| dc.description.abstract | 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 | en |
