Effect of K⁺, its Counter Anion, and pH on Sodium Efflux from Barley Root Tips

Sodium efflux from ²²Na⁺-loaded root tips root tips of Hordeum vulgare L. was markedly increased by replacing 10mM Na₂SO₄ in the washing solution by K₂SO₄ with the same electrical conductivity. This increase was inhibited by both an uncoupler and an inhibitor of oxidative phosphorylation but not by ouabain. Potassium ions did not enhance Na⁺ efflux in the presence of a rapidly absorbed counter anion, such as Cl⁻, instead of $SO42-$ . Efflux of ²²Na⁺ could also be enhanced by a low pH in the absence of K⁺; this was prevented by uncouplers, but not by an inhibitor of the mitochondrial ATPase. It seems that K⁺ indirectly enhances Na⁺ efflux. It is suggested that metabolic K⁺ uptake in excess of the counter anion results in a proton gradient across the plasmalemma (acid outside) inducing H⁺/Na⁺ antiport.