K+-Na+Exchange and Selectivity in Barley Root Cells: Effect of Na+ on the Na+ Fluxes

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Abstract
Using the compartmental analysis the unidirectional Na+ fluxes in cortical cells of barley roots, the cytoplasmic and vacuolar Na+ contents Qc and Qv, and the trans-root Na+ transport R′ have been studied as a function of the external Na+ concentration. Using the re-elution technique the effect of low K+ concentrations on the plasmalemma efflux πco of Na+ (K+-Na+ exchange) and on R' was investigated at different Na+ concentrations and correspondingly different values of the cytoplasmic sodium content Qc. The relation of the K+-dependent Na+ efflux coK+-dep to Qc or to the cytoplasmic Na+ concentration obeyed Michaelis-Menten kinetics. This is consistent with a linkage of πco, K+-dep to K+ influx by a K+-Na+ exchange system. The apparent Km corresponded to a cytoplasmic Na+ concentration of 28 mM at 0·2 mM K+ and about 0·2 mM Na+ in the external solution. 0·2 mM K+ stimulated the plasma-lemma efflux of Na+ and inhibited Na+ transport selectively even in the presence of 10 mM Na+ in the external medium showing the high efficiency of the K+-Na+ exchange system. However, πco, K+-dep was inhibited at 10 mM Na1 compared to lower Na1 concentrations suggesting some competition of Na1 with K1 at the external site of the exchange system. The effect of the Na+ concentration on Na1 influx πoc is discussed with respect to kinetic models of uuptake.