The sodium and chloride dependence of chloride secretion by the opercular epithelium

The effects of ion substitutions on the Cl− secretion rate and tissue conductance of isolated short‐circuited opercular epithelia from sea‐water‐adapted Fundulus heteroclitus were investigated. Serosal Na+ substitution had the same effect on the Cl− secretion rate that serosal Cl− substitution had on the active component of the Cl− efflux. This similarity indicated a 1:1 Na‐Cl requirement for active Cl− secretion across this epithelium, which supports the proposal of a coupled NaCl uptake mechanism at the serosal membrane of Cl− secretory epithelia. Mucosal Na+ and Cl− substitutions appeared to inhibit completely the active Cl− secretory flux. The reductions in the tissue conductance with mucosal ion substitutions suggested that this effect can be attributed to a blocking of the apical membrane Cl− conductance. These mucosal ion effects suggested a possible direct regulatory influence of the external salinity on the Cl− secretion rate and tissue conductance, which provide alternative explanations for observations with the teleost gill epithelium.