ABSTRACT We compared intracellular K
+ and Na
+ ion concentrations during cell growth and differentiation of a mouse myeloid leukemia M1 cell line. Cells undergoing mitosis had higher K
+ concentrations than quiescent cells. Treatment with a K
+ channel blocker and furosemide enhanced cell growth and produced a slight increase in the intracellular K
+ concentration. Treatment with reagents that reduced the intracellular K
+ concentation stopped cell growth. Induction of differentiation in this cell line produced a decrease in the K
+ concentration, which always was accompanied by an increase in the Na
+ concentration. Treatment with ouabain, which decreased the intracellular K
+ concentration, did not, however, induce differentiation in the Ml cell line. The data suggest that cell growth and differentiation in the M1 cells are accompanied by changes in the intracellular K
+ and Na
+ concen-trations but that the changes in the contents of these monovalent cations do not necessarily induce differentiation in this cell line.
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