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

Abstract. 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.