Stimulation of Na,K-ATPase by low potassium is dependent on transferrin

We took advantage of the fact that confluent MDCK cells can survive in a serum-free medium for several days to examine whether the upregulation of Na,K-ATPase by low K+ required serum. We found that serum was essential for low K+ to induce an increase in the cell surface Na,K-ATPase molecular number as quantified by ouabain binding assays. Further analyses identified that transferrin, not EGF or IGF-1, could simulate the effect of serum. Moreover, transferrin was also required for low-K(+)-induced increases in al-subunit promoter activity, al- and el-subunit protein abundance of the Na,K-ATPase. In the presence of transferrin, low K+ enhanced cellular uptake of iron. Inhibition of intracellular iron activity by deferoxamine (40 microM) abrogated the effect of low K+ on the Na,K-ATPase. Like deferoxamine, catalase (100 U/ml) also ablated the effect of low K+. We conclude that stimulation of the Na,K-ATPase by low K+ is dependent on transferrin. The effect of transferrin is mediated by increased iron transport and reactive oxygen species activity.