Induction of system A amino acid transport through long‐term treatment with ouabain: Correlation with increased (Na+/K+)‐ATpase activity

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Abstract
Mouse embryo fibroblast cells (C3H-10T½) and the methylcholanthrenetransformed derivative (MCA-10T½) were treated with basal modified Eagle's medium (BME) containing 10% fetal bovine serum and varying concentrations of ouabain ranging from 0.05 mM to 0.7 mM for 16 h in culture. After replacing the ouabain-containing medium with Earl's balanced salts solution, System A amino acid transport activity increased from approximately 40 to 500 pmol AIB accumulated · mg protein−1 · min−1 in the C3H-10T½ cells and from approximately 300 to 700 pmol AIB accumulated · mg protein−1 · min−1 in the MCA-10T½ cells. The (Na+/K+)-ATPase pump activity also increased form approximately 12 to 46 nmol Rb+ accumulated · mg protein−1 · min−1 in the normal cells and from approximately 20 to 42 nmol Rb+ accumulated · mg protein−1 · min−1 in the transformed cells. System A and the (Na+/K+) ATPase activity were maximally increased at approximately0.4–0.6 mM ouabain in the normal cells in contrast to the transformed cells which were maximally stimulated at a concentration of approximately 0.2 mM ouabain. This treatment with ouabain increased the [Na+]i/[K+]i as measured by atomic absorption spectroscopy, and thereby decreased the Na+ and K+ electrochemical gradients. Our data show that the internal ion gradients inverted at a lower concentration of ouabain in the transformed cells compared to the normal cells. The ouabain-induced increased in pump and System A activity shown here was used as a tool to further investigate the coordinated ion transport regulation in the control of cell growth.