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

The unidirectional Na+, Cl-, and urea fluxes across isolated opercular epithelia from seawater-adapted Fundulus heteroclitus were measured under different experimental conditions. The mean Na+, Cl0, and urea permeabilities were 9.30 x 10(-6) cm . sec-1, 1.24 x 10(-6) cm . sec-1, and 5.05 x 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 and 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 of 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 extracelluarly (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.U