Intracellular Univalent Cations and the Regulation of the BALB/c-3T3 Cell Cycle

Addition of serum to density-arrested BALB/c-3T3 cells causes a rapid increase in uptake of Na+and K+, followed 12 h later by the onset of DNA synthesis. We explored the role of intracellular univalent cation concentrations in the regulation of BALB/c-3T3 cell growth by serum growth factors. As cells grew to confluence, intracellular Na+and K+concentrations ({Na+}iand {K+}i) fell from 40 and 180 to 15 and 90 mmol/liter, respectively. Stimulation of growth of density-inhibited cells by the addition of serum growth factors increased {Na}iby 30% and {K+}iby 13-25% in early G0/G1, resulting in an increase in total univalent cation concentration. Addition of ouabain to stimulated cells resulted in a concentration-dependent steady decrease in {K+}iand increase in {Na+}i. Ouabain (100 μM) decreased {K+}ito ∼60 mmol/liter by 12 h, and also prevented the serum-stimulated increase in86Rb+uptake. However, 100 μM ouabain did not inhibit DNA synthesis. A time-course experiment was done to determine the effect of 100 μM ouabain on {K+}ithroughout G0/G1and S phase. The addition of serum growth factors to density-inhibited cells stimulated equal rates of entry into the S phase in the presence or absence of 100 μM ouabain. However, in the presence of ouabain, there was a decrease in {K+}i. Therefore, an increase in {K+}iis not required for entry into S phase; serum growth factors do not regulate cell growth by altering {K+}i. The significance of increased total univalent cation concentration is discussed.