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

A transcellular K+ current (IK) 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 (less than or equal to 15 s) mucosal pH-titration of IK indicated two titrated groups (A and B), with apparent pKA of 6 and pKB 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 PK) were conducted in order to assess possible changes in pHi 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 IK (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 IK at neutral pH; however, PCMB seemed to preferentially affect titration step B while EEDQ virtually eliminated step A. When the saturating IK 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 IK 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.