Abstract
Serum causes a large rise in the ouabain-sensitive K influx in 3T3 and other cells. It has been argued that this is secondary to the increased Na and K leaks which are also caused by serum. We have measured the K influx into 3T3 and HeLa cells at various values of intracellular Na concentration ([Na]i), and from Na-free medium in 1 and 20% (v/v) serum. Raising [Na]i increases the K influx, but saturation does not occur over the range of [Na]i studied. The estimated Km for [Na]i is around 130 mM and the Vmax some 10 times that of the resting K influx at normal (20 mM) [Na]i. Serum stimulates the K influx by a constant proportion at all values of [Na]i studied, and in Na-free medium. These results are inconsistent with previous explanations of the serum stimulation of pumping.
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