EFFECT OF SERUM ON SODIUM PUMP ACTIVITY AT VARIOUS CONCENTRATIONS OF INTERNAL AND EXTERNAL SODIUM IN MOUSE (3T3) AND HUMAN (HeLa) CELLS

¹ Department of Physiology and Pharmacology, Bute Medical Buildings, St Andrews, Fife KY16 9TS

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 K_m for [Na]_i is around 130 mM and the V_max 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.