A critical analysis of transepithelial potential in intact killifish (Fundulus heteroclitus) subjected to acute and chronic changes in salinity

We investigated the in vivo salinity-dependent behavior of transepithelial potential (TEP) in Fundulus heteroclitus (3-9 g) using indwelling coelomic catheters, a technique which was validated against blood catheter measurements in a larger species (Opsanus beta; 35-70 g). In seawater (SW)-acclimated killifish, TEP was +23 mV (inside positive), but changed to -39 mV immediately after transfer to freshwater (FW). Acute transfer to dilute salinities produced a TEP profile, which rapidly attenuated as salinity increased (0, 2.5, 5 and 10% SW), with cross-over to positive values between 20 and 40% SW, and a linear increase thereafter (60, 80 and 100% SW). TEP response profiles were also recorded after acute transfer to comparable dilutions of 500 mmol L^-1 NaCl, NaNO₃, Na gluconate, choline chloride, N-methyl-d-glutamate (NMDG) chloride, or 1,100 mosmol kg^-1 mannitol. These indicated high non-specific cation permeability and low non-specific anion permeability without influence of osmolality in SW-acclimated killifish. While there was a small electrogenic component in high salinity, a Na⁺ diffusion potential predominated at all salinities due to the low P _Cl/P _Na (0.23) of the gills. The very negative TEP in FW was attenuated in a linear fashion by log elevations in [Ca²⁺] such that P_Cl/P _Na increased to 0.73 at 10 mmol L^-1. SW levels of [K ⁺] or [Mg²⁺] also increased the TEP, but none of these cations alone restored the positive TEP of SW-acclimated killifish. The very negative TEP in FW attenuated over the first 12 h of exposure and by 24-30 h reached +3 mV, representative of long-term FW-acclimated animals; this reflected a progressive increase in P _Cl/P _Na from 0.23 to 1.30, probably associated with closing of the paracellular shunt pathway. Thereafter, the TEP in FW-acclimated killifish was unresponsive to [Ca²⁺] (also to [K⁺], [Mg²⁺], or chloride salts of choline and NMDG), but became more positive at SW levels of [Na⁺]. Killifish live in a variable salinity environment and are incapable of gill Cl^- uptake in FW. We conclude that the adaptive significance of the TEP patterns is that changeover to a very negative TEP in FW will immediately limit Na⁺ loss while not interfering with active Cl^- uptake because there is none. Keeping the shunt permeability high for a few hours means that killifish can return to SW and instantaneously re-activate their NaCl excretion mechanism.