Serum stimulates the Na+,K+ pump in quiescent fibroblasts by increasing Na+ entry.

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Abstract
Two ionophores (monesin and gramicidin) that carry Na+ into 3T3 (mouse fibroblasts) cells markedly enhance the rate of 86Rb+ uptake. Ouabain prevents both ionophores from increasing 86Rb+ uptake, indicating that the ionophores activate the Na+, K+ pump. Measurements of 86Rb+ uptake and cell Na+ and K+ over a range of monensin concentrations show that the activity of the Na+, K+ pump in 3T3 cells is limited by the supply of internal Na+ and is extremely sensitive to small changes in internal Na+. Serum rapidly enhances the rate of 22Na+ uptake and net Na+ entry when Na+ exit is inhibited by ouabain. At 0.3 .mu.g/ml, monensin increases the rate of net Na+ entry and activates the Na+, K+ pump by the same degree as serum. The stimulation of 86Rb+ uptake by serum or the ionophores has an absolute requirement for external Na+. Thus, serum appears to stimulate the Na+, K+ pump in quiescent 3T3 cells by increasing its supply of Na+.

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