Receptor binding sites for beta-adrenergic ligands on human erythrocytes

Affinity, specificity and kinetics for [3H]-DHA binding to human red cell ghost were determined by ultra-filtration. At 2 degree an apparent dissociation constant of 0.96 nM was found with maximum specific binding of 29 fmoles per mg protein. The low dissociation constant was confirmed by kinetic studies with a value of 0.86 nM. Propranolol and isoproterenol inhibited [3H]-DHA binding stereo specifically. Agonist potency (IPR greater than EPI greater than NE) indicated that human erythrocytes had an adrenergic receptor of beta-2 subtype. Isoproterenol in the presence of theophylline resulted in a concentration-dependent increase of intracellular cAMP levels in intact cells. Basal and maximal levels were 2.3 and 7.5 pmoles/108 cells respectively after 2.5 min stimulation. EC50 for isoproterenol was 0.27 microM. Propranolol shifted the isoproterenol concentration response curve to the right. The present results show that human erythrocytes possess recognition sites for beta-adrenergic ligands with binding characteristics similar to that of adrenergic receptors of beta-2 subtype. At least a small number of these binding sites are functionally coupled to adenylate cyclase.