EFFECT OF K+, ITS COUNTER ANION, AND PH ON SODIUM EFFLUX FROM BARLEY ROOT-TIPS

Sodium efflux from 22Na+-loaded root tips of Hordeum vulgare L. was markedly increased by replacing 10 mM 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 .**GRAPHIC**. 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.