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

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.