Decreased ouabain binding in cystic fibrosis fibroblasts in potassium-free medium.

We have previously demonstrated that cystic fibrosis (CF) cells show increased survival 
compared to normal cells when exposed to ouabain in medium lacking 
potassium. In this report, we show that the CF cells bind 
significantly less ouabain than the normal cells in potassium-deficient medium. Using 
age-matched normal and CF skin fibroblast strains, we show that (1) 
at ouabain concentrations < 20x10-9 M binding for both normal and 
CF cells is linear with time for 1 h and reaches 
equilibrium after 4 h; (2) at ouabain concentrations between 2 and 
20x10-9 M, the initial rate of binding for CF cells is 
approx. 70% of the normal cells; (3) under equilibrium-binding conditions, Scatchard 
analysis reveals that three different CF strains have 12, 16, and 
44% fewer ouabain-binding sites than their matched normal controls. In addition, 
studies in potassium-free medium of the inhibition of 86Rb flux (a 
K-plus analogue) into the cell by ouabain show no differences between 
CF and normal cells. We have also previously shown that in 
a low glucose, potassium-deficient medium the CF cells survived ouabain exposure 
no better than normal cells. In this report, equilibrium-binding studies with 
[3H]ouabain clearly show that CF cells bind less ouabain under these 
conditions than normal cells. These results indicate that ouabain resistance in 
CF cells is not solely a function of differences in ouabain 
binding. Furthermore, the differential ouabain killing may not be due to 
ion transport differences, but rather to as yet unknown mechanisms. CF 
cells thus appear to be unlike previously characterized ouabain-resistant mutants. 