Transepithelial potential in the Magadi tilapia, a fish living in extreme alkalinity.

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, i.e. HCO and CO) rather than Cl (112 mmol L) represents the major anion matching Na = 356 mmol L, with very low concentrations of Ca and Mg (<1 mmol L). 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 [Ca]. 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 solutions of NaHCO, NaCl, NaNO, or choline Cl, only the 350 mmol L NaHCO 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 NaHCO, the TEPs remained identical to those in comparable dilutions of Magadi water, whereas this did not occur with comparable dilutions of 350 mmol L NaCl-i.e. the fish behaves electrically as if living in an NaHCO 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 driving HCO inwards (−82.7 mV). Estimated permeability ratios are P/ P = 0.51-0.68 and $$ P_{{{\text{HCO}}_{3} }} /P_{\text{Na}} $$ = 0.10-0.33. The low permeability to HCO is unusual, and reflects a unique adaptation to life in extreme alkalinity. Cl is distributed close to Nernst equilibrium in Magadi water, so there is no need for lower P. The higher P likely facilitates Na efflux through the paracellular pathway. The positive electrogenic component is probably due to active HCO excretion. [ABSTRACT FROM AUTHOR]