Iron and magnesium exchange via the low affinity iron transporter in rabbit erythroid cells—exchange rates and the action of valinomycin, diethylstilbestrol and protein kinase inhibitors

Abstract

Evidence was presented previously that rabbit erythroid cells possess a low-affinity Fe2+ transport system which operates via the Na+/Mg2+ antiport [Biochim. Biophys. Acta 1282 (1996) 163]. This was investigated further by measurements of Mg2+ efflux as well as Fe2+ uptake by the cells and by examining the inhibitory effects of valinomycin, diethylstilbestrol (DES) and protein kinase inhibitors. Mg2+ efflux and Fe2+ uptake were measured using rabbit reticulocytes and mature erythrocytes incubated in isotonic KCl or NaCl solutions. Both processes were slower in mature cells than reticulocytes. Mg2+ efflux into KCl solution was much lower than into NaCl solution but was stimulated by addition of Fe2+ to the solution. The rate of Fe2+-stimulated Mg2+ efflux closely followed that of Fe2+ uptake in a one-to-one molar ratio. Valinomycin, DES and the protein kinase inhibitors all inhibited Fe2+ uptake from KCl solution. Valinomycin also inhibited Fe2+-stimulated Mg2+ efflux into KCl solution but markedly stimulated the efflux into NaCl. Maximal inhibition of Fe2+ uptake from KCl solution required the presence of K+, Rb+ or Cs+ ions with which valinomycin forms strong complexes. The results could not be explained on the basis of changes in cell membrane potential or cell volume. By contrast, the increase in Mg2+ efflux into NaCl solution produced by valinomycin was accompanied by cell shrinkage and production of a more negative membrane potential, either of which may be responsible for the effect. The inhibition produced by the protein kinase inhibitors indicate that phosphorylation of the transporter or an associated protein by protein tyrosine kinase is probably required to activate the transporter.