased only at mucosal pH lower than 5.4 which caused a drop in basolateral K + conductance as estimated from I - V records of the serosal membranes. These effects were much too slow to account for the fast mucosal pH effects on I K ( Rana temporaria ). Thus, we conclude that the two-step titration curves reflect solely the interaction of external H + with the mucosal side of apical membrane K + channels. Exposure to the SH-reagent PCMB, and to the carboxy-modifying EEDQ markedly reduced total I K at neutral pH; however, PCMB seemed to preferentially affect titration step B while EEDQ virtually eliminated step A. When the saturating I K kinetics were studied at different mucosal pH, protons showed a ‘mixed’ type inhibition of K + current in the range of titration step A; at pH values less than 5, protons blocked I K by competition with K + ions. These results are compatible with the presence of two K + channel populations in the apical membrane which are discernible by their different interactions with external protons and chemical modifiers.