Passive sodium movements across the opercular epithelium: The paracellular shunt pathway and ionic conductance

Abstract

The unidirectional Na⁺, Cl⁻, and urea fluxes across isolated opercular epithelia from seawater-adaptedFundulus heteroclitus were measured under different experimental conditions. The mean Na⁺, Cl⁻, and urea permeabilities were 9.30×10⁻⁶ cm·sec⁻¹, 1.24×10⁻⁶ cm·sec⁻¹, and 5.05×10⁻⁷ cm·sec⁻¹, respectively. The responses of the unidirectional Na⁺ fluxes and the Cl⁻ influx (mucosa to serosa) to voltage clamping were characteristic of passively moving ions traversing only one rate-limiting barrier. The Na⁺ conductance varied linearly with, and comprised a mean 54% of, the total tissue ionic conductance. The Cl⁻ influx and the urea fluxes were independent of the tissue conductance. Triaminopyrimidine (TAP) reduced the Na⁺ fluxes and tissue conductance over 70%, while having no effect on the Cl⁻ influx or urea fluxes. Mucosal Na⁺ substitution reduced the Na⁺ permeability 60% and the tissue conductance 76%, but had no effect on the Cl⁻ influx or the urea fluxes. Both the Na⁺ and Cl⁻ influxes were unaffected by respective serosal substitutions, indicating the lack of any Na⁺/Na⁺ and Cl⁻/Cl⁻ exchange diffusion. The results suggest that the unidirectional Na⁺ fluxes are simple passive fluxes proceeding extracellularly (i.e., movement through a cation-selective paracellular shunt). This pathway is dependent on mucosal (external) Na⁺, independent of serosal (internal) Na⁺, and may be distinct from the transepithelial Cl⁻ and urea pathways.