H⁺ Extrusion and Potassium Uptake Associated with Potential Hyperpolarization in Maize and Wheat Root Segments Treated with Permeant Weak Acids

Abstract

ABSTRACFThe rapid uptake of weak acids permnt in the unchaged form is accompanied in maize and wheat root segments by a hyperpolariztion of the transmembrane electrial potential and an increase in K uptake, suggesting a stimulation of the plasmalemma H' pump. The evalation of weak acid-induced H' extrsion must take into account the alkalinization of the medium due to the rapid uptake of the uncagd form of the acid, partially masking the proton pump-mediated extrusion of H+. The data corrected for this interference show that the lipophilic butyric acid and trimethyl acetic acid induce in maize and in wheat root segts a significant increase in 're H+ extrusion, roughly matching the ise in net K+ uptake. The presence of KV significantly incrases the rte of uptake of the weak acid, possibly as a consequence of an alkinitio of the cytosol associated with K absorption. In maize root seents, the effects of fusicoccin and those of butyric acid on both K+ uptake and H+ extrusion are clearly synergistic, thus suggesting distinct modes of action.These results support the view that the activity of the plasmalemma H+ pump is regulated by the value of cytosolic pH.We have previously reported that the treatment with permeant weak acids (such as butyric, isobutyric, trimethylacetic acids, and DMO2) induces in maize root segments a marked hyperpolarization of the PD (12, 14, 15; see also, for similar results in other materials, 1, 3, 6, 16, and 17). The finding that PD hyperpolarization is associated with an increase in the rate of K+ uptake and is correlated with the rate of penetration of the weak acid and with a decrease in cell sap pH suggested that, in these experiments, the electrogenic proton pump at the plasmalemma is stimulated by the acidification of the cytoplasm consequent to the penetration and the dissociation of the weak acid in the cytosol (compare Sanders et al. [16] for the same conclusion from experiments in Neurospora).If this interpretation is correct, the hyperpolarization associated with weak acid uptake should also be associated with an increase in proton secretion, which had not been demonstrated ' in the above-mentioned investigations. On the other hand, the demonstration of weak acid-induced changes in the rate of H+ extrusion presents some experimental difficulties due in part to the strong buffering action of the weak acids, and also, even more seriously, to the fact that the rapid uptake ofthe uncharged form of the weak acid by the tissue results in an alkalinization of the medium, which may mask the acidification due to the secretion of H+. Thus, for a correct evaluation of the 'true' HI movement across the plasmalemma, the titration values must be corrected for the changes in weak acid concentration.Starting from these premises, in the present work we investigated the changes in 'real' H+ extrusion induced in root segments by treatment with permeant weak acids. The proton extrusion values thus obtained were then compared with the results of experiments in which we invesfigated the effects of wea...

Electrical Evidence for Turgor Inhibition of Proton Extrusion in Sugar Beet Taproot

“…A transient overshoot was observed for both Em and pH upon acetate withdrawal, but an explanation is uncertain. These parallel responses to acetate by Em and pH (though observed in different tissues) support the common opinion that weak acids enhance an electrogenic proton extrusion (7,9,13,16). …”

“…Hyperpolarizations induced by weak acids are commonly interpreted to reflect enhanced proton extrusion caused by acidification of the cell interior (7,9,13,16). Additional support for this interpretation is provided by previously unreported data from oat coleoptiles.…”

“…The means and standard errors are -80 ± 2 mV at 0 M mannitol, -92 ± 5 at 0.2 M, -129 ± 4 at 0.4 M, -10 ± 9 at 0.6 M, and -136 ± 17 at 0. 8 (8,13). Furthermore, application of 10 mm acetic acid in IX (adjusted to pH 5.0) caused extensive hyperpolarizations (Fig.…”

H+-Pumping Driven by the Plasma Membrane ATPase in Membrane Vesicles from Radish: Stimulation by Fusicoccin

“…Since it has been shown that electrogenic H+ extrusion in vivo (6,15,28) well as the plasma membrane H+-ATPase in vitro (10,14,20,29) are strongly stimulated by small decreases in pH in the range of cytoplasmic pH values, these data have been interpreted as indicating that FC-induced stimulation ofelectrogenic H' extrusion is a consequence of the acidification of the cytoplasm (5,7,15).…”

“…Moreover, Romani et al (28) have shown that the effects of FC on H' extrusion, K+ uptake, and hyperpolarization of A* are clearly synergistic with those of weak acids which decrease the cytoplasmic pH, thus strongly suggesting that the two agents influence the H+-pump through different mechanisms. While the FC-induced change of the cytoplasmic pH remains open to investigation, it seems very unlikely that it represents the primary event leading to the stimulation of H+ extrusion.…”