Stimulation of bumetanide‐sensitive K+ transport in Swiss 3T3 fibroblasts by serum and mitogenic hormones

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Abstract
Rapidly growing Swiss 3T3 fibroblasts possess a bumetanide‐sensitive K+ transport system that is dependent on both Na+ and Cl− ions; a smaller bumetanide‐insensitive component of K+ transport is also present. In cells brought to the quiescent state by 8–11 days of incubation without a medium change, the bumetanide‐sensitive rate of transport was reduced by 63%; the bumetanide‐insensitive rate did not change. Removal of dialyzed fetal calf serum from the uptake medium resulted in a substantial reduction in bumetanide‐sensitive uptake in both rapidly growing cells (33% reduction) and quiescent cells (68% reduction) but had no effect on bumetanide‐insensitive uptake. Insulin was almost as effective as dialyzed fetal calf serum in stimulating bumetanide‐sensitive uptake; insulin was maximally stimulatory at 2.5 μg/ml. The combination of insulin, epidermal growth factor, and arginine‐vasopressin was maximally effective in stimulating both bumetanide‐sensitive K+ uptake and 3H‐thymidine incorporation in quiescent cells; bumetanide, however, did not interfere with the hormonal stimulation of DNA synthesis. Thus, the bumetanide‐sensitive K+ transport system is not necessary for such stimulation to occur. Furthermore, concentrations of hormones which stimulated significant levels of DNA synthesis produced no elevation in the intracellular concentration of K+. We conclude that the bumetanide‐sensitive pathway of K+ transport is modulated by serum and by mitogenic hormones, but does not play a role in the stimulation of DNA synthesis by these factors.

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