Effects of two plasmalemmaATPase inhibitors on H+ extrusion and intracellular pH in Elodea densa leaves.

lodea leaves in the dark show very little exchange of H+ with the medium in the external pH range between 5.0 and 6.0. The presence of fusicoccin and potassium in the medium markedly stimulates H+ extrusion. Fusicoccin- and K+ -induced H+ extrusion is inhibited by either erythrosin B (EB) or Na-orthovanadate, two inhibitors of H+ transporting plasma membrane ATPase. EB completely inhibits it from the first 30 min of treatment, when supplied at pH 5.5 at a concentration of 30 mmol m-3. Vanadate also inhibits H+ extrusion, this effect becoming evident only after 45 min of treatment. After this time inhibition is complete with 250 mmol m-3 vanadate but only partial for lower concentrations. In the presence of either inhibitor the intracellular pH, measured as cell sap pH, is significantly lowered. When the intracellular pH changes are determined on vacuole and, separately, on cytoplasm by the weak acid and base distribution method, acidification of both compartments is found to accompany the blocking of H+ extrusion by either of the inhibitors.