Isolated filament potentials and branchial ion fluxes in the European flounder (Platichthys flesus L.). Evidence for proton pump mediated sodium uptake

Abstract

Branchial sodium uptake, and net proton equivalent and ammonia excretion, together with isolated ®lament potentials were measured in euryhaline European¯ounder Platichthys¯esus, adapted to freshwater. External benzamil (0.05 mmol l -1 ), a speci®c inhibitor of sodium conductive channels, caused signi®cant reductions (P < 0.05) in sodium uptake and a marked decline in net proton equivalent excretion. In contrast, 0.05 mmol l -1 external 5-(N,N-dimethyl)-amiloride, a speci®c inhibitor of the Na + /H + antiport, had no effect on any of the¯uxes measured. The effect of vanadate, an inhibitor of H + -ATPases, was examined at two concentrations, 0.1 and 0.5 mmol l -1 , and both caused signi®cant reductions (P < 0.05) in sodium uptake and net proton equivalent excretion. Mean resting isolated gill ®lament potentials in freshwater-adapted P.¯esus were found to be 723.8 1.1 mV (inside negative) and became less negative in the presence of amiloride, an inhibitor of sodium transport. Using microelectrodes it was possible to measure two potentials in the ®lament: an apical potential and a transepithelial potential. The transepithelial potential was similar to that measured in vivo, while the apical potential, may be generated by the pavement epithelial cells. In this study, it appears that sodium uptake and net proton equivalent excretion are not strictly coupled but are indirectly linked probably by a proton pump arrangement, which is thought to be a mechanism for sodium uptake in trout gills. A possible role of the apical potential is discussed.