Involvement of Proton Fluxes in the K'-Na' Selectivity at the Plasmalemma; K'-dependent Net Extrusion of Sodium in Barley Roots and the Effect of Anions and pH on Sodium Fluxes

Abstract

Using excised, Na'-loaded barley (Hordeum distichon) roots, fluxes of sodium at the cellular membranes and cytoplasmic and vacuolar sodium contents were studied as a function of accompaying anions and the pH. Na' fluxes and particularly transport of sodium across the roots were substantially decreased (when the anion was varied) in the order NO,>Cl->S04-_ but the vacuolar content and tonoplast fluxes were less affected. In the presence of any anions potassium induced a transient net sodium extrusion across the plasmalemma and selective inhibition of Na' transport, again the effect being smallest in the presence of SO,--. Similarly, all Na' fluxes were reduced when the pH was decreased and K* induced a net Na' extrusion at all pH values. When related to the cytoplasmic Na' content, the highest relative rate of Na' extrusion was found at low pH. Net Na' extrusion across the plasmalemma could be induced also by an increase in [H'] from pH 8 to 4. This H'-dependent Na' extrusion was not transient and was followed by increased vacuolar Na* efflux; it did not result in selective inhibition of trans-root Na' transport. The H*-dependent Na' extrusion was smaller than K+-dependent one and was greatly stimulated when potassium was added simultaneously with acidification from pH 8 to 4, thus pH 4 does not inhibit K'-Na' exchange. The results are consistent with a mediation of K'-Na' selectivity at the plasmalemma by proton fluxes. K' stimulated the Na'' efflux also when added subsequent to an acidification and then brought about a reversal of the proton-induced enhancement of vacuolar Na' efflux. This indicates an essential role of potassium ions in the regulation of cytoplasmic pH.