Transmembrane hyperpolarization and increase of K+ uptake in maize roots treated with permeant weak acids

Treatment with weak acids (butyrate, isobutyrate, trimethylacetate, DMO) at a concentration of I mol m−3 in apical maize root segments induced a rapid, marked hyperpolarization (ca. 30 mV) of the transmembrane electrical potential, stable for at least 30 min. With butyrate, this effect increased with the increase of butyrate concentration in the medium, reaching a value of ca. 75 mV at a concentration of 5 mol m−3. Both the butyrate uptake and the hyperpolarization were roughly proportional to the pH‐regulated, undissociated/dissociated acid ratio in the medium. The butyrate‐induced hyperpolarization was reduced progressively, but was still present when K+ concentration in the medium was raised from 1 to 10 mol m−3. The hyperpolarization was accompanied by a significant increase of K+ uptake, and was almost completely suppressed by the presence of the protonophore carbonylcyanid‐p‐trichlorometoxy‐phenylhydrazone (CCCP) and strongly reduced by erytrosin B, an inhibitor of some animal ATPases and of a K+‐activated, DCCD‐ and vanadate‐sensitive Mg2+‐ATPase from plant microsomes. The hyperpolarization effect of butyrate was additive to that of fusicoccin at low, but not at high (5 mol m−3), concentrations of the weak acid. These results suggest that the intracellular pH regulates the activity of the electrogenic proton pump at the plasmalemma.