The correlation between ouabain binding and potassium pump inhibition in human and sheep erythrocytes.

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Abstract
[3H]Ouabain binding to human and sheep red blood cells was specific for receptors associated with Na/K transport. Virtually all tritium binding was abolished by dilution with unlabeled drug. Saturation levels of binding were independent of glycoside concentration and were identical to those associated with 100% inhibition of K pumping. [3H]Ouabain binding and 42K influx were measured simultaneously to correlate the degree of K pump inhibition with the amount of glycoside bound. Results by this method agreed exactly with those obtained by pre-exposing cells to drug, followed by washing and then measuring K influx. Plots of [3H]ouabain binding vs. K pump inhibition were rectilinear for human and low K (LK) sheep red cells, indicating 1 glycoside receptor per K pump site and functional homogeneity of pump sites. High K (HK) sheep red cells exhibited curved plots of binding vs. inhibition, which were best explained in terms of 1 receptor per pump, but a heterogeneous population of pump sites. External K reduced the rate of glycoside binding, but did not alter the relationship between binding and inhibition. The number of K pump sites was estimated at 450-500/human cell and 30-50/LK sheep cells. HK sheep cells had 90-130 sites/cell, of which 80-90 were functionally dominant. The number of K pump sites on LK sheep cells was not changed by anti-L, although the maximum velocity of pump turnover was increased.