Differentiation between serum stimulation of ouabain-resistant and sensitive Rb influx in quiescent NIH 3T3 cells

The addition of serum to quiescent NIH 3T3 mouse cell cultures resulted in a 10- to 20-fold increase of Rb influx which was resistant to ouabain, and only a three- to fourfold activation of ouabain-sensitive Rb influx. Stimulation of the ouabain-resistant Rb influx following serum addition reached its maximum within 2 min. The stimulation of ouabain-resistant Rb influx was a result of Vm increase while the Km for Rb was unchanged. Ouabain-resistant Rb influx, after serum addition, was resistant to amiloride and sensitive to ethacrynic acid. Replacing chloride in the medium by NO3-, CO3- and CH3COO- resulted in a drastic decrease in the ouabain-resistant Rb influx. It appeared, therefore, that the ouabain-resistant Rb influx in NIH 3T3 cells was Cl--dependent.