Effects of ouabain and isoproterenol on potassium influx in the turkey erythrocyte. Quantitative relation to ligand binding and cyclic AMP generation.

Studies have been carried out in the turkey erythrocyte to examine: (1) the influence of external K+ concentration on both [3H]ouabain binding and the sensitivity of potassium influx to inhibition by ouabain and (2) the quantitative relation between beta-adrenergic receptor site occupancy, agonist-directed cyclic AMP generation and potassium influx rate. Both [3H]ouabain binding and the ability of ouabain to inhibit potassium influx are markedly reduced at increasing external K+ concentrations, and at each K+ concentration the concentrations of ouabain required for half-maximal binding to the erythrocyte membrane and for half-maximal inhibition of potassium influx are identical. Both basal and isoproterenol-stimulated potassium influx rise with increasing external K+ concentrations. In contrast to basal potassium influx, which is 50-70% inhibitable by ouabain, the isoproterenol-stimulated component of potassium influx is entirely insensitive to ouabain. At all concentrations of K+, inhibition of basal potassium influx by ouabain is linear with ouabain binding, indicating that the rate of transport per unoccupied ouabain binding site is unaffected by simultaneous occupancy of other sites by ouabain. Similarly, the rate of isoproterenol-stimulated cyclic AMP synthesis is directly proportional to beta-adrenergic receptor occupany over the entire concentration-response relationship for isoproterenol, showing that at all levels of occupancy beta-adrenergic receptor sites function independently of each other. Analysis of the relation of catecholamine-dependent potassium transport to the number of beta-adrenergic receptor sites occupied indicates an extremely sensitive physiological system, in which 50%-maximal stimulation of potassium transport is achieved at less than 3% receptor occupancy, corresponding to fewer than ten occupied receptors per cell.