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

Branchial sodium uptake, and net proton equivalent and ammonia excretion, t ogether with isolated filament potentials were measured in euryhaline Europ ean flounder Platichthys flesus, adapted to freshwater. External benzamil ( 0.05 mmol l(-1)), a specific inhibitor of sodium conductive channels, cause d significant reductions (P < 0.05) in sodium uptake and a marked decline i n net proton equivalent excretion. In contrast, 0.05 mmol l(-1) external 5- (N,N-dimethyl)-amiloride, a specific inhibitor of the Na+/H+ antiport, had no effect on any of the fluxes measured. The effect of vanadate, an inhibit or of H+-ATPases, was examined at two concentrations, 0.1 and 0.5 mmol l(-1 ), and both caused significant reductions (P < 0.05) in sodium uptake and n et proton equivalent excretion. Mean resting isolated gill filament potenti als in freshwater-adapted P. flesus were found to be -23.8 +/- 1.1 mV (insi de negative) and became less negative in the presence of amiloride, an inhi bitor of sodium transport. Using microelectrodes it was possible to measure two potentials in the filament: an apical potential and a transepithelial potential. The transepithelial potential was similar to that measured in vi vo, while the apical potential, may be generated by the pavement epithelial cells. In this study, it appears that sodium uptake and net proton equival ent excretion are not strictly coupled but are indirectly linked probably b y 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 .