Serum stimulation of sodium transport in human fibroblasts containing low and high levels of intracellular sodium.

The relationships between intracellular sodium content, sodium transport and serum effects were investigated in human fibroblasts. In the cells with low intracellular sodium (Na+iL; 0.04 mumol sodium/mg protein), serum stimulated the sodium-potassium pump as measured by ouabain-sensitive sodium efflux and rubidium influx and also exerted a transstimulation of ouabain-insensitive sodium transport resulting in net influx. In cells with high intracellular sodium (Na+iH; 0.42 mumol sodium/mg protein) all aspects of sodium transport were increased compared to Na+iL cells. In these cells serum caused no change in sodium-potassium pump activity but significantly increased the ouabain-insensitive sodium fluxes resulting in net efflux. In Na+iL cells, serum promoted net sodium influx through an amiloride-sensitive pathway that was undetectable in the basal state. In Na+iH cells the serum-stimulated net efflux was amiloride sensitive but this pathway also contributed to a major portion of sodium transport in the basal state. This study demonstrated that sodium-potassium pump activity is directed by the supply of internal sodium and that serum can increase this supply by promoting net influx, and that serum-induced sodium transport can be modified by intracellular sodium content.