Activation by N-ethylmaleimide of a latent K+-Cl- flux in human red blood cells

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Abstract
Of the ouabain-insensitive Na+ and K+ fluxes in human red blood cells, 20-50% are mediated by Cl--dependent coupled transport (cotransport). The effect of the sulfhydryl group reagent N-ethylmaleimide (NEM) on Cl--dependent ouabain-insensitive Na+ and K+ fluxes in human red blood cells is reported. NEM altered Na+-K+ cotransport and activated a latent Cl--dependent K+ transport mode normally apparently silent. At low concentrations (0.25 mM) NEM abolished the bumetanide-sensitive Na+ efflux and had no effect, even at a 10-fold higher concentration, on the bumetanide-sensitive K+ efflux. At concentrations above 0.1 mM, NEM stimulated Cl--dependent K+ efflux that was only partially inhibited by high concentrations of bumetanide or furosemide. In experiments using Rb+ as a K+ analog, NEM activated Rb+ influx by stimulating the maximum velocity and lowering the apparent external cation affinity. The presence of chemically reactive groups in human red blood cells for both Cl--dependent K+ transport activated by NEM and Cl--dependent coupled Na+-K+ movements is suggested.