Ouabain inhibition of the sodium-potassium pump: estimation of ED50 in different types of human leucocytes in vitro.

1. We examined the effect of ouabain on the Na(+)-K+ pump of intact mononuclear leucocytes and polymorphonuclear leucocytes by using the radioactive potassium analogue, 86rubidium, as a tracer and measuring the cellular uptake of K+ (86Rb+). Na(+)-K+ pump activity was determined as the cellular uptake of K+ (86Rb+) that is sensitive to ouabain, 10(-5) mol l-1. 2. Dose-response curves for inhibition of the Na(+)-K+ pump were obtained after exposure of the cells to various concentrations of ouabain for 2.5 h when the level of pump inhibition was considered to be at steady state. 3. The ED50 of ouabain for the effect on the Na(+)-K+ pump was estimated to be 3 X 10(-9) mol l-1 in the absence of potassium. In the presence of potassium, 3 and 6.5 X 10(-3) mol l-1, it was increased by factors of 10 and 45 respectively. In the presence of potassium, 4 X 10(-3) mol l-1, the ED50 was similar but somewhat higher when determined in an artificial medium (Ringer solution) than when determined in autologous plasma. 4. The ED50 values observed in the present study were very similar to KD values reported in the literature on ligand binding of tritiated ouabain to the same types of human leucocytes.