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.
Wood
Chris M
CM
Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, FL 33149, USA. woodcm@mcmaster.ca
Grosell
Martin
M
D000064:Acclimatization; D000818:Animals; D024541:Batrachoidiformes / Q000502:physiology; D002118:Calcium / Q000378:metabolism; D002404:Catheterization / Q000379:methods; D002794:Choline / Q000378:metabolism; D004058:Diffusion; D004847:Epithelial Cells / Q000502:physiology*; D005618:Fresh Water; D023781:Fundulidae / Q000502:physiology*; D005880:Gills / Q000502:physiology*; D005971:Glutamates / Q000378:metabolism; D008274:Magnesium / Q000378:metabolism; D008564:Membrane Potentials; D009994:Osmolar Concentration; D010539:Permeability; D011188:Potassium / Q000378:metabolism; D054712:Salinity; D012623:Seawater; D012965:Sodium Chloride / Q000378:metabolism*; D013997:Time Factors
D005971:Glutamates; C476559:N-methylglutamate; D012965:Sodium Chloride; D008274:Magnesium; D002794:Choline; D011188:Potassium; D002118:Calcium