Ionic transport by the opercular epithelia of freshwater acclimated tilapia ( Oreochromis niloticus) and killifish ( Fundulus heteroclitus)

Abstract

Opercular epithelia from freshwater (FW)-acclimated Oreochromis niloticus and Fundulus heteroclitus were mounted in vitro with natural FW bathing the mucosal surface and saline bathing the serosal surface. Opercular epithelia from Oreochromis, a true FW fish, exhibited an inside-positive transepithelial potential ( V t) of +8 mV and non-diffusive uptake of both Cl − and Na +, as well as Ca 2+, from the mucosal FW media. When bathed with saline on the mucosal surface, active Na + and Cl − uptake did not occur. To test whether exposure of the mucosal surface to saline during the dissection procedure was a problem, FW osmotically compensated with mannitol was used instead as the dissection medium. However ion transport rates were not altered. Opercular epithelia from FW-acclimated Fundulus, which is more normally an estuarine rather than a FW fish, exhibited an inside-negative V t of −44 mV, 4–10-fold larger absolute flux rates of Na + and Cl −, and a 40-fold greater density of mitochondrial-rich cells than the Oreochromis preparation. However, only non-diffusive uptake of Cl − from the mucosal FW occurred, while Na + moved passively. Opercular epithelia from Fundulus acclimated to 10% seawater (SW) actively extruded Cl − when bathed with either 10% SW or saline on the mucosal side; Na + moved passively under both experimental conditions. The FW Oreochromis opercular skin is the only teleost flat epithelial preparation known to actively take up all three ions from FW in vitro and may be a good model system for studying the mechanisms of Cl −, Na + and Ca 2+ transport in FW fish gills.