Transepithelial potential in the Magadi tilapia, a fish living
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Date
2005Author
Wood, Chris M.
Bergman, Harold L.
Bianchini, Adalto
Laurent, Pierre
Maina, John
Johannsson, Ora E.
Bianchini, Lucas F.
Chevalier, Claudine
Kavembe, Geraldine D.
Papah, Michael B.
Ojoo, Rodi O.
Type
ArticleLanguage
enMetadata
Show full item recordAbstract
We investigated the transepithelial potential
(TEP) and its responses to changes in the external medium in
Alcolapia grahami, a small cichlid fish living in Lake
Magadi, Kenya. Magadi water is extremely alkaline
(pH = 9.92) and otherwise unusual: titratable alkalinity
(290 mequiv L-1, i.e. HCO3 - and CO3
2-) rather than Cl-(112 mmol L-1) represents the major anion matching
Na? = 356 mmol L-1, with very low concentrations of
Ca2? and Mg2? (\1 mmol L-1). Immediately after fish
capture, TEP was ?4 mV (inside positive), but stabilized at
?7 mV at 10–30 h post-capture when experiments were
performed in Magadi water. Transfer to 250% Magadi water
increased the TEP to ?9.5 mV, and transfer to fresh water
and deionized water decreased the TEP to-13 and-28 mV,
respectively, effects which were not due to changes in pH or
osmolality. The very negative TEP in deionized water was
attenuated in a linear fashion by log elevations in [Ca2?].
Extreme cold (1 vs. 28 C) reduced the positive TEP in
Magadi water by 60%, suggesting blockade of an electrogenic
component, but did not alter the negative TEP in dilute
solution. When fish were transferred to 350 mmol L-1
solutions of NaHCO3, NaCl, NaNO3, or choline Cl, only the
350 mmol L-1 NaHCO3 solution sustained the TEP
unchanged at ?7 mV; in all others, the TEP fell. Furthermore,
after transfer to 50, 10, and 2% dilutions of 350 mmol
L-1 NaHCO3, the TEPs remained identical to those in
comparable dilutions of Magadi water, whereas this did not
occur with comparable dilutions of 350 mmol L-1 NaCl—
i.e. the fish behaves electrically as if living in an NaHCO3
solution equimolar to Magadi water. We conclude that the
TEP is largely a Na? diffusion potential attenuated by some
permeability to anions. In Magadi water, the net electrochemical
forces driving Na? inwards (?9.9 mV) and Cl-
outwards (?3.4 mV) are small relative to the strong gradient
Citation
Journal of Comparative Physiology B Volume 182 2005Publisher
Faculty of Veterinary Medicine