Evidence for the Presence of an Electrogenic Proton Pump on the Trout Gill Epithelium

Ion transport inhibitors, amiloride, SITS, vanadate and acetazolamide, were added to the water to determine the effect of ion transfer mechanisms on the acidification of water passing over the gills. In neutral water, proton excretion causes a marked reduction in gill water pH. If water pH is 2.5 units lower than blood pH, however, then this proton excretion is inhibited and all water pH changes can be accounted for by CO₂ hydration and ammonia protonation. Proton excretion across the gills is insensitive to 0.1 mmol l⁻¹ amiloride and SITS but sensitive to vanadate, acetazolamide and water pH; thus, we conclude that proton excretion is mediated by an active proton pump on the apical membrane of the gill epithelium similar to that reported for the frog skin. Higher concentrations of amiloride (0.5 and 1 mmol l⁻¹) reduced both ammonia and acid excretion, presumably because of inhibition of Na⁺/K⁺-ATPase on the basolateral border of the gill epithelium.