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

TY - JOUR

T1 - A critical analysis of transepithelial potential in intact killifish (Fundulus heteroclitus) subjected to acute and chronic changes in salinity

AU - Wood, Chris M.

AU - Grosell, Martin

PY - 2008/8/1

Y1 - 2008/8/1

N2 - 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 PCl/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.

AB - 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 PCl/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.

KW - Calcium-dependent potential

KW - Diffusion potential

KW - Electrogenic potential

KW - Gill permeability

KW - Opsanus beta

KW - P/P ratio

KW - TEP

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U2 - 10.1007/s00360-008-0260-1

DO - 10.1007/s00360-008-0260-1

M3 - Article

VL - 178

SP - 713

EP - 727

JO - Journal of Comparative Physiology B: Biochemical, Systemic, and Environmental Physiology

JF - Journal of Comparative Physiology B: Biochemical, Systemic, and Environmental Physiology

SN - 0174-1578

IS - 6

ER -