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

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Abstract
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.