Lack of correlation between [³H]ouabain binding and Na-K ATPase inhibition in rat aorta

The binding of [3H]ouabain to intact strips of rat aorta was compared with the ability of ouabain to inhibit the uptake of 86Rb by the same preparation. When a cold temperature wash was used to process tissues after binding of [3H]ouabain, a class of relatively high affinity binding sites was found (KD = 1.2 × 10-7 M). Binding was saturable and sensitive to both ATP depletion and elevated potassium. Elevation of cytoplasmic Ca2+ levels by phenylephrine or c-AMP levels by theophylline and terbutaline had no influence on [3H]ouabain binding. Ouabain inhibition of 86Rb uptake progressed to 60% of the total 86Rb uptake at 2 × 10-3 M from a threshold of about 10-5 M. Half-maximal inhibition by ouabain occurred at a concentration of 10-4 M. The disparity between [3H]ouabain binding and inhibition of 86Rb uptake indicates that the high affinity binding site in the rat does not contribute to inhibition of Na-K ATPase function.