Transepithelial potential in the Magadi tilapia, a fish living

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