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

Treatment of apical maize root segments with weak acids (butyrate, isobutyrate, trimethylacetate, 5,5-dimethyloxazolidine-2,4-dione) at 1 mol/m induced a rapid marked hyperpolarization (c. 30 mV) of the transmembrane electrical potential, stable for at least 30 min. With butyrate, this effect increased with increasing butyrate conc. in the medium, reaching a value of c. 75 mV at 5 mol/m. 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 + conc. in the medium was raised from 1 to 10 mol/m. The hyperpolarization was accompanied by a significant increase in K + uptake and was almost completely suppressed by the presence of the protonophore carbonylcyanid- p-trichlorometoxy-phenylhydrazone and strongly reduced by erytrosin B, an inhibitor of some animal ATPases and of a K +-activated, DCCD- and vanadate-sensitive Mg 2+-ATPase from plant microsomes. The hyperpolarization effect of butyrate was additive to that of fusicoccin at low, but not at high (5 mol/m), conc. of the weak acid. It is suggested that the intracellular pH regulates the activity of the electrogenic proton pump at the plasmalemma.